JCM 2000 DSL no output

[sweatyk]

On the low side of R63 without the variac it reads 452VDC. Is that pushing it too much? I'm not sure I trust this old variac. When I used it starting out at the low end of R63 I had it dialed at about 320VDC but as I increased it I heard a strange noise somewhere in the amp that sounded like some buzzing like when fly or insect gets caught in a small place. I turned it back down and removed it. Will I be safe with the 452VDC on the low side of R63?

[mozwell]

there are 12V relays in the amp. These are run two relay coils in series from 24VDC. As you turn the mains voltage up, this 24VDC will go up. The buzz you hear is the relays trying to pull in when the 24V supply is too low. Nothing to worry about. Turn the mains supply up so you measure approx 430 to 445V at the low side of R63

[DrGonz78]

I can already predict your next piece of equipment is going to be a load resistor. This is all really great stuff to learn though!! With a load resistor you will be able to trace the signal easier without having to hook up a speaker. Keep that thought in the back of your mind and continue onward for now.

[sweatyk]

Thanks Mozwell that was most helpful. So I got the variac dialed in reading 440VDC on the low side of R63. Hook up the tone generator. Ground my probe and plugged it into my new Radio Shack mini amplifier/speaker and..................nothing. First use and the mini amp decides to not work. Took it up to the local RS store and they are out and so are the surrounding stores. 3 to 5 days to get it replaced........ARGH. It's a sign to back off and chill so that is what I will do until the replacement comes in. Just had to vent.

[mozwell]

Keep in mind, with your mini amp & speaker, when you probe various places in the amp to follow the signal, some are at a high DC voltage with the AC on top of it, like the anode resistor on a valve. Here you may have a 20VAC signal at a DC level of (say) 150V. The 150V will kill the input to your mini amp.
You MUST use a DC blocking capacitor on the input of your mini amp or you may damage it. (someone here already said this but I cant remember which post it was)
100n 600V will do the job.

You can also just measure at the gate for each tube, as here the signals will be generally ground referenced, and in the order of a few VAC only. Use the schematic to work out the best places to attach the voltage probe.

[sweatyk]

The probe I made has a .1uf @ 600v cap. The mini amp faulted prior to using it. I never had a chance to touch the circuit.

[DrGonz78]

This is the practice amp that I use for when I do signal tracing. $5 for the amp and it already paid itself off in one repair job.



My point is to just connect that probe up to any old throw away cheapo amp you can muster up on craigslist or something. Another benefit is the speaker in the 15 watt practice amp is way better than those lil tiny speakers in the radioshack units. Either way they do the job.

[sweatyk]

Dr. Gonz.....space is a big factor in my choice for using that mini amp. I really need to reorganize and get rid of some stuff.

[sweatyk]

Alright I have my test rig set up. Now here is where my lack of skills comes in. Tracing the signal via the schematic, I can find generalized areas with a schematic however tracing the path of the signal is where I get lost staring at it. I have yet to master this. Any assistance would be great. This is the learning curve for me. Thanks

[sweatyk]

Alright I have my test rig set up. Now here is where my lack of skills comes in. Tracing the signal via the schematic, I can find generalized areas with a schematic however tracing the path of the signal is where I get lost staring at it. I have yet to master this. Any assistance would be great. This is the learning curve for me. Thanks

I posted this 5 days ago and not one response. Did I say something wrong?

[The Dude]

Nope, sometimes threads get "lost in the shuffle". The last schematic posted in the thread won't download for me, so I'm unsure of what circuit we're looking at. If I knew what we were dealing with, I could be more specific. I'm also a bit unsure what your asking in the previous post. The question you posed is a bit vague and general.

From what I can gather from a quick read of the thread, you have a tone generator of some kind and an audio test probe? Insert a signal into the amp. Follow it with your test probe and see where it gets lost. At each tube stage, signal will be fed to a grid and the amplified signal will be at the plate, and then on to the next stage, and so on. Follow the schematic from tube to tube and find the place where there is no or low signal.

[Jazz P Bass]

What are we fixing?

[The Dude]

Jazz,

Near as I can tell, there is no preamp output and inputing a signal to the power amp directly there is sound. Preamp tubes have plate voltage and +-15V is there.

[drewl]

Check the 100k cathode resistor that feeds the tone stack.

The original 1/4 watt ones often opened up until they changed it to 1/2 watt.

[drewl]

Check the 100k cathode resistor that feeds the tone stack.

The original 1/4 watt ones often opened up until they changed it to 1/2 watt.