Testing a power transformer

[electra225]

Mr. Mogul asked for this. I'll be happy to share my methods. Maybe other members have their ways they would like to share.

I like to have the schematic for the device under test printed and at hand before I go further.

The first thing I do is to perform a safety check on the high voltage circuit. I measure resistance from the positive of both filter caps to common negative, typically the chassis. This is not a scientific test, but merely an indication if the filter caps in the power supply should be replaced before proceeding further. If measuring from the positives of the filter caps to chassis you see less than about 500,000 ohms, you should probably just replace the filter caps before you proceed, to prevent potential damage to the power transformer. Then, with the filter caps disconnected, check the resistance in the balance of the high voltage power supply circuit. If you see resistance in the megaohms range, chances are the filter caps are in good enough condition you can power up, then troubleshoot from that point. I know there will be those who will pooh-pooh this method as something that doesn't work. This is how I was taught, what I have always done, and it has never failed me.

The next thing I do is put a VTVM, DMM, VOM set to a low ohms setting. I put the probes of the measuring device across the power plug of the instrument I'm working on. It doesn't matter about polarity at this point, just one probe on each prong of the power plug. Then I turn the power switch to "on". If all is well, there should be continuity, only reading a few ohms. If you read infinity, or really high ohms, like in the megohm range, something in the primary circuit is open. The schematic for the device you are working on will show what components are in the primary circuit. Check each component against the schematic to find which one is open. If you get low ohms, only a few ohms, then you know the primary of the power transformer is okay. In one operation you have checked the power plug, the power cord, a fuse, if there is one, and the primary of the power transformer.

Then we go to the secondary of the power transformer. First thing I do is to pull the rectifier. Then, using my Variac, a variable AC power supply, to slowly ramp up voltage slowly. I use something to monitor current, such as a Kill-A-Watt or a dim bulb. This to show if there is a short in the secondary of the transformer or anything it powers. Pulling the rectifier will kill B+ (high DC voltage) and decrease the load on the power transformer. I power up slowly, starting somewhere around 60 volts. If all is well, no noises, no smoke, no excessive current indication, then I ramp up voltage to 75 volts and repeat the process. All being well, increase voltage in stages up to full line voltage. Tube filaments should be lit. Pilot lights should be lit. Then measure high voltage across the high voltage tap, at the pins of the rectifier socket for the plates. This high voltage should be equal. If one is appreciably higher than the other, there may be a bad winding in that tap, not good. If the voltages are okay, no noise, no smoke, filaments are lit, pilot lights are lit, it's quite likely the power transformer is okay. Replace the rectifier and see what it does with high voltage. This is your final test. By this time, the device you are working on should be operational, or at least safe enough to continue troubleshooting.

[Hi-Fi-Mogul]

Mr. Mogul asked for this. I'll be happy to share my methods. Maybe other members have their ways they would like to share.
I like to have the schematic for the device under test printed and at hand before I go further.

The first thing I do is to perform a safety check on the high voltage circuit.
I measure resistance from the positive of both filter caps to common negative, typically the chassis. This is not a scientific test, but merely an indication if the filter caps in the power supply should be replaced before proceeding further.
If measuring from the positives of the filter caps to chassis you see less than about 500,000 ohms, you should probably just replace the filter caps before you proceed, to prevent potential damage to the power transformer.
Then, with the filter caps disconnected, check the resistance in the balance of the high voltage power supply circuit.
If you see resistance in the megaohms range, chances are the filter caps are in good enough condition you can power up, then troubleshoot from that point.

Hi Greg,
I have one question.
After disconnecting the filter caps, how do I go about checking the resistance in the balance
of the high voltage power supply circuit ?
Thanks !

[electra225]

You would measure resistance from the positive of the filter cap to common negative, most likely the chassis. What you are doing in this process is to make sure there is no short after the filter caps that would load B+. With the filter caps disconnected, you are checking the balance of the high voltage circuit. If all is well, you can safely replace the filter caps. After I do that, I usually perform another safety check just to be sure.

[Hi-Fi-Mogul]

Okay, very clear, thanks.

[electra225]

I should probably add that you want to see high resistance in the B+ circuit, in the megohms. Any lower, you should check each branch of the B circuit and find the component that is causing the issue. The schematic will guide you as to which branches serve the particular circuits. Obviously, you should have the schematic at hand whenever you are working on anything electronic. Best of luck.