JCM 2204 Ceriatone build

[Oddvar]

HI.  I was wondering if anyone could hint on where to look when a JCM 800 build doesn't make any sound, exept a very low hum in the speaker when turned on.  I have gone through most of the amp many times, but doesn't seem to find the problem.  Voltage seems to be ok the places B+ etc. 
The only thing I am a bit uncertain of is the voltage coming from the transformer to the board by the rectifiere, only about 25 millivolts there...
No reaction on any pot, no sounds what so ever touching inside the amp...

I have changed all tubes as well...

[kagliostro]

http://el34world.com/Forum/index.php?topic=17701.0


Ciao


Franco

[hesamadman]

Does this layout implement both high and low input jacks?

[Oddvar]

Yes it does...(Non works)

[sluckey]

The only thing I am a bit uncertain of is the voltage coming from the transformer to the board by the rectifiere, only about 25 millivolts there...

Maybe your meter is set to measure DC volts? Change it to measure AC volts and you'll probably have about 325VAC.

[hesamadman]

If you're getting proper B+, the PT secondary has to read more than mv.


Ok. Well first thing. It's not in the first gain stage. However....it could be your input jack wiring. If wired incorrectly, you could be grounding out the signal. Those should be switched jacks and are meant to ground certain portions when not in use.


Part of my trouble shooting method that I use is, Go to phase inverter input, disconnect it, use alligater clips to an input jack, make sure your power amp is working. Then you can narrow it down to be if it's in preamp or power amp.

[Oddvar]

The only thing I am a bit uncertain of is the voltage coming from the transformer to the board by the rectifiere, only about 25 millivolts there...

Maybe your meter is set to measure DC volts? Change it to measure AC volts and you'll probably have about 325VAC.

Meter was set to DC, you are right, again.  Measure 707 v.

[Oddvar]

This is how it looks like at the moment.  I am a bit uncertain if the jacks are done right.

[sluckey]

The input jacks on the 2204 can be tricky to wire correctly due to the clever way the jack switches work. Study this schematic and insure your jacks are wired accordingly.

     http://el34world.com/charts/Schematics/files/marshall/Marshall_JCM800_2204_Lead_Series.pdf

[hesamadman]

A quick test would be to de solder the paralleled 470k/500p cap from the input jack to the gain pot. Just disconnect it from the jack not the pot. Use alligator clips on the end of your guitar cable. Plug into your guitar. Clip a wire onto the end of instrument cable to chassis. Clip a wire on the tip of the cable to the 470k/500p cap. This bypasses the first gain stage and the input jacks.

[Oddvar]

No reaction at all when I tried that now...

[silverfox]

Amp only has low hum

That is a subjective statement. I built an amp that has a low hum as soon as I turn the power switch on. No warm up time for the tubes. That is because the 60 cycle hum is being directly coupled to the output transformer at a low level.

Try this: Plug just a guitar cable into the return jack of the send unit and see if you get any buzz through the power amp. If you still don't get any sound out check your power supply voltages. I can't tell if the power supply capacitors are wired correctly or not from the picture. How experienced are you working with high voltages. There is enough present on the power supply to Kaknock you over if you contact it with your body.

If you are still not able to find the problem, start checking the power supply voltages and post them here starting with the power supply.

silverfox.

[Oddvar]

Thanks Silverfox, but no sound in the sendsection.  This is my tenth tube build, but I'm not a voltage man, more like a musician trying to understand what seperates good sound from better ones. This build seems a bit different than the others, it's making no sound what so ever, whatever I do.  The hum, when turned on is low, I can hear it when I put my ear to the speaker.  That's it. and I have gone over it many times.

[shooter]

This is my tenth tube build, but I'm not a voltage man

I'm a voltage guy NOT a musician, but I had to learn to make cool sounds from the guitar to "prove" my voltages, time to learn meters 101

[hesamadman]

Again, from here, all you have to do is narrow it down. We determined it's not in the jacks. Just de solder certain parts, clip in your guitar cable and see what happens. Disconnect the middle lug of the MV. This is your PI input. Clip your guitar cable there. Any sound? If no, look over your power amp components.

[silverfox]

Just to be sure, you want to plug the cable into the Return Jack. If that is what you did and got no sound, then in many respects the simplest half of the amp is not working. Not to say that is the source of the problem but if that's not functioning the problem is likely global. You are going to have to measure voltages at least, to simplify the troubleshooting process.

One of the greatest hazards in measuring voltage is, inadvertently touching the meter probe to a nearby terminal and causing a short-spark that scares the crap out of you. Here's an idea. Turn the amp off and unplug it. Now practice measuring the voltage at various points. Be sure to develop good practices such as: Don't lean with one hand on something while taking the measurement. Always keep one hand at your side, behind your back at best so as not to complete the circuit through your body should you contact high voltage.

After you've practiced a while, your ready to take measurements under power.

Does the power on indicator light? I would have suggested using a dim bulb tester but at this point it's moot. If this uses a rectifier tube, make sure that is the tube in the rectifier socket. Don't know what would have happened if it wasn't but thought I'd suggest it.

Welcome to troubleshooting.... Someone else is typing while I'm getting ready to post. Perhaps duplicate info.

silverfox.

[hesamadman]

Sorry. If you have fx loop, disregard my last comment. I'm not familiar with Ceriatones 2204. Mine does not have fx loop.

[MFowler]

Here is the voltage chart for the Ceriatone 2204 120v version