5F1 Champ build… bad power transformer?

[camdenh]

Hi everyone, it’s my first time posting here - thank you in advance for any suggestions!

I have a 5F1 where I think the power transformer is bad. The reason I’m doubting myself here is that this is my first amp build so it seems wildly unlikely that there’s a transformer issue, but it’s the conclusion I keep coming to.

for example the heater wires that should be putting off 6.3v — I’m getting about .7v from them (measured across both wires, of course), and the wires that supply the 5v to the rectifier I’m reading about .2v.

any thoughts on there being something I’m missing?

thanks!

camden

[Enzo]

Is your meter set for AC volts? NOT DC volts?

Look at the primary wires. Measure AC voltage right at those wires (Do not measure AC voltage at the switch or fuse or other "convenient" place.) Is there 120VAC right on those wires? If primary voltage is not getting to the transformer primary then the secondary will have no voltage either.


Or another way to test: Unplug amp from the wall, but turn the power switch ON. Now right on the prongs of the power cord, measure resistance between them. Should NOT read open. You are measuring the resistance up through the cord, through the switch, fuse and transformer primary then back down the other half of the cord. If you see open, then likely a blown fuse, a bad fuse holder, a bad power switch, an open primary, faulty wiring. What resistance SHOULD you see? I have no idea, just not open. How about something between 50 and 500 ohms.

[camdenh]

Enzo Thank you for the reply!

Yes - I am on AC volts. I run a business that gets borderline electrical so I know my way around a multimeter reasonably well.

I did what you said and measured resistance across the prongs of the power cord. I got 3.3 Ohms, which I suppose could seem kind of low, but it’s definitely not “open.”

Also, from my wall outlet, I get 123vac so I used the blue wire as opposed to brown going to the fuse holder, to support 125v (the brown was for 120v).

I appreciate your thoughts!

-Camden

[Chuck H]

Also, from my wall outlet, I get 123vac so I used the blue wire as opposed to brown going to the fuse holder, to support 125v (the brown was for 120v).

I believe he was asking you to measure for AC at the transformer primary lead connections. Pretty sure Enzo wasn't suspecting your wall outlet.

[camdenh]

Chuck H yes - and I tested the ac cable as per his directions as well. I just included the wall outlet voltage for consideration.

[Helmholtz]

The PT of my Super Champ has a primary DCR of 5R @120V.

[Helmholtz]

With a 3.3R primary DCR we know that your primary is not open.
If you don't get decent secondary voltages with all secondary wires lifted, your PT likely has shorted turns either on primary or secondary.
In any case the PT will draw excessive mains input current and should blow the mains fuse (assuming correct value/type) and/ or get hot.

[g1]

What is blue and brown? Are you talking about the AC cord, or the transformer primary wires?
Can you post a datasheet or link to the actual power transformer model you are using?

[camdenh]

Chuck H ah - I see what you’re saying now. I didn’t post that bit.

Testing the power coming into the amp via the leads on the cord was one of the first things I did - and it had proper voltage.

I’d never thought to check the resistance between prongs on the plug itself until Enzo said to do so.

[Chuck H]

You should test AC volts right on the transformer primary leads. Make sure there's wall voltage right there.

[camdenh]

Chuck H Yes I had tried that before too! I’ve been discussing today with the manufacturer and they gave me readings to confirm (certain resistances between primaries/secondaries and voltages present when powered up) and it does seem like the PT could be the issue. Waiting for another reply back from them.

[The Dude]

If you have proper input to the primary and no secondary voltages with the secondary disconnected, the transformer is bad.

[Chuck H]

Chuck H Yes I had tried that before too! I’ve been discussing today with the manufacturer and they gave me readings to confirm (certain resistances between primaries/secondaries and voltages present when powered up) and it does seem like the PT could be the issue. Waiting for another reply back from them.

What was the AC voltage read at the transformer primary leads? (Not the AC input leads)

And please spec the transformer. Supply manufacturers info if you can, please.