Hot Rod Deville burnt smell/blown fuse

[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.

[Chuck H]

Something is wrong because something burned. You should absolutely open it up and inspect it. Do you use the amp differently in any way at home than you do at a show? What's the difference? You could try to recreate the problem, but I would open it up instead.

Three things... Those amps are notorious for blowing filter caps. You may not notice the difference right away. If the bias supply went kaput your tubes would be running too hot and you may not notice right away. If a power tube blew the amp may still operate somewhat normally at lower volumes and you wouldn't notice right away.

I'm sure there are a host of other things that you may not notice right away. But if you smelled something burning, and a fuse blew, something inside blew up or burned up. You have to open it up. For the sake of safety if not for your own satisfaction.

[jadams]

I opened it up expecting to find something burnt, but everything looks good. I used the amp for about two hours today with no problems. I verified a bunch of voltage test points from the schematic. I'm stumped.

[g1]

It's possible that a power tube went bad and fried it's screen resistor. You would think that would make a noticeable sound difference, but in my experience it sometimes goes unnoticed, especially if played at higher gain and lower volume settings. After the screen resistor goes, the bad tube may no longer cause a problem as it can't do any work.
Quick test would be to remove one power tube, play, see if it makes a difference. Then put it back in and remove the other one and play. If one of them makes no difference to the sound when removed, replace it and it's screen resistor. The screen resistors are R61 & R62, 470ohm 1watt.

[jadams]

Thanks, I'll give that a shot and see what happens.

[jadams]

I checked those screen resistors(R61 and R62) and measured both at 470 ohms. Could these values drift while the amp is running hot and then return to normal after it cools.I also checked 330 ohm 5W resistors(R78 and R79). 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.

[Enzo]

Look VERY closely at the solder on the power tube socket pins. I automatically resolder those whenever I am in anay of that series of amp. Little cracks can form, and that will lose continuity with some part of the circuit, causing troube. MAy or not be your problem here, but worth checking.


No, your 470 ohm screen resistors are not hopping around in value.

[Chuck H]

I suppose it's possible that there was a temporary loss of bias due to something like a bad solder joint or a dirty pin contact that caused excess current and blew the fuses. This USUALLY take at least one power tube with it.

Have you tried removing each power tube one at a time and ear testing as prescribed?

I also mentioned earlier that you may have a bias issue. That IS something you may not notice playing for a little while with the amp down low. Do you have any way to check the bias???

Also, it occurs to me that usually when something burns the smell lingers in the amp chassis and you can detect that something has burned just taking a chassis out of the amp within a week of the failure. Is it possible that it was something else at the gig that burned up? Like crappy club wiring. And that sent the wrong voltage to your amp causing a rotten operating condition that popped the fuse.?. I've never tried running a 120V wired amp on 60V or 240V. I can guess what would happen though.

[g1]

You are looking around for something that looks burnt, but if it was an electrolytic cap that went it won't necessarily explode or look burnt. But it will stink. Any sign of anything liquid-like around any of the caps?

[jadams]

Thanks for the help guys, this is 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

[Chuck H]

In case you don't normally use them, check to see if your reverb and effects loop are working correctly. I'm thinking you may have a bad component in the power supply for the silicooties or one of the transistors may be shorted. Do replace the caps with crust on them. This particular series of amps has a bad rep for power supply caps.

The symptoms leading up to the initial failure sure sound like the power tubes lost bias. I would definitely scrutinize the bias supply as well.

This is a tough one. It's hard to fix something that isn't broken anymore. Good research and info so far. I wouldn't give up searcing for a problem though. That's a pretty major failure you had so I don't think I would trust the amp out until I found something I knew to be responsible.

[Enzo]

If you have cracks, resolder all those pins.


Note: "bias voltage" is the DC voltage on pin 5 of each power tube. The 60mv you measure at the test point is an analog for the tube idle current. Yes, it is the voltage you monitor when adjusting the bias, but it is not the bias voltage per se. SO when we say you might be losing bias voltage, what we mean is that at some point you may be losing the voltage fed to pin 5 on one or both of the power tubes. That would have the effect of causing the test point voltage to go substantially higher. But it seems to be imtermittent, so unless you check it WHEN the burning is happening, it may appear normal.

[g1]

Any sign of discoloration on the power tubes? The paint used on JJ & GT tubes (maybe other brands too) shows quite well if a tube has been overheated.

[Steve Conner]

That's a good point, the tubes may have gone into thermal runaway as we discussed in that other thread. The symptoms match up. Maybe it doesn't happen on the bench but only after several hours cranked up on a hot stage.

If the power tubes are old and heavily used, that's quite a likely scenario, and the solution is a new set of tubes.

The burning smell may have come from the tube bases (have a sniff, does it smell the same?) or from the amp's power supply section if it was overloaded by the runaway.

[Enzo]

Good point. Experienced techs know the different smells of burnt parts. Burnt transistors smell different from burnt resistors, which smell different from burnt pc board or socket.

And absolutely NOTHING else smells like burnt selenium rectifers.