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FDP Forum / Amp Mods, Repairs, and Projects / How to test a Power transformer?

Mars Hall

USA NW Indiana

Telefunkin'
Dec 11th, 2017 03:26 PM   Edit   Profile   Print Topic   Search Topic

1970 Super Reverb, the rectifier lit up like a Christmas tree. Replaced tube and fuse, the amp does not power up.

Hammond101
Contributing Member
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So. Cal. USA

Dec 11th, 2017 04:15 PM   Edit   Profile   Print Topic   Search Topic

It's a bit involved. Best to remove all of the secondary connections from the transformer to the board et al then measure the voltage at each secondary pair.

I'd most likely start with confirming mains voltage to the primaries, the two black wires and then measure the voltage at the two red leads connected to the rectifier tube socket. Pull the tubes prior to making measurements.

Be careful. You are dealing with high voltage!

Peegoo
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Close

but no guitar
Dec 11th, 2017 04:25 PM   Edit   Profile   Print Topic   Search Topic

Yep. Pull up the schematic too. It will show you what the spec voltages are for that circuit.

Mars Hall

USA NW Indiana

Telefunkin'
Dec 11th, 2017 06:38 PM   Edit   Profile   Print Topic   Search Topic

To measure if voltage is getting to the transformer, I would need to have both power and standby switches in the on position, then measure across both black wires?

If I have voltage there, I can proceed to each other pair by removing them from their destinations and measure across each?

Peegoo
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but no guitar
Dec 11th, 2017 07:53 PM   Edit   Profile   Print Topic   Search Topic

Yes. BUT! Do the following, in order.

1. With the amp unplugged, drain all stored voltage in the electrolytic caps.

2. Disconnect all of the PT's secondary wires and individually cap them with wire nuts.

3. Plug in the amp's power cable.

The primary side of the PT (two black wires) is the AC mains in. Properly wired, the hot leg (narrow blade on the AC power cable's plug) of the AC power cable runs to the amp's fuse, then the power switch, then the PT. The neutral leg (wide blade on the power plug) runs to directly to the PT.

The secondary pairs go to the rectifier, pilot lamp, and tube heaters, etc. Use wire nuts to keep those bare ends safely separated and insulated from each other as you work.

And make sure you have ground continuity from the amp's chassis through the AC mains cable to ground. It's there to save your life if the little pixies decide to take a detour. better that they run through the cable to ground than through you.

(This message was last edited by Peegoo at 10:00 PM, Dec 11th, 2017)

ejm

usa

Dec 12th, 2017 08:12 AM   Edit   Profile   Print Topic   Search Topic

Mars wrote: "Replaced tube and fuse, the amp does not power up."

What do you mean by "does not power up"?

Mars wrote: "To measure if voltage is getting ........TO.......the transformer, I would need to have both power and standby switches in the on position, then measure across both black wires?"

No.
Stop.

The power switch is on the input to the transformer. (AC voltage.)

The standby switch is on the output side of the transformer. (DC voltage.)

You do not need the standby switch on to see if voltage is getting TO the transformer.

Please don't take offense, but this makes me nervous about your abilities dealing with these high AC and DC voltages.


Hammond101
Contributing Member
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So. Cal. USA

Dec 12th, 2017 10:37 AM   Edit   Profile   Print Topic   Search Topic

Yes, the standby switch is not part of this unless you have successful voltage tests and it is found something like a shorted filter cap caused the rectifier tube failure. voltage tests should be made with no tubes and stand by can remain off.

As above, I would first check the for A/C voltage to the power transformer. This would be at the fuse holder and the mains switch then check at the two black wires.

Did you test the replacement fuse you installed in the amp?


If you have the voltage at the mains to the PT then you can proceed with the secondaries disconnect and further testing.

A safer thing may be to after checking the mains voltage, is to test for 6.3 volts A/C at the two green wires (heater wires) connected to the pilot lamp socket. Test at the pilot lamp socket connections. If you have proper A/C to the PT and no heater voltage the PT is damaged and you can stop right there. If you have heater voltage you can proceed with further testing.

All this is assuming the pilot lamp does not light and this is the reason for your "the amp does not power up" statement.

It would also be interesting to know the value of the fuse that was in the amp at the time of failure.

As ejm says safety is paramount in these tests. I would recommend using alligator clips on the ends of your test leads making all connection with the amp unplugged and powered off.

I would agree that if you have reservation or are unsure how to safely test the amplifier it should be taken to a qualified tech. Once on the bench any tech worth his salt can find the cause in short order.

(This message was last edited by Hammond101 at 06:27 PM, Dec 12th, 2017)

BbendFender
Contributing Member
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American Patriot

About as ordinary as you can get.
Dec 12th, 2017 04:20 PM   Edit   Profile   Print Topic   Search Topic

I just tested a huge transformer from a Baldwin organ. 744v on the secondary side.

(This message was last edited by BbendFender at 10:28 AM, Dec 26th, 2017)

Peegoo
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but no guitar
Dec 12th, 2017 07:45 PM   Edit   Profile   Print Topic   Search Topic

More than enough to get kilt with.

Mars Hall

USA NW Indiana

Telefunkin'
Dec 26th, 2017 04:30 AM   Edit   Profile   Print Topic   Search Topic

To clarify, what I meant by "doesn't power up", was that the pilot light nor the filament heaters would light up. At the original time of failure, the amp had the proper value 2 amp slo blo fuse.

Good news, as it turns out, the "new" fuse I installed was bad. At this point, I now have filament voltage to all tubes. This where I stopped.

Being a novice and knowing the dangers is why I asked the questions here before proceeding.

At this point, the caps do not appear to be blown or have any signs of leaking. How should I proceed at this juncture?

pdf64

UK

Dec 26th, 2017 11:08 AM   Edit   Profile   Print Topic   Search Topic

How old are the ecaps? eg original.
If so then just replace them.
Once reasonable caps are in place, remove tubes apart from the rectifier and power upin standby mode.
If the fuse holds, switch to 'operate' mode.
If the fuse holds, adjust the bias to its coldest setting and fit the power tubes.
If all ok, then set the bias for 30-35mA idle plate/cathode current, and fit the other tubes, amp should be good to go.

Mars Hall

USA NW Indiana

Telefunkin'
Dec 26th, 2017 11:47 AM   Edit   Profile   Print Topic   Search Topic

The caps were replaced about 6 years ago. Thanks for all the help!

Hammond101
Contributing Member
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So. Cal. USA

Dec 26th, 2017 03:21 PM   Edit   Profile   Print Topic   Search Topic

Very good news indeed!

Mars Hall

USA NW Indiana

Telefunkin'
Dec 27th, 2017 10:25 AM   Edit   Profile   Print Topic   Search Topic

Ok, I did just as pdf64 suggested and the amp worked perfect on the bench. However, when installed in the cabinet, it would immediately blow the mains fuse. So I removed it and again it works fine sitting on the bench.

There doesn't appear to be anything that I see from a visual scan that could be grounding out when installed in the cab. I did a chop stick test inside the chassis while the amp was playing and found no loose solder joints. I'm a little perplexed...

pdf64

UK

Dec 27th, 2017 12:44 PM   Edit   Profile   Print Topic   Search Topic

Check that the metal foil or grille screening on the underside of the cab top isn't hanging down inside the amp chassis when it's in place.
If ok, repeat the previous procedure with the chassis in the cab, to try and focus in on the issue.

JJuran
Contributing Member
******

Boston Area

Dec 27th, 2017 08:25 PM   Edit   Profile   Print Topic   Search Topic

What are we connecting for a speaker load? Same speaker and cable for both bench and cab?

Also, in either scenario (bench or cabinet), is the reverb tank connected?

Mars Hall

USA NW Indiana

Telefunkin'
Dec 27th, 2017 10:46 PM   Edit   Profile   Print Topic   Search Topic

Just my luck that I would have replaced failed components with failing components twice in a row. It did blow the fuse on the bench today. When it happened, I noticed a flash inside of the rectifier tube. Replaced it, amp works fine. Thanks again everyone!

rfrakes331K
Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Jan 12th, 2018 06:49 AM   Edit   Profile   Print Topic   Search Topic

In the past threads. Didn't someone post a Circuit for a Wheat lamp Transformer winding tester?

pdf64

UK

Jan 15th, 2018 07:24 AM   Edit   Profile   Print Topic   Search Topic

See link?

Geofex tx short tester

rfrakes331K
Contributing Member
*******

IL USA

RonHalen Jokingly He Says
Jan 22nd, 2018 08:29 AM   Edit   Profile   Print Topic   Search Topic

Thanks pdf64!

FDP Forum / Amp Mods, Repairs, and Projects / How to test a Power transformer?




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