What does lack of continuity between 5V rectifier leads mean

Steve Waltz · From: USA

I don't have continuity when I place my multimeter leads on the two wires from the power transformer that feed the heater of a 5u4. There is power getting to the heaters of that tube and it heats up. There is resistance but I don't remember what. The HV winding and the filament lines have continuity so I think I'm making the test correctly. IT seems like if there is resistance there should be continuity?

Or am I doing something wrong? I'm just trying to be 100% sure that this transformer doesn't have some minor issues such as broke down insulation somewhere that isn't causing a total failure.

Thanks,
Steve

Mike Marsh · From: Florida, USA

I believe you should be seeing a low resistance of 2-4 Ohms.

Mike
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Jim Kennedy · From: Brentwood California, USA

No continuity, also called resistance, means your winding, a coil of wire, is open, there is a break in the winding and current cannot flow, or the resistance scale you are using on your mlutimeter is the the wrong one. Choose a higher range. 0 indicates no resistance and current can flow freely. It can also indicate you are on the wrong range if you are checking low resistance. Change to a lower range. Depending on the situation 0 resistance may or may not be bad. Usually transformers have very low resistance as Mike indicated. Also, make sure you are on the right wires. Transformers have an input set of leads and an output set of leads. If your amp is a common make and model you should be able to find a schematic on the internet. And as always, you will get plenty of good advice right here on the Forum. Hope this helps.
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Get an accurate Ohm meter and check the true resistance - calibrate it to zero to take account of the resistance of the lead wires - they don't have zero resistance - it's small but not zero. As Mike said, you should have a small but positive resistance.

J Fletcher · From: London,Ont,Canada

You say "There is power getting to the heaters of that tube and it heats up." Sounds to me like it's working fine . Why do you think there might be a problem ?

Steve Waltz · From: USA

I measured resistance: heater is center tapped at 1 ohm each lead and 2ohm center. 5v winding is center tapped as well and is 49.9 ohms on one lead and 53.2 on the other with center tap at about 103 ohms. HV power is not center tapped.

So the 103 ohms on the 5v means there is a problem? I read about some test with an LED light bulb and a battery to find issues like damaged wire insulating but I can't find it now.

Reason for looking at this: buzzing power transformer and audio noise that I just can't get rid of. Is the transformer bad?

Thanks,
Steve

ajm · From: Los Angeles

Just to be on the safe side, I'm going to ask a dumb question that hasn't been asked yet.

When you are measuring the resistance, is the power to the transformer on, or off?

Steve Waltz · From: USA

Yes itâ€™s off.

If I end up replacing the PT I have a question about which to use. There is no schematic for this amp and similar amps don't have this PT. The PT was made by EPCO and there is no info out there on that. I have 420 on the plates of the two EL34 tubes. Rectifier is 5U4. I know how to figure out the MA requirements of the PT. I don't know how to know what the original PT was? I'm not sure of the correct equation for this.

Thanks,

Paul Arntson · From: Washington, USA

You say "buzzing power transformer and audio noise that I just can't get rid of".

It might be filter capacitors.
Check the filter capacitors first because if they are bad they can burn out the transformer very quickly.

Also just to be sure (rare issue) check the rectifier tube for shorts.

Once you know you have good filter caps and rectifier tube, you can continue troubleshooting.
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Steve Waltz · From: USA

All Caps have been replaced power and coupling. All resistors checked or changed. All tubes switched out. All grounds checked. All voltages checked. Biased. New 3 prog plug installed. Filament wire routing checked. Input jacks cleaned and reinstalled. Tube substitution to limit where the buzz is with no effect because it starts at start up, Speaker jack replaced..........

I didn't want to turn this into a help me find a buzz in my amp thread because I know how many options there are for that, and I've been down that road. I'm trying to limit my question to checking the power transformer.
Thanks,

Paul Arntson · From: Washington, USA

OK Sorry. Didn't understand the background

Maybe this link will help:
https://www.wikihow.com/Test-a-Transformer
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J Fletcher · From: London,Ont,Canada

So , is this a mechanical buzz coming from the transformer ? That is , is the transformer vibrating ?

Bill Sinclair · From: Waynesboro, PA, USA

Steve,
Are you certain that you're reading the heater winding and not the high voltage winding? I'm no specialist but I don't know that I've ever seen a center tapped 5V winding and yes, 2-4 ohms is typical. The 5VAC wires are probably green (edit:wrong-yellow) and should be going to terminals 2 and 8 of the 5U4 socket. The high voltage winding is usually center tapped and reads 100 ohms or higher.

J Fletcher · From: London,Ont,Canada

The 6.3 vac leads are green , usually , the 5 vac leads are yellow , usually. FWIW.
Center tapped 5 volt winding would be uncommon.

Bill Sinclair · From: Waynesboro, PA, USA

Steve Waltz · From: USA

Bill, good suggestion about having 5v and plates backwards......because I did. I was rushing and keep forgetting to take my meter off “auto” on voltage. I am getting Around 2.5 Dc volts on one of the plates So I assumed it was one side of the 5v ac heater. I thought the actual heater was hv because it was showing ac and I was thinking it was dc. I should have checked PIN numbers too.

So, now the 103 ohms on the high voltage side makes sense. The actual heater is about 5.5 Ac and the proper dc on pin 8. Hv is center tapped.

I don’t know what my meter shows around 2.5 dc volts on pin 4 but maybe that is typical or I am once again measuring incorrectly.

Thanks for the help everyone!

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