chasing an amp fault in this solid state Marshall

[pucho812]

So I am working on this Marshall vavestate bi-chorus. The clean channel works well and sounds great. The boost Channel is very low in volume.
I get the correct voltages on the tube and I do get the OD1 and OD2 switching. I got the gain pot working without issue. But after the tone stack, the contour pot and volume pot do not function. I have traced signals and I loose a lot of volume in the tone stack and it just gets progressively worse. I am not sure what is left to check. All of that is happening before any insert send and return and with the clean channel working as it should it leaves out the insert jacks as being a problem.

https://schematicheaven.net/marshallamps/stereochorusrev_2x80w_8280.pdf

 

[NoisyIndividual]

Make sure all the pots are ok and actually in circuit.
I had a smilar problem that confounded me until I realised several pots weren't actually in circuit.

Some odd corrosion event happened in storage I think.

 

[pucho812]

pots measure ok and I reflowed their joints just in case. Signal seems to drop around the time stack of the boost channel.

 

[Brian Roth]

pots measure ok and I reflowed their joints just in case. Signal seems to drop around the time stack of the boost channel.

That tone stack design does have an inherently high signal loss.....maybe 10-20 dB?

Bri

 

[pucho812]

That tone stack design does have an inherently high signal loss.....maybe 10-20 dB?

Bri

It does. I can measure that. What gets me is after the tone stack(vr 6-8). There is a tl071 but the schematic says 5534. Level going into the 71 is well beyond the 20dB drop from the tone stack and then level coming out is about the same.
My output level (vr10) and contour pot(vr9) have little effect.
So clearly I am loosing level somewhere. I just can’t seem to find it via the probe.

 

[Alva Goldbook]

Maybe someone replaced the 5534 in the past for a 741 trying to fix this same problem and failing. How's the power supply for IC1 looking? Lot of caps here, any of them passing DC? The twin T "contour" circuit could have a resistor that went haywire. It could read okay when the amp is off, but maybe once current is applied it's resistance increases. Maybe. I've seen it happen. My money is on a bad opamp.

 

[JohnRoberts]

It does. I can measure that. What gets me is after the tone stack(vr 6-8). There is a tl071 but the schematic says 5534.

In fact the TL071 is probably a better choice there. The 5534 is not unity gain stable, while used as a unity gain buffer. Maybe they changed it but didn't update the schematic.

Level going into the 71 is well beyond the 20dB drop from the tone stack and then level coming out is about the same.
My output level (vr10) and contour pot(vr9) have little effect.
So clearly I am loosing level somewhere. I just can’t seem to find it via the probe.

The schematic for IC4A and IC4B look weird... (are outputs connected together..and what's a 5201?)

JR

 

[Brian Roth]

In fact the TL071 is probably a better choice there. The 5534 is not unity gain stable, while used as a unity gain buffer. Maybe they changed it but didn't update the schematic.

The schematic for IC4A and IC4B look weird... (are outputs connected together..and what's a 5201?)

JR

I had to look up that opamp and it's an oddball. Dual opamp but with a control input (pin 1) that selects one or the other section to that common output pin 5.

 

[JohnRoberts]

I was asking rhetorically but thanks.. Something else I need to forget.

JR

 

[Brian Roth]

I was asking rhetorically but thanks.. Something else I need to forget.

JR

lol

 

[pucho812]

I post some test tone measurements this evening.

IC4 the switching opamp was looked at, I swapped it for Justin Case. I hate to shotgun parts in that regard.