chasing an amp fault in this solid state Marshall

[Brian Roth]

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.

?????? That is totally wrong. Switching should allow only ONE pathway through ONE set of volume controls and tone stacks at any given time. Switch to clean, boost controls have no effect and vice versa.

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

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

 

[Brian Roth]

What DC voltages do you see (referenced to ground/0V) on pin 1 of each 5201 as you switch between clean and boost?

Bri (glad I have a stash of straws to grasp at)

 

[rh001]

5201 should only allow one of it's opamps to pass signal at any time. by your measurements prob does seem to be in the vicinity of ic4. ic4A has unity gain. ic4B has gain of 2.4. Check to see that you're getting such gains. Also look at switching pin1...does it swing all the way from On voltage to OFF voltage? A partial swing could be the problem. If swing is correct then switch inside ic4 is likely broken.

 

[rh001]

Correction: ic4A has gain of 2.. ic4B has gain of 3.4.

 

[Admiral-dk]

As far as I remember - you will NOT see Pin1 change very much voltage vise when you change between the two halves (that has caught me a few times).

 

[james-finetune7]

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.

I'd have to agree on the op-amp replacement being the problem. No designer would spec a 5534 when a much cheaper Tl071, or an archaic 741 would do the job. The NE5534 from National Semiconductor is designed to be able to source much higher output loads, e.g. the following stage(s). Either Mouser or Digikey.com source the 5534 for a reasonable price in low quantities (1-10 units).

 

[pucho812]

So tonight’s intrigue on this amp. Ic4

It switches when I engage the switch while I am test for vdc at pin1. I can measure the changes between regular and boost. I reflowed solder and still only switches when I probe pin 1 for vdc.

 

[JohnRoberts]

Sometimes a scope probe will provide enough of a DC impedance to ground to activate a floating circuit node. Maybe check for bad solder joints. or not.

JR

 

[pucho812]

Sometimes a scope probe will provide enough of a DC impedance to ground to activate a floating circuit node. Maybe check for bad solder joints. or not.

JR

Well what’s funny is I reflowed all the joints and even tried a second opamp at ic4. Same results.

 

[Brian Roth]

So tonight’s intrigue on this amp. Ic4

It switches when I engage the switch while I am test for vdc at pin1. I can measure the changes between regular and boost. I reflowed solder and still only switches when I probe pin 1 for vdc.

Does the "switching" now restore the audio level for the non-clean mode? IOW, with the probe on pin 1, it now acts like it should when you change modes?

Check around TR4 and related parts.

 

[Brian Roth]

Well what’s funny is I reflowed all the joints and even tried a second opamp at ic4. Same results.

Another thought....IC3 also shares the same pin 1 control line with IC4. Maybe IC3 is screwing the pooch?

Bri

 

[pucho812]

Another thought....IC3 also shares the same pin 1 control line with IC4. Maybe IC3 is screwing the pooch?

Bri

Ic3 switches without issue.
Ic4 restores to normal operation if I probe pin 1 as I described above.

omg… I found it I found it.

So c29, r48 and r50 junction together. R50 series connects to the mpsa13. But at the junction of r50 and r48 they go out to pin 1 of ic3 and ic4 for switching. Continuity testing showed the r50 junction going to pin 1 of ic3 but not going to pin 1 of ic4.
The moment I added a jumper, I get the switching at ic4. Wow… looks like board damage or cracked traces causing it. So with a jumper added boom I get switching

 

[Brian Roth]

Ic3 switches without issue.
Ic4 restores to normal operation if I probe pin 1 as I described above.

omg… I found it I found it.

So c29, r48 and r50 junction together. R50 series connects to the mpsa13. But at the junction of r50 and r48 they go out to pin 1 at ic3 and ic4 for switching. Continuity testing showed the r59 junction going to pin 1 of ic3 but not going to pin 1 of ic4.
The moment I added a jumper, I get the switching at ic4. Wow… looks like board damage or cracked traces causing it. So with a jumper added boom I get switching

HURRAY!!!! That was a booger.....

I forget if you mentioned it...is the PC board made with "real deal" plastic and traces....or is it 1960's brown board and micro-thickness traces?

Bri

 

[pucho812]

HURRAY!!!! That was a booger.....

I forget if you mentioned it...is the PC board made with "real deal" plastic and traces....or is it 1960's brown board and micro-thickness traces?

Bri

It’s a modern pc board. Plastic and traces. Amp was made for 2 years back in the 1980’s or 1990’s

 

[13engrsapper]

Now that you guys are all warmed up, let's get to these 4 Ampeg SVT-CL's I have to service for a backline company here. They look like they are going to be a nightmare, lol.