Topics

Q122/222 Jfet J300 subs for 2215A scope ?


charlesterrebonne@...
 

HI All...

Got a Tektronix 2215A. Channel 1 was bad, no signal coming through. Channel 2 fine.

Found Q122 Jfet on channel 1 was a J210 part # and bad... already replaced a while back perhaps? should be orig J300 Jfet there?

After trying to sub Q122 without success with several Jfets on hand, including a NTE 133 that didnt work, decided to move Q222 (channel 2 same Jfet) to channel 1. This was found to be a J300 Jfet (never replaced?) and this made channel 1 work perfect and confirmed only prob on Channel 1 was indeed Q122.

Channel 2 is now without Q222. I tried to sub with several other Jfets I have on hand...k19...NTE132...Best result I had was a 2n3819 that works fine for about 1 minute then signal dissapears slowly. Freezing bring it all back. I suspect not a bad 2N3918, just freezing it make it behave more like a J300...that the circuit wants...

Can anyone confirm a few Jfet #s (still available? or not...I have large inventory of 70s-80s NOS parts...but no Jxxx Jfets) that will work reliably in that circuit? I think pretty much all that series of scopes used the similar input circuit. The Tek part # was 151-1124-00...

I have ordered some pn4392 Jfet but now thinking they will be no good in that circuit too...

Thanks


Ed Breya
 

Many JFET types should work fine there, but the trick is to get the right actual Idss so the DC bias conditions will be normal. Look at the Idss range specs for any candidate, versus the original J300. You can also measure the Idss of your "good" sample, and use that as reference for selecting others.

Which topology is used here? As I recall, some 2200 models may have used a totem-pole JFET follower, while most use an opamp-stabilized one. The opamp one is probably more forgiving of part variations. If this is a totem-pole, then you may have better luck replacing both upper and lower Qs with the same type. Even if they are quite different from the J300, they will be closer to each other in Idss, and should fall in a reasonable bias range.

I don't think the 2N4392 family will work - their Idss ranges are fairly high, since they're more intended for signal switching. I vaguely recall that the 2N5485/5486 family may be about right - I think these were used a lot in totem and differential matched pairs under 151-1041-XX.

Ed


toby@...
 

On 2020-11-08 11:29 p.m., charlesterrebonne@hotmail.com wrote:
HI All...

Got a Tektronix 2215A. Channel 1 was bad, no signal coming through. Channel 2 fine.

Found Q122 Jfet on channel 1 was a J210 part # and bad... already replaced a while back perhaps? should be orig J300 Jfet there?

After trying to sub Q122 without success with several Jfets on hand, including a NTE 133 that didnt work, decided to move Q222 (channel 2 same Jfet) to channel 1. This was found to be a J300 Jfet (never replaced?) and this made channel 1 work perfect and confirmed only prob on Channel 1 was indeed Q122.

Channel 2 is now without Q222. I tried to sub with several other Jfets I have on hand...k19...NTE132...Best result I had was a 2n3819 that works fine for about 1 minute then signal dissapears slowly. Freezing bring it all back. I suspect not a bad 2N3918, just freezing it make it behave more like a J300...that the circuit wants...

Can anyone confirm a few Jfet #s (still available? or not...I have large inventory of 70s-80s NOS parts...but no Jxxx Jfets) that will work reliably in that circuit? I think pretty much all that series of scopes used the similar input circuit. The Tek part # was 151-1124-00...
The Tek cross-reference confirms Idss between 10 and 30mA. Vp max 4.5v.
Gm 6000 µMho.

Linear Systems site indicates J310 equivalent in TO-92, "U/J/SST308 SERIES":
http://www.linearsystems.com/product-search-result.html?type=replacements&partnumber=j310

They're easy to deal with & I'm a happy customer.

--Toby



I have ordered some pn4392 Jfet but now thinking they will be no good in that circuit too...

Thanks





Tom Lee
 

I second the suggestion to use a J310. That's been my standard sub for general-purpose JFET buffers for a long time.

That said, I'm surprised that you didn't find joy with a 3819. The circuit is quite accommodating (necessarily) of a wide range of IDSS. The NTE133 should've worked, too; maybe it was just dead.

It's been some time since I took a close look at the 2215A's buffer, but a quick glance at the schematic says that the upper limit on acceptable IDSS is roughly 30mA. One that is around 10-15mA would be in the center of the zone, but the value is not particularly critical.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/8/2020 20:29, charlesterrebonne@hotmail.com wrote:
HI All...

Got a Tektronix 2215A. Channel 1 was bad, no signal coming through. Channel 2 fine.

Found Q122 Jfet on channel 1 was a J210 part # and bad... already replaced a while back perhaps? should be orig J300 Jfet there?

After trying to sub Q122 without success with several Jfets on hand, including a NTE 133 that didnt work, decided to move Q222 (channel 2 same Jfet) to channel 1. This was found to be a J300 Jfet (never replaced?) and this made channel 1 work perfect and confirmed only prob on Channel 1 was indeed Q122.

Channel 2 is now without Q222. I tried to sub with several other Jfets I have on hand...k19...NTE132...Best result I had was a 2n3819 that works fine for about 1 minute then signal dissapears slowly. Freezing bring it all back. I suspect not a bad 2N3918, just freezing it make it behave more like a J300...that the circuit wants...

Can anyone confirm a few Jfet #s (still available? or not...I have large inventory of 70s-80s NOS parts...but no Jxxx Jfets) that will work reliably in that circuit? I think pretty much all that series of scopes used the similar input circuit. The Tek part # was 151-1124-00...

I have ordered some pn4392 Jfet but now thinking they will be no good in that circuit too...

Thanks




Göran Krusell
 

Gentlemen,
I have followed this discussion with great interest. However, I have always had difficulty understanding how to design for optimum bias for a FET source follower. In this application, the input of Tek2215A, you want this buffer to have infinite input impedance, small input capacitance, a gain close to one, and finally low output impedance. You will have a low gm, typical for FETs, which will give you a fairly high output impedance typically 300ohm which motivates the need for an additional transistor emitter follower which will reduce the output impedance further to drive the following circuitry.

With this background I get the impression that the choice of FET is not very critical. The only parameter that you can design for is now the current Id. In this design the Id is set by R126, 200ohm, to 10mA. This means that Idss shall be 15mA or higher?

The purpose of feedback through opamp U120 is to keep Q133 emitter at +0.4V, then the circuit is balanced. This is the temperature drift compensation. Ok so far, but how does the choice of FET current Id make any difference at all?

Back to the original issue I think you have a bad FET, a bad U120 or a bad solder joint.

Göran


Ed Breya
 

I don't know about the 2215A, but the other 2200s use an opamp. The opamp (JFET input type) provides the DC/LF path.It's overall DC-balanced by the feedback, so the JFET bias is taken care of. It has to be fast enough for the bandwidth required, and the DC conditions are not too critical, as long as the loop can keep it all balanced.

Ed


Tom Lee
 

Yup, the 2215A uses an op-amp bias loop to get away from the older arrangement that required a JFET matched pair. The ~30mA max IDSS figure I quoted is from looking at the schematic. The current source transistor goes into saturation at that point. For some common-mode range, you want the JFET's IDSS to be below that maximum, hence my recommendation for something in the 15-20mA range.

If IDSS is much lower than that, the bias loop still closes fine, but the output impedance of the buffer starts to rise to the point where bandwidth might be affected.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/11/2020 07:14, Ed Breya via groups.io wrote:
I don't know about the 2215A, but the other 2200s use an opamp. The opamp (JFET input type) provides the DC/LF path.It's overall DC-balanced by the feedback, so the JFET bias is taken care of. It has to be fast enough for the bandwidth required, and the DC conditions are not too critical, as long as the loop can keep it all balanced.

Ed




charlesterrebonne@...
 

thanks to all who replied - the 2215A scope is off the bench for now but Ill get back to it soon and will report when I get this working...I am almost at the point of sticking a high quality transistor socket there and just trying some other close to J300 specs Jfets I have on hand...FYI all the parts I have tried in the Q122/Q222 position in the scope : 2N3918, NTE133, k19, NTE132 tested good out of circuit...i have some more Jfet #s in my stock I need to try.

From the Tektronix replaceable parts reg. : 151-1124-00 = Siliconix J-2400...would'nt be surprised this was not a custom design but more likely tested/hand picked J300s jfets and assigned a Tektronix custom part# from Siliconix...searching Siliconix j-2400 brings up nothing...Id suspect most input boards from that 22xx series scopes came off the assembly line with "J300" marked Jfets in those Q122/222 positions..thanks


Tom Lee
 

Instead of sticking in a socket, just measure IDSS first. I posted the current range that will work.

You may still have a problem with the bias circuit. I've never run into a 22xx scope that was anywhere near as fussy about FETs as yours. That bias circuit is quite forgiving.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/16/2020 06:35, charlesterrebonne@hotmail.com wrote:
thanks to all who replied - the 2215A scope is off the bench for now but Ill get back to it soon and will report when I get this working...I am almost at the point of sticking a high quality transistor socket there and just trying some other close to J300 specs Jfets I have on hand...FYI all the parts I have tried in the Q122/Q222 position in the scope : 2N3918, NTE133, k19, NTE132 tested good out of circuit...i have some more Jfet #s in my stock I need to try.

From the Tektronix replaceable parts reg. : 151-1124-00 = Siliconix J-2400...would'nt be surprised this was not a custom design but more likely tested/hand picked J300s jfets and assigned a Tektronix custom part# from Siliconix...searching Siliconix j-2400 brings up nothing...Id suspect most input boards from that 22xx series scopes came off the assembly line with "J300" marked Jfets in those Q122/222 positions..thanks




Jeff Kruth
 

I dont know if this helps, but I have a small stash of metal can J310 type.Jeff Kruth In a message dated 11/16/2020 9:35:56 AM Eastern Standard Time, charlesterrebonne@hotmail.com writes: 
thanks to all who replied - the 2215A scope is off the bench for now but Ill get back to it soon and will report when I get this working...I am almost at the point of sticking a high quality transistor socket there and just trying some other close to J300 specs Jfets I have on hand...FYI all the parts I have tried in the Q122/Q222 position in the scope : 2N3918, NTE133, k19, NTE132 tested good out of circuit...i have some more Jfet #s in my stock I need to try.

From the Tektronix replaceable parts reg. : 151-1124-00 = Siliconix J-2400...would'nt be surprised this was not a custom design but more likely tested/hand picked J300s jfets and assigned a Tektronix custom part# from Siliconix...searching Siliconix j-2400 brings up nothing...Id suspect most input boards from that 22xx series scopes came off the assembly line with "J300" marked Jfets in those Q122/222 positions..thanks


Tom Lee
 

As I've said a couple of times, that circuit is very tolerant of FET parameters. It was designed to avoid the expense of matched FETs, so I very much doubt that any selection was involved here.

Are you certain that you are inserting these various JFETs correctly? Pinouts are not standardized. Perhaps you are assuming that they are.

Again, that circuit is just not fussy. Your problem lies somewhere else.

Tom

Sent from my iThing, so please excuse the terseness and typos

On Nov 16, 2020, at 6:35, charlesterrebonne@hotmail.com wrote:

thanks to all who replied - the 2215A scope is off the bench for now but Ill get back to it soon and will report when I get this working...I am almost at the point of sticking a high quality transistor socket there and just trying some other close to J300 specs Jfets I have on hand...FYI all the parts I have tried in the Q122/Q222 position in the scope : 2N3918, NTE133, k19, NTE132 tested good out of circuit...i have some more Jfet #s in my stock I need to try.

From the Tektronix replaceable parts reg. : 151-1124-00 = Siliconix J-2400...would'nt be surprised this was not a custom design but more likely tested/hand picked J300s jfets and assigned a Tektronix custom part# from Siliconix...searching Siliconix j-2400 brings up nothing...Id suspect most input boards from that 22xx series scopes came off the assembly line with "J300" marked Jfets in those Q122/222 positions..thanks





Ed Breya
 

It may be good to rethink and summarize what's going on with these parts. Here's my take on it, presuming I haven't missed something.

1. There is one "good" original J310 that works properly in either channel. This indicates the circuits for both are likely OK, as long as the "right" JFET characteristics are present.

2. The main issue is finding another JFET - a J310 or similar one - that has the "same" characteristics as the good one. Alternative part trials so far have been unsuccessful.

3. The input circuit of the 2215A is of the opamp-stabilized type, so should be quite tolerant of JFET DC characteristics.

4. The "proper" part is 151-1124-00 = Siliconix J2400, which apparently is not a standard commercial part.

5. You have to get the pinout right, and we're assuming a TO-92 package.

6. Measuring Idss

Recommendations:

1&3. You can be pretty confident that both channel circuits are OK. However, it's possible that one may have a minor fault or part out of tolerance such that it's OK with the good J310, but another part that's close and should work, doesn't quite. For now I think it's safe to say both are OK, and can be determined once a "right" part is found.

2. All along, the discussions have been mostly about getting Idss right - this is still the case, for DC conditions, one way or another. If you can find a VHF-class JFET (more on this in #5) that you can select to be close to the good J310, then it should work.

4. Let me explain a little about Tek and selected parts. In the old days, when particular characteristics were needed, if they could be selected out from stocks of generic parts at reasonable yield, Tek often did this in-house, and a dash-number was assigned to the base part number. This assumed also that the generic parts were used in sufficient quantity that those not selected out (usually the -00) would be used up elsewhere. Later, as the real cost of all the in-house selection and inventory and tracking complications became evident, there was a big push to reduce this sort of stuff. One way is to have the vendor pre-select for the desired characteristics, out of their much larger volumes, and assign a custom part number. This can be done if you're a large enough customer, and you pay a little more for the parts to cover the vendor's cost of doing it. The benefit is that you get what you need, under a single part number, and you know the true arm's-length cost. So, the supposition that J2400 is a custom part number is likely correct - it's selected by the vendor from one of their generic types, and sold only to Tek.

5. The pinout is important - more than just getting it connected right. What you want is a TO-92, with the source lead in the middle. VHF parts typically have this arrangement to minimize proximity and capacitance between gate and drain leads. The exception is if it's for common-gate topology, where you'd ideally want the gate in the middle, and it's RF-grounded, and shields the drain from source. So, in choosing possible candidates for substitution, first they should be listed as "VHF amplifier" in the application highlights. Then look at the pinout, and pick only those with the source on the middle pin. The drain and gate on the outside don't matter - you can always flip them around, but of course be sure what's what when installing.

6. For Idss, the easiest is to use a curve tracer and measure the original good one - it's the gold standard, so take good care of it. If you don't have a curve tracer, you can rig up a simple bias circuit for checking and comparing. The best would be to measure the DC bias conditions in the actual working circuit. The gate is assumed at zero, so all you need are the source and drain voltages, and you can estimate the drain current. Then set up a resistor bias scheme that gives the same conditions if the same JFET were present. Note that this is not an Idss measurement (unless the source happens to also be at zero) - it's even better - an actual in-circuit test that can be done on the bench to sort the best parts to try in the real thing. In reality, the vendor likely sorted for a certain Idss range at a certain drain voltage, which you don't know, but an in-circuit equivalent should be as good or better. There is a simple proxy for Idss, that you can use to pre-sort parts, rejecting those that are very unlikely to land close. Just measure the "on" resistance of the drain-source (with gate tied to either), with an ohmmeter, compared to that of the gold standard. I think you'll find that the switching type JFETs will be quite low (10-50 ohms), and the small, slow ones (like for high-Z DC amplifiers) quite high (over about 200 ohms). The VHF and HF ones are typically in the mid-range around 50-200 ohms.

The idea of putting in a temporary socket is good too, but a little awkward working down in the guts. That will certainly tell what works and what doesn't.

That's all for now. Good luck.

Ed


dave G8SFU
 

Can I add a further thought to Ed Breya's suggestions.
By all means use Ed's procedure to get the best possible spec for the required fet.
But then try and match a pair of new ones. It seems to me much more likely to work than matching a 40 year old one with a 'new' one. Regards.  Dave.  

⁣Sent from BlueMail ​

On 17 Nov 2020, 20:58, at 20:58, "Ed Breya via groups.io" <edbreya=yahoo.com@groups.io> wrote:
It may be good to rethink and summarize what's going on with these
parts. Here's my take on it, presuming I haven't missed something.

1. There is one "good" original J310 that works properly in either
channel. This indicates the circuits for both are likely OK, as long as
the "right" JFET characteristics are present.

2. The main issue is finding another JFET - a J310 or similar one -
that has the "same" characteristics as the good one. Alternative part
trials so far have been unsuccessful.

3. The input circuit of the 2215A is of the opamp-stabilized type, so
should be quite tolerant of JFET DC characteristics.

4. The "proper" part is 151-1124-00 = Siliconix J2400, which apparently
is not a standard commercial part.

5. You have to get the pinout right, and we're assuming a TO-92
package.

6. Measuring Idss

Recommendations:

1&3. You can be pretty confident that both channel circuits are OK.
However, it's possible that one may have a minor fault or part out of
tolerance such that it's OK with the good J310, but another part that's
close and should work, doesn't quite. For now I think it's safe to say
both are OK, and can be determined once a "right" part is found.

2. All along, the discussions have been mostly about getting Idss right
- this is still the case, for DC conditions, one way or another. If you
can find a VHF-class JFET (more on this in #5) that you can select to
be close to the good J310, then it should work.

4. Let me explain a little about Tek and selected parts. In the old
days, when particular characteristics were needed, if they could be
selected out from stocks of generic parts at reasonable yield, Tek
often did this in-house, and a dash-number was assigned to the base
part number. This assumed also that the generic parts were used in
sufficient quantity that those not selected out (usually the -00) would
be used up elsewhere. Later, as the real cost of all the in-house
selection and inventory and tracking complications became evident,
there was a big push to reduce this sort of stuff. One way is to have
the vendor pre-select for the desired characteristics, out of their
much larger volumes, and assign a custom part number. This can be done
if you're a large enough customer, and you pay a little more for the
parts to cover the vendor's cost of doing it. The benefit is that you
get what you need, under a single part number, and you know the true
arm's-length cost. So, the supposition that J2400 is a custom part
number is likely correct - it's selected by the vendor from one of
their generic types, and sold only to Tek.

5. The pinout is important - more than just getting it connected right.
What you want is a TO-92, with the source lead in the middle. VHF parts
typically have this arrangement to minimize proximity and capacitance
between gate and drain leads. The exception is if it's for common-gate
topology, where you'd ideally want the gate in the middle, and it's
RF-grounded, and shields the drain from source. So, in choosing
possible candidates for substitution, first they should be listed as
"VHF amplifier" in the application highlights. Then look at the pinout,
and pick only those with the source on the middle pin. The drain and
gate on the outside don't matter - you can always flip them around, but
of course be sure what's what when installing.

6. For Idss, the easiest is to use a curve tracer and measure the
original good one - it's the gold standard, so take good care of it. If
you don't have a curve tracer, you can rig up a simple bias circuit for
checking and comparing. The best would be to measure the DC bias
conditions in the actual working circuit. The gate is assumed at zero,
so all you need are the source and drain voltages, and you can estimate
the drain current. Then set up a resistor bias scheme that gives the
same conditions if the same JFET were present. Note that this is not an
Idss measurement (unless the source happens to also be at zero) - it's
even better - an actual in-circuit test that can be done on the bench
to sort the best parts to try in the real thing. In reality, the vendor
likely sorted for a certain Idss range at a certain drain voltage,
which you don't know, but an in-circuit equivalent should be as good or
better. There is a simple proxy for Idss, that you can use to pre-sort
parts, rejecting those that are very unlikely to land close. Just
measure the "on" resistance of the drain-source (with gate tied to
either), with an ohmmeter, compared to that of the gold standard. I
think you'll find that the switching type JFETs will be quite low
(10-50 ohms), and the small, slow ones (like for high-Z DC amplifiers)
quite high (over about 200 ohms). The VHF and HF ones are typically in
the mid-range around 50-200 ohms.

The idea of putting in a temporary socket is good too, but a little
awkward working down in the guts. That will certainly tell what works
and what doesn't.

That's all for now. Good luck.

Ed



Tom Lee
 

Hi Dave,

Keep in mind that this is a 60MHz scope, and that the circuit in question is a simple buffer. I don't see how adding a new constraint of matching helps to fix it. I still think that the OP either has a collection of JFETs that Murphy has arranged to have extremely off-spec IDSS (hence my advice to simply measure it; it's trivially easy to do), or hasn't gotten the pinouts quite right (there is not a single standard pinout). It is also possible that there is a separate hardware problem that is making one channel excessively finicky. It's straightforward to eliminate or verify possibilities 1 and 2, so if it's neither 1 nor 2, the OP should move on to 3. It's not hard to debug the bias loop, so if he eliminates 1 and 2, getting through 3 should be fairly quick.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/18/2020 01:55, dave G8SFU via groups.io wrote:
Can I add a further thought to Ed Breya's suggestions.
By all means use Ed's procedure to get the best possible spec for the required fet.
But then try and match a pair of new ones. It seems to me much more likely to work than matching a 40 year old one with a 'new' one. Regards.  Dave.

⁣Sent from BlueMail ​

On 17 Nov 2020, 20:58, at 20:58, "Ed Breya via groups.io" <edbreya=yahoo.com@groups.io> wrote:
It may be good to rethink and summarize what's going on with these
parts. Here's my take on it, presuming I haven't missed something.

1. There is one "good" original J310 that works properly in either
channel. This indicates the circuits for both are likely OK, as long as
the "right" JFET characteristics are present.

2. The main issue is finding another JFET - a J310 or similar one -
that has the "same" characteristics as the good one. Alternative part
trials so far have been unsuccessful.

3. The input circuit of the 2215A is of the opamp-stabilized type, so
should be quite tolerant of JFET DC characteristics.

4. The "proper" part is 151-1124-00 = Siliconix J2400, which apparently
is not a standard commercial part.

5. You have to get the pinout right, and we're assuming a TO-92
package.

6. Measuring Idss

Recommendations:

1&3. You can be pretty confident that both channel circuits are OK.
However, it's possible that one may have a minor fault or part out of
tolerance such that it's OK with the good J310, but another part that's
close and should work, doesn't quite. For now I think it's safe to say
both are OK, and can be determined once a "right" part is found.

2. All along, the discussions have been mostly about getting Idss right
- this is still the case, for DC conditions, one way or another. If you
can find a VHF-class JFET (more on this in #5) that you can select to
be close to the good J310, then it should work.

4. Let me explain a little about Tek and selected parts. In the old
days, when particular characteristics were needed, if they could be
selected out from stocks of generic parts at reasonable yield, Tek
often did this in-house, and a dash-number was assigned to the base
part number. This assumed also that the generic parts were used in
sufficient quantity that those not selected out (usually the -00) would
be used up elsewhere. Later, as the real cost of all the in-house
selection and inventory and tracking complications became evident,
there was a big push to reduce this sort of stuff. One way is to have
the vendor pre-select for the desired characteristics, out of their
much larger volumes, and assign a custom part number. This can be done
if you're a large enough customer, and you pay a little more for the
parts to cover the vendor's cost of doing it. The benefit is that you
get what you need, under a single part number, and you know the true
arm's-length cost. So, the supposition that J2400 is a custom part
number is likely correct - it's selected by the vendor from one of
their generic types, and sold only to Tek.

5. The pinout is important - more than just getting it connected right.
What you want is a TO-92, with the source lead in the middle. VHF parts
typically have this arrangement to minimize proximity and capacitance
between gate and drain leads. The exception is if it's for common-gate
topology, where you'd ideally want the gate in the middle, and it's
RF-grounded, and shields the drain from source. So, in choosing
possible candidates for substitution, first they should be listed as
"VHF amplifier" in the application highlights. Then look at the pinout,
and pick only those with the source on the middle pin. The drain and
gate on the outside don't matter - you can always flip them around, but
of course be sure what's what when installing.

6. For Idss, the easiest is to use a curve tracer and measure the
original good one - it's the gold standard, so take good care of it. If
you don't have a curve tracer, you can rig up a simple bias circuit for
checking and comparing. The best would be to measure the DC bias
conditions in the actual working circuit. The gate is assumed at zero,
so all you need are the source and drain voltages, and you can estimate
the drain current. Then set up a resistor bias scheme that gives the
same conditions if the same JFET were present. Note that this is not an
Idss measurement (unless the source happens to also be at zero) - it's
even better - an actual in-circuit test that can be done on the bench
to sort the best parts to try in the real thing. In reality, the vendor
likely sorted for a certain Idss range at a certain drain voltage,
which you don't know, but an in-circuit equivalent should be as good or
better. There is a simple proxy for Idss, that you can use to pre-sort
parts, rejecting those that are very unlikely to land close. Just
measure the "on" resistance of the drain-source (with gate tied to
either), with an ohmmeter, compared to that of the gold standard. I
think you'll find that the switching type JFETs will be quite low
(10-50 ohms), and the small, slow ones (like for high-Z DC amplifiers)
quite high (over about 200 ohms). The VHF and HF ones are typically in
the mid-range around 50-200 ohms.

The idea of putting in a temporary socket is good too, but a little
awkward working down in the guts. That will certainly tell what works
and what doesn't.

That's all for now. Good luck.

Ed




dave G8SFU
 

Hi Tom, my thinking went thus::

I understood from previous posts that there were a pair of identical fets. I similarly understood they were "selected".

If these two statements are correct I stand by my original suggestion.
I should admit I have not worked on a 2215 and base my opinion on reading the thread, not looking at the actual circuit.

Regards dave
⁣Sent from BlueMail ​

On 18 Nov 2020, 10:17, at 10:17, Tom Lee <tomlee@ee.stanford.edu> wrote:
Hi Dave,

Keep in mind that this is a 60MHz scope, and that the circuit in
question is a simple buffer. I don't see how adding a new constraint of

matching helps to fix it. I still think that the OP either has a
collection of JFETs that Murphy has arranged to have extremely off-spec

IDSS (hence my advice to simply measure it; it's trivially easy to do),

or hasn't gotten the pinouts quite right (there is not a single
standard
pinout). It is also possible that there is a separate hardware problem
that is making one channel excessively finicky. It's straightforward to

eliminate or verify possibilities 1 and 2, so if it's neither 1 nor 2,
the OP should move on to 3. It's not hard to debug the bias loop, so if

he eliminates 1 and 2, getting through 3 should be fairly quick.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/18/2020 01:55, dave G8SFU via groups.io wrote:
Can I add a further thought to Ed Breya's suggestions.
By all means use Ed's procedure to get the best possible spec for the
required fet.
But then try and match a pair of new ones. It seems to me much more
likely to work than matching a 40 year old one with a 'new' one.
Regards.  Dave.

⁣Sent from BlueMail ​

On 17 Nov 2020, 20:58, at 20:58, "Ed Breya via groups.io"
<edbreya=yahoo.com@groups.io> wrote:
It may be good to rethink and summarize what's going on with these
parts. Here's my take on it, presuming I haven't missed something.

1. There is one "good" original J310 that works properly in either
channel. This indicates the circuits for both are likely OK, as long
as
the "right" JFET characteristics are present.

2. The main issue is finding another JFET - a J310 or similar one -
that has the "same" characteristics as the good one. Alternative
part
trials so far have been unsuccessful.

3. The input circuit of the 2215A is of the opamp-stabilized type,
so
should be quite tolerant of JFET DC characteristics.

4. The "proper" part is 151-1124-00 = Siliconix J2400, which
apparently
is not a standard commercial part.

5. You have to get the pinout right, and we're assuming a TO-92
package.

6. Measuring Idss

Recommendations:

1&3. You can be pretty confident that both channel circuits are OK.
However, it's possible that one may have a minor fault or part out
of
tolerance such that it's OK with the good J310, but another part
that's
close and should work, doesn't quite. For now I think it's safe to
say
both are OK, and can be determined once a "right" part is found.

2. All along, the discussions have been mostly about getting Idss
right
- this is still the case, for DC conditions, one way or another. If
you
can find a VHF-class JFET (more on this in #5) that you can select
to
be close to the good J310, then it should work.

4. Let me explain a little about Tek and selected parts. In the old
days, when particular characteristics were needed, if they could be
selected out from stocks of generic parts at reasonable yield, Tek
often did this in-house, and a dash-number was assigned to the base
part number. This assumed also that the generic parts were used in
sufficient quantity that those not selected out (usually the -00)
would
be used up elsewhere. Later, as the real cost of all the in-house
selection and inventory and tracking complications became evident,
there was a big push to reduce this sort of stuff. One way is to
have
the vendor pre-select for the desired characteristics, out of their
much larger volumes, and assign a custom part number. This can be
done
if you're a large enough customer, and you pay a little more for the
parts to cover the vendor's cost of doing it. The benefit is that
you
get what you need, under a single part number, and you know the true
arm's-length cost. So, the supposition that J2400 is a custom part
number is likely correct - it's selected by the vendor from one of
their generic types, and sold only to Tek.

5. The pinout is important - more than just getting it connected
right.
What you want is a TO-92, with the source lead in the middle. VHF
parts
typically have this arrangement to minimize proximity and
capacitance
between gate and drain leads. The exception is if it's for
common-gate
topology, where you'd ideally want the gate in the middle, and it's
RF-grounded, and shields the drain from source. So, in choosing
possible candidates for substitution, first they should be listed as
"VHF amplifier" in the application highlights. Then look at the
pinout,
and pick only those with the source on the middle pin. The drain and
gate on the outside don't matter - you can always flip them around,
but
of course be sure what's what when installing.

6. For Idss, the easiest is to use a curve tracer and measure the
original good one - it's the gold standard, so take good care of it.
If
you don't have a curve tracer, you can rig up a simple bias circuit
for
checking and comparing. The best would be to measure the DC bias
conditions in the actual working circuit. The gate is assumed at
zero,
so all you need are the source and drain voltages, and you can
estimate
the drain current. Then set up a resistor bias scheme that gives the
same conditions if the same JFET were present. Note that this is not
an
Idss measurement (unless the source happens to also be at zero) -
it's
even better - an actual in-circuit test that can be done on the
bench
to sort the best parts to try in the real thing. In reality, the
vendor
likely sorted for a certain Idss range at a certain drain voltage,
which you don't know, but an in-circuit equivalent should be as good
or
better. There is a simple proxy for Idss, that you can use to
pre-sort
parts, rejecting those that are very unlikely to land close. Just
measure the "on" resistance of the drain-source (with gate tied to
either), with an ohmmeter, compared to that of the gold standard. I
think you'll find that the switching type JFETs will be quite low
(10-50 ohms), and the small, slow ones (like for high-Z DC
amplifiers)
quite high (over about 200 ohms). The VHF and HF ones are typically
in
the mid-range around 50-200 ohms.

The idea of putting in a temporary socket is good too, but a little
awkward working down in the guts. That will certainly tell what
works
and what doesn't.

That's all for now. Good luck.

Ed







Tom Lee
 

Hi Dave,

I earlier posted that I doubt very much that any detailed selection went on, other than a crude sort for IDSS. The whole point of that design was precisely to eliminate the cost and bother of matching!

-- Tom

Sent from my iThing, so please excuse the terseness and typos

On Nov 18, 2020, at 6:12, "dave G8SFU via groups.io" <djk302=zoho.com@groups.io> wrote:

Hi Tom, my thinking went thus::

I understood from previous posts that there were a pair of identical fets. I similarly understood they were "selected".

If these two statements are correct I stand by my original suggestion.
I should admit I have not worked on a 2215 and base my opinion on reading the thread, not looking at the actual circuit.

Regards dave
⁣Sent from BlueMail ​

On 18 Nov 2020, 10:17, at 10:17, Tom Lee <tomlee@ee.stanford.edu> wrote:
Hi Dave,

Keep in mind that this is a 60MHz scope, and that the circuit in
question is a simple buffer. I don't see how adding a new constraint of

matching helps to fix it. I still think that the OP either has a
collection of JFETs that Murphy has arranged to have extremely off-spec

IDSS (hence my advice to simply measure it; it's trivially easy to do),

or hasn't gotten the pinouts quite right (there is not a single
standard
pinout). It is also possible that there is a separate hardware problem
that is making one channel excessively finicky. It's straightforward to

eliminate or verify possibilities 1 and 2, so if it's neither 1 nor 2,
the OP should move on to 3. It's not hard to debug the bias loop, so if

he eliminates 1 and 2, getting through 3 should be fairly quick.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/18/2020 01:55, dave G8SFU via groups.io wrote:
Can I add a further thought to Ed Breya's suggestions.
By all means use Ed's procedure to get the best possible spec for the
required fet.
But then try and match a pair of new ones. It seems to me much more
likely to work than matching a 40 year old one with a 'new' one.
Regards. Dave.

⁣Sent from BlueMail ​

On 17 Nov 2020, 20:58, at 20:58, "Ed Breya via groups.io"
<edbreya=yahoo.com@groups.io> wrote:
It may be good to rethink and summarize what's going on with these
parts. Here's my take on it, presuming I haven't missed something.

1. There is one "good" original J310 that works properly in either
channel. This indicates the circuits for both are likely OK, as long
as
the "right" JFET characteristics are present.

2. The main issue is finding another JFET - a J310 or similar one -
that has the "same" characteristics as the good one. Alternative
part
trials so far have been unsuccessful.

3. The input circuit of the 2215A is of the opamp-stabilized type,
so
should be quite tolerant of JFET DC characteristics.

4. The "proper" part is 151-1124-00 = Siliconix J2400, which
apparently
is not a standard commercial part.

5. You have to get the pinout right, and we're assuming a TO-92
package.

6. Measuring Idss

Recommendations:

1&3. You can be pretty confident that both channel circuits are OK.
However, it's possible that one may have a minor fault or part out
of
tolerance such that it's OK with the good J310, but another part
that's
close and should work, doesn't quite. For now I think it's safe to
say
both are OK, and can be determined once a "right" part is found.

2. All along, the discussions have been mostly about getting Idss
right
- this is still the case, for DC conditions, one way or another. If
you
can find a VHF-class JFET (more on this in #5) that you can select
to
be close to the good J310, then it should work.

4. Let me explain a little about Tek and selected parts. In the old
days, when particular characteristics were needed, if they could be
selected out from stocks of generic parts at reasonable yield, Tek
often did this in-house, and a dash-number was assigned to the base
part number. This assumed also that the generic parts were used in
sufficient quantity that those not selected out (usually the -00)
would
be used up elsewhere. Later, as the real cost of all the in-house
selection and inventory and tracking complications became evident,
there was a big push to reduce this sort of stuff. One way is to
have
the vendor pre-select for the desired characteristics, out of their
much larger volumes, and assign a custom part number. This can be
done
if you're a large enough customer, and you pay a little more for the
parts to cover the vendor's cost of doing it. The benefit is that
you
get what you need, under a single part number, and you know the true
arm's-length cost. So, the supposition that J2400 is a custom part
number is likely correct - it's selected by the vendor from one of
their generic types, and sold only to Tek.

5. The pinout is important - more than just getting it connected
right.
What you want is a TO-92, with the source lead in the middle. VHF
parts
typically have this arrangement to minimize proximity and
capacitance
between gate and drain leads. The exception is if it's for
common-gate
topology, where you'd ideally want the gate in the middle, and it's
RF-grounded, and shields the drain from source. So, in choosing
possible candidates for substitution, first they should be listed as
"VHF amplifier" in the application highlights. Then look at the
pinout,
and pick only those with the source on the middle pin. The drain and
gate on the outside don't matter - you can always flip them around,
but
of course be sure what's what when installing.

6. For Idss, the easiest is to use a curve tracer and measure the
original good one - it's the gold standard, so take good care of it.
If
you don't have a curve tracer, you can rig up a simple bias circuit
for
checking and comparing. The best would be to measure the DC bias
conditions in the actual working circuit. The gate is assumed at
zero,
so all you need are the source and drain voltages, and you can
estimate
the drain current. Then set up a resistor bias scheme that gives the
same conditions if the same JFET were present. Note that this is not
an
Idss measurement (unless the source happens to also be at zero) -
it's
even better - an actual in-circuit test that can be done on the
bench
to sort the best parts to try in the real thing. In reality, the
vendor
likely sorted for a certain Idss range at a certain drain voltage,
which you don't know, but an in-circuit equivalent should be as good
or
better. There is a simple proxy for Idss, that you can use to
pre-sort
parts, rejecting those that are very unlikely to land close. Just
measure the "on" resistance of the drain-source (with gate tied to
either), with an ohmmeter, compared to that of the gold standard. I
think you'll find that the switching type JFETs will be quite low
(10-50 ohms), and the small, slow ones (like for high-Z DC
amplifiers)
quite high (over about 200 ohms). The VHF and HF ones are typically
in
the mid-range around 50-200 ohms.

The idea of putting in a temporary socket is good too, but a little
awkward working down in the guts. That will certainly tell what
works
and what doesn't.

That's all for now. Good luck.

Ed










Ed Breya
 

There may be some confusion over "selecting" versus "matching," due to the different input amp topologies also included in the discussion. In this case, it's an opamp-stabilized system, with only one JFET up front, for each channel. There is no need for matched JFETs, but apparently there is a need for the JFETs used to fall in a narrower Idss range than the generic part types they come from. It looks like the official Tek part number used is provided by the vendor from J310s or similar, selected for some range of Idss according to Tek's spec for this part. The problem is that we don't know the selection criteria applied, but we do know the one good original J310 found in the circuit appears to work, so finding another VHF JFET that has similar Idss should work too. We don't know if the original is an ideal example - it may be at an edge of the desired range, or right in the middle, but at least getting close to whatever it is should be in the ballpark.

Ed


Tom Lee
 

Ed,

I’ve analyzed the circuit. Any IDSS below about 30mA will satisfy the loop, as I’ve noted twice before. If much lower than that, dynamics may suffer, but the upshot is that there’s a very wide window.

It’s that accommodating nature that leads me to suspect that the OP may have other problems.

Tom

Sent from my iThing, so please forgive brevity and typos

On Nov 18, 2020, at 12:15, Ed Breya via groups.io <edbreya=yahoo.com@groups.io> wrote:

There may be some confusion over "selecting" versus "matching," due to the different input amp topologies also included in the discussion. In this case, it's an opamp-stabilized system, with only one JFET up front, for each channel. There is no need for matched JFETs, but apparently there is a need for the JFETs used to fall in a narrower Idss range than the generic part types they come from. It looks like the official Tek part number used is provided by the vendor from J310s or similar, selected for some range of Idss according to Tek's spec for this part. The problem is that we don't know the selection criteria applied, but we do know the one good original J310 found in the circuit appears to work, so finding another VHF JFET that has similar Idss should work too. We don't know if the original is an ideal example - it may be at an edge of the desired range, or right in the middle, but at least getting close to whatever it is should be in the ballpark.

Ed





dave G8SFU
 

Sorry I got the impression from earlier in the thread that these were a totem pole pair of fets .
My bad
Must be more careful to look at the manual.

Regards dave

⁣Sent from BlueMail ​

On 18 Nov 2020, 20:19, at 20:19, Tom Lee <tomlee@ee.stanford.edu> wrote:
Ed,

I’ve analyzed the circuit. Any IDSS below about 30mA will satisfy the
loop, as I’ve noted twice before. If much lower than that, dynamics may
suffer, but the upshot is that there’s a very wide window.

It’s that accommodating nature that leads me to suspect that the OP may
have other problems.

Tom

Sent from my iThing, so please forgive brevity and typos

On Nov 18, 2020, at 12:15, Ed Breya via groups.io
<edbreya=yahoo.com@groups.io> wrote:

There may be some confusion over "selecting" versus "matching," due
to the different input amp topologies also included in the discussion.
In this case, it's an opamp-stabilized system, with only one JFET up
front, for each channel. There is no need for matched JFETs, but
apparently there is a need for the JFETs used to fall in a narrower
Idss range than the generic part types they come from. It looks like
the official Tek part number used is provided by the vendor from J310s
or similar, selected for some range of Idss according to Tek's spec for
this part. The problem is that we don't know the selection criteria
applied, but we do know the one good original J310 found in the circuit
appears to work, so finding another VHF JFET that has similar Idss
should work too. We don't know if the original is an ideal example - it
may be at an edge of the desired range, or right in the middle, but at
least getting close to whatever it is should be in the ballpark.

Ed






Ed Breya
 

For some reason I've been referring to the original working JFET as a J310, but looking back all the way to the OP, I'm not sure what it is - all kind of parts are floating around this thread. So Charles, can you identify the one official, working JFET that I've been referring to?

The reason I ask is that I looked up the J310, and found it is a very heavy beast - VHF/UHF, with Idss 24-60 mA - way too big and fast for this little scope amplifier, and way too big for Tom's estimated max of 30 mA for circuit operation. The correct part must be something else more "normal," among many possible choices. The J309 may work, but even it seems a bit much.The J300 seems about right, and the J210 or preferably J211 may be a good option. Also, I mentioned the 2N5485 and 2N5486 earlier, which I'm pretty sure were used a lot under various Tek part numbers. I think especially the 2N5486 was used extensively in scope input amplifiers.

Anyway, there are types that it could be, and types it shouldn't be.

Ed