Marshall 8100 Valvestate Volume issue on Distortion channel

[jmarshall]

Hi, I have a Marshall 8100 Valvestate 100V. I got it for free as it kept blowing fuses. I replaced the transistors TR8, TR9, and TR10 and the amp worked, the clean channel is perfect, but the distortion channel is not very loud. I can't play the clean channel above 4, but I have to turn the distortion channel all the way up to be able to comfortably hear it.

I'm wondering if someone could give me some guidance in tracking the problem down. I am no electrical engineer, but am handy with a soldering iron and a multimeter. I have checked the board for cracked solder joints and have re-soldered suspicious looking ones already.

The only other funny thing I noticed (by accident!) was that when the amp is on the distortion channel and I touch C1 or R2, then LED's 6 and 7 light up very brightly and LEDs 4 and 5 light up slightly and there is a buzzing noise from the transformer area. With the amp on the clean channel, and doing the same thing, only LEDs 4 and 5 light up slightly and the buzzing is much quieter.

What are the LEDs on teh board for, and what should I check next? I'm using the schematics from guitarampheaven for the 8100 (pc0689p and pc0689). Thanks!

[jmarshall]

I noticed that C1 and R2 are tied to the input jack, so I put a patch cable into the input jack and when I touch the end the same thing happens.

Also, I noticed that the IC9 is pretty hot to the touch.

One more thing, the 12ax7 tube doesn't glow at all, is it supposed to glow like in a conventional tube amp? It gets slightly warm, and I know the tube doesn't function the same in this SS amp as they do in an all-tube amp.

[Enzo]

The LEDs are diodes, in this case, the fact they make light is irrelevant. They are used as clippers to increase distortion and provide some limits. It is normal they light up.

Look at the schematic page 2 upper right. The heaters of the tube get their current from the main power transformer through a pair of 180 ohm 7 watt resistors. R112,113. They should be pretty easy to find. Either one open?

[jmarshall]

Thanks for the reply, I've read a few other posts regarding this amp that you've contributed to. I really appreciate the help.

I measured these two resistors, although they aren't open they only read 101-105 ohm. Could this be contributing to the low distortion channel though, or is that more likely to be another component, as the clean channel is (very!) loud when turned up?

I was actually quite surprised at the sound of this amp last night playing a littel bit through the 'crunch' clean channel!

[jmarshall]

Any more advice? I've went over the board again and resoldered any suspicious looking joints.

Should the cement block resistors be off so much in value?

[Enzo]

Those resistors will measure fine if you remove them from the circuit. They measure low because of all the circuitry around them. If they were truly low, then you tube would glow brighter than normal, not darker.

You need to determine if heater current is getting to the tube. If not, then why not. It is a very basic circuit.