Blowing my power fuse

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MadHatter_Lurch

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Hey I am having an issue. Tonight when everyone got here to practice, I had my amp warming up, When I turned the standby off the amp turned off. I had played it about 20 minutes earlier just fine. The power fuse was blown. I replaced the fuse and it blew as soon as I took the amp off standby. The only new thing I've done to it today different is hooked a cry baby before the pre-amp. I do a lot of modding to my amp, but have not messed with the power section. The only in-head modds I've done is cap swaps on the tube board and Input cap. If you have an thoughts or questions, I'm all ears

Thanks

Peace love and hat
 
Hmmm...I am an electronics idiot so it sounds like you know more than I do. Are the power tubes good? Maybe try another outlet in the wall?

In a house I used to own, if too many things were plugged in on the same circuit it would spike the current on another and blow up light bulbs. The issue just appeared one day after a year of doing everything the same way. Otherwise, check the connections that you changed on your own to make sure that you don't have a bad solder point.
 
Yeah, I replaced the cord and everything, but all the other amps and PAs are fine, only my favorite went bad XP a power spike is not out of the question, but it would have to be one that at least the PA could handle without tripping a breaker or blowing a fuse. I have checked my work and it looks good, plus it doesn't make sense. The issues seem to be in the power section. The closest my mods get to that is the Tube board. Also, no tube failure light came on as it did when I blew a tube before. The 6550s are a couple months old biased to mid-range according to the Owners manual. I just don't know

*Scratches hat*
 
When one of my 34's was going bad, no tube indicator light came on but it would not hold its bias and it would randomly cut the power, volume, and fuzz out on me.

Otherwise, make sure the cab is still good. Maybe you are not getting a load from the speakers? Again, it sounds like you know a lot more about what you are doing than I do. Good luck!
 
Well it acts like you cut power from the chord to the whole amp and blows the fuse. It is not like I am just losing the power to my speakers or tubes the whole amp goes off just like you flipped the switch
 
You could run it half power and swap the pairs of tubes to see if one of the power tubes has gone bad. That's what I would do. 90% when I had issues like that it was a bad power tube.
 
Possibly a shorted power tube even though a tube fuse didn't blow. Please verify:
1) Power one/Standby off = Fuse does not blow.
2) Switch standby to play = main power fuse blows?

If this is what happens, do this:
1) Remove all 4 power tubes.
2) Install new main fuse.
3) Turn power on. Fuse does not blow.
4) Switch standby to play. If the fuse blows, your amp is broken and will need to be repaired. If the fuse does not blow, this would indicate one of your power tubes is shorted.
5) Turn standby and power off. Put one power tube back in. Repeat the above steps. If the fuse blows, that is a bad power tube. Remove the bad tube and put in a different one. If the fuse does not blow, put in the next power tube and repeat the procedure. When you put the "bad" tube back in the fuse will blow.
 
Awesome tips, Bruce! Any specific position the tube ought to be in? I'm guessing V4 (first power tube after the PI)?

Lurch's dad checked out the problem and thought that (in the event it's not tubes blowing the fuse), that it was a bad triac (correct me if I'm wrong, Lurch). Any thoughts on that? If I got the name wrong and you're wondering what I'm talking about, it's the gray box attached to the chassis next to the filter caps.
 
all right I will go try that now! it did not make sense to me why it would be the tubes till you explained it. I will be back momentarily! Thank you very much.

I believe that is what dad called a Triac. It has 4 plugs on it and is bolted to the chassis. He apparently fixed a similar problem to mine a while back by replacing that component. Well I'm off to test my amp! my wonderful amp in oz!
 
First, do what Bruce said.
The part you are describing in the amp is a full-wave rectifier bridge.
A Triac is a solid state AC switch. It's not that.
Check you cap work. Use a strong light and look at the boards for any trace of arcing.
 
Welllllllllllllllllll.... I hooked the amp up with no power tubes and turned it on. I left the shell off so I could see the circuitry. As soon as I touched the standby, QC9A a plug on the tube board with a brown wire from the the smaller transformer lit up the night sky. It shot sparks as my fuse blew. I had this problem about 6 months ago when I had hooked the amp up to an incorrect speaker load and ran a bass through it. That time two resistors exploded and I also upgraded some caps on the board. The amp has worked great till now, but I watched the sparks shoot from it. Recently, I have noticed a bit of rust on my transformers. If you have any thoughts please please let me know. Thanks


Sad Hat
 
DO NOT remove rust from the transformer. Rust is an insulator, so if the laminations are rusty, leave them alone. If you remove the rust you'll
compromise the magnetic field of the tranny.
The board is carbonized from the arching which acts like and is a short.
Either you need to get a new board or, if you know what you're doing,
the burnt up part of the board has to be cut away and filled with epoxy.
See this link from the work I did on my RM50.
http://mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair
Look at all the pictures.
 
Very good photos. It does look like that may be what I have to do. What kind of Epoxy did you use and how did you clear the board without damaging the leads? Thank you for the information.
 
I very carefully cut away the burned material with a cutter on a Dremel tool. It's a two layer board with conductors sandwiched in the middle.
You have to be very careful not to cut any copper traces.
I used Duco 5 min. epoxy. I looked up the fact sheet for it on the company website. Any epoxy you use has to be an insulator, have a high di-electric (insulating) strength.
 
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