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Tubes and Hybrids / Re: Marshall 8080 combo blowing fuse and more
« on: June 23, 2018, 12:14:03 PM »
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.
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.