Re: Troubleshooting and repairing a 7704A instrument

Albert Otten


Not included is the 155-0078, the big hybrid at the output of the vertical amplifier. This one
may be still withheld as a trade secret?
? There is a lot of info about 155-0078-xx (M84) versions at Tekwiki and also a schematic.

What I find is that two transistors in the differential output drive, U4356
and U4366, get very hot, even after having squelched the 50 Mhz oscillation.
I question if this is normal? Time will tell.
The whole area around that output stage gets quite hot. After half an hour with the sweep free running at 1 ms/div I measured about 55-60 C with an IR thermometer and 60 C with a P6058 temperature probe right at the top of the Q4365 an Q4366. P6058 reading not corrected for measurement influence, so slightly >60 C in reality.


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