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Marshall 8080 combo blowing fuse and more

Started by Psabin8951, April 11, 2018, 09:36:42 AM

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Ok pinout issues resolved and TR10 now replaced.
TR readings now are:-

TR4 B 0.01v C 0.01v E 0.07v
TR5 B 3.33v C 0.01v E 3.73v
TR6 B 4.35v C 7.76v E 3.8v
TR7 B 8.21v C 0.58v E 3.47v
TR8 B 0.53v C 8.41v E 2.14v
TR9 B 0.2v   C 0.53v E 0.08v
TR10 B 0.08v C 0.02v E 1.42v
TR11 B 1.59v C 0.03 E 1.76v
TR12 B 1.54v C 1.55v E 1.76v

Jazz P Bass

It appears as if the B- rail is missing.

J M Fahey

Please check that Tr10 collector is insulated from heatsink and chassis.
Did you use mica+grease or a Silpad there?
Collector shorted to ground would kill -V .

Or: check for continuity between Tr10 collector and -V terminal at main filter caps (C61) and/or - end of rectifier bridge (Br102/DB1).

Or maybe negative-pointing part of bridge (Br102/DB1) is open.


Thanks for the fast reply.
I used a mica sheet plus grease.
Collector shows no connection to heatsink or chassis on meter.
There is continuity back to -V on C61 and DB1

J M Fahey

OK, let´s separate power supply and amplifier and measure.

With a sharp boxcutter cut tracks leading from +V and -V to amplifier at points 01 and 02.
Cut a narrow 1mm strip or so and remove copper , later you can scratch soldering mask on both sides and rejoin with a drop of solder or a little bit of wire.

1) Measure +V and -V to ground at points 04 and 06 .
If normal (35/40V ) , also measure +/- 15V rails at ungrounded ends of ZD1 and ZD2 .

2) with amp OFF measure resistance to ground from points 03 and 05.
Both ways and using resistance (say 200 ohm or 2k) and diode scales.
Open circuit is no problem, I´m trying to find some continuity to ground which shorts raill voltage.

I also suspected some open track or connection, but you confirmed
QuoteThere is continuity back to -V on C61 and DB1
so that seems to be working.


I had an 8040 that had similar issues.
Picked it up for $10 a market. First thing I realised researched / worked out was that the output transistors were probably blown as I could still get a signal through the preamp.

Once I'd replaced those, if I recall correctly it blew fuses . Measured the voltages and worked out that something was pulling down the voltage on the preamp. Replaced the preamp transistors and still happened. Inspected. Carefully resistors around the power rail for the preamp and replaced any that looked the slightest discoloured, and may have replaced capacitors too. Don't recall.

Anyway that fixed it. Other things that were wrong were a broken channel switch that I then replaced. Found after awhile of playing it would start to cut out. Hoped it wasn't the problem that killer the amp before. Tested it with cable in the preamp out / power amp in and eventually carefully managed to squash the jack contacts so that they had a strong contact when there was no cable in it.

Tried other speakers but found that the original speaker suited the amp best ( what do you know). Fixed the rattles in the cab with the amp chassis with foam strips that you put on the bottom of doors, no more rattles or buzz. Added a jack for speaker out and did s couple of mods to reduce the gain from the gain control and Merlin's mods for cathode followers from his preamp book.

In the end worked out pretty well. Was going to keep it but too many amps, kind of surprised myself at how well it cane out and that I was able to work out the faults and fix each one.

Sold it for probably half the going second hand price of one in good nick. To someone who is now very pleased with it.

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