Mesa Mark 4 lead mode hum

[Zouto]

Hi, Mesa Mk.4 has a bad hum in lead mode only, all other modes are ok. All caps are replaced with fresh TAD and Panasonic caps, except the three blue radial 8K2uf/16V caps, which I could not remove from that crammed pcb. All tubes are fresh TungSols, sound is fine on all other modes, lead mode sounds fine, but with a real bad hum. Pulling V3 disengages lead mode and hum. Plate voltages on V3 are similar to those on schematics. So what to check to fix that hum?
Thanks a lot for hints and help and stay well and healthy
Zouto

[Justin Thomas]

Probably obvious, but did you try a new tube in V3, besides just pulling it? Or several, for that matter? Even new tubes can be noisy.

Justin

[Zouto]

All tubes are new, I also tried different tubes for V3 for no bettering.

[Jazz P Bass]

Schematic link is dead.
Here: mk4a.zip

[Zouto]

Schematic link is dead.
Here: [ATTACH]58313[/ATTACH]

THANKS!!!

[nickb]

You just know it's going to be those 8200uF caps, don't you?

Try pulling V1 when in lead mode and see what happens to the hum. That will narrow down the source. If you didn't try a sub for V1 try it now.

To check the heater supply measure the DC volts between pins 4 and 5 of V1. If it's not 12.6 give or take a pinch then that would indicate a problem. If off, check the input to the voltage regulator VR12, a 7812 I'd guess. The input needs to be at least 15.6V. The design seems pretty marginal as I think it may struggle to produce that using 1N5392 regular diodes, better to use schottky's. That means a failing cap will quickly lower the voltage and give you ripple on the heater power supply.

That said, there are high gain amps that have AC heaters and don't have hum so this could be a problem but not the problem.

[Zouto]

Nick, thanks, pulling V1 reduces the hum in lead mode a little bit, I get 11.8 DC Volts on V1 between pins 4/5, 13.8 Volts on input of VR 12. Switching from Rhythm2 to Lead manually by the rotary switch produces loud popping and crackling noises, switching the same by footswitch gives popping. Might be some weirdness caused by Mesa‘s LDR/ switching matrix jungle.
Servicing the impenetrable thicket of a Mesa MK 4 pcb in times of Corona is quite a bit too much....

[nickb]

So pulling V1 makes just a small difference but the main hum is still there, have I got that right? In that case, talking only about lead mode, I doubt the hum is coming from the DC heater supply crummy though it is, since if it were V1 would make a big difference. As an experiment, if you have a bench DC power supply you can feed the V1 heater with 12.6V DC, get the polarity the same and you don't have to disconnect the exisiting supply. Then you know you have a clean supply and can tick that off for sure. The task on improving the internal DC supply can be left to another day if desired.

Your rotary switch might just need a spray with contact cleaner as I don't see any fundamental reason it should sound different from other switches. Again, not the hum issue.

So it's not V3 or the heater supply. So what about the node 'C' power that feeds V3 and the reverb driver. Do you think maybe the reverb is more hummy than usual> Maybe try a cap in parallel with the 30uf 500v on node 'C'.

Try shorting the grids of V3 to a nearby ground ( C333 negative end ) i.e try it with pin 7 then pin 2 to narrow it down some more.

[Zouto]

Reverb is not hummy, shorting V3‘s grid pin 2 turns down lead drive, signal and hum. I put a 100uf/500V in parallel with the 30uf/500V cap at C node for absolutely no hum reduction at all. So what to check next??? I suspect some complicated LDR switching matrix defect, absolutely easy to find, check and fix, as can be seen....

[Zouto]

Any ideas how to fix it???

[nickb]

Can't give a fix until we have narrowed down where the problem is. We're getting there.
Does the hum go away if you turn the lead drive pot to zero?
Does the body of that pot have a good contact to ground?
What happens to the hum if you short out R214 (sheet 1, between V1a and V1b)?
I also wonder if hum is getting into the signal path via LDR8 from it's LED. The power for that LED is on sheet 9 top left. It could be that 15uf ( try one in parallel), or the 15V footswitch power and that brings us back to the 8200uF caps.

PS: You could test the hum from the LDR8 idea by putting it in lead mode and shorting out the LDR side of LDR8.

I just noticed that the low voltage power supply design is nuts. The upper side of the 6.3VAC winding will be elevated to about 8VDC, the other side will be 6.3VAC with an 8VDC offset. The point being that the addition of the DC power side has completely negated the effect of the 100 ohm heater balance resistors.

[Pedro Vecino]

Shorting V3 grid pin 2 should be the same as setting the gain control to zero. Is that so?
Going backwards you could short R214 (100K) to see if the same thing happens. And repeating the operationthe in V1B grid pin 7.
That way you could delimit the area in which the problem is generated.
Before that I would remove the 12AX7 Tung Sol in sockets 2 and 3 and use either Chinese or JJ tubes. 12Ax7 Tung Sol are sometimes prone to strange behaviors regarding hum when the heaters are AC powered.

EDIT: I didn't read Nickb's message. We have posted at once.

[Zouto]

Can't give a fix until we have narrowed down where the problem is. We're getting there.
Does the hum go away if you turn the lead drive pot to zero?
Does the body of that pot have a good contact to ground?
What happens to the hum if you short out R214 (sheet 1, between V1a and V1b)?
I also wonder if hum is getting into the signal path via LDR8 from it's LED. The power for that LED is on sheet 9 top left. It could be that 15uf ( try one in parallel), or the 15V footswitch power and that brings us back to the 8200uF caps.

PS: You could test the hum from the LDR8 idea by putting it in lead mode and shorting out the LDR side of LDR8.

I just noticed that the low voltage power supply design is nuts. The upper side of the 6.3VAC winding will be elevated to about 8VDC, the other side will be 6.3VAC with an 8VDC offset. The point being that the addition of the DC power side has completely negated the effect of the 100 ohm heater balance resistors.

Shorting R124 reduces hum quite a bit, swapping tubes makes no difference. Same on shorting #8’s LDR side. The hum sounds indeed like unbalanced AC heaters. In the other modes Rh1 and Rh2 it is low, but also still audible. So to go for fixing unbalanced AC heaters caused by poorly designed low voltage supply? What a drag...

[nickb]

Shorting R124 reduces hum quite a bit, swapping tubes makes no difference. Same on shorting #8’s LDR side. The hum sounds indeed like unbalanced AC heaters. In the other modes Rh1 and Rh2 it is low, but also still audible. So to go for fixing unbalanced AC heaters caused by poorly designed low voltage supply? What a drag...

I've got a lot of questions. Please take your time and don't miss any.

Did shorting LDR8 reduce the hum or not? Your answer is not clear. I'm taking it as it didn't help.
Never did get an answer to the effect of turning the gain pot down and checking it's body is grounded. So...?
How loud is this hum? Is just audible or so loud it's unplayable?
Put it into hum mode and measure the ACV across the speaker.
Did is suddenly start happening or has it always been link this?
Do you have your guitar plugged in when this happens?
What happens if you unplug it?
What about if you are plugged in and turn the guitar's volume down to zero?
Would you say this is 50/60Hz or 100/120Hz hum? Your profile doesn't tell us where you are so I don't know your mains frequency. Maybe a good time to update it.

The heater circuit is not easily fixable. Being unbalanced isn't necessarily a problem given V1 and V3 have a DC supply. I doubt that is the issue as there are plenty of these out there, assuming your amp amp is behaving differently from normal, which I'm starting to doubt.

[Zouto]

Checking heater voltages on power tubes, I get 6.8 Volts DC on pins 2 and 8, similar values when tubes pulled. Checking AC Voltages it‘s 6.1 Volts AC on pin 7 and 0.6 Volts AC on pin 2.