7934 Disassembled

Karin Johnson

Hi All:
I thought you might want to see what a Tek 7934 looks like when it is taken all apart. The photo was uploaded to the Photos section of this list. This photo was taken 2 days ago, and since then I've made good progress in putting is all back together. All of the front panel switches and cables are connected correctly and scoped out. The top and bottom are mated together, but the remaining cables between top and bottom have yet to be connected.
I am taking this very slowly and so far so good. Of course the final test will be, does it work. On the faulty power supply issue that started all of this madness, there is also some good progress.

It turns out there was a short on one of the main switching transistors. Q40. There is a small insulated bushing that fastens this transistor to the heat sink. The heat sink is not at ground potential, it floats at the negative terminal of the bridge rectifier which is fed from the AC line. the external finned heat sink is at ground but there is a a mylar insulator sheet between the external fins and the power supply sub-chassis. It took me several days to work up enough courage to start removing components. It also turns out that my previous post about a faulty programmable unijunction transistor was INCORRECT. The symptom overall was that the inverter would not start. Troubleshooting the power supply showed that with a negative lead on TP46 and the positive lead on the cathode of CR38 the voltage would never rise above 10 volts DC. I am using a 7A22 differential plug in with my Tek 7603 to make these measurements. I finally lifted the cathode end of CR49 and the path to the short was detected, because now the voltage on the cathode of CR38 would rise above the previously mentioned 10 volts. It was also confirmed with ohm meter readings.
All of this troubleshooting is being done with a variac feeding the AC line to the 7934 power supply. The variac is very important to limit the line voltage and prevent the supply from going into tick mode. All of the loads are disconnected while I am testing.
One very important thing that I didn't pay to much attention to, caused me to overlook these transistors in the first place. The troubleshooting steps listed in the service manual, specifically indicate to remove Q34 and Q40 and test them separately, I did in circuit ohm meter tests and it did not show the short. The short is actually a high resistance short, just enough to prevent the starting circuit from working, but not enough to detect via in circuit ohm meter testing.
Lesson for self. Read the service manual and follow it religiously.

Onward and Upward,


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