Hot Rod Deville Blown Fuse/Burning Smell

[jadams]

Hey all,

At a gig last night my amp started sounding a bit off, then there was a burning smell and then the fuse blew. Today I removed all of the tubes, turned it on and there was no burning smell or blown fuse. I then put the tubes in, turned it on and no problem. Then I plugged a guitar in and played for awhile with no problems. This would be a lot easier to troubleshoot if the problem would present itself again. Any help will be greatly appreciated. Thanks.

[Reeltarded]

Have you been inside there? Check the area around the output tube sockets. Inspect each resistor carefully, and look for evidence of arcing in on and around the sockets.

[jadams]

Thanks for the quick reply. Everything looks fine inside. No scorched components or burns on the board. The only other issue is that there is a fairly loud hum with the treble on ten, if i back off its fine.

[Structo]

Hmm, I hate intermittent problems, they can be hard to trace.

That being said, don't those amps use PCB's?

Were you running it hard when it crapped out?

I would look around the power supply section, particularly around any power resistors.

Some of these amps get so hot that the solder can melt around some resistors and then the resistor comes a bit loose.
Then when it cools the solder solidifies allowing it to run again but it will be a bad connection or cold joint.

[jadams]

Everything seems ok inside. I verified a bunch of voltage test points from the schematic and played it or had it powered on for 2+ hours. I'm stumped.

[honeydip]

Like Tom States, there are 2 Big Power resistors 5w that Provide the +-16v that Heat up a Lot.

Watch out for Bad Soldes Joints especially ob those two resistors.

Honeydip

[martin manning]

Some good info here: http://www.justinholton.com/hotrod/ The commom DeVille-DeLuxe problem with the too-hot power supply resistors is under repair help -> randomly switching channels.

[jadams]

Thanks guys. Would it be wise to replace those resisters with something that can take a little more heat?

[xtian]

Thanks guys. Would it be wise to replace those resisters with something that can take a little more heat?

Definitely. This is one of the first things to fail in this amp. I got mine for $200 because these resistors failed. Easy fix.

[jadams]

I pulled the board out and checked those 330 ohm 5W resistors. There was some discoloration of the copper trace in the pcb but both resistors measured around 330 ohms. At this point I can only assume that bad solder joints are to blame. Any idea why these would cause the amp to blow the fuse and smell burnt. I can't find any significant burnt areas on the pcb which is surprising because it really stunk.

[Prairie Dawg]

Unless you can validate that the power tubes are free from shorts, you're spinning your wheels on this problem. Anything that'll cause a mains fuse to blow is drawing significant current.

[butwhatif]

change output tubes, and verify proper bias,

[jadams]

Thanks for the help guys, this is a more detailed description of what happened and what I've done so far to troubleshoot:
1. After playing for about two hours my tone started to lack highs.
2. Within 5-10 minutes I noticed a significant volume loss.
3. Seconds later there was no sound, and as I approached the amp there was a strong burnt odor.
4. I unplugged the amp and finished with my back-up amp.
5. The next day I opened up the amp expecting to find something obviously burnt inside, but all I found was a bad fuse.
6. I used two different sets of tubes (including the pair in at the time of failure) and both sounded great, however I didn't try them one at a time. I played for about an hour, then left it ion for an hour to see if anything would happen. Nothing.....
7. I checked the bias voltage and it was at 65mV which is where I had set it.
8. I've checked the values of every resistor on the board, all are ok.
9. I've checked voltages from the schematic and all values seem reasonable.
10. I looked at the solder joints under magnification and I noticed a couple of pins had tiny cracks in the joint and all had little solder on the pins.
11. The trace under the two 330 ohm 5W power resistors(R78, R79) appears darker, as if it got hot. There is a similar spot on the trace under R97, but all three resistor values are ok.
12. Two of the filter caps(C35, C36) have a small amount of crusty white ooze on the + side and the voltages match what is indicated on the schematic. I've already ordered new filter caps to replace these, though I don't think they are the culprit.

Thanks again guys your help is much appreciated. Here is the schematic for reference http://support.fender.com/schematics..._schematic.pdf

[husky]

65ma for 2 tubes I hope?
I fixed a lot of those amps at Fender. The main cause of failure I saw was the large dropping resistor for the switching supply where they pull from high voltage (stupid). It gets so hot it unsolders itself from the traces on the board and then heals when it cools down. The solution I used was to mount a dale style power resistor to the chassis and take it off the PCB. Why they couldn't splurge on an extra tap on the power transformer I'll never know ! Probably to save $1.00

Thanks for the help guys, this is a more detailed description of what happened and what I've done so far to troubleshoot:
1. After playing for about two hours my tone started to lack highs.
2. Within 5-10 minutes I noticed a significant volume loss.
3. Seconds later there was no sound, and as I approached the amp there was a strong burnt odor.
4. I unplugged the amp and finished with my back-up amp.
5. The next day I opened up the amp expecting to find something obviously burnt inside, but all I found was a bad fuse.
6. I used two different sets of tubes (including the pair in at the time of failure) and both sounded great, however I didn't try them one at a time. I played for about an hour, then left it ion for an hour to see if anything would happen. Nothing.....
7. I checked the bias voltage and it was at 65mV which is where I had set it.
8. I've checked the values of every resistor on the board, all are ok.
9. I've checked voltages from the schematic and all values seem reasonable.
10. I looked at the solder joints under magnification and I noticed a couple of pins had tiny cracks in the joint and all had little solder on the pins.
11. The trace under the two 330 ohm 5W power resistors(R78, R79) appears darker, as if it got hot. There is a similar spot on the trace under R97, but all three resistor values are ok.
12. Two of the filter caps(C35, C36) have a small amount of crusty white ooze on the + side and the voltages match what is indicated on the schematic. I've already ordered new filter caps to replace these, though I don't think they are the culprit.

Thanks again guys your help is much appreciated. Here is the schematic for reference http://support.fender.com/schematics..._schematic.pdf

[jadams]

Thanks for the info. Are you referring to the pair of 330 ohm 5W power resistors? Could a failure of those resistors cause the problem I described?
Thanks again.