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.

 

[Alva Goldbook]

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.

View attachment 140219

Looks like the 741 has a different pin out. I think we found the problem.

 

[Brian Roth]

Looks like the 741 has a different pin out. I think we found the problem.

View attachment 140238

How did a 741 enter this discussion? Pucho mentioned finding a TL071 in place of the 5534 shown in the schemo, but those should be pin compatible in this application. However, a 741 is also pin compatible...shrug.

The 5201 is used in two very distinct locations and they control the switching between the clean and "overdriven" signal path.

Bri

 

[Alva Goldbook]

How did a 741 enter this discussion? Pucho mentioned finding a TL071 in place of the 5534 shown in the schemo, but those should be pin compatible in this application. However, a 741 is also pin compatible...shrug.

The 5201 is used in two very distinct locations and they control the switching between the clean and "overdriven" signal path.

Bri

Ah. Looks like you posted the wrong pin out for a 5534 above.

 

[Brian Roth]

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.

R19 and R20 ahead of IC1 (the "5534" or "TL071" in our discussions) is an approx 15 dB attenuator. IC1 itself is a unity gain buffer.

Now why VR10 has "little effect" is baffling. It should go from full off to some nominal value.

 

[Brian Roth]

Ah. Looks like you posted the wrong pin out for a 5534 above.

View attachment 140242

We are were talking about a 5201 and I linked the pinout earlier in the thread....NOT a 741/5534/TL071. A pair of 5201's (IC3 and IC4) handle the switching between clean and overdrive. The schematic shows a 5534 (Pucho found a TL071) for IC1 and is merely a buffer for the tone stack in the overdrive path.

Bri

 

[heatwalk]

Have you checked continuity between pot pins and wherever they are running? Not this amp in particular but Ive been stumped before where a trace cracked in the middle of a run not at the joint. Or instead of using science give it some chopstick whacks if you can get it to behave intermittently and zero in on a location

 

[pucho812]

Ok
Tonight’s measurements.
With my test signal of 1k at 0dBu

At the r17 land c16 junction I get +2 in the od1 position and +20dBu with od2 switched in. Vr5 works adjusting gain.
With all 3 tone stack pots adjust full clockwise along side gain at full clockwise the same 1k signal measures -20dBu in od1 and -3dBu in od2 position at the output, pin 6 of ic1. I get the same measurements at pin 3 of ic1.
At r22 I measure -22dbu(od1) and -5(od2) with same conditions as before and vr 9 full clockwise.
At the other end of r22 with those same conditions I get -35dBu(od1) and -16dBu(od2).
Same conditions + vr10 at full clockwise:
R55 measures -37(od1) and -20(od2). Vr10 Will adjust this.

With the clean channel
If I have all the pots at full clockwise I measure -20dBu at pin 5 of ic4.

With the boost channel I get -60dBu(od1) and -53dBu(od2) at pin 5 of ic4.
With all the pots at full clockwise I measure -35dBu(od1) and -18dBu(od2) of the boost channel. at pin 7 of ic 4

All pots test ok out of circuit and I get continuity of each pot pin to where they connect in the circuit.

 

[Brian Roth]

Lots to digest.

"At the r17 land c16 junction"....typo? Actually C10?

One end of R55 is fed from the wiper of VR10 via coupler C35. Other end of R55 connects to ground, so that reading is bogus.

Seems like the probs are in that VR9 and VR10 sector. But I'm tired here at 1:13 AM! lol

Bri

 

[pucho812]

Lots to digest.

"At the r17 land c16 junction"....typo? Actually C10?

One end of R55 is fed from the wiper of VR10 via coupler C35. Other end of R55 connects to ground, so that reading is bogus.

Seems like the probs are in that VR9 and VR10 sector. But I'm tired here at 1:13 AM! lol

Bri

Yes c10. Typo.


Further testing measuring pin5 of ic 4 and get clean and boost channels without switching. So I can adjust level changes by adjusting gain on clean and boost.

Pin7 of ic 4 yields -35(od1) and -18o(od2) Dani respectively,

This is consistent with previous measurements and the level drop at R22.