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Bassman 400 power issue

Started by markorock37, May 06, 2013, 01:04:08 PM

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markorock37

QuoteDoes this mean +70V at TP4 and -70V at TP5?
How can you have +70V at TP4 which is a negative point?
Please confirm.
Sorry I mixed those up...-70 at TP4 and +70 at TP5 like it should be. I'll recheck what you have requested.
Quotewhat is the mains voltage where you are?
110 volts USA
QuoteWhere are you placing the negative/blackmultimeter probe?
To ground at the chassis AC ground

markorock37



P5 (TP1) - 54VAC    52.9VAC
P6 - 0V                    0VAC
P7 (TP2) - 54VAC     51.9VAC

TP3 - +72VDC         +70VDC
TP4 - -72VDC           -70VDC

P8 (TP5) - 23VAC       43.2VAC measured this 3 times
P9 (TP6) - 23VAC        22.4VAC

TP7 - +30VDC            +56.2VDC
TP8 - -30VDC              -55.9VDC  measured also 3 times

0VDC between P8 and P9 unplugged.

R161 is 11.2 ohms






Enzo

OK, P8 is twice what it should be, so pull the wire off post #8, and measure AC volts on that wire as it hangs in mid air.   Leaving the black probe on chassis ground.   If you still get 40-some volts,the transformer has some odd issue.  If you now get 22vAC, then the transformer seems to work.

If the transformer alone is OK, then I guess we have to look at the fan circuit as involved somehow.  If so, first thing would be to disconnect the fan itself.


Just a thought, idea is wondering if somehow the center tap and one end got reversed on the low voltage winding.   The schematic shows only 5 wires on the secondary and only two on the primary.  Does your transformer have only 7 wires?   It shows the red/yellow wire as the shared center tap lead.  ANy chance your transformer has separate center tap wires that are combined on one post?

The way this theory would work is if an end wire was grounded, then there still would be 22v from ground to one end (really the center tap now), but the other end would be the whole winding away, or 44v.  I am trying to rationalize something reasonable that would cause the numbers you are getting.   Or if the wires LOOK OK, it is still possible the tranny was made with a miss-connection inside.

Then again it is only an idea.

Roly

Which leaves us scratching our heads wondering a) what sort of single fault could cause this situation to arise, or b) how did this amp get so far from the factory with exploding regulators without anybody actually noticing?   ???

There simply has to be more to this than a loose nut.   :loco

Isolate the fan rectifier, D8-D11, and retest voltages, particularly P8 (TP5).
If you say theory and practice don't agree you haven't applied enough theory.

g1

Quote from: Roly on May 09, 2013, 10:38:47 PM
Isolate the fan rectifier, D8-D11, and retest voltages, particularly P8 (TP5).
I think you mean D108-D111, also C116.

But before this, check what Enzo asked in reply #17.

Roly

Quote from: g1 on May 09, 2013, 11:07:30 PM
Quote from: Roly on May 09, 2013, 10:38:47 PM
Isolate the fan rectifier, D8-D11, and retest voltages, particularly P8 (TP5).
I think you mean D108-D111, also C116.

But before this, check what Enzo asked in reply #17.

Yup.  We just had an eclipse and I'm a bit confused.   :duh
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

#21
So basically somebody disconnected those transformer wires for some reason and reconnected them the wrong way.

Not *all* of them, just transposing 2 may make a mess out of it.

Worst case, I'd follow (roughly) the following path:

1) disconnect all secondaries and leave them in the air.

2)with amp ***unplugged*** (turned OFF is not enough) I'd draw the transformer and colour wires, for extra safety would stick a small "flag" (maybe car painter's paper tape) with a letter to each, as in A/B/C/D/E and measure which wire has continuity with each other.
Just that. It will confirm or discard the theories about transformer internal failure or wis-winding or mis-labelling.
Post results here.
Mention each wire color and letter .

Tasks 3/4/5/etc. will follow, depending on what we find first.

EDIT: FWIW I'm sort of agreeing with Enzo's hunch ("sort of" only because it still needs confirmation through measurement) and I'm asking incredibly boring questions just to not leave it open to the slightest doubt.


markorock37

Transformer wires are correct. No voltage on P8 or P9 to ground unplugged.
No doubled wires on posts.
Only 5 secondary wires.
I have to take board back out to disable fan rectifier, will try to get to later tonight. Fan does come on as soon as amp is turned on.
The floating piece of metal was found on D112 of the fan circuit just a FYI.

g1

Quote from: markorock37 on May 10, 2013, 01:21:20 PM
Transformer wires are correct. No voltage on P8 or P9 to ground unplugged.
We need to know the AC voltage at the wires that attach to P8 and P9.  Not DC voltage, and not at P8 or P9.
Disconnect the brown and brown/white wires from P8 and P9, connect your meter to the wires.
What is the AC voltage from brown to brown/white wire?  AC volts from brown wire to chassis ground?  AC volts from brown/white wire to chassis ground?

markorock37

Voltages between P8 (brn) and P9 (brn/wh) is 0VAC.  P8 to ground is 0VAC.
P9 to ground is 22.4VAC.
A = red wire
B = red/yellow
C = red/white
D = brown
E = brown/white

A has continuity with all but D
B has continuity with all but D
C has continuity with all but D
D has no continuity with all
E has continuity with all but D


Enzo

And there it is.

The open winding leaving D with no continuity to the rest and zero volts output.  Just how that resulted in the one pin having twice voltage I'll have to think about.   But the transformer is bad.


Now sincve the transformer has to come out anyway to replace, you might as well try to fix it.   It is POSSIBLE the problem is in the lead wires and not th windings.  The transformer inside is wound with enamel coated wire, not the colored is=nsulated wires you see coming out.  Inside will be some point where the lead wire is soldered to the winding wire.   There may be solder terminals you can see from the outside, or they may be buried.  We need to go inside enough to see where th brown wire connects to determine if the connection to it has broken, or if the end of the winding wire snapped off right at that point.

And we cannot yet rule out the crimped-on female push connetor on the wire end.  Check that joint.

If you have to, you may need to slice open the fish paper (heavy cardboard) around the transformer middle.

Hey, what is there to lose?

Roly

Quote from: EnzoThe open winding leaving D with no continuity to the rest and zero volts output.  Just how that resulted in the one pin having twice voltage I'll have to think about.   But the transformer is bad.

Maybe I'm just going senile, but I can't see how just the D=brown wire being open alone could result in 50-odd volts and exploding regulators - that is, I suspect that the brown winding being open is a result, not a cause.  :headscratch:   ???
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

We may not have the cause and effect figured out, but it definitely is what is wrong at this point.

markorock37

Well whatever shorted did cause quite a show, I am not surprised. I'll take a look at that tranny. The overvoltages with an open winding doesn't make much sense to me either.

J M Fahey

Yes, the low voltage area should *still* have roughly +/-30V, only half wave rectified and with more hum/ripple but that's survivable .

I bet someday between Factory and you getting it in your hands and it was messed with.

That's why I asked you to identify and measure the *wires* themselves (which always are what they are) and not the "P" terminals, which may receive anything, if miswired.

You already posted what you read on Brn (0V) and Brn/Wh (22V) "relative to ground" but as a last check I ask you to please reread and post voltages with all 5 secondaries connected to nothing (not even ground) and relative to RdYel  .

Seems redundant but we are chasing ghosts here.

"They should measure the same as before".

Yea, famous last words ;)