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 “ 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
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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