JCM 800 build..... No Sound at all...

[colinaae]

Hey everyone.

So I have been picking away at my first build. And this is the second time I've built it since it didnt work the first time. I had no sound when plugged in so I figured it was an output thing. Just for fun I got a second output transformer to make sure. I still had no sound. I'm having a hell of a time trying to pin point what the problem is? The next thing I want to look at and what I need help with is.... If the filter can caps are bad or if the choke is bad will that create this problem? I'm just stumped and I don't want to replace every component if I just knew a little more with your help.

Please can someone help me so I can finally play this thing

Colin

[ampgeek]

Welcome Colin!

Start by taking a handful of high rez pictures of the inside and outside of the amp and posting them up here.

Next, measure and record the voltages at all tube pins and report them here.

That will get the collective mind gig churning here and will likely identify the issue(s).

And..yes...the last thing that you want to do is to start "shotgunning" component replacement.

Cheers,
Dave O.

[statorvane]

Getting the input jack wiring can be confusing the first time (and 2nd and 3rd...).

[menger66]

Post some pix after that its simple these guys will help you get it going thats what were all here for to help and be helped, most guys need a little help with there first build, and most guys start with simpler project too get more versed in amp building lingo with pix and some basic voltages you'll start too see where you went wrong and start to sort it out. mark

[colinaae]

Ok thanks guys.

I have a bit of time tonight to take a few pics and the voltage readings. I will post the pics ASAP.
I appreciate the help so far.

Colin

[colinaae]

Hey guys

I should say this. When ever I turn the amp on without the tubes in the 500mA fuse blows right away. I see it ARC at the fuse and I also see it ARC where the red wires hook up to the diode from the power transformer. ill Add the pic of where it ARCs.

Colin

[Jana]

...

[Stevem]

Blowing the HT fuse when all the tubes are out of the amp is new one on me after my 42 years of repair work I gotta say!!
I am sorry to say this but the wiring around the output tubes is a mess, and that photo does not help much, but I do think I see a issue.
Your red bias test points and resistors that are hanging in that photo seam to have a problem.
The resistor goes from pin 8 and 1 of the the output tube to ground if you are using EL34 tubes, and the take off point for the bias test point should be on the tube side of the resistor.
If the non tube side of that resistor is not grounded, than you`ve got problems.
It also looks like the stripped end of the red wire going to the fuse holder is contacting the lead of one of those resistors, as in short circuit.
If the output tubes do not have a
there cathode ground they are going to red plate, pull masive current thru the OT and maybe toast it, along with killing the tubes if powered up long enought.

[martin manning]

The nut or screw head (in the other picture) is at ground potential, and too close to the red HT lead. Once it arcs, a carbon track can be created and it will happen more easily. Maybe you can temporarily replace the metal screw, nut and stand-off with nylon parts and get on to solving other problems.

[colinaae]

Hey guys.
Thanks to Martin manning you we're absolutely correct. The screw head was arcing off the red wire. Now that the 500mA fuse isn't blowing I can finally get my reading. They seem low but maybe someone can find where I have went wrong. And I am still getting no sound at all.
Here are my reading:

V1- 1- 56.3VDC
2- 0
3- 0
4,5- 3.4VAC
6- 48.6VDC
7- 0
8- 0
9- 3.4VAC

V2- 1- 48.9VDC
2- 0
3- 0
4,5- 3.4VAC
6- 48.8VDC
7- 47.7VDC
8- 0
9- 3.4VAC

V3- 1- 46.6VDC
2- 5.5mVDC
3- 5.5mVDC
4,5- 3.4VAC
6- 45.5VDC
7- 9.7mVDC
8- 5.5mVDC
9- 3.4VAC

V4- 1- 0
2- 3.4VAC
3- 45.2VDC
4- 44.9VDC
5- 55.2VDC
6- 44.5VDC
7- 3.4VAC
8- 0

V5- 1- 0
2- 3.4VAC
3- 43.9VDC
4- 43.7VDC
5- 55.2VDC
6- 43.4VDC
7- 3.4VAC
8- 0

That's what I got for reading. I already appreciate the help you guys have given me. So if you can find where I went wrong I will try anything you throw at me.

Thanks a million

Colin

[ampgeek]

Making progress!!

Yep......very meger VDC across the board.

Measure the VAC right where the PT secondary hits the rectifier diodes and let us know what you find. One DVM lead on one side and the other lead on the other side. Again..AC voltage there.

Also measure the DC voltage to ground right where the two diode strings come together.

This ought to tell us how healthy your PT secondary is and whether or not your full wave rectifier string is doing its thing.

Good luck,
Dave O.

[xtian]

Yeah, your voltages are very wrong. Time to start with some basics. Remove all the tubes.

First, measure voltages at the power tube sockets. Pins 3 should have full B+, on the order of 400+ volts DC. Also, pins 5 should have your NEGATIVE bias voltage, in the range of -30 to -50 v DC.

Let's see if we can get that far.

[tictac]

Dude.... that is one sloppy looking mess. Why not look at some other examples of neat looking builds posted on this forum, then gut that thing and start over. Take your time and do a neat looking layout and lead dress.

Then you'll have something to be proud of and that we can troubleshoot better....

I wouldn't feel safe working inside an amp like that, and don't like the fact that you're poking around in there near lethal voltages with wires going all over the place....

TT

[rp]

Maybe God is testing us?

Dude....

+1 to everything tictac said. Even if you do get it running what's the point? It'll be an amp holding on to life by a single hair. I don't think it'll survive a trip across the room much less out of the house. Not only is it real dangerous to you but you risk burning some place down. Very best senario, and very likely, you'll be loosing a few hundred bucks in parts.

We all have to start somewhere, but this is electricity and you have to start with some discipline. Do what tictac said, web is full of people documenting step by step their Marshall builds. Just follow one. Neat, short and clean - take one wire, one part, one solder joint, one stage, at a time. You can wire a 747 or the space shuttle if you approach it like that.

I pulled these off the web, here's some very nice Marshall JTM types to give you an idea. Just google image for hundreds of more images and layouts of the JCM800 or 2204 for you to follow.

[rp]

and, check out the TAG light bulb limiter stickie.