Re: How to troubleshoot a faulty 7B92A?

Mark Vincent


I am glad you are making progress. The schematic shows that when the div. knob is pushed (usual operation), that grounds the 74112 through the 68 ohm resistor (R861) J & K inputs making the Q outputs static as you see. When it is pulled, the 74112 oscillates. The function table shows the states of the inputs and outputs. From the table, this IC appears to be working correct.

See if you have the 400mV pulses at the collector of Q358. This is waveform 7 on <3>. You can pull and reinsert the short coax between J260 and J270. The inner contact could be dirty. The trig. sens. pot may need to be adjusted. The TD bias pots may also need adjusting a bit. There should be a pulse at DA at ,5V d-c.

Now back to the other side of the plug-in on the interface board. Check the waveforms 1, 2, 3 and 5-11 on page <5>. See if you have them there. Also check the transistors there with the number mentioned earlier if you have not already done so. The leakage in these will be from collector to emitter as a diode check. An analogue meter will have the needle vertical on the Rx1 scale. Usually the same number that has a leakage of the 0367 in something, the others likely will be leaky as well. The manual will tell you the Q numbers.


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