Topics

465B won't trigger


blackholenulled
 

Have a 465B in pretty good shape.

I can get a trace and waveform on the display, but it doesn't seem to trigger. The waveform will just roll sideways.

I have a paper repair manual, although I need to check if the serial number range matches.

I haven't cracked the case yet. My first thought is to check the supply voltages. Although I should probably do a more thorough operational check which should isolate the problem.

I don't have any calibration equipment. Just a DMM and the built in calibrator on the front. So I'm not inclined to make any internal adjustments unless absolutely necessary.

Any thoughts where to start?


 

There is a verification section in the service manual that will help you determine if there are any problems. And help you understand the instrument.

I would suggest you open it up and do a good visual inspection and to acquaint yourself with the unit.

Then you might use compressed air to blow out any dust. This will help with cooling as the dust is a good insulator.

Use your DMM to check the low voltage power supplies against the specs in the service manual. Use the AC Volts setting to check the ripple on the supplies.

Let us know what you find.


Regards

----- Original Message -----
From: "blackholenulled" <lorn@...>
To: <TekScopes@groups.io>
Sent: Monday, April 23, 2018 6:40 PM
Subject: [TekScopes] 465B won't trigger


Have a 465B in pretty good shape.

I can get a trace and waveform on the display, but it doesn't seem to trigger. The waveform will just roll sideways.

I have a paper repair manual, although I need to check if the serial number range matches.

I haven't cracked the case yet. My first thought is to check the supply voltages. Although I should probably do a more thorough operational check which should isolate the problem.

I don't have any calibration equipment. Just a DMM and the built in calibrator on the front. So I'm not inclined to make any internal adjustments unless absolutely necessary.

Any thoughts where to start?


Tom Gardner
 

On 23/04/18 23:40, blackholenulled wrote:
Have a 465B in pretty good shape.

I can get a trace and waveform on the display, but it doesn't seem to trigger. The waveform will just roll sideways.

I have a paper repair manual, although I need to check if the serial number range matches.

I haven't cracked the case yet. My first thought is to check the supply voltages. Although I should probably do a more thorough operational check which should isolate the problem.

I don't have any calibration equipment. Just a DMM and the built in calibrator on the front. So I'm not inclined to make any internal adjustments unless absolutely necessary.

Any thoughts where to start?
You don't say whether it worked before, nor your experience with scopes in general.

Before cracking the case I would try waggling all relevant controls (trig source, level etc) to see if there is an oxide buildup that can be dislodged. Don't forget to see if it triggers on "line".

Inside the case, the first thing is to check the power supply voltages and ripple.

Look for any obvious problems on the relevant boards - unattached connectors, overheating resistors, cracked caps, dislodged transistors. You might try waggling the transistors in case there is some oxide on their contacts.

Then the manual has a section on how the circuit ought to work, then a faultfinding section, and an adjustment section. You will see there are trimpots "near" the tunnel diodes that may need tweaking.

Before tweaking any trimpot, mark its position so you can return it to the original position and/or see how far you needed to tweak it.


blackholenulled
 

Sorry, I should have been more specific. New to the forum.
I have a Bachelor of Science in Electrical Engineering, but I'm working as a Software Engineer (software pays the bills). I do the hardware as a hobby so I don't get too rusty.
My strong suit is digital, not so much analog. I've used all sorts of oscopes including digital and some time in the past analog. In some respects I prefer the analog scopes which is why I have the 465B. I'm comfortable reading schematics, using test instruments, taking things apart (and sometimes getting it back together!), and soldering, including some microsoldering. I've done lots of computer repair on the side. I know enough not to stick my fingers in the high voltage section. So this should be good for you guys since you can talk high level and I'll know what's going on and we can focus on the specifics of the repair.

I've had the scope for several years but it's just been sitting on the bench. Once in a while I power it on. The trigger problem has been there ever since I can remember, I just haven't had time to work on it. It's showing signs the controls are dirty or oxidized. After it warms up the display shrinks a little bit, but I'll cross that bridge when I get to it (checking the supplies may show up that problem anyway).

So these are all good points everyone gave. Thanks!

Ok so last night I only had time to do a performance check per the service manual. Since I don't have a CT-3 signal pickoff adapter nor a working function generator (next project) as specified in the manual I just used the probes and the built in calibrator. Then I just went through the settings they gave and the check tables.

A trigger doesn't work at all in any mode. B trigger works mostly. B trigger coupling DC worked on Normal trigger source, and marginally on Ch 1 and Ch 2 (had a hard time getting the level to adjust). B trigger coupling AC, LF and HF reject work on Norm, Ch1 and Ch 2.

According to my notes External triggering didn't work for either A nor B. I did remember to move the bnc end to the relevant external trigger input when testing it, and used the built in calibrator as the source. It seems odd to me that the B external trigger wouldn't work, considering the other B trigger sources work. I'll recheck that. Maybe user error.

I didn't try line triggering probably because it wasn't in the test matrix. I'll go back and check that.

A Trigger View only ever showed a flat line. I had no idea this thing even had trigger view. I presume this is to show the trigger input?

So according to the schematic it appears A triggering section is separate from the B triggering section. Whatever the fault is in the A triggering section will be common to all the A triggers.

Thanks! I'll keep you posted on what I find.

Lorn
KK4KRI

tmillermdems
Apr 23 #147148

There is a verification section in the service manual that will help you determine if there are any problems. And help you understand the instrument. (ok found that)

I would suggest you open it up and do a good visual inspection and to acquaint yourself with the unit. (next opportunity)

Then you might use compressed air to blow out any dust. This will help with cooling as the dust is a good insulator. (good idea. I fried a graphics card one time because of too much dust).

Use your DMM to check the low voltage power supplies against the specs in the service manual. Use the AC Volts setting to check the ripple on the supplies. (next opportunity. I have a true RMS meter so it should show ripple properly).

Let us know what you find. (see above, and more to come on the voltages.)


Regards

On Mon, Apr 23, 2018 at 11:57 pm, Tom Gardner wrote:


On 23/04/18 23:40, blackholenulled wrote:
Have a 465B in pretty good shape.

I can get a trace and waveform on the display, but it doesn't seem to
trigger. The waveform will just roll sideways.

I have a paper repair manual, although I need to check if the serial number
range matches.

I haven't cracked the case yet. My first thought is to check the supply
voltages. Although I should probably do a more thorough operational check
which should isolate the problem.

I don't have any calibration equipment. Just a DMM and the built in
calibrator on the front. So I'm not inclined to make any internal adjustments
unless absolutely necessary.

Any thoughts where to start?
You don't say whether it worked before, nor your experience with scopes in
general. (see above)

Before cracking the case I would try waggling all relevant controls (trig
source, level etc) to see if there is an oxide buildup that can be dislodged. (exercised the controls a bit. There's signs they're dirty, which improved with exercise, but A trig doesn't work regardless. See test results above.)
Don't forget to see if it triggers on "line". (I'll check that tonight)

Inside the case, the first thing is to check the power supply voltages and
ripple. (next opportunity)

Look for any obvious problems on the relevant boards - unattached connectors (good idea!),
overheating resistors, cracked caps, dislodged transistors. You might try
waggling the transistors in case there is some oxide on their contacts. (next opportunity. Pictures I've seen, many of the transistors are socketed. Will I be expecting the trigger transistors feeding the IC to be socketed? (I'll probably find this out anyway when I crack the case))

Then the manual has a section on how the circuit ought to work, then a
faultfinding section, and an adjustment section (found the performance check and the theory. I need to look again at the fault finding section). You will see there are
trimpots
"near" the tunnel diodes that may need tweaking.

Before tweaking any trimpot, mark its position so you can return it to the
original position and/or see how far you needed to tweak it. (good idea!)


Doug Blodgett <photopro@...>
 

There is an old manual out there on the internet about how to troubleshoot Tek scopes.  A Google search should uncover it quickly.  I have used it to troubleshoot scopes and it is quite amazing how much information you can get about your problem without any test equipment at all.  Most of it is done from the front panel controls on the scope itself.  Good luck.


From: blackholenulled <lorn@...>
To: TekScopes@groups.io
Sent: Monday, April 23, 2018 6:51 PM
Subject: [TekScopes] 465B won't trigger

Have a 465B in pretty good shape.

I can get a trace and waveform on the display, but it doesn't seem to trigger. The waveform will just roll sideways.

I have a paper repair manual, although I need to check if the serial number range matches.

I haven't cracked the case yet. My first thought is to check the supply voltages. Although I should probably do a more thorough operational check which should isolate the problem.

I don't have any calibration equipment. Just a DMM and the built in calibrator on the front. So I'm not inclined to make any internal adjustments unless absolutely necessary.

Any thoughts where to start?


JJ
 

Doug is talking about this: "Troubleshooting your Oscilloscope". You should
read through this - it tells you how to use the scope itself to narrow the
problem down. I recommend it highly. It's designed for technicians to
narrow down Tektronix scope problems quickly through the front panel
controls. The only way to do that is to use the scope to debug itself
according to them.

I don't think you need the exact serial number for the 465B to use the
service manual - they are all pretty much the same.

http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Best,
John

On Tue, Apr 24, 2018 at 5:56 PM, Doug Blodgett <photopro@...> wrote:

There is an old manual out there on the internet about how to troubleshoot
Tek scopes. A Google search should uncover it quickly. I have used it to
troubleshoot scopes and it is quite amazing how much information you can
get about your problem without any test equipment at all. Most of it is
done from the front panel controls on the scope itself. Good luck.


From: blackholenulled <lorn@...>
To: TekScopes@groups.io
Sent: Monday, April 23, 2018 6:51 PM
Subject: [TekScopes] 465B won't trigger

Have a 465B in pretty good shape.

I can get a trace and waveform on the display, but it doesn't seem to
trigger. The waveform will just roll sideways.

I have a paper repair manual, although I need to check if the serial
number range matches.

I haven't cracked the case yet. My first thought is to check the supply
voltages. Although I should probably do a more thorough operational check
which should isolate the problem.

I don't have any calibration equipment. Just a DMM and the built in
calibrator on the front. So I'm not inclined to make any internal
adjustments unless absolutely necessary.

Any thoughts where to start?








HBcubed
 

Actually there are two different manuals for the 465B. The early 465B were actually "engineering models" pretty much made by hand (the better of the two types many believe) and followed by the "production 465B.
If I'm not mistaken , one of the great things about the early 465B was that the xistors "plugged in" to the circuits using neoprene sockets while the later production models had them soldered in to the circuits with the 'plugables" being obviously much easier to troubleshoot

One manual covers S/N B059999 and down with other covering S/N B060000 and up. I have CD of both,
There are some significant differences between the two so unless you have the right manual many things don't match...

I repaired/ refurbished all 400 series scopes for quite a while. Must say that of all of them my favorite far and away is the 465B.. a really GREAT scope....well worth putting in top shape .... On your trigger problem.... there are tunnel diodes used in the heart of the trigger ckt... they are "weird " little devices that have unique electrical characteristics that are perfect for use in a trigger ckt... could be your problem... just a guess


blackholenulled
 

Suhweet! Thank you so much! This is like gold!

Lorn
KK4KRI


ArtekManuals
 

What is like gold? There is nothing from the prior to know what your referring too
Dave
NR1DX

On 4/25/2018 11:12 AM, blackholenulled wrote:
Suhweet! Thank you so much! This is like gold!

Lorn
KK4KRI


--
Dave
Manuals@...
www.ArtekManuals.com


blackholenulled
 

What I have found so far on the 465B trigger problem:
Serial number B061387, service manual says B060000 and up.
Removed case and did a very brief physical inspection. Very clean, no dust. No obvious burned parts so far, no burn smell. Most transistors seem to be socketed.

Voltages: input selector is set to 104-126. I know the power company sometimes keeps switching between 120 and 125 because my UPS complains about that.

Used an old Radio Shack (yeah I know, can't afford a Fluke) DVM with true RMS capability.

Voltage test point, reading, ripple, comment
+110 unreg 164.7 Vdc .425 Vac dc voltage seems high?
+110 111.9 2.8 mV
+55 54.8 1.3 mV
+15 14.95 1.4 mV
+5 5.00 1.4 mV
-8 -7.98 1.4 mV

So that all seems nominal.

Schematic 6, A and B Trigger Generator board
Set the front panel controls for DC measurement per the schematic.
Test Point 43, which is not marked on the board layout picture, but is marked on the schematic:
Should read zero volts, reads -.1mV so I would call that zero (input to Q7324A and B).

U7331
Pin, expected Vdc, actual
1. -0.8, -1.1mV
2. 0, .761 V
3. 0, .761 V
4. 4.9 actual (tied to +5 through 1k resistor)
5. -8, -7.95
6. -1.0, -.959 (close enough)
7. ground, -.5mV (effectively zero)
8. 10.6, 10.72
9, 10.6, 9.73
10. 0, -39.7 mV
11. 10.5, 10.68
12. -0.9, -.852
13. 7.1, 7.18
14, 0, .7 mV
15. 0, .7 mV
16. -0.8, -204.6 mV

I'm thinking based on these numbers the answer must be staring me in the face. Pins 2, 1, and 16 aren't correct, so I'm thinking either a U7331 problem or possibly the A trigger view circuit which hangs off of 1 and 16 may have a problem. I'll check the voltages in the trigger view circuit next opportunity, and maybe pop out Q7424 and Q7422 to isolate the chip from the trigger view circuit and see if anything changes on 2, 1 and 16.

I didn't write down the numbers but a quick probe of some of U7131 on the B trigger side showed expected voltages, especially on 2, 1, and 16.

You guys are an amazing resource! I knew I came to the right place. I also hope this thread turns out to be useful to someone else.

Lorn
KK4KRI


Michael
 

I have two 465s. I have repaired one. They way it is constructed does not lend itself to be easily repaired. One must be patient with repairs on a 465. Also be very careful about moving boards around. I love these scopes when they work. I use at least one daily to repair electronic equipment. It was a great functional design, not the best design to allow access to parts.


Fabio Trevisan
 

Hello Lorn,
I also own a 464 and mine also had a problem triggering... And while looking at your description I looks like deja vu to me.
On mine one of the FET transistors of the input circuitry were leaky...(it had about 200kOhms impedance between source and gate, while reverese biased. The good ones measure Infinite, or very high Megohms.
The leak developed a voltage drop on the 1M resistor at their gates and make the output voltage to have a huge offset of... wow! 70mV...(that's 1/10th of what you're getting).
70mV offset was enough to take the preamplifier IC so off that trigger would only work by adjusting the level potentiometer near its end of range and, still, after warming up, the offset would grow just a bit more and then even taking the Level potentiometer to the end was not enough to make it trigger.
On yours it seems the offset is some 0.7V, so no wonder why it doesn't trigger.
Your course of action is exact and removing the FET transistors will allow you to rule-out if the positive offset is coming from the IC, or if it's coming from the buffer stage into the IC.
It's likely, however, that if you just leave pins 2 and 3 opened, as they will get when FETs are removed, you will be fooled by false voltage readings at pins 2 and 3, due to the input pins need some biasing.
So, after you remove the FET ICs, connect a resistor of - say - 51R between pins 2.3 of the preamp IC to ground.
The impedance at the other side of this balanced input differential amplifier is roughly 50Ohm, so biasing pins 2,3 with 50Ohms will make the input perfectly balanced.
In this condition, pins 2 and 3 must read a low voltage, close to 0V. By no means it can be more than 50mV... as I told, my 464 had 70mV and that was enough to screw up the triggering.
Alternatively, you can check the FET input buffer's offset, by taking the IC out and measuring voltage directly at pins 2.3 of the IC socket...
Don't worry, taking the IC out is safe... nothing that's past of the IC will burn because the IC isn't present. (I had this same doubt back then).
Set the Trigger input to EXT, coupling to DC, and ground the Ext. input, so that you're sure that you're not feeding DC into the input buffer, and its output should be very close to 0V.
If the transistors are perfectly matched and thermally even, in theory, the DC offset of this input buffer is exactly ZERO.
Good luck,
Rgrds,
Fabio


blackholenulled
 

Ok, cool. So I need to isolate whether the input transistors are leaky or whether there's a problem with the chip.

The transistors are socketed but the chip is not. So it's not practical to casually take the chip out. So I'll try removing the transistors and put in the 51 ohm on 2 and 3 as you suggest. Even if I can't take the chip out, this should tell me something. If the voltages are all nominal then, that would point upstream to the transistors.

While I have the transistors out I can ohm them out to check for a leak. The B side transistors should be identical and for the most part the B side works, so I could also swap those transistors and/or use them for comparison when ohming.

I'm still suspicious about the readings on pins 1 and 16. It will be interesting to see if they change when biasing 2 and 3 with the resistor.

Thanks!
Lorn
KK4KRI

snippet of the previous measurements:
U7331
Pin, expected Vdc, actual
1. -0.8, -1.1mV
2. 0, .761 V
3. 0, .761 V
4. 4.9 actual (tied to +5 through 1k resistor)
5. -8, -7.95
6. -1.0, -.959 (close enough)
7. ground, -.5mV (effectively zero)
8. 10.6, 10.72
9, 10.6, 9.73
10. 0, -39.7 mV
11. 10.5, 10.68
12. -0.9, -.852
13. 7.1, 7.18
14, 0, .7 mV
15. 0, .7 mV
16. -0.8, -204.6 mV
<< end snippet


On Wed, Apr 25, 2018 at 11:46 am, Fabio Trevisan wrote:


Hello Lorn,
I also own a 464 and mine also had a problem triggering... And while looking
at your description I looks like deja vu to me.
On mine one of the FET transistors of the input circuitry were leaky...(it had
about 200kOhms impedance between source and gate, while reverese biased. The
good ones measure Infinite, or very high Megohms.
The leak developed a voltage drop on the 1M resistor at their gates and make
the output voltage to have a huge offset of... wow! 70mV...(that's 1/10th of
what you're getting).
70mV offset was enough to take the preamplifier IC so off that trigger would
only work by adjusting the level potentiometer near its end of range and,
still, after warming up, the offset would grow just a bit more and then even
taking the Level potentiometer to the end was not enough to make it trigger.
On yours it seems the offset is some 0.7V, so no wonder why it doesn't
trigger.
Your course of action is exact and removing the FET transistors will allow you
to rule-out if the positive offset is coming from the IC, or if it's coming
from the buffer stage into the IC.
It's likely, however, that if you just leave pins 2 and 3 opened, as they will
get when FETs are removed, you will be fooled by false voltage readings at
pins 2 and 3, due to the input pins need some biasing.
So, after you remove the FET ICs, connect a resistor of - say - 51R between
pins 2.3 of the preamp IC to ground.
The impedance at the other side of this balanced input differential amplifier
is roughly 50Ohm, so biasing pins 2,3 with 50Ohms will make the input
perfectly balanced.
In this condition, pins 2 and 3 must read a low voltage, close to 0V. By no
means it can be more than 50mV... as I told, my 464 had 70mV and that was
enough to screw up the triggering.
Alternatively, you can check the FET input buffer's offset, by taking the IC
out and measuring voltage directly at pins 2.3 of the IC socket...
Don't worry, taking the IC out is safe... nothing that's past of the IC will
burn because the IC isn't present. (I had this same doubt back then).
Set the Trigger input to EXT, coupling to DC, and ground the Ext. input, so
that you're sure that you're not feeding DC into the input buffer, and its
output should be very close to 0V.
If the transistors are perfectly matched and thermally even, in theory, the DC
offset of this input buffer is exactly ZERO.
Good luck,
Rgrds,
Fabio


Fabio Trevisan
 

hi Lorn,
On your previous writing:

The transistors are socketed but the chip is not. So it's not practical to
Look carefully... it's more likely it's socketed than not.
Tek used a lot a kind of socket that is made of individual socket pins that go directly on the board... (on really wide PCB holes),
To bare eyes, it's very easy to overlook it.
When I was still new to my 464 I also thought they were soldered.
Rgrds,
FT


Fabio Trevisan
 

Lorn...
Ah, in time... about your writing:

On Wed, Apr 25, 2018 at 12:28 pm, blackholenulled wrote:

I'm still suspicious about the readings on pins 1 and 16. It will be
interesting to see if they change when biasing 2 and 3 with the resistor.
Since between pins 2,3 and pin 1 there's only the base-emitter junction of the IC's input transistor... if the input is at 0,7V or so, it's expected that pin 1 will rise to about 0V, instead of staying at the nominal -0.8V or so... This even testimony in favor of the IC, that its input transistor isn't defective.
As for pin 16, since this is a differential input pair, the upper side being heavily (postively) biased, it's natural that it will bring the emitter of the complementary xsistor up... In fact, this is exactly what the differential input pair is meant to do.
One may be fooled by the fact (so often found on Oscilloscope's Vertical and Trigger amplifier circuits, ICs or discrete) that the transistors don't have their emitters tied together to a single "long tail" resistor and thus, not spotting right away a differential amp., but Y connected emitter resistors and Delta connected are mutually convertible and the set is indeed a differential input pair.
If one is heavily driven into conduction, the other will be driven into cutoff from its emitter.
R
FT


blackholenulled
 

I popped out the transistor pair in the downstream A trigger view circuit and the voltages didn't change, so that leaves out that circuit as causing problems.

So I did like Fabio said and pulled out the preamp transistor pair, and biased the input to the chip with two parallel 100 ohm resistors to ground. Now we're getting somewhere. The input was close to zero (don't remember now exactly) and 1 and 16 were -.755 volts. So it seems the chip works.

Ohmed the transistors but that didn't tell me anything specifically about their condition. So since the B trigger side has an identical pair of preamp transistors, I swapped those into the A trigger side. Voltages were -30 mV on the input to the chip, and -.755 at 1 and 16. Tested the trigger and now I can get a trigger! I also have the A trigger view.

The chip isn't coming out unless it were to turn out to be bad. It's very soldered complete with solder fillets.

So somehow the original preamp transistors are bad, or at least one is. I just need to source them, which so far the internet is telling me they are obsolete (of course). Any ideas anyone?

Manufacturer part number: SF50031, which comes back to 2N5245 N channel JFET, which is obsolete. It's late and I haven't tried to cross reference that yet.

As a side effect the trace seemed to have some ac coupled into it. It hasn't done that before but I'm wondering if it's because the case is off?

Thanks Fabio and everyone else!

Hello Lorn,
I also own a 464 and mine also had a problem triggering... And while looking at your description I looks like deja vu to me.
On mine one of the FET transistors of the input circuitry were leaky...(it had about 200kOhms impedance between source and gate, while reverese biased. The good ones measure Infinite, or very high Megohms.
The leak developed a voltage drop on the 1M resistor at their gates and make the output voltage to have a huge offset of... wow! 70mV...(that's 1/10th of what you're getting).
70mV offset was enough to take the preamplifier IC so off that trigger would only work by adjusting the level potentiometer near its end of range and, still, after warming up, the offset would grow just a bit more and then even taking the Level potentiometer to the end was not enough to make it trigger.
On yours it seems the offset is some 0.7V, so no wonder why it doesn't trigger.
Your course of action is exact and removing the FET transistors will allow you to rule-out if the positive offset is coming from the IC, or if it's coming from the buffer stage into the IC.
It's likely, however, that if you just leave pins 2 and 3 opened, as they will get when FETs are removed, you will be fooled by false voltage readings at pins 2 and 3, due to the input pins need some biasing.
So, after you remove the FET ICs, connect a resistor of - say - 51R between pins 2.3 of the preamp IC to ground.
The impedance at the other side of this balanced input differential amplifier is roughly 50Ohm, so biasing pins 2,3 with 50Ohms will make the input perfectly balanced.
In this condition, pins 2 and 3 must read a low voltage, close to 0V. By no means it can be more than 50mV... as I told, my 464 had 70mV and that was enough to screw up the triggering.
Alternatively, you can check the FET input buffer's offset, by taking the IC out and measuring voltage directly at pins 2.3 of the IC socket...
Don't worry, taking the IC out is safe... nothing that's past of the IC will burn because the IC isn't present. (I had this same doubt back then).
Set the Trigger input to EXT, coupling to DC, and ground the Ext. input, so that you're sure that you're not feeding DC into the input buffer, and its output should be very close to 0V.
If the transistors are perfectly matched and thermally even, in theory, the DC offset of this input buffer is exactly ZERO.
Good luck,
Rgrds,
Fabio


Fabio Trevisan
 

Hello Lorn,
Great news indeed!... So your problem is narrowed down to exactly the same cause of mine: One, or both, of the input FET Transistors bad.
And I did exactly what you did, meanwhile I couldn't source another pair... I cannibalized the B sweep trigger.
In fact, there's very little impact to it, because 90% of the times, we use the B Sweep in the "STARTS AFTER DELAY" mode... and this mode doesn't use the triggering input buffers or the trigger amplifier whatsoever...

As for the source of the transistors you wrote:
Any ideas anyone?
Manufacturer part number: SF50031, which comes back to 2N5245 N channel JFET, which is obsolete. It's late and I haven't tried to cross reference that yet.
Not quite right!
I don't know from where you got the 2N5245, but according to Tektronix_Xref_sm.pdf available at Archive.org (https://archive.org/details/tektronix_Tektronix_Xref_sm), part number 151-0042-00 is found at page 7-7.
This part number is a characterized and matched pair of 2N4416 (which is probably also obsolete, but not that hard to find).
From the "stock" 2N4416 whose IDss can vary from 5 to 15 mA, those ones are selected to a narrower IDss range of 10 to 15mA, and the pair need to be matched within 0.5mA between each other.

Even if you cannot get a matched pair, as long as they're not too different, you can zero them out by tweaking the value of R7325 or R7327 (or both).
This will compensate for the different Vgs that will develop across each one of the **not matched** xsistors' source resistors, in order to get the output voltage as close to 0V as possible,
The fact they're not matched will have the down side their IDss will also drift differently with temperature so, tweaking the resistors solves the matching problem at one given temperature but as the temperature changes, they will drift apart from each other...
But since this is **only** triggering (I can see Howard Vollum revolving in his grave now), it's not that critical!. And you can always put the non-matched pair on the B sweep trigger, which is rarely used.

Good luck with your 465B, This is my dream 465 scope, with Alternate Delayed time base instead of the least useful "MIX" mode that's present on the regular 465s.
I have a 464 with DM44 multimeter (it has a weak CRT) and just recently got a 7623A with 7B53A timebase, so I'm probably married to the "MIX" mode for the foreseeable future.
I'll probably sell my 464 and watch out for a good 465b, or even a 468 (which is a 465b + digital storage).

Rgrds,

Fabio


blackholenulled
 

Fabio:
Yeah, I was hoping for "been there, done that", so you've been a big help!

So this is interesting what you're saying about the cross reference. The transistors I pulled out are 3 pin TO-92 case devices. The data sheets I'm finding for 2N4416 show it as a four pin device. I wonder if there is a three pin variant?

I need to compare the data sheets but there are indications from other sources that 2N4416 and 2N5245 are electrically equivalent.

So how I arrived at 2N5245:

https://www.wbparts.com/rfq/5961-00-436-3317.html

Which lists as a cross reference 2N5245. It came from military specs, so I figure if it's good enough for the military, it's good enough for my purposes.

This discussion indicates 2N5245 and 2N4416 are equivalent:
https://www.vintage-radio.net/forum/showthread.php?t=67673

I figure if I can't purchase a matched pair I can buy a handful and do some matching using this article:
http://www.firstwatt.com/pdf/art_matching.pdf

So the Tektronix cross reference you linked is a great resource. I'm just puzzled as to why Tektronix would list a four pin device rather than three pin, unless it turns out there is a three pin variant of 2N4416 that used to be available. (well, quick search on the internet indicates there are TO-92 case variants, but more common is TO-72 four pin for the 2N4416).

What I need to do is remove the heat sink from the transistor pair and see what the markings are.

Lorn
KK4KRI

On Thu, Apr 26, 2018 at 01:31 pm, Fabio Trevisan wrote:


Hello Lorn,
Great news indeed!... So your problem is narrowed down to exactly the same
cause of mine: One, or both, of the input FET Transistors bad.
And I did exactly what you did, meanwhile I couldn't source another pair... I
cannibalized the B sweep trigger.
In fact, there's very little impact to it, because 90% of the times, we use
the B Sweep in the "STARTS AFTER DELAY" mode... and this mode doesn't use the
triggering input buffers or the trigger amplifier whatsoever...

As for the source of the transistors you wrote:
Any ideas anyone?
Manufacturer part number: SF50031, which comes back to 2N5245 N channel
JFET, which is obsolete. It's late and I haven't tried to cross reference that
yet.

Not quite right!
I don't know from where you got the 2N5245, but according to
Tektronix_Xref_sm.pdf available at Archive.org
(https://archive.org/details/tektronix_Tektronix_Xref_sm), part number
151-0042-00 is found at page 7-7.
This part number is a characterized and matched pair of 2N4416 (which is
probably also obsolete, but not that hard to find).
From the "stock" 2N4416 whose IDss can vary from 5 to 15 mA, those ones are
selected to a narrower IDss range of 10 to 15mA, and the pair need to be
matched within 0.5mA between each other.

Even if you cannot get a matched pair, as long as they're not too different,
you can zero them out by tweaking the value of R7325 or R7327 (or both).
This will compensate for the different Vgs that will develop across each one
of the **not matched** xsistors' source resistors, in order to get the output
voltage as close to 0V as possible,
The fact they're not matched will have the down side their IDss will also
drift differently with temperature so, tweaking the resistors solves the
matching problem at one given temperature but as the temperature changes, they
will drift apart from each other...
But since this is **only** triggering (I can see Howard Vollum revolving in
his grave now), it's not that critical!. And you can always put the
non-matched pair on the B sweep trigger, which is rarely used.

Good luck with your 465B, This is my dream 465 scope, with Alternate Delayed
time base instead of the least useful "MIX" mode that's present on the regular
465s.
I have a 464 with DM44 multimeter (it has a weak CRT) and just recently got a
7623A with 7B53A timebase, so I'm probably married to the "MIX" mode for the
foreseeable future.
I'll probably sell my 464 and watch out for a good 465b, or even a 468 (which
is a 465b + digital storage).

Rgrds,

Fabio


Chuck Harris
 

The fourth pin is just a connection to the
case, for consistent shielding. You can safely
clip it close to the case.

-Chuck Harris

blackholenulled wrote:

Fabio:
Yeah, I was hoping for "been there, done that", so you've been a big help!

So this is interesting what you're saying about the cross reference. The transistors I pulled out are 3 pin TO-92 case devices. The data sheets I'm finding for 2N4416 show it as a four pin device. I wonder if there is a three pin variant?


Renée
 

in this instance the 4th pin usually connects to the case for shielding, as it is not a dual gate device.
(depending on the environment (other uses- noisy places)  adding the shielding may be necessary, but not this time.)
Renée

On 2018-04-27 07:23 AM, blackholenulled wrote:
Fabio:
Yeah, I was hoping for "been there, done that", so you've been a big help!

So this is interesting what you're saying about the cross reference. The transistors I pulled out are 3 pin TO-92 case devices. The data sheets I'm finding for 2N4416 show it as a four pin device. I wonder if there is a three pin variant?

I need to compare the data sheets but there are indications from other sources that 2N4416 and 2N5245 are electrically equivalent.

So how I arrived at 2N5245:

https://www.wbparts.com/rfq/5961-00-436-3317.html

Which lists as a cross reference 2N5245. It came from military specs, so I figure if it's good enough for the military, it's good enough for my purposes.

This discussion indicates 2N5245 and 2N4416 are equivalent:
https://www.vintage-radio.net/forum/showthread.php?t=67673

I figure if I can't purchase a matched pair I can buy a handful and do some matching using this article:
http://www.firstwatt.com/pdf/art_matching.pdf

So the Tektronix cross reference you linked is a great resource. I'm just puzzled as to why Tektronix would list a four pin device rather than three pin, unless it turns out there is a three pin variant of 2N4416 that used to be available. (well, quick search on the internet indicates there are TO-92 case variants, but more common is TO-72 four pin for the 2N4416).

What I need to do is remove the heat sink from the transistor pair and see what the markings are.

Lorn
KK4KRI

On Thu, Apr 26, 2018 at 01:31 pm, Fabio Trevisan wrote:

Hello Lorn,
Great news indeed!... So your problem is narrowed down to exactly the same
cause of mine: One, or both, of the input FET Transistors bad.
And I did exactly what you did, meanwhile I couldn't source another pair... I
cannibalized the B sweep trigger.
In fact, there's very little impact to it, because 90% of the times, we use
the B Sweep in the "STARTS AFTER DELAY" mode... and this mode doesn't use the
triggering input buffers or the trigger amplifier whatsoever...

As for the source of the transistors you wrote:
Any ideas anyone?
Manufacturer part number: SF50031, which comes back to 2N5245 N channel
JFET, which is obsolete. It's late and I haven't tried to cross reference that
yet.

Not quite right!
I don't know from where you got the 2N5245, but according to
Tektronix_Xref_sm.pdf available at Archive.org
(https://archive.org/details/tektronix_Tektronix_Xref_sm), part number
151-0042-00 is found at page 7-7.
This part number is a characterized and matched pair of 2N4416 (which is
probably also obsolete, but not that hard to find).
From the "stock" 2N4416 whose IDss can vary from 5 to 15 mA, those ones are
selected to a narrower IDss range of 10 to 15mA, and the pair need to be
matched within 0.5mA between each other.

Even if you cannot get a matched pair, as long as they're not too different,
you can zero them out by tweaking the value of R7325 or R7327 (or both).
This will compensate for the different Vgs that will develop across each one
of the **not matched** xsistors' source resistors, in order to get the output
voltage as close to 0V as possible,
The fact they're not matched will have the down side their IDss will also
drift differently with temperature so, tweaking the resistors solves the
matching problem at one given temperature but as the temperature changes, they
will drift apart from each other...
But since this is **only** triggering (I can see Howard Vollum revolving in
his grave now), it's not that critical!. And you can always put the
non-matched pair on the B sweep trigger, which is rarely used.

Good luck with your 465B, This is my dream 465 scope, with Alternate Delayed
time base instead of the least useful "MIX" mode that's present on the regular
465s.
I have a 464 with DM44 multimeter (it has a weak CRT) and just recently got a
7623A with 7B53A timebase, so I'm probably married to the "MIX" mode for the
foreseeable future.
I'll probably sell my 464 and watch out for a good 465b, or even a 468 (which
is a 465b + digital storage).

Rgrds,

Fabio