Hot power transformer

[anton]

Hello, long time lurker, first time poster. I have recently tried to breathe new life into a Peavey classic 30 combo with some reasonable success.
Bought this used from a local pawn shop. Problem was pilot light on, no sound. So I thought nice little project, let the fun begin.
Took it home, pulled the chassis out, speaker and started a full renovation on the tolex(just cleaned it with soap and a stiff brush + elbow grease).

Out came the meter.No voltage on secondaries, voltage on primary ok. Discovered an open primary with the meter so i began ringing around for a replacement. Most places i rang ( i live in Australia) either dont have an off-the shelf transformer to suit or had to get one from Peavey. So it was left with either a rewind or
get one made. Quotes i was getting back ranged from 350-450 dollars. ridiculous i thought for a $200 amp.

So i was ready to contact Peavey USA for a 240v unit but would have been at least a 3 week wait so after reading about the thermal fuses on this forum, Id
thought id open it up and take a peek. Noticed the plastic bell cover on the fuse side had heat discoloration so wasnt very encouraging. Sliced down the paper with an exacto and dug out the fuse. I replaced it with a 133C, put it back together and powered it up, unloaded. ( how does one mend the fishpaper, btw?)

With a finger feel, noticed it was warming up ever so slightly. Thought nothing of it. Loaded it up and checked all the voltages. B+, B++, heaters, screen, bias and plate. Discovered an open 15ohm 5watt R66 resistor, so i replaced and all looked good.

Two power tubes were missing when I bought it, so ordered a quad set from a local reseller. Power the unit up, checked the voltages again, all good. After
a while, heard a loud pop and lost sound. The 1A fuse had blown and one of the tubes went out. Pulled the tube out and noticed a ever so slight crack on pin 1. Had a spare on hand so installed. What would have caused a brand new tube to crack and blow? faulty at manufacture? Im not a ham-fisted dude, so im dismissingoperator error on this. Contacted reseller and he agreed to replace it.

But ive noticed the PT gets extremely hot after idling for about 10 mins. So hot that you cant hold your finger on it for more than a second, spit evaporates in 6 secs.
There is no smell, no loud hum and no crackles or pops. Is this normal? Im so paranoid, that I dont want to leave the amp powered-on, unattended, incase it catches on fire. If I suspect short windings, would R.G's short tester pick up the most minor short? (ie adjacent winds ).

Are there any other tests i can do on the PT to give it a clean bill of health? What should the current draw be like on an unloaded PT?

[Prairie Dawg]

I've got a gadget here called a Watt Minder that would be just the ticket for seeing what the unloaded current draw is. I guess the question is "How hot is hot?" An inexpensive infrafred thrermometer ought to be available at your end that would give you a number. I'm a little concerned about the original thermal fuse taking French leave-was your replacement the same value? What is the value of the line fuse you have in the primary? Reason I ask is sometimes people put in higher rating fuses when a transformer is failing.

[anton]

Im looking around for a infra red thermometer to at least record the temp over a 18 hour period to see how high it goes. The fuse according to microtemps website was rated 141C. The closest down from that i found was 133C. As for the rest:

Primary AC line fuse = 2A
secondary HV = 1A
secondary heaters = 10A ceramic.

checked them all before powering up and then used a bulb limiter for the initial startup. Im tempted to install a resetable thermal fuse inline with the primary mounted on the TF laminates before i start using it. the only thing is, im thinking this thing will reset like crazy though, annoying me no end.

I might install a small smoke detector (or hack one) inside the amp driving a relay to open the AC when it goes off. Just hope its not too late when smoke is detected.

[Enzo]

Two thoughts:

Go to RG Keens Geofex page and look up his simple transformer tester. Unplug the amp from the wall, and inside, disonnect the two pairs of wires from the circuit board. Now the power transformer is not connected to anything else. DO the test.


Alternatively, disconnect the two secondary wires - red ones and yellows, I think - and insert an AC ammeter in the mains. Power switch on. We are measuring the amount of AC current the transformer draws all by itself. Should be about zero. If it sits there drawing any current it may be shorted.

A convenient way to insert the meter would be remove the mains fuse and connect the ammeter in its place.

A gross short will blow fuses. But if the transformer has like one shorte3d turn, it will still function, but get real hot. Not good.

[jrfrond]

Thermal fuses open for a reason. When this happens, replacing the thermal fuse and putting the transformer back into service is generally NOT a good idea, and at least here in the USA, is against electrical code.

[Prairie Dawg]

I went down in the basement and retrieved my Kill-a-watt model P4400 electrical widget and tried it out with my repurposed Champion 600. Works like a charm and shows current draw up to 15A, line voltage, watts, frequency and KWH-not needed here. Available from Amazon for about $18.00 and well worth the price of admission when current draw is an issue as it may be in this case. For the price it's a handy tool to add to the collection. I haven't figured out yet if I can blow it up so there's always that to consider.

Thought the name was a Watt-minder but I'm a little forgetful in my dotage.

Amazon.com: P3 International P4400 Kill A Watt Electricity Usage Monitor: Home Improvement

[anton]

Two thoughts:

Go to RG Keens Geofex page and look up his simple transformer tester. Unplug the amp from the wall, and inside, disonnect the two pairs of wires from the circuit board. Now the power transformer is not connected to anything else. DO the test.

Okay. The chances of sourcing a neon globe here in the LED world are slim, next to none. I was tempted to pull my soldering irons neon until i looked down and saw another neon bulb. My amps neon bulb. Unpluged from the wall, pulled out the yellow and red secondary taps from the board and ran 12v dc across the live and neutral of the plug. Clipped a 1kohm across the existing pilot light resistor (680k) and toggled the on/off switch and observed the brilliant red momentary flashes.

Therefore we conclude the PT does not have major shorts . Still sceptical whether this picks up minor shorts but we press on...

Alternatively, disconnect the two secondary wires - red ones and yellows, I think - and insert an AC ammeter in the mains. Power switch on. We are measuring the amount of AC current the transformer draws all by itself. Should be about zero. If it sits there drawing any current it may be shorted.

A convenient way to insert the meter would be remove the mains fuse and connect the ammeter in its place.

Also did this and connected ammeter as suggested, via the mains fuse. Again secondary pulled out, transformer was drawing 200mA. PT was getting barely warm. A bit higher than ambient.
Connected everything up - with the tubes glowing, its drawing 440mA.

So, the short circuit tester says one thing, the unloaded current draw says another. When I get my hands on a temp probe, Im going to monitor it over the next few days and might even get a bimetal fuse and mount it close by, if not on the laminates. In the mean time, I'll hook it up to my strat and belt it to kingdome come.

Thermal fuses open for a reason. When this happens, replacing the thermal fuse and putting the transformer back into service is generally NOT a good idea, and at least here in the USA, is against electrical code.

I would think similar codes apply here in Australia as well. Doesnt deter backyard mechanics from servicing their own vehicles and doing the unscrupulous acts, but i hear what you're saying. I wouldn't sleep right knowing i might potentially pass on a hazardous piece of equipment to another person. If I cannot establish the transformers condition beyond any reasonable doubt and to the best of my ability, it stays with me.

can someone please msg me a price from Peavey spares for a 220-240v transformer to suit a Peavey Classic 30 ? I emailed them two weeks ago but have not heard since. And probably dont expect to for another 2 weeks. Id skype them but Im normally in bed sleeping during their BH.

[anton]

I went down in the basement and retrieved my Kill-a-watt model P4400 electrical widget and tried it out with my repurposed Champion 600. Works like a charm and shows current draw up to 15A, line voltage, watts, frequency and KWH-not needed here. Available from Amazon for about $18.00 and well worth the price of admission when current draw is an issue as it may be in this case. For the price it's a handy tool to add to the collection. I haven't figured out yet if I can blow it up so there's always that to consider.

Thought the name was a Watt-minder but I'm a little forgetful in my dotage.

Amazon.com: P3 International P4400 Kill A Watt Electricity Usage Monitor: Home Improvement

handy little gadget to keep on eye on those carbon emissions. might look for an AU one. cheers.

[g1]

Were all secondaries disconnected when you measured 200mA through the primary? If so, I'd say that PT is shot.

[anton]

Were all secondaries disconnected when you measured 200mA through the primary? If so, I'd say that PT is shot.

Seems a bit high for copper and core losses, I suppose. Actually, would the pilot light would draw that amount? cause it's shunted across the primary and hadn't bothered disconnecting it. ive put the chassis back in the amp so too late to measure again.

[Enzo]

pop the bulb out.