JCM 2000 DSL no output

[Drewline]

Have you checked & replaced C46. We always replace it with a 22pf 2kV.

[sweatyk]

Have you checked & replaced C46. We always replace it with a 22pf 2kV.

I haven't but it appears that it was replaced at one time in it's life. Based on the burnt area on the PCB it met a violent end. Could this have an affect on the current symptom?

[The Dude]

IF your power amp is working correctly and we're still chasing a preamp problem, C46 wouldn't have anything to do with it. It is a good idea, nonetheless to check this cap whenever you open up one of these amps.

[Drewline]

I wouldn't rule it out. The easiest way to check it would be to remove it. It is there to stop oscillation.

[sweatyk]

Just got thru building my audio test probe. Signal generator next or just might use an app on my phone.

[sweatyk]

OK time for dumb questions. First I'm going to use a signal generator app from my phone. So does it matter what frequency I use as long as I can hear it? And when tracing a signal I'm assuming the amp has to be powered on? I only ask because I have never done this before.

[g1]

Any audible frequency is fine. 1K is fairly annoying, I normally use 400hz, but you could go even lower if you want.
To trace signal, you power up, run the signal into the amp with a level usually around 100mV AC at the input jack. Set up the controls to normal use type settings. The follow along the signal path with your tracer.
Some places will normally have drops in level, so you will need to have an idea from the schematic.

[sweatyk]

One last newbie question before I begin. If the power tubes are removed is it necessary to have a speaker load on the amp while it's on? I've always plugged in regardless just to be safe but it is a curiosity.

[Jazz P Bass]

With the power tubes removed, there is not a requirement to have a load hooked up.

Best to keep in mind that transformers work on a Volts AC basis.
With the power tubes removed, there will be no 'change' in the primary winding of the OT.

[DrGonz78]

When operating the amp without output tubes is the one time you will not be required to connect up a load. Please note that the B+ voltage will be higher with the output tubes removed. Monitor the B+ to make sure it does not exceed the voltage rating on your filter caps. I like to monitor the B+ in this instance while gradually turning up the line voltage via a variac.

Edit: Jazz beat me to the answer... lol. Looks like I need a cup of coffee to get moving faster

[sweatyk]

Hmmmm wasn't considering the B+ on the filter caps. I might just put the power tubes in and hook up a speaker just to be on the safe side. I hadn't been able to do that because of the height of the power tubes until yesterday when I decided to build an amp stand so that the tubes would clear and I could set it upside down.

[sweatyk]

Just a quick check on one side of R60 with the power tubes out I'm getting about 512VDC.......a little too much for the filter caps to handle huh? If I had a variac I could dial it down some right?

[g1]

Yes, you could dial it down with a variac, but you may not need to.
See if R63 is dropping the voltage to 450V or less. If so, you are ok.
The earlier filter stages on either side of R71, are totem pole arrangement. So you have two 315V caps in series that can handle 630V.
The filter stages after R63 are only 450V, so that is where you may have an issue.

[sweatyk]

Well it wont be an issue now. I contacted a friend that actually had a variac and told me to come get it. It looks like it came over on the Mayflower but that's OK as long as it works. So what voltage should I keep it under and from what point?

[g1]

Keep the low side of R63 below 450V.
Don't trust the labelled numbers, verify with your meter.
You don't want to reduce the voltages too much, low heater voltage or op amp supply voltages can cause dropouts which will make signal tracing difficult.