Bass 400+ all tubes red plating

[tizianod]

Hi, I'm trying to repair a bass 400+ but i'm having trouble finding the issue. A friend of mine asked me to inspect his amp because last time he tried to play on it, he noticed only one row of output tubes was working. I gave the amp a good cleaning and inspection and fired it up on the variac. The fan and the pilot light starts to work. So far so good... The heater wires for the preamp tubes start to glow. At this point I'm thinking there's nothing wrong with the amp... but then... Nothing from the power tubes. I have 12 dead power tubes and i'm starting to get a dull popping noise from the speaker. The noises start to get louder and louder after a couple of seconds.

After a thorough inspection against the schematic I noticed that resistors R311 & R312 are missing. So in the past these got bad and somebody decided to just leave them out of the circuit... I soldered new resistors into place (100ohm/2watt) and when I start up the amp, still in standby mode, everything seems to work just fine. The heater wires of the power tubes start to glow. But when I switch the amp off standby mode, I hear a loud pop and the amp starts to buzz. Immediately followed by all power tubes red plating. I tried to measure the voltages on pin 3 and 5 but I get zero results.

I noticed someone has been inside the amp before. They already changed a filter cap and screen grid resistor in the past. So now I'm thinking what could be causing this.. It's a bias issue so what should I look for next... Screen grid resistors? Grid leak resistors? Grid stopper resistors? Diode? Bad filter caps? The resistors seem to look just fine. I'll give everything another inspection try to see if I missed something... But I would love to hear the opinions from you guys, maybe I'm missing something?

 

[jediguitarist]

I know nothing about Boogie repair, but I watched this guys vids.

https://www.youtube.com/watch?v=yDoI8rgd3K8

He mentioned red plating on a Mesa Boogie DC3.
Maybe it can help you solve the issue with the 400+

 

[user 84685]

Check you got -60volts across R103?
If not, there is your issue.
The other two resistors hold the heater supply around zero volts.

Not keen on the gaffa tape!

 

[user 41014]

the amp starts to buzz. Immediately followed by all power tubes red plating

Red plates on all power tubes, not 1/2?

The 1st thing I would do with mesa hot plates is check the coupling caps for the bias circuit. Are they 400V or 630V and is there a short?

While checking continuity it doesn't hurt to probe all the film caps and filters for shorts going for the low hanging fruit 1st. I'd also check the MOV(s) on the mains while at it for protection's sake.

I see a screen CC replaced with a film R (and resistors that don't look original throughout) so someone's had issues before. As such make sure the fuse is not over the correct A. Someone on an amp I bought put a 10A fuse rather than replacing the shorted .1uF (should be 630 poly-met) coupling cap. All the old Marks had 400V and I'm not sure if they upgraded to 630V by the Bass 400+.

 

[user 84685]

All valves red plate, very unlikely for both coupling capacitors to fail together.

 

[user 41014]

All valves red plate, very unlikely for both coupling capacitors to fail together.

The blue and gold:
I count 7 mesa labelled 220uF filters. Someone pullled one and replaced with gold so it does not look to be extra. But I'm with you on the duct tape. It only looks right when combined with baling wire (hay bales) holding the chassis together.

If it was me I'd tidy up the filters with 8x the same. The coupling caps may have very low odds of going bad together but it is a known fault point if they are original 400V spec'd and need upgrading either way if not 600 or 630V.