Mark III rebuild troubleshooting...no output!!!

I have basically rebuilt a Mark III. I changed the output transformer, all of the caps, any resistors that were a little out of spec, new tubes etc.
But it has come to a pretty frustrating puzzle.

I have tested the voltages and I think everything seems right. 515v on the B+, 5.2v on the GZ-34 pins 8 and 2, 6.64V on the green filament heaters, and it is biasing fine to 1.56v. But still I am getting no output.

Going through with an audio probe I find that the signal is good all the way until the 6550's coming from board points 1 and 3. The audio signal on pins 3 and 4 of each 6550 is a hum/buzz though and not anything close to the incoming audio signal. Should it be? Is this evidence of bad tubes? I have gone as far as buying a second set of tubes kt88's and still get the same result(they aren't matched though). Even swapping the rectifier tube has no result.

The only things I have left to suspect seem unpromising. The power transformer has a blue and white wire instead of a red/black one going to the diode. Also the diode was updated sometime previous to my ownership. But if the amp is biasing right and voltages seem good I can't see how either of those could be a problem.

Edit: I neglected to say that the output jacks are something I'm suspicious of. This Mark III is wired with 1/4" jacks. Should the ground from the OT be connected to each jack, to another ground, and then go to the speaker?

Any help would be huge.
Steve

the ground speaker post and the negative, usually black, of the OPT, must be connected to chassis star ground, (and ONLY the ground speaker post) so should the B+ ground and driver pcb ground, all should go to the common chassis star ground. And if you're using a 3 wire mains cord, that ground too should be connected to the same star ground!
So, connect the negative of the OPT to the ground/black speaker post, then run a heavy gauge wire, usually the same gauge as the speaker wire, to chassis star ground.

Hey thanks Montanaway...everything you've described is what I assumed and how the amp is. I only doubted on the output ground because the output leads are already so close to grounded anyway and in my experience jacks are often the problem is you get no output but everything tests ok.

I'm going to go over everything for the millionth time i guess, hopefully I'll find the culprit. Still it's odd that everything tested good and has been triple checked yet im getting ZERO output.

Sometimes it's also the little things like checking your speaker connections, interconnects, source (is on and works with other devices), preamp (is on and works with other devices), etc.

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To answer Corndog: I checked and always check that stuff religiously.

Petercapo: Those are the kind of questions I was hoping for. This is a great forum and it's the only thing keeping my frustration/anxiety from boiling over right now. I've been fixing guitars all night and I'm not feeling fresh enough to do all those tests but tomorrow I will get back to you and add some pics as requested. I must admit its looking pretty sexy with those orange drops.

I used the proper replacement OT from triode electronics. It was the power transformer that has a couple of different colored wires, which is the correct model number as well, but instead of red wires from the PT they're grey. The Red/Yellow is just yellow and the red/black is blue and white or black and white and possibly faded over the years.

In response to PeterCapo Voltage are as follows:
Choke to pin 8 of rectifier: 520vdc and 6vac which slowly rises
Choke to B+: 510 VDC and similar to the previous reading in AC..now at 6.4vac and still climbing slowly
Eyelet 4 -511vdc
Eyelet 5 - 477vdc
eyelet 6 - 436 vdc
Bias diode cathode - .17vac
Diode Anode - fluctuating between .250vdc and .365 vdc
Bias caps -
#1(the one after the diode) is rated 50uf at 75vdc and its voltage readiing is the same as the bias diode cathode(theyre connected)
0vdc and 0vdc on the other side to ground.
#2 after the 1k resistor rated at 68uf 400v has -58vdc and 0vac on the negative side and zeros on the plus side which goes to ground

Sending pics to Bob Latino now.

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The can cap was replaced with separate caps and is wired up correctly. The voltage ratings are increased. I do have a schematic and a wiring diagram and although the caps change the overall appearance dramatically , the amp has been brought back to stock as faithfully as possible. The only exceptions being those two bias caps. Also the switch is on the bottom of one pot, and only the volume pot has been kept in the tone circuit.

I'll check on the bias cap not being connected to the diode.

What voltages should I expect to see on the bias caps?

Also i was hopinh bob might have some idea whats going on with the blue and white and yellow wires coming from the power transformer. Is this something that he's ever seen? Ive retraced and checked every connection in this amp and I believe it's correct. The only mysteries to me are those two wires.

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PeterCapo:

I don't blame you for seeing what looks quite different from the stock Mark3. You are correct on many things. But let's be clear on this; I have digested and stared at both the original schematic and wiring diagram and completely rebuilt it on that.

When I received it, it had been modified significantly and had an entire tone circuit which I tore down. The "extra tube" you're seeing is the preamp socket but they tried to rewire it as...well something messy. So I have returned that to stock, its just mounted on the other side because that what they did.

The only pot that's doing anything here now is the volume pot which is as simple as it gets as a shunt to ground. The other pot which appears connected is only an on/off power switch. A removable fuse holder has been put in place and a 3 prong plug.

Also the negative of the first cap is connected to the negative of the diode correctly upon checking it.

The Power transformer says " DYNACO 464003 P782" and should be the P782 inside I assume, but its become the point of doubt now for me with those odd colored wire in place of what should be there. I guess I may have to order a replacement but first I think I'll test it according to bob's info.

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I'd love to believe that the red/black or in this case the blue/white which may be faded should go to the diodes cathode and fix my problems, but doesnt the diagram say otherwise? the diagram says red/black to the anode.

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Maybe it's time for a complete rebuild. Strip it down. Clean up the connections. Maybe get some fresh sockets. Check the PT to make sure the raw raw voltages are what they're supposed to be. Consider replacing that choke. etc.

It's a lot easier to troubleshoot when you build it back up from scratch. Take your time.

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Yeah that's the diode alright. One of the few remaining "inherited" parts in the machine. It's an AB1 or ABI 6637. The positive is going to the blue/white wire and the negative is going to the negative of the 50uf cap.

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I have checked it several times now due to you suspecting it being backwards, and my answer is the same. The end marked with a plus goes to the blue/white or red/black and the negative goes to negative of the bias cap. That is correct isn't it???

I haven't been able to find anything on it either. It seems to be out of production. I'm ordering some parts soon for another project and was thinking I might as well throw in a new diode.