bias issue

[daz]

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What is the ac voltage at the HV winding? Can you post the PSU schematic?
What is the ac ripple on the B+ ?
Bad solder at the filter caps could give you weird problems like this, but I'm assuming there is a speaker connected and no bad hum?

About 700VAC as i recall. I had the PSU board out but put it back in to test something. But i checked the filter caps and thier solder joints and they seem fine. There IS a buzz but it's not like hum due to cap issues, more like a 1-3k buzz or thereabouts.

[g1]

Well, if you have 350Vac on each side AT the diodes, I would think the PT is ok.

[daz]

Well, if you have 350Vac on each side AT the diodes, I would think the PT is ok.

I agree, but something is keeping it over 100VDC low at the plates and screens. Could it be the OT? I don't know what else to check in the PSU that i can see would possibly cause that. I checked the rectifier, the choke, and R2,3,9 and 8. Tho R9 may be suspect because the meter settled at 220k on the other 3 after a few minutes attached in circuit while R9 settled at 143k. But then i just realized thats part of the preamp supply.

[g1]

What is your ripple (AC volts) at the plate supply?
With all power tubes removed, what is the DC at the plate supply?

[daz]

What is your ripple (AC volts) at the plate supply?

150vac....seems odd. Does this mean theres a bad cap? I never thought to measure ac at the plate b4.

With all power tubes removed, what is the DC at the plate supply?

It went down to 325vdc at some point the other day, so now it's 325 w/tubes and 350 w/o.

[g1]

150vac....seems odd. Does this mean theres a bad cap? I never thought to measure ac at the plate b4.

Bad cap (or connection/trace) or shorted rectifier diode.
I'm surprised there was not a major hum problem.

[daz]

Bad cap (or connection/trace) or shorted rectifier diode.
I'm surprised there was not a major hum problem.

Thanks G-one. I guess i'll just shotgun the PSU caps because none seem bad as far as i can test them in circuit. Every diode on the PSU board reads good. But in any case i will have to pull the board and go for it. I just wanted to get a good idea before i did because this board is unbelievable tedious to remove. More connectors than you've seen in your life time and a hole in the middle that all the PT wires come thru and all bundled and tied going all over the place. A frucking nightmare at best ! But sounds like you are quite sure so i'll go at it in a bit here, tho if i do end up having to shotgun the caps i will have to order them. Thanks much !

[Jazz P Bass]

You know you could simply parallel a cap to the B+ & see if it helps.
No rooting, yanking or pulling.

[daz]

You know you could simply parallel a cap to the B+ & see if it helps.
No rooting, yanking or pulling.

Not really....you have to see the construction on this thing ! You can't even look at a component's pathway to determine where you could get access to it's legs. trillion bundled wires, board so crowded you can't see or get to anything, etc. I just removed the 4x4" PSU board.....took me about 45 minutes to unhook maybe 75-100 connectors and label them all.

anyways, checking suspect components out of circuit now and will have to decide whether to shotgun the caps. They aren't shorted and seem to read ok.

[daz]

Well, i believe you nailed it G-one. Tho it remains to be seen for sure, but i pulled the 2 main caps for the plates and they had black leakage on the positive terminals of both. I'll have to pull the rest now, but i think that had to be it. I just wish there was a local source for these ! (450v....hard to find anything but low V locally)

[daz]

I got the caps, replaced them all, no joy. In fact, not it also sounds even worse with a extra not like a harmonizer every time i hit any note. That sounds cap relater to me, but they are new, correct voltage and MF values, and triple checked that they are correctly oriented for polarity. I wonder if the double sided board didn't get solder all the way thru, but to take that PSU out again to check that...oh god !!!!

[g1]

Sounds like there may be a connection failing between the filter caps and the B+ line. It seems to be getting progressively worse, maybe from board flexing? It went from 350V to 325V and now you have the harmonizer note.
As you say, it could be a feed through for the double sided board, but it could also be a cracked pad or trace.
Resistance checks from main filter caps may help, to the ground connection, between the 2 caps and to the B+ line.

[daz]

Sounds like there may be a connection failing between the filter caps and the B+ line. It seems to be getting progressively worse, maybe from board flexing? It went from 350V to 325V and now you have the harmonizer note.
As you say, it could be a feed through for the double sided board, but it could also be a cracked pad or trace.
Resistance checks from main filter caps may help, to the ground connection, between the 2 caps and to the B+ line.

yeah, thats what i'm gonna do. But it's a freakin' nightmare. Takes about an hour to remove it and label about 100 connectors. This thing is a torture device more than an amp. They apparently thought they'd never have to be repaired when they designed this monstrosity. But you know, i never even checked the voltage after i got it back together. I will tomorrow. But i was so bummed with the worsening sound after going thru all this it didn't even occur to me.

[daz]

Done....it was indeed the caps and the issue once i replaced them was indeed the board thru holes were not getting solder thru them. There are 3 caps of 6 in the the PA supply who's negative belonged at ground and one was because it didn't physically need solder thru the board to make contact. the other 2 were not making ground contact. I now see what they MODDED the original caps ! When i removed them i noticed there was magnet wire like pickup windings use that was wound around the cap legs and soldered there to give them more diameter so that they would make contact with the inside of the hole. What i don't get is with the legs on the replacement caps even tho thinner, why would they not make contact no matter how i tried even after giving the joint heat and solder for much longer than normal. I was afraid of killing the traces, but it should have eventually made contact on at least one of the 2. Crazy man ! Why is my question !

In any case, amp works fine now, all voltages back. Thanks to all who replied, especially the guy who nailed it, G-one. Thanks a bunch ! I do these repairs for friends so i don't get paid. So when something becomes such a hair puller it's very frustrating, and i appreciate the help. I have a LOT of musician friends, and sometimes i wish i didn't ! But they're mostly poor as most musicians are, so who do they come to when they have issues....

[g1]

Glad you got it sorted out and did not succumb to the frustration!
However, what you are referring to as a "mod" sounds like the actual "feed through" or via (Via (electronics) - Wikipedia, the free encyclopedia) of double sided circuit board. Are you sure it was wire and not a (ribbed) eyelet? They will often rip out of the board if you don't have the component perfectly desoldered.
Without the via it is very difficult to get the traces on opposite sides of the board to reconnect, sometimes you have to run a little wire through along with the component and solder the wire to the trace on either side of the board.
If the fault returns at some point, you will know what to do.
If you did not ever resolder the caps before you removed them to test, it is possible the original fault was only a bad solder connection. Because you can't see the connection on the component side of the board, a visual inspection is not enough for double sided boards. If in doubt, resolder.
Through hole repair kits are available to repair boards where the eyelets get yanked out.

(Edit: I'm probably not using the term "via" properly, but it was all I could come up with to illustrate the point. Where component leads go through the board, "plated through hole" is probably the correct term. "vias" are used to connected traces on opposite sides of the board without component leads going through them.)