Mains Fuse blowing up

[fabiomayo]

Ok... Here it is...

I couldn't find any shorts to ground on the power supply.
I disconnected the PT secondary (the high voltage, red ones) and
the light bulb was still glowing strong. I measured the voltage anyway:
~25v AC. That's odd, isnt it? Outlet was around 126v... I'll get back to
disconect the rest...

All the best...

Oh... BTW Allynmey, the fuse blows instantly when turnin the mains on,
with the amp still on standby...

[fabiomayo]

Ok. I disconected every single secondary, including bias, except for
the grounds and the light is still shining. I got around 326mV AC on the
heaters, 286mV on the rectifier heater, 590mV on the bias winding to
ground and ~25V AC on the high voltage winding. I'm not sure on the
math so I don't know if this is okay with the 100w light bulb...

I did previously tested the filter caps but lost my notes. I only recall
that according to the GeoFx debug section these may not be faulty.
Is there a good way to test them? I own a good (Fluke) but no cap
meter...


Any ideas?

I should say that I was already thinking of rebuilding this amp myself,
kick some of the options out (tube rectifier, triode/pentode, some of the
preamp switches - it has them all, make the post-OD tone stack internal),
redo the wiring.
Why? Both for kicks and because it doesn't sound as "dumbly" as I hoped.
Sounds good, don't get me wrong. Perhaps the speakers aren't helping.
I'd only do this if I could ensure the good trannies...


Ohhh... BTW, this amp has 110v and 220v primary windings but this part
also doesn't appear to be at fault...

[Bob-I]

Ok. I disconected every single secondary, including bias, except for
the grounds and the light is still shining.

Right there you've got a problem. Either a short on the primaries, or a shorted power xformer.

I got around 326mV AC on the
heaters, 286mV on the rectifier heater, 590mV on the bias winding to
ground and ~25V AC on the high voltage winding. I'm not sure on the
math so I don't know if this is okay with the 100w light bulb...

With a 100W light and no load, you should see close to the same voltage as without the light. This reenforces my statement above.

I did previously tested the filter caps but lost my notes. I only recall
that according to the GeoFx debug section these may not be faulty.
Is there a good way to test them? I own a good (Fluke) but no cap
meter...

If you disconnected the secondaries, then the caps are out of the ckt. They may still be bad, but until you fix the first problem you can't troubleshoot this.

Check out the primary connections. It sounds to me like a bad PT. Sorry for the bad news.

[Zippy]

... or a short on the downstream side of the power switch. I've seen solder splatter do horrible things.

[fabiomayo]

Thanks, Bob-I. That's what I figured. As I said before, this amp is
difficult to work on, very busy chassis. I'll just resolder and simplify
the primary connections (use only the 110v, no ground switch) and
we'll see...

Thanks a lot, guys!

[heisthl]

To me this sounds like good news, it would be rare for the primary side of the power transformer to burn up and short - just disconnect the primary wires and see what the bulb does - I'm betting it's still bright and you have a problem with the AC getting to the primary (fuse holder, power switch, acessory outlet, voltage selecter, etc.) something shorting to the chassis.

[Allynmey]

Nevermind the voltages with the Centertaps not grounded. You won't have a ground reference and the voltages will be way out of wack.

Allyn

I think bad PS. Disconnect the primaries fro the power and see if the bulb is still bright. I doubt it will be.

Allynmey

[heisthl]

I'm wondering if this amp has the .047 cap to ground (located somewhere on the hot side of the power cord AC line to chassis ground point) either hardwired or on a ground switch and the cap has shorted. Alternatively it might have a MOV (located between the hot and ground of the powercord AC lines) for spikes that instead of blowing up open has shorted. Either of these possibilities would explain the sudden change from working to not working with no notice.

[fabiomayo]

I'm wondering if this amp has the .047 cap to ground (located somewhere on the hot side of the power cord AC line to chassis ground point) either hardwired or on a ground switch and the cap has shorted. Alternatively it might have a MOV (located between the hot and ground of the powercord AC lines) for spikes that instead of blowing up open has shorted. Either of these possibilities would explain the sudden change from working to not working with no notice.

You might have nailed that in the head. It does have the cap connected
on a 3-way ground switch. I just came from work. I'll get back soon.

BTW I have a question. A VERY newbie one
What's MOV?

Thanks for the help people! A LOT!

All the best

[Zippy]

MOV - metal oxide varistor. Used to guard against spikes in supply voltage by breaking down when a threshold voltage is exceeded. Typically placed across the mains.

[Pete]

- If your "ground switch" is 3 way (not 2 way) a shorted cap would probably have no effect (good or bad), when switched to center position.
- You may be able to spot a MOV if you look for a thing that looks a little like a large ceramic cap somewhere near where AC power (like the line chord) comes in. Typically across the rear AC outlet if you have one.

[fabiomayo]

Thanks, guys. There's no MOV here. I think Kidding...

Anyway, I removed the ground switch from the circuit and still I got a very
bright light. Still have all secondaries disconnected.

Right now, I've removed the 110/220v switch and it appears to be ok.
I'm having a hard time understanding the transformer input wires.

I'll get back...

Thank you!

[fabiomayo]

Thanks, guys. There's no MOV here. I think Kidding...

Anyway, I removed the ground switch from the circuit and still I got a very
bright light. Still have all secondaries disconnected.

Right now, I've removed the 110/220v switch and it appears to be ok.
I'm having a hard time understanding the transformer input wires.

I'll get back...

Thank you!

[fabiomayo]

Ok, guys. One more time.

I stripped everything form the voltage input. Here's is what I got:
A power cord with a 100w light bulb in series in the hot lead.
A 3A fuse in series in the hot AC input followed by a power switch.
Then direct to the PT. All secondaries are disconnected (6.3v heaters,
high voltage winding, 5v rectifier heater, bias).

I resoldered everything and checked for continuity and shorts in those
connections, all's ok. Turned the amp on and the light bulb is still
glowing strong. Here's what I measured:
8.40V on the PT primary
25.6V on the high voltage secondary
333mV on the heaters winding
290mV on the rectifier heaters


Can I make a simple association, like this:
input 120v = 6.3v heaters
input 8.4v = .441v heaters

If so, this voltages doesn't seem to be far off...

I'm really lost here... Not implying that I was ever found anyway


Any ideas?

[Bob-I]

I resoldered everything and checked for continuity and shorts in those
connections, all's ok. Turned the amp on and the light bulb is still
glowing strong. Here's what I measured:
8.40V on the PT primary
25.6V on the high voltage secondary
333mV on the heaters winding
290mV on the rectifier heaters

There's got to be a short somewhere, or the light bulb wouldn't light and the voltages would check out ok. You've only got 8.4V on the primary, that should be 110V.

As long as you're sure you're measuring AC voltage, your PT is gone. Get a new PT.

Can I make a simple association, like this:
input 120v = 6.3v heaters
input 8.4v = .441v heaters

If so, this voltages doesn't seem to be far off...

The question is WHY is the primary only 8.4V? It must be a short in the primary dragging the AC in down.

I'm really lost here... Not implying that I was ever found anyway


Any ideas?

Yep, get a new PT.