Marshall 8080 Issues

[misterc57]

Picked up an 8080 for my next project. Issue was no sound.

Using a 440 hz signal, I traced the issue to the effects loop. Signal stopped between IC5 and IC6. I placed a jumper cable in the effects send/return jacks. Now I had
a signal all the way to C17.

I placed the chassis back into the amp. Hooked up the speaker and reverb tank. With jumper cable in effects loop...

Issues are;

Clean channel, when strumming it sounds like the speaker is rattling like crazy, harder the strum, the worse the rattle.
Gain channel, hardly any sound at all, occasionally there is sound with rattle.

All pots, switches, jacks have been cleaned.

Every IC I tested had +15 VDC at pin 8 and -15 VDC at pin 4. However I did not test every IC.
12ax7 had 199 VDC at plate fed from R110 and 115 VDC at other plate fed by R109.

Other tests;

Using another amps pre-amp signal fed into the 8080 return jack, the 8080 sounds fine, the speaker sounds fine with no rattles.

Using the 8080 as a pre-amp to another amp, all I get is a loud ground hum in the other amp. I hear no guitar signal.

Could all these issues be related to the effect loop problem?

Are there other components that could be at fault?

Thank you in advance! Mark

[nickb]

Picked up an 8080 for my next project. Issue was no sound.

Using a 440 hz signal, I traced the issue to the effects loop. Signal stopped between IC5 and IC6. I placed a jumper cable in the effects send/return jacks. Now I had
a signal all the way to C17.

I placed the chassis back into the amp. Hooked up the speaker and reverb tank. With jumper cable in effects loop...

Issues are;

Clean channel, when strumming it sounds like the speaker is rattling like crazy, harder the strum, the worse the rattle.
Gain channel, hardly any sound at all, occasionally there is sound with rattle.

All pots, switches, jacks have been cleaned.

Every IC I tested had +15 VDC at pin 8 and -15 VDC at pin 4. However I did not test every IC.
12ax7 had 199 VDC at plate fed from R110 and 115 VDC at other plate fed by R109.

Other tests;

Using another amps pre-amp signal fed into the 8080 return jack, the 8080 sounds fine, the speaker sounds fine with no rattles.

Using the 8080 as a pre-amp to another amp, all I get is a loud ground hum in the other amp. I hear no guitar signal.

Could all these issues be related to the effect loop problem?

Are there other components that could be at fault?

Thank you in advance! Mark

The only things than can kill the signal around the fx loop is R48 22K being open, the FX Wet/dry control VR1 and bad joints. With the jumper removed have you tried rotating that control both ways ?

The hum when using the FX send suggests that the ground is open. Check R20 10 ohm.

8080 Schematic

I suspect the rattle is caused by something else but best not to jump to conclusions. Do you have a scope?

[misterc57]

I have rotated the wet/dry pot VR11, both with and without the jumper in place. I have measured resistance to ground on R47 and R48 varying VR11 and can see that the pot is working.

I have checked all resistors in circuit around the send/return jacks. All are close to schematic except R44 and R20 (both 47K resistors) which both read 29.7K.

I have pulled the board up and inspected all solder around the send/return jacks. Everything looks clean.

Only odd things I have noticed are

1. It looks like some work may have been done around the on/off switch. A jumper may have been added, and can't tell if there is some discoloration around R101 and R102. Both those resistors read 330 ohms.

2. C44 should be a 4n7 capacitor but instead is a resistor reading 337K, and it does not look like this was replaced by someone.

I have attached some pictures.

I can get a o-scope, but have not used them much. What did you have in mind for the scope?

Thank you!

[nickb]

I have rotated the wet/dry pot VR11, both with and without the jumper in place. I have measured resistance to ground on R47 and R48 varying VR11 and can see that the pot is working.

I have checked all resistors in circuit around the send/return jacks. All are close to schematic except R44 and R20 (both 47K resistors) which both read 29.7K.

The problem with measuring thing in circuit is that there are often parallel paths. In this case R43, R46 and VR11 are throwing it off. Anyway, since a signal into the return works OK we have no need to probe there.

I have pulled the board up and inspected all solder around the send/return jacks. Everything looks clean.

Only odd things I have noticed are

1. It looks like some work may have been done around the on/off switch. A jumper may have been added, and can't tell if there is some discoloration around R101 and R102. Both those resistors read 330 ohms.

The discoloration is normal. Lets not worry about the ugly-looking link on the power switch since it's doing it's job. It's not our issue.

2. C44 should be a 4n7 capacitor but instead is a resistor reading 337K, and it does not look like this was replaced by someone.

I think you'll find that is a capacitor, not a resistor. Sometimes they look like that. Nothing to to with this anyway.

I have attached some pictures.

I can get a o-scope, but have not used them much. What did you have in mind for the scope?

Thank you!

I was thinking about tracing the rattle but we might be able to get there using your signal tracer and DC measurements. We'll see.

For the send hum do this - measure the resistance from the the sleeve of the send jack to the sleeve of the return jack. It should be very close to 10 ohms.

[Edit: If that is OK it might a bad lead or perhaps you just have the gain of the listening amp up too high. Normal signal levels at this point in the circuit are quite high - a volt or so.]


For the open signal path do this. Set VR11 in the middle measure the resistance from the -ve end of C16 to R48, one end should be<1 ohm and the other ~13K. Now measure from the higher resistance end (from last measurement) of R48 to pin 6 of IC6. It should also be about 13k.

Another test you can try, with VR11 still in the middle touch your finger on the metal blade of a small screwdriver and touch the blade against either end of VR11. You should hear some hum in the speaker - you might need to raise the master volume VR13 to hear it. The hum should be about the same in both places.

On the rattle, report the DC at

IC1 pin 7
IC1 pin 1
IC2 pin 6
IC3 pin 5
IC4 pin 6
IC5 pin 5
IC6 pin 7
IC6 pin 1

....these should all be close to zero.

[misterc57]

Chassis is out right now. Here are some of the results with power off.

>>For the send hum do this - measure the resistance from the the sleeve of the send jack to the sleeve of the return jack. It should be very close to 10 ohms.

10.3 ohms

>>For the open signal path do this. Set VR11 in the middle measure the resistance from the -ve end of C16 to R48, one end should be<1 ohm and the other ~13K.

Yes, .3 ohms and 12.5K ohms on R48 ends.

>>Now measure from the higher resistance end (from last measurement) of R48 to pin 6 of IC6. It should also be about 13k.

13.97K

Will try other tests tomorrow night when I have more time to hook it back up to power and speakers, etc.

Very much appreciate the help! Mark

[misterc57]

Ok, reporting the DC referenced to chassis ground at

IC1 pin 7; -14.1 V
IC1 pin 1; -13.7 V

IC2 pin 6; -1.8 mV
IC3 pin 5; +0.12 mV
IC4 pin 6; -2.5 mV
IC5 pin 5; +4.28 mV
IC6 pin 7; +30.4 mV
IC6 pin 1; -1.78 mV

If these are all suppose to be close to zero I have big problems at IC1.

Also, I can hear an audio signal going into IC1A pin3, but nothing at pin 2 and 1.

If IC1 needs replacement does it have to be a specific TL072?

Thank you!

ps. Another point of interest is this time I have an audio signal all the way to C17 without inserting a jumper cable at send/return jacks.

[Enzo]

IC1 should not have -14v on those output pins. First check pin 8 to make sure +15 is getting there. If the IC has -15 on pin 4 and is missing +15 at the other corner, then for sure, all the voltages in it will be skewed off to -15 land. If you DO have +15 on the IC, then the IC is bad.

Notice just below IC1, upper left area, is the isolated power feed to the IC. They use 47 ohm R64, R65 in series with the 15v feeds. If one of those resistors opens, you lose voltage to the IC. Note this arrangement is ONLY for IC1.

A "specific" TL072? TL072 IS a specific IC. I am sure yours are in a DIP package, DIP-8, or an inline package. so don't order surface mount SOIC package, if that is what you mean. I don't think TI makes an inline package any more anyway. Brand doesn't matter.

[nickb]

As, Enzo but also check that pin 1 is close to zero in case R2 is open.

The resistance test demonstrates that everything is connected and parts are working in the FX send/return path so i don't know why you are having issues there. Anyway, Fix IC1 and we'll go from there.

[misterc57]

IC1 pin4 = -14.8V
IC1 pin8 = +14.8V

R64, R65 and R2 all measure to spec.

I will locate a TL072 and replace IC1. Could be several days before my next update on this.

Thank you! Mark

[Deadear]

I don't know where you are at but I walked into this store and bought the TL072 IC. Fixed my 8080 it was a hisser.SAYAL Electronics Web Site - 11/18/2015 7:14:30 PM

[misterc57]

Replaced IC1. Everything is working now, amp sounds great. Thank you to all for helping me isolate the issue!

Mark

[Deadear]

Good news seems to be common with that amp. That is three I know of that ic 1 went out. Rock ON