Marshall valvestate 8080 crunch sounds farty

[Rickybocker]

I have been repairing a valvestate 8080 and seems like the crunch setting still doesn't sound right. It sound kind of splatty or farty for lack of a better term.


So far I have replaced the IC1 and IC2 and IC5. The diodes in the clipping circuit.
The crunch/clean switch and all the pots in the clean stage were replaced.

I saw some DC offset coming out of IC1 and also saw that only one diode in the crunch circuit would light up barely. I saw that the signal coming out of IC2 was getting loaded down.

After getting new ICs and the diodes, everything seems to be working, but when I turn the crunch mode on channel one and turn the gain up then it has the fart distortion.

I'm stumped and wondering if that is just how these amps sound. The clean channel and channel 2 sounds pretty decent.

Any suggestions?

 

[tschrama]

Post DC values of the opamp pins. That can give a clue abour what is going on.

 

[Rickybocker]

Ok thanks. I'll get those measured ASAP and post. Assuming you want IC1, IC2 and IC5 only?



During low level testing last night It sounded as though the clean channel was finally at the correct output level. But today at high volumes, the clean channel is back to be very low compared to the 2nd channel.

I have to have the crunch engaged on channel one to get the clean to be fairly audible.

Post DC values of the opamp pins. That can give a clue abour what is going on.

 

[ricksdisconnected]

diet related

 

[Rickybocker]

Post DC values of the opamp pins. That can give a clue abour what is going on.

Ok I tested with and without a signal generator. DC V with COM to ground.

Idle no signal

IC1
1 = 0.2mv
2 = 0.2mv
3 = - 0v
4 = -15.5v
5 = -0v
6 = -0.3mv
7 = -0.3mv
8 = -15.42v

IC2
1 = -14.4v
2 = 2.5V (started dropping while measured)
3 = - 0v
4 = -15.8v
5 = 0.62mv
6 = -2.3V (started dropping while measured)
7 = 15.7v
8 = 14.82v


IC5
1 = 15.5v
2 = 1.7mv
3 = 4.3mv
4 = -15.9v
5 = 3.9mv
6 = 1.9mv
7 = 0v
8 = 15.7v


Signal generator 1k sine wave 0.5vpp

IC1
1 = 0.6mv
2 = -0v
3 = - 0.2mv
4 = -15.5v
5 = -0.3mv
6 = -0.5mv
7 = -1.3mv
8 = -15.42v

IC2
1 = -14.4v
2 = 8.5V (started dropping while measured)
3 = - 0v
4 = -15.8v
5 = 0.62mv
6 = 3.3V (started dropping while measured)
7 = 15.5v
8 = 14.72v


IC5
1 = 15.5v
2 = 1.7mv
3 = 4.8mv
4 = -15.9v
5 = 3.9mv
6 = 1.9mv
7 = 0v
8 = 15.6v

 

[thetragichero]

IC1
8 = -15.42v

that doesn't look right! typing error? should be the positive supply rail for the chip. would expect about +15v here

that aside, since it wasn't mentioned: have you changed the preamp tube in it?

 

[Rickybocker]

that doesn't look right! typing error? should be the positive supply rail for the chip. would expect about +15v here

that aside, since it wasn't mentioned: have you changed the preamp tube in it?

Ya. Sorry those are supposed be positive. Ive swapped the preamp and no change. From my understanding of the circuit, the preamp is only used on channel 2.


It appears channel 1, is just going through IC1 for preamp gain. The crunch button changes gain of the op amp by switching in a different resistor then also passes the signal into a diode clipping circuit to create distortion.

IC2 is the variable gain controlled with pot.
I have removed R30 to check the output of IC2 before it hits IC5 switching op amp. It had a good output signal but once I connected it back to IC5 the signal seemed reduced. Like IC5 was loading IC2 down.

I calculated there should be a gain of 6 on IC1 in clean mode and gain of 26 in crunch mode. I'm only seeing half that. With 0.5vpp signal I'm getting 1.5Vpp and then in crunch mode getting 5.5vpp.

I've verified resistors in the circuit and I guess I need to recheck caps for loss.

 

[tschrama]

It is difficult to interpret the IC voltages without a Schematic and IC part numbers/or pin-functions.

 

[thetragichero]

standard 8 pin op amps. schematics i glanced at yesterday showed tl072 but could also be 4558 depending on what marshall got cheaper that week

 

[tschrama]

Are you sure? IIRC there are switching opsmps in these amps!

 

[Rickybocker]

Posted the schematic.

IC1 is a TL072
IC2 is a TL071
IC5 is a M5201

Uploaded data sheet for M5201

Online data sheet for TL072 and TLO71. File was too large to upload.

https://www.ti.com/document-viewer/...B7-95FE-95EB38023472#TITLE-SLOS080SLOS0809617

 

[Rickybocker]

Bump

 

[Rickybocker]

Ok so I have gone through all the caps in the clean/crunch channel. There was a slight DC offset at about .3V pk. I saw that the decoupler caps for the IC1 on the 15V rails were slightly out of spec. I replaced those and few other ceramics that were suspect. This really improved the farty sound on the crunch. No it only goes farty with crunch setting on and gain fully up.


At this point I have checked and or replaced every component in the channel one stages. But I'm still not getting the expected volume out of the clean. I can kick on the channel 2 and it is very loud.

When watching the waveform on my scope, I can see that the signal actually goes down with more gain. and up with less gain. Just slightly. At low levels the clean sounds pretty great.

I'm wondering if anyone else has heard this with the 8080. Maybe its an inherent design flaw. Since I have rebuilt the entire circuit I'm not sure what else it can be.

Does anyone know a good chat board for technical help?