Bassman 400 power issue

[markorock37]

I am working on a Fender Bassman 400 ss bass head, its is blowing voltage regulators to bits. I replaced the 7815 and 7915 regulators and it blew 'em again. I have now replaced them once more along with all the diodes nearby. Instead of lifting one leg to test each I just went ahead and replaced all D100-111. Q102 was also blown and replaced. Ready to start it up on the limiter, anything else I should check beforehand?

[Enzo]

Let's be systematic.

You have common 7815/7915 Vregs.   It takes quite a bit to blow them apart, which seems to be your description.  Are they actually blowing apart?  Or are you being dramatic, and they are just failing?

In any case, they are either being fed something bad, or something bad is happening on the output side.  SO take them off the board and power up without them.  Your schematic should say there is about 30v at the Vreg input pin, so is there?  And is it clean DC?

The output pin of each should now have zero volts.  But if something out in the amp is shorting a high voltage rail to them, that would cause damage.  So:  is there any unwanted voltage on those output pin holes?

The Vregs ought to shut themselves down when overloaded, but...  SO power off, no Vreg installed, is the output pin hole shorted to ground?

Are they in the right places?  The 7915 is the one right on the edge of the board, right next to the transformer wires.   If you reversed them, it will cause blow ups.


Q102 failure is hard to associate with the other stuff.

Look right below Q102 on the print, that note is important.  The fan circuit is not ground referenced.  Make sure not to connect grounded test probes or anything to it, unless you know what you are doing.

[markorock37]

Its literally blowing them apart. I hit the rafters in my garage with a piece of flying debris. I'll take out the regulators and get some numbers. I have them in the right spot btw. I did pull out a chunk of metal, a welded chassis nut for the case that had broken off. Who knows what damage that thing did in there.

[Roly]

   Actually exploding regulators together with a failure of Q102 suggest to me wildly excessive supply voltage.  Knowing the amp history would be helpful because I'm thinking things like the amp is wired for 110V and being plugged into 240V, or that somehow the main +/-70V supply has been misconnected to the +/-15V board.  It's hard to see how one loose nut alone could cause damage that would cause both regs to actually explode.  These are typically quite tough little critters and it takes some serious abuse to get them to fail, much less explode.  Voltage measurements per Enzo should move us forward.

[markorock37]

That's why I changed all the rectifier diodes in case it was AC coming down the path, but I'll admit I'm still learning these things. I'll get some voltages posted soon.

[J M Fahey]

Yes.
To the point of being redundant, recheck that you have 23VAC from each TP5/TP6 to ground, +30VDC/-30VDC at TP7/8 to ground, and, *without* the regulators installed, :
a) you have 0 V at TP5/TP10 and
b) after that, if 0V present, turn amp off, wait for discharge, and only then measure resistance to ground from TP5/10, using both the resistance scale (say, 200 ohms and 2K)  and repeat using the diode scale, with the read lead always on the "most positive" side meaning from "+15" to ground for TP5 and from ground to "-15".

Looks redundant, or a waste of time, but we are chasing ghosts here

And by the way, an LM7xxx with 30V on one side, 15V on the other and the typical low load that's meant by a preamp dissipates "nothing" , so we must find what's exploding them.

[markorock37]

Voltage regs:
U101
Pin 1 - +55.9V
Pin 2 - 0
Pin 3 - 0

U102
Pin 1 - 0
Pin 2 - -56V
Pin 3 - 0

Power to voltage regs but nothing on the output side.
I will check the other test points per Mr. Fahey

[Enzo]

Well there is your problem, you have 55v going into those poor things, no wonder they popped.

Just a hunch... Look at TP3 and TP4.  They should be 70v each, but are they closer now to 30v?

Verify the transformer wiring.  Is the brown wire from the transformer on post  P8?  And the brown/white wire on P9?

Also verify the plain red wire is on P5, and the red with yellow stripe is on P6, and the white with red stripe is on P7.


Unless you have a 120v amp plugged into 240v mains, the only other explanation for voltage that high on your power supply is mixed up transformer wires.

[Roly]

OUCH!   xP

Since the power tranny doesn't seems to have an international primary (and unless you are plugging a 110V amp into a 240V outlet) then it has to be the secondary high and low voltage wires to the rectifiers mixed up.

[markorock37]

I didn't remove any of the board wires, but doublechecked anyway, all wiring is correct per schematic and board labeling. I measure +70/-70 at TP4 and 5. Resistance check from TP5 to ground is continuity.

[markorock37]

Bad transformer?

[Enzo]

That would be a very odd transformer problem.   Anything is possible, but in general, transformers fail with shorted turns, which makes them run very hot, or blows fuses or drops voltages way low.   Next on the list would be open windings, on the primary side that results in a dead unit, on the secondary side it results in a missing voltage.  Having a transformer produce 20v more than it should on a 30v circuit would be something I have not seen.

On the other hand I am not sure how else to explain it.

[J M Fahey]

I measure +70/-70 at TP4 and 5.

Does this mean +70V at TP4 and -70V at TP5?
How can you have +70V at TP4 which is a negative point?
Please confirm.

Resistance check from TP5 to ground is continuity.

How can you have continuity (which is a low resistance value) and have -70V at the same point?
Which by the way is a positive point?

Where are you placing the negative/blackmultimeter probe?

[g1]

 Is R61 ok?
A couple of points I'll throw in: 1)  Those 35V caps (C108 & C109) should be exploding if they're getting 50V on them, or at least bulging.
2)  The two transformer windings are common to the fan circuit, maybe something weird there?

But for quick check, disconnect P8 & P9 (brown and brown/white), what AC voltage do you measure between those wires?

[Roly]

Having a transformer produce 20v more than it should on a 30v circuit would be something I have not seen.

That would be just a little bit remarkable.

Those 35V caps (C108 & C109) should be exploding if they're getting 50V on them

My thought also.


@markorock37 - what is the mains voltage where you are?

We need to confirm some voltages to ground;

P5 (TP1) - 54VAC
P6 - 0V
P7 (TP2) - 54VAC

TP3 - +72VDC
TP4 - -72VDC

P8 (TP5) - 23VAC
P9 (TP6) - 23VAC

TP7 - +30VDC
TP8 - -30VDC

I'm currently at a loss to understand where +/-56VDC could be coming from.