Mesa Mark 4 lead mode hum

[nickb]

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.

Exactly. That ain't balanced! But it's by design so we can leave it as in the 'could be improved but not the issue category'.

[Zouto]

Nick, thanks for your help. Shorting LDR8 reduced hum, but it was still audible. The actual lead mode hum is too loud, plugged in I get a good guitar signal accompanied by a loud constant hum. This hum started increasing for about a year.
Turning down my guitar‘s volume does not reduce it, just the guitar’s signal, the hum remains unaltered.It‘s a 200hz hum, as I checked with my Peterson strobe tuner app. Unplugging my guitar removes pickup hum only, lead mode hum remains. All controls on 5, I measured across the 8 Ohms speaker output
in Rhy1 : 7.5 mV AC plus occasional loud and short cracking noise, low audible hum
in Rhy2: 4.3 mV AC
in Lead : 311,6 mV AC, Voltage slowly rising, constant hum and occasional loud and short cracking noise.
In Rhy 1 and 2 , other than in lead mode, the signal can be turned down completely by the guitar‘s volume control.

[nickb]

311.6mV is very loud.

It's hard to pinpoint this but I think part of the hum is from the LDR and the rest from the heater supply.

I would replace the 8200uF caps and while you are it change the two diodes to Schottky barrier types. The diodes are currently 1N5392 1.5 amp DO-41 sized with a voltage drop of 1.4V at max current. I suggest MBR3100G or MBR340G. These have a voltage drop of about 0.5V. That with the new caps should give you enough voltage on the 12V regulator to make to work properly and so give you clean heater power. I also suggest you replace that 15uF on page 9 top left to reduce the ripple to the LEDS. Stick as high a value as you can physically fit in there. LDRS respond very slowly and so should naturally filter out ripple but some will remain. The act the shorting it make a difference might well be an indication that the LDR is failing so since it's such a PITA to pull the main board I think you should replace LDR8 too.

No promises but I'll cross my fingers for you.

PS: If you did want to fix the heater supply you'd have to add a 12V transformer and use that to generate the DC 15V supply (PSU cct mods required). It may not be worth the effort.

[Zouto]

Here some pix of crammed horror. Instead of 6800uf the built-in caps are 4700uf types, a new Vactec VTL5C9 (LDR8) is hard to find. I am looking for reliable substitute. I spoke to a friendly Mesa service technician, who told me that troubleshooting and fixing this (PITA) amp will be a time-consuming and serious costly affair, better sell it as broken and buy a new/other one. I think Mesa amps are not“built for comfort“ (as Howlin‘ Wolf once sang…) and easy servicing, other than Vintage Fender amps.

[nickb]

Search for "VTL5C9 XVIVE" . You'll find them easily enough. People keep remaking them in various guises. Not hard to make one yourself from an LED and LDR tucked into lightproof sleeving (heatshrink).

Fix or repair it's up to you. Very occasionally, when dealing with obscure issues on hard to work on amps, I've taken all the guts out and rewired on the bench to that is possible to power up and access every component.

[Zouto]

Thanks Nick, pulling the board goes far beyond the scope of my duty and my possibilities (it´s a friend‘s amp), having it repaired is too expensive... so the amp will remain as is, just another frustrating messy MesaBoogie wreck. So I‘d never go for MesaBoogie again, sustainability and long life-time is definitely not their strength and purpose.