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12-22-2023, 03:47 AM
Hi Guys
Everyone has their own style and approach for repairing amps. personally, I hate repairs and take a bit of a paranoid approach to it to reduce damaging anything further than what is already broken.
I ALWAYS REMOVE OUTPUT TUBES FIRST
For the VK100, since the symptom is no sound, I would do a resistance check of the OT primary and secondary. Make sure the output jack is open.
You should see steady DC resistance readings for all windings. Any ambiguity likely means there is an open winding and the OT is defective.
You can measure the screen-stops if you suspect them, but they are likely okay.
I do an incremental power-up through the Power Limiting Safety Socket. Connect the negative meter probe to circuit ground using an alligator test lead. This frees one hand for voltage measurements and you can have the other on the power switch for a quick turn-off if required.
Since you already know this amp does not blow fuses, did you check to make sure none are already blown? Do the tubes light up? Assuming those things are okay, measure the voltages on the power tube sockets to see that all the voltages are present, of the correct polarity, and of the correct magnitude. With limiting from the PLSS, all the voltages will be low but proportional. If all the 9-pin tubes are in place, limiting will be modest as the heaters need something close to their rated voltage.
If you have a scope and sine generator, feed a signal into the effects return and see if signal passes to the output sockets pin-5 (grid). You could similarly feed a signal into the guitar input and scope through to the PA, adjusting EQ for a good sine wave rather than using "instrument" settings. Seeing the sine wave retain its shape especially through the PA tells you a lot about the functionality of the circuit.
If everything is satisfactory up to this point, you can power down, install the power tubes, preset the bias for maximum negative voltage on pin-5 (this amp lacks bias controls), then power up with a bit less limiting. With the load on the output open, you should be able to see a good sine wave there, and it should clip evenly positive and negative as the Volume is turned up and down.
It is dreadful that there is no bias adjustment! Poor design. Gives no opportunity to hum-balance the output stage for best tone.
You can attach a speaker and try out the amp. Usually I connect a bench load before that and just sweep the volume up and down to see that the amp can drive the load.
With an older amp like this, solder connections are the main culprit for most symptoms. Solder pads for large leads, say for power resistors, will definitely need to be resoldered NOT simply "touched up". The latter is a sure way to degrade the connection further and guarantee a cold joint sooner than later. For PVs, the insulation-displacement connectors are a weak point, especially for heater power connections. I would solder those terminals to improve reliability.
Everyone has their own style and approach for repairing amps. personally, I hate repairs and take a bit of a paranoid approach to it to reduce damaging anything further than what is already broken.
I ALWAYS REMOVE OUTPUT TUBES FIRST
For the VK100, since the symptom is no sound, I would do a resistance check of the OT primary and secondary. Make sure the output jack is open.
You should see steady DC resistance readings for all windings. Any ambiguity likely means there is an open winding and the OT is defective.
You can measure the screen-stops if you suspect them, but they are likely okay.
I do an incremental power-up through the Power Limiting Safety Socket. Connect the negative meter probe to circuit ground using an alligator test lead. This frees one hand for voltage measurements and you can have the other on the power switch for a quick turn-off if required.
Since you already know this amp does not blow fuses, did you check to make sure none are already blown? Do the tubes light up? Assuming those things are okay, measure the voltages on the power tube sockets to see that all the voltages are present, of the correct polarity, and of the correct magnitude. With limiting from the PLSS, all the voltages will be low but proportional. If all the 9-pin tubes are in place, limiting will be modest as the heaters need something close to their rated voltage.
If you have a scope and sine generator, feed a signal into the effects return and see if signal passes to the output sockets pin-5 (grid). You could similarly feed a signal into the guitar input and scope through to the PA, adjusting EQ for a good sine wave rather than using "instrument" settings. Seeing the sine wave retain its shape especially through the PA tells you a lot about the functionality of the circuit.
If everything is satisfactory up to this point, you can power down, install the power tubes, preset the bias for maximum negative voltage on pin-5 (this amp lacks bias controls), then power up with a bit less limiting. With the load on the output open, you should be able to see a good sine wave there, and it should clip evenly positive and negative as the Volume is turned up and down.
It is dreadful that there is no bias adjustment! Poor design. Gives no opportunity to hum-balance the output stage for best tone.
You can attach a speaker and try out the amp. Usually I connect a bench load before that and just sweep the volume up and down to see that the amp can drive the load.
With an older amp like this, solder connections are the main culprit for most symptoms. Solder pads for large leads, say for power resistors, will definitely need to be resoldered NOT simply "touched up". The latter is a sure way to degrade the connection further and guarantee a cold joint sooner than later. For PVs, the insulation-displacement connectors are a weak point, especially for heater power connections. I would solder those terminals to improve reliability.
