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Marshall Valvestate 8040

Started by MasterVolume, June 22, 2014, 05:55:00 AM

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MasterVolume

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.
Using Tapatalk

phatt

Your preamp rails should be +/-12Volts or very close, the fact they are low tells you there is an issue somewhere like failing chip maybe pulling rails low.
Obviously IC3 could need replacing.

Also Check R55 and R59 for signs of heat stress, they normally run warm but if something shorts after that point they get very hot.
That voltage reg setup consisting of Resistor, Zener and Cap is a common fail point in these circuits.
Others here will help soon I'm sure.
Phil.

MasterVolume

I picked up a new tl072 and 2x 12v Zeners + 2x 4700uf 50v caps to replace the old c50 and c53.

Thanks Phil
Using Tapatalk

phatt

Can I suggest that while you have IC3 out, fire it up and check you have normal rail voltages as the problem maybe a combination of faults.
Phil.

Jack1962

1st the voltage on those TLO72 should be around -/+14 to 16 volts (at least they have been on all of them I have worked on) VR1 should not have any DC voltage on it . check your caps I'm sure the 1 before it is bad at least , replace IC3 , as far as the 9 volts goes it's a issue but not a severe on TLO72's will operate wil a voltage as low as 5 volts.

MasterVolume

Replaced the 12v Zeners and upgraded the 2200uf caps to 4700uf . The problem is still there. I can only think it is the ic at fault. Will check the rail voltages when I get it out
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MasterVolume

I have replaced the TL072 IC3 and the voltages have returned to the expected values. The chip must have been damaged.

So now to replace the TDA1514a and the missing channel select switch.
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MasterVolume

Got it working, gave it a half hour work out and it's now working great.
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Roly

Quote from: MasterVolume on June 29, 2014, 05:41:02 AM
Got it working, gave it a half hour work out and it's now working great.

Excellent!   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

MasterVolume

A little down the track now and I finally replaced the channel switch and all is not well.

After a few minutes at reasonable volume it fades in and out. So I removed the switch. Last night I tested it for about 20-30 minutes at low volumes then it just cut out completely.

Powered it of then on and it works. What is the issue? Overheating? Too high voltage ? Or dry solder. I had to move the board around quite a bit to get the switch in and the power filter caps which I replaced with larger ones got moved a bit so I will have to re check that.

Any ideas
Using Tapatalk

g1

  Try it with a patch cord between the FX send and return jacks.

MasterVolume

Pulled it apart and reflow all solder joints. Put leads in the fx loop several times. Fixed the standoff's and added draft seal around all are areas that contact the cab to stop vibration of the chassis.

Seems to work well now though I haven't had a chance to test it a hi volume at any length of time.

Though I have fixed everything in place well and would not want to pull it apart again the things I would look to do would be change the gain pot so it has an even range over mild overdrive and adjust the contour / mid cut for a more subtle amount.

I have changed the speaker to a celestion silver series which I thin helps too.
Can get some on the edge of overdrive and warm mover drives with the contour on nothing and the bass and treble rolled back.
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MasterVolume

More trouble.... Having not touched the amp for several days. I've had a chance to crank it and I'm now getting a weird ringmod square wave like synths distortion occurring.

It happens on both channels when the volume is high. It also happens with a signal from a digitech rp150 into the line in.

So the issue must be the power stage. Voltages on the chip seemed ok. Any ideas how to troubleshoot this.
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Roly

Quote from: MasterVolumeweird ringmod square wave like synths distortion

As a synth player I can imagine what you mean, but as a tech I'm a bit at a loss to think what might cause this.

Every time a new fault appears like this you have to treat it as a whole new fault in the same methodical way from the ground up.

This amp uses current feedback, R65, and a fault around here could potentially cause some sort of output stage instability.

The Reverb control, VR10, should be at minimum, but if it had a faulty ground you could be hearing the reverb pan.

Does it respond to a thump on the top?

My next move would be a powerful light and a lens, and go over all the joints and tracks around the power amp looking for any sign of a dry joint or a hairline crack in a trace (typically right next to a solder pad).
If you say theory and practice don't agree you haven't applied enough theory.