st70 blowout

[gongli]

hi

i was running an st70, it was fun there was a bit of hum and a slight odor but i assumed it was dust or something, it was shelved for an unknown amount of time. then there was no sound, and then white smoke coming out! i turned it off, but it doesnt turn on anymore. i opened it, here's a photo of the choke c354 which seems to be messed up because there is waxy stuff right under the c354 choke iron core inductor on the bottom "tray"

im a complete neophyte, is the presence of waxy stuff under the choke conclusive that the c354 is what burned out? do i need a new c354 in any case?

finally how do i setup a work area so i can turn the amp upside down while its running to multimeter whats going on on the underside (where the c354 is located) without damaging the tubes?

thanks for any help

gong

[SY]

It's unlikely that the choke failed all on its own. I'd be looking at rectifiers and filter caps, too.

[diesel_tech]

Well first off, have you replaced the filter caps, the electrolytic ones? Test these first, Using a cap checker is highly recomended, you can use a DVM, make sure there is NO leakage. Not even in the millions of ohms, if there is, they need to be replaced, and measure for continuity of the filter choke. Let us know what you find out.

[stokessd]

Also check the resistance of the choke wires (either it doesn't matter) to the case. There should be infinite resistance to the case.

The choke won't fail on it's own, something downstream is shorting if it's a choke problem. The choke is really tough, it has a high mass to absorb heat. You can pound on it pretty mercilessly and it doesn't mind.

Sheldon

[kevinkr]

I would say that every component in the power supply needs to be checked. The choke hasn't necessarily failed, almost all I have seen had wax goop on the bottom chassis plate and were fine - in fact it is more likely that the power transformer did.

It's a very bad idea to take a tube amp that has sat unused for umpteen years and just plug it in. It is very likely that the electrolytics at the minimum need to be reformed and more likely in fact that they need to be replaced.

Usually the first section in the can (cap) fails due to the long term heating effects of the ripple current in it. Once the cap becomes leaky this can increase the load current sufficiently that the power transformer can fail if the cap doesn't first.

Remove the rectifier tube, replace the fuse, apply power - you should see the filaments in the remaining tubes glowing brightly after about 30 seconds or so. Then measure the voltage at the 5AR4 from pins 4 and 6 respectively to ground. IIRC the voltage should be around 360Vrms. Any significant difference one to the other or fuse blowing indicates a problem, and in most cases it will be a short in the high voltage secondary. (Other possible causes include shorted filament wiring. IMO this is unlikey as the amp was working beforehand.)

If the above test is successful carefully check the choke's dc resistance with a good meter, IIRC the resistance should be somewhere between 50 ohms and 75 ohms. Measure from either lead to chassis ground, if the choke is good after the caps charge there should be a very high resistance reading or open. (Best to disconnect the caps before measuring in any case you'll have to replace the can anyway.)

Finally the selenium bias rectifier and caps must be replaced.

Other things to check and replace: all coupling caps on the driver pcb.. Not unusual for the earlier versions to have leaky black cats and the like. Carbon comp resistors are not stable over time, replace any that are out of tolerance. Tubes...

All of the parts you need to fix this amp can be found here:

http://www.triodeelectronics.com (No affiliation)

[dsburg]

test

[ray_moth]

The most likely culprit, as already stated by several others, is a failed electrolytic smoothing capacitor. These deteriorate with age, and failure in old ones is very common. Failure can sometimes take the form of a short-circuit within the capacitor, which would lead to overloading of smoothing choke and transformer (the humming noise) and their possible incineration (the smoke). Rectifiers can also be quickly destroyed by a short-circuit.

In an old unit, it's always safer to replace all electrolytic capacitors with modern parts (not NOS!), whether or not they show signs of deterioration You just cannot trust old electrolytics, and 'collateral damage' can result when they fail, i.e. they take other things with them, as appears to be the case with your ST70. If you're lucky, it might just be a blown fuse.

Whatever you do, PLEASE don't power it up again until you have inspected the amp, found all failed and damaged parts, and repaired/replaced them as required. That includes the wiring.

[gongli]

hi

thanks everyone for the information. i replaced the fuse, took out the output tubes, and then powered up. driver and power tubes were glowing for about 15 secs then i powered down. then i powered up again, then 5secs later there was white smoke from the pa-060 transformer and then the fuse flashed and blew.

later i checked resistance from the quad electrolytic cap; it was never really constant but generally decreasing, but in the megaohm range when 2Mohm selector was there, but briefly i could get readings in the 200kohm range also. in any case the resistance decreased from 1.5Mohms to .1Mohms in about 10-15 seconds or so.

smoke from the power transformation must be bad; how do i check it overall? it seems quite complicatedly connected to everything else...

the leads from the choke are kind of "hidden" is there a good way to check the resistance through it?

does the smoke from the transformer allow any conclusions about what's wrong in general?

as a neophyte im a little reluctant to replace all caps though i know i should do it; but my skills are such that its not clear whether i would do a good enough job to justify the replacement yet.

-gong

[SY]

Originally posted by gongli
hi

then i powered up again, then 5secs later there was white smoke...

does the smoke from the transformer allow any conclusions about what's wrong in general?

It means you're the Pope now.

Anyway, the electrolytic cap that everyone has been encouraging you to replace is likely to be the culprit. The smoke from the transformer may mean your troubles have gotten more expensive; ST70 transformers run $100-150 or so for the part.

This is high voltage circuitry, so if you're going to try to fix it yourself, make sure you understand how to safely work with it and, preferably, get someone with more experience to keep an eye on you. Staying alive is the first requirement for good sound.

[dsavitsk]

Originally posted by SY
Staying alive is the first requirement for good sound.

And for staying the Pope