Marshall JCM2000 TSL Hum

[u.rimle]

TSL100 XLR hum

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

Con19 gives clean sound!
pulling V4 has no impact
channel vol. has no influence. I go in a MACKIE DL1608. This time I connected to a battery powered VOX Escort, which was also humming on XLR out.
ACV between pins 2 and 3 is 2-3mV.

I will try another IC1.


Other ideas?
salute
urs

[nickb]

Other ideas?
salute
urs

You didn't try pulling CON20 as suggested.

Why change IC1 when you've that shown by pulling CON19 that the problem is before it? So you pull V4 and the hum is the same? That tells us the problem is between V4 and CON19.

[u.rimle]

You didn't try pulling CON20 as suggested.

Why change IC1 when you've that shown by pulling CON19 that the problem is before it? So you pull V4 and the hum is the same? That tells us the problem is between V4 and CON19.

I pulled CON20: no reaction


I think we have a misunderstanding: I connected at CON19 the amp (a battery powered and an AC powered) and there was no hum. that means that the issue has to be after after CON19 resp. CON1 on the rear board. Maybe I damaged the IC1 during soldering.

What could have been happened a long time ago: maybe the phantom power was on on the mixer one time. could that damage something?

Urs

[nickb]

Yes, we are at cross purposes. The debugging steps given assumed that you kept your amp plugging in to the emulated out. I think it unusual for a chip to cause hum. I'd be looking for a ground connection problem in the JCM2000 between those two boards.

That said you're getting 2-3mV of hum on a line level output. That's about -50dB signal to hum ratio. Not great but not awful either. Did it change or was it always like this?

[u.rimle]

Yes, we are at cross purposes. The debugging steps given assumed that you kept your amp plugging in to the emulated out. I think it unusual for a chip to cause hum. I'd be looking for a ground connection problem in the JCM2000 between those two boards.

That said you're getting 2-3mV of hum on a line level output. That's about -50dB signal to hum ratio. Not great but not awful either. Did it change or was it always like this?

The hum is awful, like a real (external) ground loop!
I bought the amp new in April 1999. It was perfect, no hum on XLR out. around mid 2000 the issue occurred and I changed to an DSL 212. Actually a JTM312 from 1996, which is very silent on XLR out!

[nickb]

Then I suggest you repeat the debugging steps but leave your amp connected to the XLR.

[u.rimle]

Then I suggest you repeat the debugging steps but leave your amp connected to the XLR.

STRIKE! I changed the IC1 again, but this time by a NOS LM348N from STmicroelectronics. \m/ all good now!

nickb: thx a lot for your support/comments!

Urs