Cooked power tranny? (Vibrolux Reverb)

[slidincharlie (Carlo P)]

I seem to have frequent troubles with power trannies...
While I haven't yet solved the heater supply issue in my Silvertone 1482, I have another problem in my '79 Fender Vibrolux Reverb.
This amp used to work fine. All of a sudden, after two or three turn on/off cycles at a gig, it wouldn't turn on anymore.
Same at home: after on/off switching two-three times the amp doesn't turn on any longer and the PT is very hot. At last it hasn't turned on at all.
While the power cord delivers the correct AC voltage, there is no voltage from the PT secondaries: neither high tension nor 5V or 6.3V heaters.
I have also bypassed the VAC selector (this is the export version of the amp) in case it was shorting to no avail.
How do I tell if the power switch got broken or if rather there's a short inside the PT? Is there any chance that the voltage is interrupted somewhere before getting to the PT?
I attach a sketch of the power supply section of this very amp version.

[olddawg]

First off unplug the amp and measure the resistance across the power switch open and close. It should read open in the off position and a direct short in the on position of course. Sometimes the switch contacts carbonize from arcing and become resistive or the switch simply brakes. If the PT is getting hot there is a good chance that it is simply shorted internally. Pull the tubes and measure the secondary taps and/or float the secondary taps and measure the AC. Anything shorted after the secondaries could be pulling the power supply down. You don't want to turn it on like that much. If the PT is not already shorted it will be in short order if you are drawing excessive current.

[Steve Conner]

Carlo, are you sure the mains at your home is 220 and not maybe 380?

If the PT were shorted, it would hum loudly, smoke, and/or blow your mains fuse. You have an open circuit, not a short, and that means a bad mains switch, bad fuse holder, or a loose wire, maybe even inside the PT.

Broken power switches often spark and work intermittently. You'll sometimes hear terrible crackles and bangs through your speaker when you wiggle the switch a bit. I'd just replace it temporarily to check. Fuses can also go intermittent, have you tried a new fuse?

You can also connect a neon lamp (with built-in resistor!) to the PT primary terminals, and see if it goes out when the problem manifests. This is an example of "divide and conquer" troubleshooting: if the amp dies but the light stays on, the problem is inside the PT. If the light goes out when the amp dies, the problem is in the mains wiring before the PT.

And finally when it goes dead have you tried the "calibrated fist" :-)

[slidincharlie (Carlo P)]

Is a four-lug power switch like this a common feature? I've never seen one but in this amp... I don't know between which lugs I should measure resistance...
Anyway I'm too tired now... yawn... going to bed...
Thank you guys...
See ya tomorrow...

[Dave Curtis, dB AudioTech]

This may be one that has the Thermal Protector (Klixon switch; they put 'em in the exports) integral to the tranny. When it gets up to a certain temperature, the Klixon opens up. Opposite when it cools off. Is that what's happening? Shutting itself off? Anyway, "ring out" the tranny windings and see what the various winding resistances are (unplugged, of course). If your primary is open and the secondaries look OK, it's possible the Klixon isn't closed. It's not easy to get at... If the tranny was getting real hot real fast, you might have an issue in the circuit that's stressing the PT.

BTW, confirm that the brown from the cord is neutral and the blue is hot; it's supposed to be the other way around. If your wall voltage is 110V from either one to ground then you have no neutral and it's a non-issue, If you measure 220 from brown to blue and brown to green and nothing from green to blue, then the blue is neutral and shouldn't be fused; swap the brown and blue. The power switch opens both the neutral and the hot, so it has 2 separate poles (also an export feature).

[Steve Conner]

The electrical system in Europe is different, because we use distribution at 3-phase 400V, and every home gets one phase and neutral, so 230V. We don't do that silly 240v centre-tapped thing.

Blue should always be neutral: there should be very little voltage on it with respect to green, and the full 220 should appear on brown.

Unless Italy is one of the places where you can insert the plugs either way round... this being the reason why the export amps have the double-pole switch.

I didn't know that export amps had a thermal cutout inside the transformer. If that's what it is, then that's almost certainly what's cutting out, and you should investigate whether the PT is being overloaded.

[slidincharlie (Carlo P)]

Unless Italy is one of the places where you can insert the plugs either way round... this being the reason why the export amps have the double-pole switch.
I didn't know that export amps had a thermal cutout inside the transformer. If that's what it is, then that's almost certainly what's cutting out, and you should investigate whether the PT is being overloaded.

Yes, Italy is one of those countries... So I have no way to ensure that the hot wire from the mains is fused or not.

And here comes the crucial question, which is of very broad interest: how do I test if the PT is overloaded?

[MWJB]

"And here comes the crucial question, which is of very broad interest: how do I test if the PT is overloaded?" Excessive load on any of the secondaries will normally result in a noticable voltage drop on the remaining secondaries, e.g. too much current draw on the B+ will cause your heater voltage to drop & vice versa. Anything that pulls your B+ down more than 20vdc from your datum (all tubes installed & moderate plate current on power tubes), or pulls your heaters under 6.2/6.3VAC would warrant investigation.

Placing an ammeter accross the standby, then plugging into the wall & powering up will give you total dc current draw accross the B+ winding, then break it down tube by tube, to see if it's excessive & possibly highlight a problem should something other than your power tube plates be drawing a lot of current.

[pdf64]

Checkout this thread
http://music-electronics-forum.com/t14249/
Peter.

[slidincharlie (Carlo P)]

I have a few thing to clarify and to update.

1) The amp did never shut off while it was on. It rather seemed to work fine for a few on/off cycles (each cycle about 15-30 minutes), then after say 3 or 4 such cycles it did not turn on any longer,

2) In two cases the failed turn-on was due to a blown fuse (1A slo-blo).

3) After about 40 minutes of normal operation the PT gets hot, say around 45-50°C (113-122°F). Yet the amp keeps on working.

4) Today I realized during my test that the reverb driver tube had glowing plates. I replaced it with another tube and there was no more glowing... but also when I put the old tube back in place there was no glow this time.

5) The plate voltage is totally in specs (460V) and the power tubes draw around 60% of max dissipation at idle.

6) The rectifier heaters read around 5.4 VAC (measured across pins) but the tubes filament read at 7 VAC: why such a high tension?

7) As soon as I turn the standby switch on I can hear some pops and crackles through the speaker (no sparks inside the tubes though). Such noises go away after a couple minutes.

Said that, the amp plays beautifully. I have tested it all the afternoon with several on/off cycles and it seems to work. Yet two blown fuses and the high heater voltage tell me that something is wrong inside the amp. This amp has stayed inactive for about two years now.
Do I have to do the current test across the power switch as per MWJB advice? What current am I expected to read at startup?

[olddawg]

How old are your output tubes? How old are your filter caps?

[pdf64]

What's your power line voltage? What voltage is the power selector set to?
Does the next setting up get your heaters closer to 6.3V?
1A sounds a bit low for your line fuse, I would see how it goes with 1.5A slo blo. The ceramic types are best at withstanding surges.
Peter.

[slidincharlie (Carlo P)]

The power tubes and most preamp tubes came with the amp, I think they are the original tubes (late 70's). The rectifier was bought NOS a few years ago. The filter caps are Sprague Atoms installed 6 or 7 years ago.
The mains voltage is around 235 V. I have now switched the selector from 220 to 240 V, and the heaters are now closer to specs. The B+ is now around 430 V, while the original schematic calls for 460 V (this is the reason why I used to select the 220 V actually).
The power tranny get hot like before. I don't know if it's too hot actually. Is there a sort of standard temperature limit that should not be exceeded?
Re: fuse. I've always used fuses as small as half the original 110 V mains value because most (all?) Fender schematics of modern amps call for "x" A value with 110 VAC and "x/2" A with 220 VAC. Am I wrong on this?

[Dave Curtis, dB AudioTech]

The power tranny get hot like before. I don't know if it's too hot actually. Is there a sort of standard temperature limit that should not be exceeded?

They can get pretty hot. I'd say if it goes much over 100C, you got a problem.

Re: fuse. I've always used fuses as small as half the original 110 V mains value because most (all?) Fender schematics of modern amps call for "x" A value with 110 VAC and "x/2" A with 220 VAC. Am I wrong on this?

You're correct. Twice the voltage (push), half the current (flow), same power (work). Take an amperage reading of the AC; see what the amp's actually drawing while in use. Dumb Q: Using Slo-Blo fuses, right?

Sorry if I got off track with the thermal protector, I guess I didn't fully understand the fault. You can see one on this print that came with my '76 Pro Reverb: http://www.schematicheaven.com/fende...-bmstr_rev.pdf (I'm pretty sure that's the schema I submitted to them).

The CSA Thermal Protector was not fitted in my amp, and probably isn't in yours, either.

Once I get my amps set up, they'll usually stay powered up until I'm all done; I'll just leave the volume pedal down during breaks.

[slidincharlie (Carlo P)]

Thanks you Dave.

They can get pretty hot. I'd say if it goes much over 100C, you got a problem.

Ok. I should buy a thermometer to get the temperature then.

Sorry if I got off track with the thermal protector, I guess I didn't fully understand the fault. You can see one on this print that came with my '76 Pro Reverb: http://www.schematicheaven.com/fende...-bmstr_rev.pdf (I'm pretty sure that's the schema I submitted to them).
The CSA Thermal Protector was not fitted in my amp, and probably isn't in yours, either.

There's a CSA Thermal Protector in my amp's schematic, too... but I can't see it inside my amp. Probably it is not there.

You're correct. Twice the voltage (push), half the current (flow), same power (work). Take an amperage reading of the AC; see what the amp's actually drawing while in use. Dumb Q: Using Slo-Blo fuses, right?

Yes, slo-blo fuses of course.

Re: current reading. Should I clamp the probes of my DMM (set to AC current) across the power switch? May I rather clamp one probe on the fuse holder (lateral lug) and touch one switch lug with the other probe while the amp is on, or will I get sparks and create danger? I ask because there is not enough room between lugs to clamp the probes (this is a four-lug power switch, remember). I attach a photo of the area.