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Topics - MasterVolume

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Amplifier Discussion / Marshall Valvestate 8040
« on: June 22, 2014, 05:55:00 AM »
Hi all.

I've been working on a Marshall 8040 that I picked up from a market for $10. I was told it did not work.

After inspecting it I found that someone had attempted work on it before. The TDA1514a was sitting loose not even soldered in to the PCB. There was the smell of something heating up. For the time being I have removed it and also have a replacement ready to drop in.

Here is the schematic  http://www.drtube.com/schematics/marshall/8040-iss8.pdf

I noticed that the two 2200uf caps c50 and c53 had dry joints and have replaced all but one electro cap c27 in the reverb line. I also replaced the 6v8 zenner ZD3 as I measured that it was not dropping any voltage across it.

Unfortunately there was still a warm smell and touching each opamps all were cool except IC3 a TL072 that is the first and second stage for the clean channel.

Measuring the voltage on each pin the v+ and v- were both around +/-9v however there is around 5-6v on both pins 6 & 7. There is also around 6v on each leg of VR1 the gain control for the clean channel. I found that this control didn't work when I ran a guitar into the input and the preamp out into a headphone amp. The channel select switch is missing perhaps broken off from a fall and the boost channel is on by default, but the clean channel can be selected by footswitch.

I'm hoping someone can help point me in the right direction for further troubleshooting.

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