JCM800 with low power, distorted output

[BrianS]

So I've got this mid 1980's 2204 jcm800 on the bench right now that has a low power, distorted output signal.  The distortion sounds good, like rock and roll distortion, but there is virtually no clean signal, and with everything on ten this thing would not be as loud as a little vox AC4TV.

So, when I opened it up, I found that the power supply was messed with and the PT is not original and is not the same specs as an original.  It took me a while to figure out that most of the "mods" were to get this thing to work with the new PT.  So, after futzing around with the power supply, I find that I have the correct voltages throughout the amp, but I still have the low, distorted output.

I also swapped in all new tubes and end up with the same result.  Pots, jacks, switches and tube sockets are all cleaned, too.

I get the feeling that an o-scope would really come in handy right now, but I do not have one.  Maybe I need to unhook the OT and do some diagnostic tests on it?  It is the original Drake.

Any common component failure that would cause the low output problem?  Trouble shooting advice will be greatly appreciated!

Brian

[John]

If it were me I'd try using a listening amp- it's under Doug's Library of Information page. Or do a forum search and see how other's have done theirs. You just need to find where the signal is going to ground, I *think*. If there were multiple mods done by the last guy, a wiring error somewhere is very likely. Hope this helps!

[BrianS]

I do have a listening amp that I built shortly after finding this site, and I'll give it a try.  I'm just thinking that it won't show me how much each stage is amplifying, and I'm not sure I will be able to hear the distortion through my tinny little computer speaker. 

We'll see if it tells me anything.  Thanks, John!

[SILVERGUN]

Take a look at the input jacks...

I once had a low input jack tab that was not making great contact, and since the high input signal goes through the low input jack I had a greatly reduced signal.

Almost blew my ears out when I found it...I had a signal gen. feeding the high input side and chopsticked the faulty tab and WEEE

Good luck Brian

[phsyconoodler]

Check the screen resistors and any effects loop jacks if this amp even has an effects loop.
Also make 100% sure the negative feedback wire is connected.The amp will go nuts without it and either have very low output or none at all.
   If you have a bias meter what is the current running throught the power tubes?
Also you might want to remove the output transformer wires and measure the resistance from the center tap to each leg.If it's wonky you have a bad OT.Not uncommon but after youve checked everything else.

[BrianS]

Thanks for the tips!

I did use my listening amp today and I seem to be getting good signal up until I hit the cathode follower that drives the tone stack.  When I listen there, the signal...so it seems to me...seems weak and distorted...but I'm not convinced I'm hearing a problem...but maybe.

Poking around in this area of the circuit, I don't see or measure any issues so far.  Whatever the problem is, it's not obvious to me.

I think if I had the proper test equipment I could track this one down faster.

I installed the 1 ohm resistors on the power tubes so I could bias the amp and I have the tubes running at right around 38 milliamps each.  The screen grid resistors are fine and the B+ seems normal...I'll certainly have to check the negative feedback wire as I haven't confirmed it's hooked up correctly.

This circuit is so simple, you'd think the problem would just jump out at you, but it's not.  I'll also have to unhook and check the OT.

Thanks again!

[John]

Check the components and solder joints right around your cathode follower then. A cold joint might look fine, but give you problems. When your injecting and listening to the signal, whack all the joints and components with a plastic chopstick of some sort, hopefully something shows up!

[jjasilli]

Use troubleshooting flow charts, e.g.:  http://www.geofex.com/ampdbug/ampdebug.htm

[BrianS]

uffda...tried all the suggestions...nothing yet.

I had an idea, but not sure if it is wise or would be useful:

-Could I basically disconnect the entire preamp right before the master volume and then inject a line level signal there, in order to just test the power amp section?  I recently did this with a solid state amp and it worked well.

It looks like the entire preamp section is connected to the master volume/power amp via the wiper of the treble control.

 

[jjasilli]

Yes, you could inject signal anywhere; or send signal to a Listening Amp, a/k/a signal tracer.  See Doug's Library of Information. 

[BrianS]

Just tested the pre and power sections separately...the problem is in the power section somewhere without question. 

First I separated the pre and power sections at the treble control and hooked an ipod up to the master volume...distorted signal at all master volume settings and low output.

Second, I hooked the preamp up to a 100 watt solid state power amp.  Plugged my guitar in...whoa!!  Nice, clean signal and very, very loud.  My power amp does not have a volume control as of yet, so I could not crank the Marshall preamp without blowing my shop apart and loosening my neighbors fillings.

Now I'm going to concentrate on checking components around the phase inverter as well as look into a more thorough test of the OT.  It passed the basic ohm test as suggested above by phsyco.

[jjasilli]

 

[BrianS]

After some more thorough testing, I determined that the OT was shot.  If I'm reading right, there is a short in the core.

Replaced the OT with one from Doug and the amp is up and running again.

An O-scope & signal generator would have helped me solve this problem much faster. 

[Cliff Schecht]

Take a look at the input jacks...

I once had a low input jack tab that was not making great contact, and since the high input signal goes through the low input jack I had a greatly reduced signal.

Almost blew my ears out when I found it...I had a signal gen. feeding the high input side and chopsticked the faulty tab and WEEE

Good luck Brian

This cracked me up! I've scared the living crap out of myself a handful of times doing this same thing.

After some more thorough testing, I determined that the OT was shot.  If I'm reading right, there is a short in the core.

Replaced the OT with one from Doug and the amp is up and running again.

An O-scope & signal generator would have helped me solve this problem much faster. 

Not necessarily, I probably would have noticed this with just a multimeter. Plate voltages will be unbalanced and you can confirm by measuring the OT resistance and/or output tube current draw on each side of the OT. Glad you figured out the issue though!