Traynor YBA-3 Power Xformer making arching noise, Is it about to fry?

[Enzo]

If the PT were shorted, it would freak out even in standby. Yours only freaks out when the standby switch is turned on. Your caps are exxploding. That is not likely the fault of the transformer, all the transformer does is make AC voltages. If you replaced the bad caps and the new ones blew up, then either the voltage across them (wwhich you could measure) is WAY above their rating

ORRRRRRR

you have a bad rectifier sending AC (reverse voltage ) into the caps. I like that second suspect the best.

[CoopDaKill]

OK, replaced the 1N4007 diodes, with 1N4002's. Thats all I had. The old ones tested OK on meter. Changed that 80uf-450v cap that was smoking previously. Check other caps. Put in new fuse, Took out power tubes, Plugged amp into lightbulb dimmer and fired it up. Took some voltage readings, seemed OK, So, I put the power tubes back in and plugged into to wall outlet. Took some voltage readings. The biggest voltage issue I noticed is: I'm getting +590v on pin 3 of the output tubes. Schematic says +540v. I know line voltage is a little higher, as compared to yesteryear, but +50v difference is alot. What should I do I'm only running the amp (out of standby) long enough to take voltage readings. Thanks.
Coop

[g1]

If you are taking your readings real quick, the power tubes may not be warming up. The voltage may come down some as they start to conduct properly. Also the bias setting (idle current) will affect the pin 3 voltage, the colder the amp is biased, the higher the plate (pin3) voltage will be.
Any decent EL34's should be able to handle that kind of voltage, it's the 450V caps you need to worry about. Check that none of them have more than 450V across them.
Because the fuse only blew after playing for 5 minutes, I'm inclined to think you have a bad power tube that is shorting under use. Either that or a leaky coupling cap or bias problem.
Watch for any sign of an output tube turning red or bright flashes, if so, try to shut down before blowing more fuses.

[CoopDaKill]

Changed a few more resistors. One of the carbon comps drifted badly. Amp is starting to stablize, I think. But, I'm getting about 584v on R37. Schematic says 540v. All other 80uf-450v caps are close to normal voltages.

This amp has a bias adjust pot, So, what would be an ideal bias voltage? EL34's. Also,

This amp is missing the tube cooling fan, that mounts in the side of the head cabinet. Am I correct in saying it's a 117v AC fan? I'd like to find a replacement. Thanks Again
Coop

[Gtr_tech]

Its a 120vac fan. It doesn't need to stir up a hurricane inside the chassis, just move some air around so you don't get too much heat build up.

I'd replace those HT rectifiers with 4007s. 4002s are not gonna cut it.

[Ted]

44V difference from 540V is about 8%.

The amp was designed for a wall voltage of 110V. Modern US mains voltage is closer to 120V (which is ~ 9% higher).

So 584 sounds about right.

You really can't set the bias in any meaningful way by measuring the bias voltage.

Best bet is to measure the DC resistance of the O/P Transformer primaries from centre tap to plate (having first powered the amp down, disconnected it at the wall and discharged power electrolytics) . Then with the amp powered up you can measure the voltage across them and work out the plate current of each tube by Ohms Law. You can work out the tube dissipation by measuring the plate voltage of each tube and multiplying it by the current you just calculated. You probably want to see your EL34s dissipating between 14 and 17W at idle.

If you didn't understand what I just wrote. Don't do it. Take it to someone who knows how to do it.

If you did: Be careful. There's nearly 600V on both ends of those windings and the centre tap. Touching DC voltages that high can do rather more than spoil your day.

[CoopDaKill]

Its a 120vac fan. It doesn't need to stir up a hurricane inside the chassis, just move some air around so you don't get too much heat build up.

I'd replace those HT rectifiers with 4007s. 4002s are not gonna cut it.

Thanks guys. I'm grateful for the valuable knowledge that you have given.

Will you educate me as to why the 1N4002 diodes should be replaced with 1N4007 diodes, in this circuit?

[Ted]

1N4002s are rated at 100V. 1N4007s at 1000V. You actually want diodes that are rated at at least twice the B+ voltage as that is the reverse voltage that will appear across them from the transformer. You can put diodes in series to achieve the desired voltage rating. (You can see from the schematic that the original design used series diodes). Call me paranoid, but I'd be inclined to use two 1N4007 per side for an HT Voltage of 580V. Or use something rated at 1.5kV like a BY448