What causes this?

[496dart]

I built an amp about 3 years ago (kind of a jcm 800) . This thing has performed flawlessly. I sold it to a buddy last year. Yesterday he brought it back to me today because it went up in smoke---literally ! he was playing thru it and it billowed a ton of smoke and popped the main amp fuse. I took it apart today, disconnected all the secondary wires on the PT and put a new fuse in it. I powered up and it pops the fuse instantly. Im assuming the power trans has internally shorted. So...is there a cause to this or is it just a bad tranny ?

[rathmann]

I had the same symptoms happen to a Mig 50 a few years ago. The smoke came from the caps. It happened twice within a couple of months.

It's been fine ever since replacing the PS caps. Transformer is fine.

[stokes]

If you disconnected the PT leads and the fuse pops,it can only be the PT at that point,since it is the only thing in the circuit.Whether or not anything else is shot is a matter to be determined,but if the PT is the only thing on the other end of the AC line and the fuse blows you can be sure the PT is shorted.I would suspect that there may be other damage i.e. rectifier,power filter,these could have gone south and taken the PT with them.

[496dart]

i checked the other parts in the power supply . The 2 ,50/50 caps test ok , the only way i have to test them is connecting a 9v battery to them and a voltmeter, then i remove one of the leads to the battery and watch how fast the cap drains.

[ken]

It sounds to me like you blew your rectifier or your 50x50 power caps. IMHO charging the caps with a battery and measuring discharge rate just won't work, because the test voltage on the caps just isn't high enough. A cap can measure good under a low voltage, and still leak at 'real' (450+VDC) voltage.

Replace these caps with F and T caps if you can get them, I blew too many JJ's
but never a F and T.

Ken

[Enzo]

If it pops fuses right now and the amp smokes, I would be looking for hard failures, not hidden ones. If the caps are not dead shorts, they are probably OK. That is no guarantee of course. If rectifiers are shorted, replace them. etc. If the tranny is bad, replace it and bring it up on a variac while watching current. Then start reconnecting secondaries one at a time and watching current. Leave the tubes out at first. And those old output tubes are certainly suspects.

ANd before ordering a new tranny, make sure there is not a little cap across the mains where it comes in. A little noise suppressor cap shorted across the mains will blow fuses just as dramarically as a bad tranny. Such caps don't usually fail, but they do now and then.

[496dart]

one of the can caps looks as if the top has "bubbled up" . i will replace that one for sure. Who sells the F&T caps?
enzo, there is no noise suppresor cap. the rectifiers all test ok on the meter

[Enzo]

Oh well, sounds like tranny then. If connecting the primary to 120 causes fuses to blow with no secondary connections, ther is no other choice.

[496dart]

Well, today i replaced the PT and the cap that had "grown" . The cap is a 50/50uf JJ 500v . My B+ is 450v . Everything looks good , volts/ bias .I will attach a pic of the caps--do you think the one is too close to the power tubes in the pic?

Ive been blasting the amp for about 15 minutes and eveything looks good!

I hooked up power to the bad tranny to see what reading it put out. It was supossed to put out 350v and the meter showed 40v on the sec.

[496dart]

update:

ive had the amp idling now in run mode. The OT is cold, however the power Trans is getting fairly hot. This is idling for about 15 minutes . Normal ?

[stokes]

It shouldnt get very hot.It is a sign something is drawing too much current.It sounds like you had a blown cap the first time.Are you sure one of the caps down the rail isnt shorted also?If the top of the other cap was "bubbled up" I would think the shorted cap was what blew the first tranny.Could be there is a short in the caps somewhere.Something aint right the tranny should not get hot unless it is being strained by too much current,most likely caused by a leaky cap.

[496dart]

as in the filter caps? Ill try to check the other 2 50/50 caps tonight. the power supply/section,etc.. is copied from a 2203 Marshall.

Edit --i idled the amp for a good 30 minutes and felt the PT. Hot but not too hot to hold on to. I also idled my 5150 next to it. The 5150 pt got hot too, not quite as hot as my other. I think if i had a shorted cap it would have acted up by now ?

[stokes]

Yeah,we're talking filter caps.Might not be completely shorted but just leaky at this point.Cant be 100% sure from where I sit,since I cant feel how hot it is.I am assuming it is unusually hot since you are questioning it.A normal PT gets warm but hot is a sign of too much current.Too much filiment current will cause a PT to run hot but that doesnt seem to be the case here since you had a blown filter cap in the first place.I would check everything in the power rail and be sure you are not drawing more current than the transformer is rated for.

[Enzo]

PTs do get warm, and some get uncomfortably hot to the touch. Yes, in normal operation. Not all do. It is just a matter of experience as to when it is "too" hot.

[496dart]

thanks. Yea-its warm/hot, but i can sit there with my hands on it ... Doesnt seem more than 5 or 10 degrees hotter than my 5150 PT.

This is a stupid question, and im still trying to learn: Where exactly along the B+ rail am i supposed to check current draw? Ive just popped the fuse in my meter 3 times......
still learning...