Blown Power Transformer?

[dbun]

Fired up my AC15CC last night and got a 'bzzzzz' then dead. Pulled the fuses and sure enough the mains fuse had blown. Popped a new one in and pulled all valves, to check if a valve had died, and got the same thing.

I then disconnected the PT secondaries from the circuit and measured the voltages on the taps. I got from tap-to-tap around 580VAC and from tap-to-ground 250VAC and 270VAC. Does these seem a little low to you??

Last time my amp did the same thing it was a blown PT, but the tap-to-ground voltages were 300VAC and 80VAC, so I knew instantly that the PT had gone. I replaced it with a Mercury Magnetics one, so I'm really hoping the PT hasn't gone again, as I'm in Australia and sending it back will be a real pain.

Haven't had time to dig further, to see what else could be causing it, but want to get peoples opinions on the PT first.

Also, if the PT has blown again, does this point to a fault in the circuit somewhere, or have I just been unlucky twice.

[WaltC]

that (250-0-270) doesn't sound that far off to me... but your PT has other secondaries that could be causing the problem as well. Check the 5V (there is a rectifier tube right? I can't remember <G>) and 6.3v secondaries too, 'cause a short there can have the same result.

[dbun]

thanks for the reply.

No rectifier in this amp, it's the newer CC range, which has solid state rectifier.

I'll have a look at the other secondaries, but I don't think there is a short there, as it's only blowing the mains fuse, not the fuses on the other secondary taps.

[jay42]

A blown PT will get hot fast if given the chance to operate.

[Prairie Dawg]

Fired up my AC15CC last night and got a 'bzzzzz' then dead. Pulled the fuses and sure enough the mains fuse had blown. Popped a new one in and pulled all valves, to check if a valve had died, and got the same thing.

I then disconnected the PT secondaries from the circuit and measured the voltages on the taps. I got from tap-to-tap around 580VAC and from tap-to-ground 250VAC and 270VAC. Does these seem a little low to you??

Last time my amp did the same thing it was a blown PT, but the tap-to-ground voltages were 300VAC and 80VAC, so I knew instantly that the PT had gone. I replaced it with a Mercury Magnetics one, so I'm really hoping the PT hasn't gone again, as I'm in Australia and sending it back will be a real pain.

Haven't had time to dig further, to see what else could be causing it, but want to get peoples opinions on the PT first.

Also, if the PT has blown again, does this point to a fault in the circuit somewhere, or have I just been unlucky twice.

The blown fuse is telling you there's a short somewhere.

I'd start by doing what you did. If that doesn't blow the fuse, connect it to the board and remove all the tubes. If it then blows the mains fuse, lift the filament wiring and see what happens. If it still blows the fuse you're down to the B+ rail.

While you're in there bear in mind that B+ goes right into the output transformer and out both sides. If there's a short on one side or the other of the output then that could blow the mains fuse. A simple test from the plate connection to where B+ goes in should tell the tale on the primary side of the output.

Then, it's down to looking for shorts and chafed wires. Think bent over socket pins too near a ground point. Also look for signs of heat and arcing.

Let us know what you come up with.

[dbun]

Thanks for the help so far guys.

I'd start by doing what you did. If that doesn't blow the fuse, connect it to the board and remove all the tubes. If it then blows the mains fuse, lift the filament wiring and see what happens. If it still blows the fuse you're down to the B+ rail.

So far this is the situation:
- All valves pulled.
- With the B+ secondaries lifted and all other secondaries still connected, the amp fires up fine, without any fuses blowing.
- When I connect the B+ back to the circuit, I hear an audible 'bzzzz' and then the mains fuse blows.
- The way the fuse blows is different to last time, in the sense that I get an audible buzz then the fuse blows, whereas last time the fuse blew literally as soon as I flicked the switch. Maybe this is leaning towards a faulty OT??

So this must point to the B+ rail as having a fault somewhere, so I'll start hunting down there.

Thanks for the tip on checking the output transformer. Can I test this by lifting the B+ to the output transformer and seeing if the amp blows a fuse, or are there easier ways of going about this?

Link to schematic if it makes it any easier for people:
http://www.guitarpug.com/wp-content/uploads/guitarpug/2008/06/ac15cc1_schematic_only.pdf

[Prairie Dawg]

Thanks for the help so far guys.



So far this is the situation:
- All valves pulled.
- With the B+ secondaries lifted and all other secondaries still connected, the amp fires up fine, without any fuses blowing.
- When I connect the B+ back to the circuit, I hear an audible 'bzzzz' and then the mains fuse blows.
- The way the fuse blows is different to last time, in the sense that I get an audible buzz then the fuse blows, whereas last time the fuse blew literally as soon as I flicked the switch. Maybe this is leaning towards a faulty OT??

So this must point to the B+ rail as having a fault somewhere, so I'll start hunting down there.

Thanks for the tip on checking the output transformer. Can I test this by lifting the B+ to the output transformer and seeing if the amp blows a fuse, or are there easier ways of going about this?

Link to schematic if it makes it any easier for people:
http://www.guitarpug.com/wp-content/uploads/guitarpug/2008/06/ac15cc1_schematic_only.pdf

the only reason I mentioned that is because I had a Supro one time that had one side of the output transformer primary shorted to ground. Not real common but worth checking.

What I would do would be to test from plate to plate and either plate to center tap (power off, of course). That should validate the primary windings.

Let us know what you find.

[phsyconoodler]

A bad rectifier diode will blow the fuse.

[donnyjaguar]

I think Psycho has it. What you experienced is the typical failure mode of a rectifier diode!

[dbun]

Thanks a lot for the replies everyone.

A bad rectifier diode will blow the fuse.

I'll check the rectifier diodes and report back. Couldn't look at the diodes last night, as I didn't have time to lift one side from the circuit.

What I would do would be to test from plate to plate and either plate to center tap (power off, of course). That should validate the primary windings.

Let us know what you find.

Tested plate to center tap and get around 130ohm. From plate to plate I get around double that, so I don't think there's a short on the primary side of the OT.

I'm pretty sure it's not the PT now, as the reading of 250-270V on the secondary taps is pretty reasonable. I usually take my readings after the rectifier, where it's around 320V, so the low reading had me worried for a bit, until I realized I was reading at different nodes!

I'm really hoping my output valves are okay, as I really liked those NOS Tesla Hex Plates!!!

[dbun]

Tested the rectifier diodes and they tested fine. Looks like the search continues!

Could it be the valves sockets themselves?

[Prairie Dawg]

Tested the rectifier diodes and they tested fine. Looks like the search continues!

Could it be the valves sockets themselves?

I've heard at least one report of a shorted socket on a Vox....can't remember if it was a CC but now you're in the area where you should see signs of arcing. It won't be long now.

Think of it this way. Your problem is located in a part of the universe that is only about 18 x 24 inches. As you eliminate each system-filament, rectifier windings, output transformer-you're narrowing down the places for the trouble to hide.

I expect a great AHA! when you locate the problem.

;)

[dbun]

okay....I did a few more tests on the transformers just to be sure.

On the PT, testing each tap to chassis (ground), I get a reading that is pretty much open circuit, which is expected, as it indicates no shorts to center core.

On the OT, I get a reading of several hundred kOhms on the primary, which is good. However, on the secondary, I get a reading of 0.6ohms from tap to chassis. This is with and without a load connected. I'd expect (I think) to see open circuit here with no load connected, or with a load connected the resistance of that load, as you'd pick up the load through the speaker jacks. I had a variable resistance box hooked up, and even with the resistance wound up to several hundred kOhm, still got a reading of 0.6ohm.

This to me, unless I've mistaken how the secondary side of the OT operates, looks like the secondary side of the OT has a short. Does anyone agree with this??

I re-measured the impedance of the speaker, just to check if the speaker may have been faulty and got 6.0ohms (for an 8ohm speaker), so the speaker tests okay. I did this because I know if you fire up a valve amp with no load, you can damage the OT, so I wanted to be sure that the speaker hadn't died and taken out my amp as a result.

[Jerry]

Your amp produces a hum/buzz with no output tubes? A friend of mine had a Fender Hot Rod amp that did that, it was the output transformer that was bad. Can you disconnect the output transformer from the circuit easily to see if that is the cause of blowing fuses?

[dbun]

Your amp produces a hum/buzz with no output tubes? A friend of mine had a Fender Hot Rod amp that did that, it was the output transformer that was bad. Can you disconnect the output transformer from the circuit easily to see if that is the cause of blowing fuses?

There's only a quick buzz before the mains fuse blows.

Yeah, my next test was going to be disconnecting the from the circuit and seeing if the fuse still blows.

However, I cut the resistors and rectifier diodes out of the power circuit during initial tests, so I have to put them back in before I can test any further, not sure when I'll get time to do that.

At least a blown OT will give me an excuse to upgrade to a Mercury Magnetics!!

[Prairie Dawg]

okay....I did a few more tests on the transformers just to be sure.

On the PT, testing each tap to chassis (ground), I get a reading that is pretty much open circuit, which is expected, as it indicates no shorts to center core.

On the OT, I get a reading of several hundred kOhms on the primary, which is good. However, on the secondary, I get a reading of 0.6ohms from tap to chassis. This is with and without a load connected. I'd expect (I think) to see open circuit here with no load connected, or with a load connected the resistance of that load, as you'd pick up the load through the speaker jacks. I had a variable resistance box hooked up, and even with the resistance wound up to several hundred kOhm, still got a reading of 0.6ohm.

This to me, unless I've mistaken how the secondary side of the OT operates, looks like the secondary side of the OT has a short. Does anyone agree with this??

I re-measured the impedance of the speaker, just to check if the speaker may have been faulty and got 6.0ohms (for an 8ohm speaker), so the speaker tests okay. I did this because I know if you fire up a valve amp with no load, you can damage the OT, so I wanted to be sure that the speaker hadn't died and taken out my amp as a result.

As it happens I have a known good Princeton Reverb output transformer here-which should be close to what you have- and I got out my multimeter and insulation tester. With the multimeter the primary measures 270 ohms from blue to brown, and 135 each way to the center tap. The secondary measures 0.8 ohms DC. Hooking up the insulation tester showed me around 250 megohms from either side of the primary to ground at 1 kv. This one's an IRT2 that I picked up at a yard sale for five bucks. It's great for finding high resistance shorts.

I think if you have not done so yet, lift the secondary leads so you know they're isolated. If you still get what amounts to a dead short from either side to ground you may well have isolated the problem.