Marshall 6100 all levels "slow" starup

[Yelly]

Hi there,

I have a 6100 LM here, it works and everything, but has a funny "fault":
Nothing happens until either channel volume is at 9-10 o´clock (or 2-3 outta 10).
On the clean channel, it´s almost dead until volume comes above 2, ch.2 and 3 are very low volume but more audible because of the gain.
I did these test with everything else at max, all channels.
It´s like the mu is too low to drive the PI until a certain volume, and I know that the loop and ICs are in between the PI and preamp. All voltages, low as high, are normal.
When above the breaking point, things start working fine, although there might be a possible attenuated gain going on. I don´t think the ch2. C-mode sound as gainy as I remember, also ch.3 seems less metal-drive.

Certainly not how the volumes should work, what I recall from other 6100 amps I´ve tried, does anybody have a clue or seen this before?

Wild guess, a semi-bad IC attenuating the signal, but any thoughts would be much appreciated

[Yelly]

I just tried swapping ICs around, tried a new 072 also, but no difference.

[g1]

Exactly which 072 did you replace?
Suggest you look at the 5201 switching op amps.

[Yelly]

Exactly which 072 did you replace?
Suggest you look at the 5201 switching op amps.

I changed IC203 072 in the loop with a brand new. I also swapped input 5201 IC201 and 202 with the ones in the FX SEND section, 204/205, but if one of them are bad it might still attenuate signal regardsless which position it is in. I actually only tried with loop set to parallel, so sound came through no matter if 204/205 was pulled.
On monday, will try bypassing the loop section entirely, see what happens.

[TimmyP1955]

I don't have a schematic for that, so.... Check all grounds, and all supply voltages

[Jazz P Bass]

Marshall 6100

6100 schematic.

[The Dude]

Have you tried inserting a cable in the loop? You might just have a bad switching jack.

[Yelly]

Have you tried inserting a cable in the loop? You might just have a bad switching jack.

That would not be the problem as in parallel mode the signal passes through 100%. I also doubt it could be the serial/parallel switch. It also sounds nothing like the typical bad contact in a loopjack.
Thank you for suggesting anyway

[Yelly]

Today I tried bypassing the loop, partially. Went from ch.1 out to Return, by connecting Red/screen to a female jack and used a jackcable from that to Return, same slow volume reaction. If I move my applied signal from my generator to the Return, volume is pretty high.
Looking at the input on the amp, 100mV goes into V1A and out comes 2V, but it´s very low entering V1B unless I turn up Volume to at least 10 o´clock. Then I get about the same outputlevel as if I use Return. Everything is grounded, all cathodes connected to their resistors and all values ok, cathode DC is couple of volts as normal, I´m starting to think this amp is just the way it is. As far as I know, all 6100 models through the years used the same A1M pots, with same logarithmic turnratios. Seems odd though, I can crank up the Master to full, and I still have to go past 9-10 o´clock on each channel gain/volume to just get minimum sound. Usually this happens already on 8 o´clock, or "1".
Last resort: test with another 6100, will try that soon.

Appreciate any thoughts on what could be "wrong". Thank you for you time

[g1]

Sorry, I wasn't paying enough attention to your original post and thought we were talking about volume settings of 9 or 10 but you actually said 9 or 10 o'clock (2 or 3 on the pot).
I think this could well be normal for this amp (if all else sounds good). As you say, best to try comparing to another 6100 or scouring forums for others complaining of this issue.