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Author Topic: Marshall 8080 combo blowing fuse and more  (Read 258 times)

Psabin8951

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Marshall 8080 combo blowing fuse and more
« on: April 11, 2018, 09:36:42 AM »

I have a poorly Marshall 808 combo come my way.
It was blowing the internal fuse when received and by isolating the circuit I know the +15v and -15v areas of the circuit are ok as is the tube heater portion which leaves the power amp stage. With R84, 85 and TR8 removed from the circuit it powers up fine.
With R84 reintroduced again it powers up fine.
With R85 added too, that resistor severly overheats but doesnt blow fuse.
If TR9 is out of circuit R85 doesnt overheat.
With R85 back out and TR8 refitted the fuse blows.
Can anyone point me to what is likely to be the cause?
I have attached a pic of the circuit in question
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g1

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Re: Marshall 8080 combo blowing fuse and more
« Reply #1 on: April 11, 2018, 01:34:05 PM »

I've attached a better copy of schematic.
Did you check the transistors?  TR8 & TR10 good?
TR6,7, & 9 good?
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J M Fahey

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Re: Marshall 8080 combo blowing fuse and more
« Reply #2 on: April 12, 2018, 05:37:23 AM »

Agree.
Resistors overheat because *something else* is applying too much voltage to them or passing too much current through them.
First suspects are shorted transistors so check those suggested by G1.
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Psabin8951

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Re: Marshall 8080 combo blowing fuse and more
« Reply #3 on: April 17, 2018, 11:51:54 AM »

Ok after a delay due to tonsilitis im back on this. TR4 - TR12 now checked. All ok except TR8 which has now been replaced.
Fuse instantly blowing now.
Any suggestions where to go from here?
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g1

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Re: Marshall 8080 combo blowing fuse and more
« Reply #4 on: April 17, 2018, 05:52:06 PM »

There should be no load (speaker) connected to the amplifier, is this the case?
Do you have a light bulb limiter?  (see here: https://www.ssguitar.com/index.php?topic=2093.0).
Something is turning on TR8 too hard, and appears to be related to the TR6 area, as disconnecting R84 was making a difference.
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Psabin8951

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Re: Marshall 8080 combo blowing fuse and more
« Reply #5 on: April 18, 2018, 08:09:49 AM »

Correct, there is no load currently connected.
I haven't got a bulb limiter but can construct one fairly quickly.
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Psabin8951

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Re: Marshall 8080 combo blowing fuse and more
« Reply #6 on: Today at 03:19:49 AM »

So what now guys?
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