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Messages - MasterVolume

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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.


Tubes and Hybrids / Re: Homemade Tube amp blowing fuses
« on: March 05, 2017, 05:33:47 PM »
Hi i recently had hard to troubleshoot issues with an amp I had tweaked.

In the end I think it was solder shorting to ground one of the filter caps or dropping resistors.

After rebuilding bridge rectifier with new diodes on a tag board, the amps working great again.

good luck....

Sent from my iPhone using Tapatalk

Amplifier Discussion / Re: Marshall Valvestate 8040
« on: December 31, 2014, 10:03:46 PM »
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.

Amplifier Discussion / Re: Marshall Valvestate 8040
« on: December 25, 2014, 04:31:48 AM »
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.

Amplifier Discussion / Re: Marshall Valvestate 8040
« on: November 30, 2014, 04:25:18 PM »
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

Amplifier Discussion / Re: Marshall Valvestate 8040
« on: June 29, 2014, 05:41:02 AM »
Got it working, gave it a half hour work out and it's now working great.

Amplifier Discussion / Re: Marshall Valvestate 8040
« on: June 26, 2014, 07:45:42 AM »
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.

Amplifier Discussion / Re: Marshall Valvestate 8040
« on: June 25, 2014, 08:28:03 PM »
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

Amplifier Discussion / Re: Marshall Valvestate 8040
« on: June 23, 2014, 06:59:07 AM »
I picked up a new tl072 and 2x 12v Zeners + 2x 4700uf 50v caps to replace the old c50 and c53.

Thanks Phil

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

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.

The G100 has a very full Bass sound. It is super loud when run at full volume but can sound very hard. I'd suggest getting speakers with very high efficiency so you don't need to push the power amp at all.
Make sure you get the bias checked too.

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