Hey there,
I have this very old osc. It was acting out on the very low freq range and I tried to repair it, fiddling around I broke it even more.
Output now shows NOTHING (Very small DC on the main output). I cannot figure out what could be wrong with it. I have the schematic and data for this device. Can someone please help me think this through? Where do I start?
I have this very old osc. It was acting out on the very low freq range and I tried to repair it, fiddling around I broke it even more.
Output now shows NOTHING (Very small DC on the main output). I cannot figure out what could be wrong with it. I have the schematic and data for this device. Can someone please help me think this through? Where do I start?
What did you fiddle with? Have you done the basics- check power suppy voltages, check for oscillation at the oscillator circuit (Volume control clockwise pin) etc. . . Do you have an oscilloscope to check with?
Thank you for trying to help.
Trying to solve the very low freq range distortions, the last thing I did before the oscillator went DC, was try to trim R202 (both A and B) by pulling them both out and installing a 1k trimmer instead of them. Ever since, output is very close to DC null (no indiction of oscillations at all, no matter what voltage is set at the output knob or freq. multiplier is selected). By the way, the 1 vrms output is also giving me DC but not zero.
Its like its not oscillating anymore. At all.
I checked Q201 (the FET receiving the oscillation signal) and everything seems DC In all pins. I pulled it out and check the device itself in a transistor tester and it seems to be OK. The SM indicates 11 volts at the collector of Q201 (it was present and fine when everything was humming) and now its on 13.4v.
Power supply seems OK. Both 22v and -22v are present and very stable.
Sure, I have a scope. I don't understand why and how everything stopped oscillating, and how to go about finding the fault.
Trying to solve the very low freq range distortions, the last thing I did before the oscillator went DC, was try to trim R202 (both A and B) by pulling them both out and installing a 1k trimmer instead of them. Ever since, output is very close to DC null (no indiction of oscillations at all, no matter what voltage is set at the output knob or freq. multiplier is selected). By the way, the 1 vrms output is also giving me DC but not zero.
Its like its not oscillating anymore. At all.
I checked Q201 (the FET receiving the oscillation signal) and everything seems DC In all pins. I pulled it out and check the device itself in a transistor tester and it seems to be OK. The SM indicates 11 volts at the collector of Q201 (it was present and fine when everything was humming) and now its on 13.4v.
Power supply seems OK. Both 22v and -22v are present and very stable.
Sure, I have a scope. I don't understand why and how everything stopped oscillating, and how to go about finding the fault.
Those components set DC bias it seems and the Jfet is a switch engaging them at HF. The range seems to be 2.5K to zero. It should function anyway. Check for shorts, opens, broken traces first. There are a bunch of very sophisticated tricks in that instrument to get it working in all the ranges making understanding of it all difficult. Maybe an open around L201 could cause the symptoms. What is the voltage at the base of Q202 and Q203? They should be almost the same and close to the voltage at FO.