Re: Help with (another) broken 7B92 please


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).


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 “ 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
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
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

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