PV Session 400 LTD...again

[Pryde]

Schematic:
http://assets.peavey.com/literature/...s/80344000.pdf

So I fixed up this old PV Session 400 LTD for a friend several months ago (with a lot of help from Enzo and others here) and it worked great for a few months and now another problem has resurfaced. A brief history:
-Full cap job replacing every lytic on the board (even the little 2uf couplers)
-New dual-diode in power section
-Replaced 2 microphonic ceramic caps in the preamp that were causing distortion at high levels


My friend is a gigging lap-steel player and plays this thing LOUD all the time. So, he brought it back with the amp making a horrible distortion ONLY when cranked up past 5 or so. I suspected the vibration was causing a faulty solder joint to act up so I gave it a couple whacks and heard some pop/crackle confirming this. Then the amp blew a fuse and that was that.

So, installed another fuse and it blew again. Checked all output transistors (diode test) and all solder joints on their housing and all seams good.

I don't have a variac but I can bring power to the amp with my bulb limiter without blowing the fuse. a 60w bulb glows very bright!

Any direction would be greatly appreciated. I know their is a loose joint or shorted part somewhere as its blowing the fuse.
Thank you!

[Sowhat]

I hate it when stuff like that happens!

What you do is up to you of course but if it was me with egg on my face I would be methodical and trace up. By that I mean disconnect the power supply from the amp and test the PS first. If it no longer blows fuses then it's in the amp and so on. Check things piece by piece. It's obvious that you are drawing too much current and have a short somewhere, that much is obvious but exactly what is not so obvious seeing that you have already been mucking around inside there, my bet is it's something you have already repaired, maybe a stray wire clipping laying across power and ground or something like that.

[Pryde]

I hate it when stuff like that happens!

What you do is up to you of course but if it was me with egg on my face I would be methodical and trace up. By that I mean disconnect the power supply from the amp and test the PS first. If it no longer blows fuses then it's in the amp and so on. Check things piece by piece. It's obvious that you are drawing too much current and have a short somewhere, that much is obvious but exactly what is not so obvious seeing that you have already been mucking around inside there, my bet is it's something you have already repaired, maybe a stray wire clipping laying across power and ground or something like that.

So disconnect the PT leads from the power switch first?

[Sowhat]

So disconnect the PT leads from the power switch first?

Well, I don't know if that will bear any fruit. What I'm talking about is the +50 and -50 volt lines going from the power supply to the amplifier.
Your supplied schematic don't show the power supply but I imagine it's the typical center tapped power transformer with a bridge rectifier and a big cap or two thing unless it's a switcher. I guess it's probably a separate board, module or terminal strip arrangement from the amplifier board. The +50 volts is probably a red wire and the -50 volts is probably a green wire but that may not always be the case, tough to give a definitive answer without examining the insides because I've never seen one. Maybe post picture of the chassis insides and I can identify these features for you.

[Enzo]

No one used switchers back then, 1974. Power is utterly basic, CT winding, bridge, two caps.

[Sowhat]

No one used switchers back then, 1974. Power is utterly basic, CT winding, bridge, two caps.

Yeah, I hear you there Enzo. I never checked the vintage of his amp but 1974, yeah, linear for sure. Many of my customers refer to their equipment as being old because it was made in 1999.

[Pryde]

The amp is a 1977.
I will try to get some photos up and describe the PT windings when I do. As a side note I noticed the mains ground (green) was not a very secure bond mounted to a somewhat loose rivet in the chassis. I relocated it to the PT mount/bolt and resoldered but still the same issue. Never is that easy is it?

So it appears my course of action is to trace through the power line and see were I might be popping a fuse.

[Pryde]

Here are a couple pics of the PT wiring:

2 red twisted: each terminates to separate molex connector on power amp board (see photos)
yellow: to filter caps
yellow/blue: to power switch
yellow/black: to power switch
black: to power switch
blue/red: no connection
red/black: no connection

So would I just unplug the 2 red molex connectors from the board, then power up to see if fuse blows? This would isolate problem to pcb then if fuse is ok correct.

[Enzo]

Yes.

[Sowhat]

Here are a couple pics of the PT wiring:

2 red twisted: each terminates to separate molex connector on power amp board (see photos)
yellow: to filter caps
yellow/blue: to power switch
yellow/black: to power switch
black: to power switch
blue/red: no connection
red/black: no connection

So would I just unplug the 2 red molex connectors from the board, then power up to see if fuse blows? This would isolate problem to pcb then if fuse is ok correct.

From what little I can see by those pictures disconnecting those two molex connectors will not accomplish what I am talking about. It appears that the power supply is a part of that board only in the presence of the 4 diodes between those connectors, those two connectors simply stitch the transformer and filter caps (the 6800uf 63volt ones) to the bridge rectifier diodes on the board. That board appears to have more going on in it than just supplying power. I can see the output transistors adjacent to the board to the right along with the thermal overload sensor... that must be the output heatsink. Wires from the output transistors go from their sockets to that board directly and those big rectangular white resistors are probably the emitter resistors. As I say that board has multiple purposes, not just power supply duty. It looks like the +-50volt connections are those wires that go from the output transistors to that board... Notice how some of those wires are red and some are green? that's probably the +50volts and -50 volts respectively.

I would need a wider shot where I can see mor of the chassis layout to really make sure I'm telling you the correct things... afterall you don't want to make things worse.

[Pryde]

There is no separate power supply board, only 2 boards total: main amp board and a preamp board. I will try to unhook the molex connectors (red twisted wires) from the main amp board and see what happens.

BTW:
Here is an online photo of what this one looks like inside

[Enzo]

Those Molex connectors hook the caps to the board, and maybe the transformer secondary as well.

Pull them off, then check the four main rectifiers for shorted - the four things between the molexs.

It is unlikely, but check the filter caps for shorts as well. Those are not original, so ought to be new enough, but...

Check resistance to ground from the mounting screws of each power transistor. There should be no continuity, we are checking to see if the insulating wafers are OK.

The power transistors? They need not to be shorted in any of the three combinations of leads.

[Sowhat]

I see your big picture now and those separate red and green wires going from the amp board to the preamp board are the +-50 volts I've been talking about. Disconnect them and you will take the entire preamp out of the picture narrowing your search. Enzos suggestions are sage advise but those are not the only things that can cause a power rail short in this amp, they may be the primary suspects but not the only ones.

[Pryde]

Pulled the 2 molex connectors from the main board and got no blown fuse with power up so problem is "somewhere" after filter caps.

checked rectifiers, no shorts
checked filter caps, no shorts (Enzo I installed them previously)
checked all transistor mounting screws with no continuity to ground
Power transistors only have 2 leads (2N-3773) and they all check ok with diode test.

Should I pull the boards and start looking underneath for shorts then ???

[Sowhat]

Power transistors only have 2 leads (2N-3773) and they all check ok with diode test.

TO3 power transistors and actually all transistors have 3 leads but one of them may not be apparent at first. The case of the transistor is the missing third lead. You need to check between the case and lead one in both directions, the case and lead 2 in both directions and from lead 1 to 2 in both directions to make sure they are not shorted.

Checking only the two leads only checks the emitter/base junction. That's why Enzo wanted you to check the transistor case to ground for a short, that would be a collector to ground short... very very bad thing which can be caused by a bad mica or capton insulator between the transistor and heatsink.