So far for the obvious damage. After replacing these parts, it still did not work however. Checking the supply voltages: it has two 5V rails, one is used for the front panel display, the other however, independent of the first, is used to supply the actual signal generation part. It sat at 1.8V. R131, the big 5W resistor, got really hot. It had a voltage drop of over 12V across it. First instinct was to check the voltage regulator U6. It checked out fine. So something was pulling this rail low.
U23, the CA3046 transistor array to the bottom left, wasn't properly soldered in. So this was one of the first things I fixed. I of course socketed it, which meant that taking it out of the circuit was a no brainer. Not the fault.
Q13 and Q14 also checked out fine. Next instinct was U14, the SN75107 line reciever. I've seen a lot of bad 75 series parts. Taking it out of circuit didn't bring the rail back to 5V either.
Q13 and Q14 also checked out fine. Next instinct was U14, the SN75107 line reciever. I've seen a lot of bad 75 series parts. Taking it out of circuit didn't bring the rail back to 5V either.
It can run on both AC as well as DC. To achieve this it rectifies the AC into DC and then uses the DC in a very early switch mode power supply. The connections that go to the board to the left only go to two rectifier diodes that were placed on there. The board on the right generates an ~8 kHz switching signal to drive T1, T2 which then drive the transformer
Except the oscillator was dead as a dodo.
Except the oscillator was dead as a dodo.