Help with (another) broken 7B92 please


Bruce Atwood
 

7B92, S/N B052568, No Sweep, Help!

Since it has been almost a year since my 7B92 arrived in my new-to-me 7904 I’ve decided that it is time to make it work. I’ve re-posted with a new title since it may have been confused with “..no repeating sweep”

The first question might be is it normal for the Hold-Off circuit to free run? I have not found a configuration where it does not free-run at a frequency determined by timing switches 7, 8, 9 and 10.

Test setup
‘92 installed in right-most slot of a 7603. No vertical plug-ins installed. (The 7603 has been tested with a 7B53A in the right-most slot and a 7A26 in the left-most slot.) The side covers have been removed from the ‘92. The side covers are removed from the 7603 giving good access to the main trigger board and the sweep board.
The external trigger is driven by the cal-out of a 7904 set to 1kHz and 0.4 volts. The termination is off. Trigger is set to Auto, DC coupling, External. The “Alt” and “Level” controls are pushed in. Sweep speed set to 20 uSec/division. The “Triggered” light is off when powered-up but come on and stays on if single-sweep is selected.

U310 Voltages, taken with Fluke 115, Waveforms observed with 7409/7A26/7B53A combo:
Pin # name Voltage
1. +TD input 1.159
2. -TD input 0.232
3. + TD Auto 0.331
4. - TD Auto 4.890
5. -5V -4.980
6. Auto timing 0.022
7. Tgd Lamp 4.602
8. HO timing 2.239 + Hold-Off ramp, ~4V P-P, 40 uSec
9. 0.001
10. HO out 0.289 + 0.5 usec 6V pulses at HO frequency (25kHz)
11. SS ready 4.663
12. 0.013
13. 0.216
14. Reset timing 4.716
15. Reset start 0.762
16. 0.074
17. SWP disable 1.686
18. Lockout input -0.006
19. Auto Mode 0.001
20. +5V 4.889
And:
TP 392 4.367V
TP 321 1.429V
C 1.61V + Hold-off pulses
E 0.115V + Hold-off pulses
CR740 (TD) ~1.5uSec, ~600mv pulses at Hold-Off ramp freqeuncy
Q754 base 0.747V
Q748 base 0.790V


Miguel Work
 

Do you have pulses at TP321?


Bruce Atwood
 

7B92, S/N B052568, No Sweep, Help!

Thanks Miguel,

TP321, and R765, have ~60mv, ½ usec pulses. So pulses but probably not functional. I went back and checked carefully on all pins of U310 and found, in addition to those noted below, several volt 200 nsec pulses on 16 and 17. All are at the same frequency and in phase with the HO ramp.



Since it has been almost a year since my 7B92 arrived in my new-to-me 7904 I’ve decided that it is time to make it work. I’ve re-posted with a new title since it may have been confused with “..no repeating sweep”

The first question might be is it normal for the Hold-Off circuit to free run? I have not found a configuration where it does not free-run at a frequency determined by timing switches 7, 8, 9 and 10.

Test setup
‘92 installed in right-most slot of a 7603. No vertical plug-ins installed. (The 7603 has been tested with a 7B53A in the right-most slot and a 7A26 in the left-most slot.) The side covers have been removed from the ‘92. The side covers are removed from the 7603 giving good access to the main trigger board and the sweep board.
The external trigger is driven by the cal-out of a 7904 set to 1kHz and 0.4 volts. The termination is off. Trigger is set to Auto, DC coupling, External. The “Alt” and “Level” controls are pushed in. Sweep speed set to 20 uSec/division. The “Triggered” light is off when powered-up but come on and stays on if single-sweep is selected.

U310 Voltages, taken with Fluke 115, Waveforms observed with 7409/7A26/7B53A combo:
Pin # name Voltage
1. +TD input 1.159
2. -TD input 0.232
3. + TD Auto 0.331
4. - TD Auto 4.890
5. -5V -4.980
6. Auto timing 0.022
7. Tgd Lamp 4.602
8. HO timing 2.239 + Hold-Off ramp, ~4V P-P, 40 uSec
9. 0.001
10. HO out 0.289 + 0.5 usec 6V pulses at HO frequency (25kHz)
11. SS ready 4.663
12. 0.013
13. 0.216
14. Reset timing 4.716
15. Reset start 0.762
16. 0.074 2V 200 nsec pulses
17. SWP disable 1.686 3 V 200 nsec bipolar pulses
18. Lockout input -0.006
19. Auto Mode 0.001
20. +5V 4.889
And:
TP 392 4.367V
TP 321 1.429V
C 1.61V + Hold-off pulses
E 0.115V + Hold-off pulses
CR740 (TD) ~1.5uSec, ~600mv pulses at Hold-Off ramp freqeuncy
Q754 base 0.747V
Q748 base 0.790V


Miguel Work
 

I´m waiting to receive one 7B92 that I have bought cheap beacuse I need pulse diodes, and this time base has 6 fast TD diodes.

I don´t understand what you neen to know.
what does it mean free run hold off?

It seems that this time base don´t have a variable hold off control.


Roger Evans
 

I am not sure I understand what is happening so these are no more than comments.

Have you set the P308/P309 jumper for 'non 79xx' mainframes?

If you set the trigger mode to normal then do the repeating hold off pulses stop?

The voltage on TP321 is suspiciously high. Apart from the 51R resistor R765 it is equal to the voltage across the gating tunnel diode CR750. Germanium tunnel diodes normally have a voltage drop of 50-100mV when armed and around 500mV when triggered. GaAs tunnel diodes go as high as 1.1V when triggered. The parts list says that CR740 and CR750 are Silicon diodes - I have never seen a Silicon tunnel diode, but a Google search says they do exist and would have an on state voltage drop around 700-800mV.

Regards,

Roger


Miguel Work
 

I use a simple x y octoupus to check in board these diodes

I have fix recently a 7T11 with bad TD outuput diiode, I replaced it with a 10mA russian diode and it now works.

https://groups.io/g/TekScopes/photo/249051/2975689?p=created%2C%2Cs51%2C20%2C2%2C0%2C0

Some bad TD diodes has high resistance, this can explain the high voltage that you see

https://groups.io/g/TekScopes/photo/260881/3326552?p=Created%2C%2C%2C20%2C2%2C0%2C0


Miguel Work
 

https://groups.io/g/TekScopes/photo/260881/3326552?p=Created%2C%2C%2C20%2C2%2C0%2C0

Horizontal is in volts, Vertical 10mA/div

Good diode has a resitance <10 ohms, in this example 4 ohms in reverse, bad diode > 20 ohms

Regards!


Ozan
 

Hi Bruce,
1.4V across CR750 is not a good sign, it is most likely in open circuit. You can short across CR750 with an ammeter and make sure the current is not excessive (I think parts list says ~ 10mA) because of some other fault. If that looks good and you have a tunnel diode available you can replace CR750. Another option is to carefully swap with CR950. Diode CR750 is main trigger, CR950 is delayed trigger. With this change main trigger should work, then you can install the replacement diode in CR950 location later.


Regarding your question about hold-off: In this state of the circuit most likely ramp is railed at the top (i.e. TP408 is at max) and Q398 is not resetting the ramp. As soon as hold off ends (Q349 turns off) HO start is triggered again because ramp is already at its final voltage. That is why you see hold off ramp running and it is independent of timing switches (ramp is not reset so there is no setting dependent ramp).


Ozan

On Mon, Oct 25, 2021 at 01:25 PM, Bruce Atwood wrote:


7B92, S/N B052568, No Sweep, Help!

Thanks Miguel,

TP321, and R765, have ~60mv, ½ usec pulses. So pulses but probably not
functional. I went back and checked carefully on all pins of U310 and found,
in addition to those noted below, several volt 200 nsec pulses on 16 and 17.
All are at the same frequency and in phase with the HO ramp.



Since it has been almost a year since my 7B92 arrived in my new-to-me 7904
I’ve decided that it is time to make it work. I’ve re-posted with a new
title since it may have been confused with “..no repeating sweep”

The first question might be is it normal for the Hold-Off circuit to free run?
I have not found a configuration where it does not free-run at a frequency
determined by timing switches 7, 8, 9 and 10.

Test setup
‘92 installed in right-most slot of a 7603. No vertical plug-ins installed.
(The 7603 has been tested with a 7B53A in the right-most slot and a 7A26 in
the left-most slot.) The side covers have been removed from the ‘92. The
side covers are removed from the 7603 giving good access to the main trigger
board and the sweep board.
The external trigger is driven by the cal-out of a 7904 set to 1kHz and 0.4
volts. The termination is off. Trigger is set to Auto, DC coupling,
External. The “Alt” and “Level” controls are pushed in. Sweep speed
set to 20 uSec/division. The “Triggered” light is off when powered-up but
come on and stays on if single-sweep is selected.

U310 Voltages, taken with Fluke 115, Waveforms observed with 7409/7A26/7B53A
combo:
Pin # name Voltage
1. +TD input 1.159
2. -TD input 0.232
3. + TD Auto 0.331
4. - TD Auto 4.890
5. -5V -4.980
6. Auto timing 0.022
7. Tgd Lamp 4.602
8. HO timing 2.239 + Hold-Off ramp, ~4V P-P, 40 uSec
9. 0.001
10. HO out 0.289 + 0.5 usec 6V pulses at HO frequency (25kHz)
11. SS ready 4.663
12. 0.013
13. 0.216
14. Reset timing 4.716
15. Reset start 0.762
16. 0.074 2V 200 nsec pulses
17. SWP disable 1.686 3 V 200 nsec bipolar pulses
18. Lockout input -0.006
19. Auto Mode 0.001
20. +5V 4.889
And:
TP 392 4.367V
TP 321 1.429V
C 1.61V + Hold-off pulses
E 0.115V + Hold-off pulses
CR740 (TD) ~1.5uSec, ~600mv pulses at Hold-Off ramp freqeuncy
Q754 base 0.747V
Q748 base 0.790V


 

I have a general question about the diagnostic approach to this instrument: how are people figuring out what should be seen at a given test point?

There are lots of test points on the sweep board, but I don't see any central guidance in the service manuals for what I should measure at them. The service manuals for the 7B92 seem awfully short on diagnostic guidance, unlike some Tek service manuals I have used.

-- Jeff Dutky


Harvey White
 

I can give some general answers, but perhaps not as helpful as you'd want.

Considering the trigger circuits:  A good approach is to take the input waveform, compare to a level, then produce a waveform that is above or below a threshold.  For slope, if the trigger is > than the threshold, it's + going, if not, it's minus going.

At this point, you might have used a fast comparator (with hysterisis), or possibly a tunnel diode (which also provides hysteresis).  This comparator output (possibly a square wave) is differentiated to produce a very fast rise time pulse.

The way the older scopes worked (and this may still be valid), was that there would be a coupled monostable multivibrator, think a self triggering flip flop that could be in one of two states (for bias), one is not running, the other is self triggering.

Putting a trigger pulse into the multivibrator causes it to trigger, which then makes the ramp generator run.  With the ramp running, the ramp cannot be triggered again until the ramp is over, and a holdoff period (often variable) has elapsed.

So the question for the multivibrator is threefold:

1) is it biased so that the trigger pulse cannot trigger it? (ain't running)

2) is it biased so that the trigger pulse is high enough to trigger the the sweep? (triggered mode)

3) is it biased so that without a trigger, it will retrigger itself (auto mode)

The general trigger diagnostics path is to put in a known waveform, and taking the waveforms in the schematic (if any), trace them through.  You should see trigger level, trigger shaping, and then the generation of a fast trigger pulse.

NB: I had a telequipment scope that had a very fragile trigger threshold.  The cause was an open 10 pf capacitor that differentiated the trigger pulse.  Putting a capacitor across that gave a trigger pulse with sufficient amplitude to trigger the sweep.

I don't think that the sweep mechanism varies much in concept (but does in implementation) from 7000 series plugin to another. What I have described may be more suitable to the old 500 series tube scopes, but the concepts should be close.

Once the trigger pulse is there, you need to look at the sweep generator to see if it works at all.  If it does, then that's one thing, if not, then likely the ramp generator is never triggered (or perhaps it is never reset).

Harvey

On 10/26/2021 9:13 PM, Jeff Dutky wrote:
I have a general question about the diagnostic approach to this instrument: how are people figuring out what should be seen at a given test point?

There are lots of test points on the sweep board, but I don't see any central guidance in the service manuals for what I should measure at them. The service manuals for the 7B92 seem awfully short on diagnostic guidance, unlike some Tek service manuals I have used.

-- Jeff Dutky





Ozan
 

On Tue, Oct 26, 2021 at 06:13 PM, Jeff Dutky wrote:


I have a general question about the diagnostic approach to this instrument:
how are people figuring out what should be seen at a given test point?

There are lots of test points on the sweep board, but I don't see any central
guidance in the service manuals for what I should measure at them. The service
manuals for the 7B92 seem awfully short on diagnostic guidance, unlike some
Tek service manuals I have used.

-- Jeff Dutky
--------

For me it is usually based on analysis of the circuits. If you want to hear a more detailed answer please DM me and I can send you details on how I would approach guessing couple of test points.

Ozan