How to test a 155-0155-00 or 155-0120-00 from a TEK 455? #file-notice


James55
 

Olá,

After getting the LV rails in order on this old scope, there is no vertical deflection. The trace sweeps horizontally just fine.
Looking in the service manual, and following the troubleshooting chart, it says to 'check preamplifier and switching circuits', which has either a 155-0155-00 or 155-0120-00, 40-pin IC at the centre of the circuitry.

Does anyone know of any way to test this IC, or is it just a case of confirming that all the pin voltages covered in the manual are present and correct?


The service manual can be found here...

https://groups.io/g/TekScopes/files/455%20Service%20Manual.pdf

...and the relevant schematic is located on pages 197 and 198


James


Ozan
 

On Tue, May 10, 2022 at 07:52 PM, James55 wrote:


Olá,

After getting the LV rails in order on this old scope, there is no vertical
deflection. The trace sweeps horizontally just fine.
Looking in the service manual, and following the troubleshooting chart, it
says to 'check preamplifier and switching circuits', which has either a
155-0155-00 or 155-0120-00, 40-pin IC at the centre of the circuitry.

Does anyone know of any way to test this IC, or is it just a case of
confirming that all the pin voltages covered in the manual are present and
correct?
Measuring pin voltages is the easiest path. Keep in mind if a voltage is wrong it doesn't automatically mean the IC is bad, voltage could be off because of some external fault too. If you report the voltages (if anything is off) someone from the group may be able to help you with the next steps.
Ozan


James55
 

A quick update.


So, after taking the readings, it became clear that the Channel 1 input was way off.

Following the schematic just now, revealed a dual JFET to check out, Q4124, only to find that it's missing!

For sure that won't be helping. :)


Where I'll find one of those in Brazil remains to be seen. Maybe there is an equivalent, or some sketchy workaround that can be bodged for now?


James


 

James,

On the bright side, the missing JFET can't be malfunctioning!

-- Jeff Dutky


 

I see one for sale on eBay. Sphere also appears to have a 151-1090-04 that may be suitable.

The Tek RPR (transistors) says that the appropriate replacement part is 151-1115-00, a Nat Semi part with Mfr PN SF93007, which I'm having trouble finding. Sphere did have a selection of JFETs recently, you might try asking Walter if they have the part.

-- Jeff Dutky


James55
 

Hi Jeff.

I like your attitude.
The bright side is a good place to focus.

Looking on eBay, the cheapest is around $25 inc P&P, which makes me laugh as I only paid $15 for the whole scope.
It was bought for parts, as it came with a completely smashed case. At the time it never occurred that it might actually be repairable...

One question is, does it make any difference if a xxx-xxxx-00 is used instead of a xxx-xxxx-02?



James


 

James,

from the Common Design Parts Catalog, page 7-8, the differences between the 151-1090-xx parts are:

151-1090-00 Idss: 7-25 mA, VP: max 5V, Gm: 6500 uMOHS, Ig: 100 pA, offset/drift: 50mV/50uV/degC in 20Ω totem pole
151-1090-02 Idss: 7-25 mA, VP: max 3.5V, Gm: 6500 uMOHS, Ig: 100 pA, offset/drift: 50mV/50uV/degC in 20Ω totem pole
151-1090-04 Idss: 7-25 mA, VP: max 3.5V, Gm: 6500 uMOHS, Ig: 100 pA, offset/drift: 50mV/50uV/degC in 20Ω totem pole
151-1115-00 Idss: 7-25 mA, VP: max 5V, Gm: 6500 uMOHS, Ig: 100 pA, offset/drift: 50mV/50uV/degC in 20Ω totem pole

so it sure looks to me as if you could use a -00 in place of a -02, unless there's some effect of the higher VP that I don't understand (probably: there's lots I don't know). The fact that the recommended replacement part (the 151-1115-00, as indicated in the RPR-transistors) has the same specs as a 151-1090-00 suggests that you could use a -00 without any problems.

I did notice that folks are selling the -00 parts at much lower prices than the -02 part, but I had not made this comparison yet.

My feeling is that any dual JFET is better than no JFET. I wonder how sensitive the circuit is to the specific characteristics of this component, or which characteristics are most important. I also wonder if you could get away with two discrete JFETs (of appropriate specs) strapped together for thermal equilibrium.

-- Jeff Dutky


James55
 

Looking online, there are no clear options here in Brazil and looking abroad seems to be a minimum of $30, which right here, right now, is beyond my means.

Therefore, the Dual JFET will be added to my 'Parts Needed' list and I'll pack the scope away for now.



Thanks again for the help. Progress has been made.


James


 

James,

In the spirit of Any JFET is Better Than No JFET I would seriously consider finding two single JFET parts with similar or compatible specs and installing them with some heavy wire holding the two bodies together. Unless I am completely misunderstanding the specs (which I might be) it looks like a pair of 2N5486 might work. They are cheap and plentiful in this neck of the woods (i.e. the continental USA, which I know is not where you are). they are an RF amplifier part, which probably means that they fast enough for use in a 455. The real trick would be to get their characteristics closely matched.

I'm sure I read about people replacing dual FETs with two single FETs strapped together to get them thermally equalized, so it's probably worth a try here.

I would hope that one of the more knowledgeable members of this group would step in and correct me if I am wrong.

-- Jeff Dutky


Michael A. Terrell
 

If they have a flat face that touch each other I would try a single dron of
crazy glue. I used to see small flag style heatsinks for pairs of TO92
packages which would help, as well.

On Fri, May 13, 2022 at 8:10 PM Jeff Dutky <jeff.dutky@...> wrote:

James,

In the spirit of Any JFET is Better Than No JFET I would seriously
consider finding two single JFET parts with similar or compatible specs and
installing them with some heavy wire holding the two bodies together.
Unless I am completely misunderstanding the specs (which I might be) it
looks like a pair of 2N5486 might work. They are cheap and plentiful in
this neck of the woods (i.e. the continental USA, which I know is not where
you are). they are an RF amplifier part, which probably means that they
fast enough for use in a 455. The real trick would be to get their
characteristics closely matched.

I'm sure I read about people replacing dual FETs with two single FETs
strapped together to get them thermally equalized, so it's probably worth a
try here.

I would hope that one of the more knowledgeable members of this group
would step in and correct me if I am wrong.

-- Jeff Dutky






James55
 

Ok, that is possible.

The 2N5486 seems to be readily available here and I'm off to the eletronics shops in a bit. The only issue to resolve would then be the pin connections.
I say this because the 151-1090-02 is a six pin metal device, however there are only five sockets on the board. According to the manual, pins 4 and 5 are connected together. In fact there is another dual JFET on the other channel, connected this way, it's just buried under the channel 2 selector mechanism, but that'll help.

If it is a case of 'any JFET is better than no JFET' then I've also seen metal-can 2N3955's and 2N3958's online over here for less than 10 dollars. I did check their datsheets, but the couldn't see the same ratings as the Tek device.
For example.. 'Vp? and 'Offset Drift' ?

Thank you for the help


James


Tom Lee
 

Vp is "pinchoff" voltage (otherwise called the threshold voltage in MOSFETs; same thing, electrically). JFETs are natively in the on state; you have to pull the gate voltage below that of the source to turn it off. This is not a well-controlled parameter, unfortunately. If you look at Vp values on datasheets that bother to give a value, you'll see quite a range. Depending on how fussy the circuit is, you may have to do some sorting on your own to find a suitable device pair.

Offset and offset drift are measures of mismatch, so you won't find such values on a datasheet for single devices.

JFETs are symmetrical devices, so you can exchange source and drain freely. Just get the gate pins to the right place.

Good luck!

--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 5/14/2022 04:39, James55 wrote:
Ok, that is possible.

The 2N5486 seems to be readily available here and I'm off to the eletronics shops in a bit. The only issue to resolve would then be the pin connections.
I say this because the 151-1090-02 is a six pin metal device, however there are only five sockets on the board. According to the manual, pins 4 and 5 are connected together. In fact there is another dual JFET on the other channel, connected this way, it's just buried under the channel 2 selector mechanism, but that'll help.

If it is a case of 'any JFET is better than no JFET' then I've also seen metal-can 2N3955's and 2N3958's online over here for less than 10 dollars. I did check their datsheets, but the couldn't see the same ratings as the Tek device.
For example.. 'Vp? and 'Offset Drift' ?

Thank you for the help


James




Tom Lee
 

In the Interfet datasheet for the matched-pair JFET part numbers you named, the offset (and drift) are given. I would expect any part that calls itself a matched pair to provide some data about the degree of mismatch they accept. The datasheet calls them slightly different names.

Here's a partial primer on JFET specs to help you decode things in such cases:

1) Offset voltage is a measure of mismatch. Its formal definition is that it is the amount of input voltage you need to apply to achieve a balanced (zero) output. Random JFETs of the same part number but coming from different lots or vendors can have volts of offset. Surface-mount JFETs taken from adjacent spots on a reel match much better than that (e.g., 100mV or better).
2) Offset voltage drift means what you'd think it means.
3) Vp, the pinchoff voltage, is the voltage required to make the drain current very small (~0)
4) IDSS (saturation drain current), is the drain current you get when you make VGS = 0.
5) Transconductance, gm, is the ratio of a change in drain current to a change in gate-source voltage. It's measured in siemens (S). Practical JFETs will gm values in the milli- or microsiemens (mS or uS).
6) Ciss is the input capacitance (typically measured with VGS = 0, and VDS some nonzero DC value (e.g., 5-10V))
7) Crss is the drain-gate capacitance (typically measured with the same DC conditions as Ciss)

When looking for substitute parts, good candidates are ones with compatible DC specs (Vp, IDSS and gm not too far from the center of Tek's specs), and with not significantly greater capacitances.

I hope you're able find (or make) a suitable replacement (if, of course, you do verify that the FET is the problem). If you have more questions, feel free to ask. There are lots of folks here who can help.

--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 5/14/2022 11:01, Tom Lee wrote:
Vp is "pinchoff" voltage (otherwise called the threshold voltage in MOSFETs; same thing, electrically). JFETs are natively in the on state; you have to pull the gate voltage below that of the source to turn it off. This is not a well-controlled parameter, unfortunately. If you look at Vp values on datasheets that bother to give a value, you'll see quite a range. Depending on how fussy the circuit is, you may have to do some sorting on your own to find a suitable device pair.

Offset and offset drift are measures of mismatch, so you won't find such values on a datasheet for single devices.

JFETs are symmetrical devices, so you can exchange source and drain freely. Just get the gate pins to the right place.

Good luck!

--Tom


toby@...
 

On 2022-05-14 9:02 p.m., Tom Lee wrote:
In the Interfet datasheet for the matched-pair JFET part numbers you named, the offset (and drift) are given. I would expect any part that calls itself a matched pair to provide some data about the degree of mismatch they accept. The datasheet calls them slightly different names.
Here's a partial primer on JFET specs to help you decode things in such cases:
...
When looking for substitute parts, good candidates are ones with compatible DC specs (Vp, IDSS and gm not too far from the center of Tek's specs), and with not significantly greater capacitances.
In case it saves anyone time, Linear Systems is a good place to look for Tek-compatible dual matched JFETs.

--T

I hope you're able find (or make) a suitable replacement (if, of course, you do verify that the FET is the problem). If you have more questions, feel free to ask. There are lots of folks here who can help.
--Cheers,
Tom


James55
 

Hi Tom and apologies for the delayed response.

Thank you so much for your input. It all helps. That Interfet datasheet will come in handy, I'm sure.


There is a street of electronics shops out in the suburbs of the city where I currently am, and one specialises in obsolete parts, so the plan is to pay a visit later this week. I have a list of various transistors and capacitors, which were not even easily available online here in Brazil.

I'll keep everyone posted as to how the JFET story unfolds.


James


HF
 

Hi James,
I had a good outcome replacing the 151-1042-00 JFETs in my 465B's trigger circuit with J211s. So, shopping for JFETs with similar characteristics to the originals can work. Assuming you find a cheap substitute, buy 10 and use the ones that match.
Cheers
Halden VE7UTS


James55
 

An update.

After traipsing around all the known electronics shops here, I was able to find the grand total of ONE kind of JFET. The BF246A.

So I connected two of them up as best I could understand how the original would have been, only to find that the +95v has dropped to +12v, so there is no longer any trace on the CRT.


The other LV rails and fuses all seem to be ok, however It's late here now, but I shall see what went wrong in the light of day tomorrow. Pretty sure the pins were not connected correctly, but we'll see.


James


toby@...
 

On 2022-05-18 10:35 p.m., James55 wrote:
An update.
After traipsing around all the known electronics shops here, I was able to find the grand total of ONE kind of JFET. The BF246A.
So I connected two of them up as best I could understand how the original would have been, only to find that the +95v has dropped to +12v, so there is no longer any trace on the CRT.
The other LV rails and fuses all seem to be ok, however It's late here now, but I shall see what went wrong in the light of day tomorrow. Pretty sure the pins were not connected correctly, but we'll see.
The figures of merit on JFETs, and therefore the part, matter greatly (for more on this, The Art of Electronics has an unseemly amount to say).

If you can pin down the type you need, I can order from Linear Systems and forward to you, if you can't order directly. Not sure if Sphere Research has a suitable type, but if so, I can order there too.

--Toby


James


Mark Vincent
 

James,

How about a 2N5911? This is a dual FET. I think it is a 1032 instead of the 1042 as you have.

The jumper could be off the 95V supply. Check that. The other thing I think of is the filters for that supply went bad. Going to 3.3 or 4.7mfd 200V would be fine if you replace them.

Mark


James55
 

With daylight, and a fresh morning head on my shoulders, it turned out that Ithe +95v was missing because I had knocked over a transistor whilst removing and replacing the vertical module. Doh!

The good news is that the scope now has vertical deflection... Woohoo!

So this actually answers the initial thread question. The 155-0155-00 chip IS working.
Obviously, the current configuration of BF246's can't be used, however this was just to confirm where the problem lay. I have no idea what happened to the original JFET, but it's great that the 455 is essentially working, as one thing particularly cool about this scope is the fact that it can run off of a 12v car battery. Handy!

@Toby

Thanks for the input. I still have much to learn, but it has become clear on different projects that certain components are critical, whereas others, not so much. I do want this scope working properly so I'll do more research and see what the options are.
After the saga of trying to locate a replacement here, you may very well be taken up on your kind offer - so don't be surprised if you get a pm.

@Mark

Hi Mark,
Actually, the missing JFET is a '1090'. This is a 455, not the 466 scope. I did see that the 2N5911 was mentioned but couldn't find it on any outlet over here, hence doing a temporary 'MacGyver' job...


@All

Many thanks once again for the help and suggestions. 'Invaluable', I think is the most appropriate word here.
It makes a nice start to a day, when you resolve a problem which you've had for a while and expended a fair amount of energy into resolving. Now where was that perpetual-motion machine I was working on...?


All the best.

James


Keep up the good work