Marshall Valvestate 8080 dead effect loop

[RobinPlas]

Hello all,

This is my first post after reading and using this forum for a long time. I love it, there a a lot of really helpful posts here! Thanks for that! Anyway, I have two issues I haven't found a solution for. I thought I might get some help here.

I'm using a Marshall Valvestate 8080 as a practice amp. Unfortunately the effect loop is dead. Both the pre- and the post amps are working great. Whenever I turn the effect loop master knob from dry towards wet it produces a very loud pop over the speaker and killing the signal completely in the progress. The signal only comes back when completely in dry mode. Obviously the effect loop has died on me.

I would like to fix this myself? I'm new to electronics myself, but I'm more than willing to learn. How can I narrow down what part is broken? I have a volt meter.

Thanks in advance for any advice!

Robin

[Jazz P Bass]

Are you using the effects loop itself?
With nothing inserted into the jacks, a full clockwise rotation of the Dry/ Wet pot will kill the signal.
Try running just a jumper cord from In EFF Send to Return & see if it still pops.

[km6xz]

With or without a signal in the return jack, the wet/dry balance control should not create a pop, or kill the dry signal completely until set to the center or wet portion of the wiper travel. Considering both symptoms, check for DC on the out pin 1 of IC6. It should not have it. Replace the IC if there is more than a couple millivolts. If the IC is bad, with an offset on pin1 the only position normal signal would be found is when the effects balance is full in the dry position, even a slight rotation from 100% dry would create a high DC offset and no signal in the dry/effects summing amp post the bridge summing network.
If there is no DC offset on IC6 pin 1, time for some more in depth measurements, report back.

[RobinPlas]

Thanks for the replies.

I performed the check. Since this is a first for me, I'll describe what I did because I easily could have done it wrong. Please correct me if I did.

I powered the amp up with 220V. I connected the black wire from the multimeter to the chassis of the amp, this should be ground. According to my research Pin 1 should be the one where the dot (dimple?) is on the IC. I connected the red wire to this one. When I dial in 20V on the multimeter I get a reading of 14.37 when the effect loop knob is turned to dry. Slowly turning to 100% wet will increase this value to 14.45.

This tells me there is too much voltage on the output of the IC?

[km6xz]

It appears that you found the problem. Replacing IC6 should get the return amp working again and get rid of the pop.
Good luck.

[RobinPlas]

One final question: do you know where to get the correct IC (NMJ072BD)? I cannot seem to find this item anywhere.

[Jazz P Bass]

NJM072 is the part #.
Link:[url]http://www.findchips.com

[RobinPlas]

Jazz P Bass and km6xz thank you both for your replies! I owe you a beer.

Cheers!

[km6xz]

The NJM072 is the same part as the common TL072, a dual internally compensated opamp. Even Radio Shack should have it, or any parts store. Most shops have dozens or hundreds in parts inventory. You can also use a low noise compatible part NE5532 that will plug into the same circuit.