Need Help with Polytone MiniBrute Solid State Guitar Amp

[Lovguitar]

Hello,

A friend of mine is joining me at a benefit performance for a child
with a very serious medical condition in November. At rehearsal he
told
me that his Polytone Minibrute is blowing fuses, and was using a little

Freedom Amp someone let him borrow.

My friend is a student with very little money and had been told by a
local tech that it would probably cost $400 to analyze and fix the
Polytone amp and
he should just dump it.

I volunteered to take a look at it. I am good at repairing tube amps
but have not worked on solid state stuff. Using my tube approach, I
would look for shorts, bad power tubes, bad bias supply, arcs on the
sockets, check the rectifier, diodes, power
transformer, caps.

In this case, I'm thinking I should start with the power supply --
rectifier and diodes. Probably the power transistors next. In looking
at the circuit boards, I observe a metal film resistor right next to
the bridge rectifier that has a burnt looking area on the circuit board
beneath it, as though it has been heating up. Bridge rectifier getting
more suspicious?

Any advice?

Oh, one more thought. Does anyone know what size fuse is supposed to
be in this amp? It has no fuse right now and I don't know if my friend

has been using the proper replacement fuse in the amp. Wouldn't it be
something if the fuse just blew because it was tired and he has been
replacing it with undervalued fuses?

Please help in any way you can. Value of fuse, schematic, advice in
order of troubleshooting -- all appreciated.


Best regards,


Paul

 

[Homer J Simpson]

In this case, I'm thinking I should start with the power supply --
rectifier and diodes. Probably the power transistors next. In looking
at the circuit boards, I observe a metal film resistor right next to
the bridge rectifier that has a burnt looking area on the circuit board
beneath it, as though it has been heating up. Bridge rectifier getting
more suspicious?

Any advice?

The AC output from the transformer is a guide to the correct DC output (say
1.2 times). Track it to each point in the system. Check all transistor
voltages - the base should be about 0.2 VDC higher than the emitter.

Anything make no sense?

 

[Arfa Daily]

Hello,

A friend of mine is joining me at a benefit performance for a child
with a very serious medical condition in November. At rehearsal he
told
me that his Polytone Minibrute is blowing fuses, and was using a little

Freedom Amp someone let him borrow.

My friend is a student with very little money and had been told by a
local tech that it would probably cost $400 to analyze and fix the
Polytone amp and
he should just dump it.

I volunteered to take a look at it. I am good at repairing tube amps
but have not worked on solid state stuff. Using my tube approach, I
would look for shorts, bad power tubes, bad bias supply, arcs on the
sockets, check the rectifier, diodes, power
transformer, caps.

In this case, I'm thinking I should start with the power supply --
rectifier and diodes. Probably the power transistors next. In looking
at the circuit boards, I observe a metal film resistor right next to
the bridge rectifier that has a burnt looking area on the circuit board
beneath it, as though it has been heating up. Bridge rectifier getting
more suspicious?

Any advice?

Oh, one more thought. Does anyone know what size fuse is supposed to
be in this amp? It has no fuse right now and I don't know if my friend

has been using the proper replacement fuse in the amp. Wouldn't it be
something if the fuse just blew because it was tired and he has been
replacing it with undervalued fuses?

Please help in any way you can. Value of fuse, schematic, advice in
order of troubleshooting -- all appreciated.


Best regards,


Paul

It's not at all unusual for resistors in the power supply to run hot enough
to discolour the board. However, the bridge could be short circuit. Assuming
a packaged bridge rather than 4 individual diodes, set your meter to ohms
range, black lead to bridge " - " and read to each of the " ~ " pins. If no
short, red probe to bridge " + " and again, measure to each " ~ " pin. That
will check each individual diode in the package. Note that a short reading
across an individual diode or package limb, could be a shorted ceramic cap
that they tend to put acros them - I've had it many times.

The way that the fuse is blown is a good indicator of whats's causing the
problem. If it is totally vapourised and blackened, the problem is very
nearby - possibly the bridge or one of the main smoothing caps, or maybe a
collector/emitter shorted output device. If it blows with a flash, but
doesn't blacken the glass, then the problem is further downstream - may be a
regulator transistor or somesuch. A "soft" blow where the centre of the fuse
wire just melts without much fuss, usually takes more looking for, as it
just indicates that somewhere in the amp is drawing a bit too much current,
probably the output stage, but more of a drive problem than a direct short.
I wouldn't expect a fuse on the power side of the unit to be more than 1 amp
BUT, in answer to your question about the wrong value of fuse being fitted
by your friend, it might actually be the wrong *type* that he's fitting. Any
primary circuit fuse should be a " T " rated type ( delay ). The transformer
inrush current as the magnetic flux in the core builds over a few cycles,
will blow an " F " rated fuse, even of the correct value, every single time
you switch on, until you run out of fuses to put in it ...

Arfa