Marshall JCM2000 TSL Hum

[AtomicMassUnit]

I am working on a TSL100 with a hum problem. It's got what sounds like a ground loop hum that gets going after about 10 minutes of being on. With an instrument (or cable) plugged in, it's significantly worse, and unaffected by the volume controls. I'll have to check again but I think it's worst on the clean channel. I read a few threads here and elsewhere about C46 being burnt, and it was. I replaced that with a 1kv cap of the same value, and it didn't fix the problem. Any ideas?

[Enzo]

C46 runs between pin 3 and 4 of the end tube - plate and screen. Make sure the screen resistor for that tube is not open. For that matter check all four.

Isolate the problem, pull the PI tube. Still hum?

[AtomicMassUnit]

C46 runs between pin 3 and 4 of the end tube - plate and screen. Make sure the screen resistor for that tube is not open. For that matter check all four.

Isolate the problem, pull the PI tube. Still hum?

Pulling PI tube silences the hum completely. Replaced tube with another, still there.

[Enzo]

OK, so the trouble seems not to be in the power tube stage, and any C46 connection is in the past. This also means the problem is either before the PI tube or right at it. leave the PI and pull the other small tubes. Still hum? Moving from the PI back towards the input, start adding the small tubes back in one at a time. Looking for what stage adds in the hum.

Everyone hates all those little cables going from here to there - like all the hoses under the hood of your car. But you can use them to troubloeshoot. Looking at the schematic, CON11 is the input to the PI. With the PI installed, unplug CON11. Hum stay or go? I do realize the drawing can be very confusing to follow, but you can use these little connectors like that all over the place.

And don;t forget the solid state parts, they have their own low voltage power supplies, so make sure the ICs have good CLEAN +15 and -15v.

[Amp Kat]

Check all of the grounds to and make sure the connectors are good and tight and check to see if the input jack is grounded from sleeve to chassis and not open.

[AtomicMassUnit]

Enzo, removing all the preamp tubes (except PI) has no effect on the hum. Pulling CON11 makes the hum way louder.

Amp Kat, the input jack is well grounded.

[davetutt]

by far the easiest way to see whats going on is to split the amp at the send and return sockets on the back. plug an open lead into either the send or return and if the hum goes away then it is in the pre-amp stages. If it doesn't go away then the issue is due to the standard problem with the output stage. Search for my notes on here for a guide.

That's a good start.

If you don't have thermal bias issues or you haven't got damaged output valves (tubes) caused by the same issue then prime candidates are the HT reservoir capacitors that can dry out due to the high heat level under the metalwork. Most are only 85 degree types and realistically they should be 105 degree types.

Dave

[TimmyP1955]

Check the hum balance resistors.

[u.rimle]

Hello,
I have a hum problem with TSL100 at the XLR out only. I changed main board, iss.20 now, and all tubes. I had this hum before. It is not a ground loop problem. Maybe internally?
Any ideas?
Cheers

[Jazz P Bass]

More than likely, IC1 on the Output Board is the culprit.

tl10-62-02 Output Bd.pdf

[Reader]

More than likely, IC1 on the Output Board is the culprit.

[ATTACH]50802[/ATTACH]

the attachment seems to be truncated as only a portion of the circuit is showing

[Jazz P Bass]

I cropped the image to show the circuit in question.

[u.rimle]

TSL100 XLR hum

More than likely, IC1 on the Output Board is the culprit.

[ATTACH]50802[/ATTACH]

I am with you. make sense. I will change the IC1 and post the result. Thx

[u.rimle]

TSL100 XLR out hum

I am with you. make sense. I will change the IC1 and post the result. Thx

Changing the IC1 didn't help. you can hear the hum even with the amp in standby mode. disconnecting con1 it's quite.
I will check the signal that is coming from connection con19. maybe IC5a?

Any other ideas?
Cheers Urs

[nickb]

Keep following it back. After CON19 you can try CON20.

What is the effect of the pulling out the phase inverter V4?

What effect do the master volumes have on the issue? What are you plugging it into?

Exactly how much hum are we taking about? Measure the ACV between pins 2 & 3 of the XLR