Mesa Bass 400 - Blown from Speaker Cable Not Plugged In Fully

[MikeBoneman]

Hello, I accidentally blew my Bass 400. I plugged things in, warmed up at a low volume. Few minutes into first song there was a spark and burnt smell. I realized what had happened, speaker cable was not 100% pushed into my cab.

I examined and saw a blast mark around a resistor (100 ohm I believe) near the second power tube, resistor broke in half when touched. This resistor seems connected to the wire connecting to R311 which got burned off and was hanging loose. My understanding based on a little research is this is related to the heater circuit. Fuse was still intact but had burns on the glass. The board didnt look scorched anywhere else with a visual.

Based on other pics, it appears that half the caps on this were replaced with some gray ones.

I will probably need to take this to a tech but loathe to think about the potential costs! Any thoughts on what could be wrong? I can fix a resistor and probably reconnect to 311, and replace some tubes. I have access to repair and testing tools but anything beyond a little soldering is probably out of my league.

Any thoughts on the possible collateral damage (for example, could all tubes get blown by this?) or insights on what could have happened under the circumstances (like if speaker wasnt fully plugged in, would it cause power tubes to overheat and maybe short the circuit?).

 

[japetus]

Well, I know you don't wanna hear it as you already stated...but take it to a tech. Better yet bite the bullet and send it to Mesa and you will have an amp for the rest of your life.

 

[Magic Dragon]

R311 and R312 are 100 ohm resistors for the artificial center tap on the heater circuit. The fact that one is blown, and appears to be a 2 watt resistor, indicates that likely one or more tubes are shorted to a filament. The resistor blew and hopefully protected a transformer winding.

That amp needs a tech to go through it and:
1. Test all the tubes
2. Test the remaining center tap resistor (or just change them both out).
3. Test the screen grid resistors
4. Test the control grid resistors
5. Test both the power and output transformers
6. Test all the electrolytic caps (and likely replace them all). The Blue Mesa/Sprauges are 38 years old according to the date codes stamped on them.

That amp is probably overdue for a servicing. It won’t be cheap. I’d send it back to Mesa because they will fix it 100%. I would guess it will cost around $400 provided the power and output transformers are not blown.

 

[Purgasound]

Hopefully it's not bad other than replacing what you can see. Plenty of Fenders and Peavey's have had the artificial heater CT fry before and replacing it usually solves the problem.
However, we're dealing with Mesa so things are not always so clear cut. I also had a heater issue with a 400 and it cascaded into a catastrophic board failure. The entire PCB near the heater traces became conductive. Even lifting the heater traces off the PCB entirely didn't help because the damage to the board was done and B+ was partially shorting somewhere and drawing too much current. Could never pinpoint it. Ended up scrapping the board altogether.
Hopefully it doesn't end this way but even a good tech can run around in circles on these when it's an oddball failure. Doesn't help that the board is also mislabelled so if you install capacitors according to their orientation printed on the PCB you'll release a lot of magic smoke.
Cheers and good luck. Definitely want to use a light bulb limiter on startup and a thermal scope isn't a bad idea either. The main sign I had too much current draw was the low resistance cement wirewound resistor in the power supply was getting hot enough to melt the solder holding it in place. Had I known to look there first it could have saved me hours of troubleshooting.

 

[MikeBoneman]

Good news was able to fix it. We were able to look at it and carefully test things (we did use a lightbulb limiter). Basically repaired the blown resistors and fixed the lead on the wire that got melted off. Seemed like everything else was spared but it was that heater circuit that went out.

I replaced a pair of power tubes (one of them would not light up on standby and could have possibly been why the circuit blew).

I will probably change the caps next since half of them were replaced at one point (odd that it was half of them!) and the originals are 35 years at this point.

 

[Rocker68]

Well done . Way to dodge the bullet .