Marshall Guitar Amp Transistors overheating - help

[Eeyore]

Fakes aren't just reserved for Ebay.......I've gotten fakes from MCM
recently. Last I checked they were still offered in spite of me
telling them they were junk and sending them back. Bean counters at
work.....

Report them to the genuine manufacturer.

Graham

 

[JosephKK]

Correct.

Not correct. No correctly designed amp blows up at no load (possibly
excluding some high power transmitters).

 

[Eeyore]

Not correct. No correctly designed amp blows up at no load (possibly
excluding some high power transmitters).

I have the evidence that says otherwise. You're welcome to argue whether it was
'correctly designed' though.

Graham

 

[Phil Allison]

"Joseph Kook Krackpot"

Not correct.

** It is a perfectly correct comment in this context - you tenth witted
ass.

No correctly designed amp blows up at no load (possibly
excluding some high power transmitters).

** Totally spurious rubbish.

Piss off - you ridiculous, ASD fucked cretin.



...... Phil

 

[Kevin Aylward]

Not correct. No correctly designed amp blows up at no load (possibly
excluding some high power transmitters).

As already been eluded to, "correctly" designed valve/tube guitar amps are
liable to fail if their load is removed. This is because, the transformer
acts a bit like a current transformer. That is, a current is being fed into
the primary, and a current therefore wants to flow out of the secondary.
Hence, the output voltage raises to try and force a current into an infinite
load. There is usually a resister to try and mitigate this, say 500ohms, but
it is not practical to include a resister low enough to solve this issue. In
principle, one could design a protection circuit, but I have never seen this
in a comercial product. Typically one gets the valve bases cracking...

www.kevinaylward.co.uk
www.blonddee.co.uk
www.anasoft.co.uk - SuperSpice

 

[Phil Allison]

"Kevin Aylward"

As already been eluded to, "correctly" designed valve/tube guitar amps are
liable to fail if their load is removed. This is because, the transformer
acts a bit like a current transformer. That is, a current is being fed
into the primary, and a current therefore wants to flow out of the
secondary. Hence, the output voltage raises to try and force a current
into an infinite load. There is usually a resister to try and mitigate
this, say 500ohms, but it is not practical to include a resister low
enough to solve this issue. In principle, one could design a protection
circuit, but I have never seen this in a comercial product.

** Never seen the plate to ground diodes fitted in Boogie, Peavey, Music Man
and Ampeg valve amps - then ??

Typically one gets the valve bases cracking...

** Only if the OP tranny insulation is not first.




..... Phil

 

[Kevin Aylward]

"Kevin Aylward"


** Never seen the plate to ground diodes fitted in Boogie, Peavey,
Music Man and Ampeg valve amps - then ??

Not since I switched over to using transistor guitar amplifiers.

www.kevinaylward.co.uk
www.blonddee.co.uk
www.anasoft.co.uk - SuperSpice

 

[Neil]

** Have a very careful look at R29 ( 0.33 ohms 7 watt ) and make sure the

soldering of the legs is OK.

This resistor is CRITICAL as it conducts speaker current to ground and
supplies a take off point for the POSITIVE feedback loop.

If the soldering is cracked,  the amp will oscillate at supersonic
frequency -  probably intermittently.

.....   Phil

Sorry its been a while since my last message!

I've just popped R29 out of circuit to test. Allowing for the error in
my DVM (doesn't go to Zero) the resisitor value is fine (as are the
other 0.33 ohm resistors).

All solder joints seem oaky and theres no sign of cracks or burning.

An earlier message suggested the MJF122 darlington so I popped this
out too and there are no short circuits. With the dvm set to diode
function on the ohm scale and the neg probe on the centre pin and pos
probe on left pin the reading is 0.73 & 0.64 with the pos probe on the
right pin.

 

[Stephen Cowell]

An earlier message suggested the MJF122 darlington so I popped this
out too and there are no short circuits. With the dvm set to diode
function on the ohm scale and the neg probe on the centre pin and pos
probe on left pin the reading is 0.73 & 0.64 with the pos probe on the
right pin.

Measure the bias currents into the 'base' leads... somehow,
somewhere, there's too much, usually a leaky, perforated
driver.
__
Steve
..

 

[Neil]

Appologies for the delay getting back to the thread - I thought I
posted at the weekend but it doesnt appear to have uploaded.

Anyway, I removed transistor MJF122 as someone suggests it might be
faulty. With a DVM set to diode function on the ohm scale and the +ve
probe on pin 2 there's no reading on pins 1 & 3. With the -ve probe on
pin 2 pin 1 reads 0.74 & pin 3 reads 0.66. With the -ve probe on pin 1
and the +ve probe on pin 3 there is no reading, and reversed the
reading is 1.03

I popped out the 0.33 ohm resitor and the value is fine, as are the
rest of them and the solder joints too

I have an oscilloscope - can anybody advise where and what signals I
should be looking for please?

Thanks in anticipation.