The AM Forum

A Thordarson T-21M65 has come into my shack.  Tag says 300-500 watts.  I'll try to attach a pic.

I was not able to see it in service so reluctant to sell it on without being sure it's OK.

Please forgive what might be a naive question:  Is there a way to test it?

I'm newly returned to the hobby after a 50 year absence and my AM transmitter experience is limited to assembling a DX-60 in 1964.

Thanks,

--Brad   W1BCC

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Nice iron, congrats!

There are many ways to test the transformer. Of those, I like to start with the safe and simple ones first and work up. For example, begin with using an ohmmeter to check continuity of the windings (and lack of between them and the case). A wiring chart is a must for this of course.

After that, run tones from a signal generator through it at a low level (0 to +10dBm) and measure the voltages on the various windings to check for shorted turns then sweep it with a range of frequencies. You will likely find that a loading resistance will need to be added to the secondary winding under test for the flattest frequency response. I use a potentiometer for this.

Some suggest connecting a line cord to the primary and plugging it into an outlet. Great 60Hz test, but this idea skeeves me. I'd only do this with a 40W lamp and fuse in series with the hot lead, then there's no issue if something arcs over inside. The secondaries are tested for relative AC voltages.

A high voltage power supply is useful for checking insulation with the B+ lead though a voltmeter on one of the windings and the common (ground) on other windings or the case.  Meter readings will indicate the condition of the insulation at those voltages.

6.3 volt fill xforner for thr BCG Skeve test.


KLC

Eventually you should test it at rated volts.

There are a series of tests that can be done but some of the tests need special equipment which is not typically on every bench. I always use a variac and put 100 vac across 1/2 of the primary. Bring it up on a variac and watch to see if there is any signifacant current flow. If so, its shorted. If, in fact, there was any arcing going on, the transformer is no good anyway. Measure other side of primary, it should be 100 vac also. center tap to taps on primary, should read the same ac volts. Measure the secondary while doing the ac test on the primary. Using a formula, figure the impedence. Next test is a hi-pot. Rule of thumb is twice the rated voltage plus 1000. Its nice to have a 10kv hi-pot tester. Test from each winding to each winding and each winding to ground. Final test is a PI test to insure the insulation is ok. This is what I do when a transformer comes into the shop. Most mod and output transformers have so many turns on the primary that 100 vac will not harm a thing. If it does, the transformer was bad in the begining, nothing lost.
Regards,
TRS
Gary

When Gary says "100volts" on both sides, that's more likely "about the same on both sides",
it will vary a bit depending on the actual turns ratio.

I like to use a 10volt AC signal in, to determine the turns ratio - it makes the math easier. :D

                         _-_-bear

First thing I’d do is put it out in the hot sun for a few days or in an oven set at about 150 or 160 F for as long as mummy will let you.  Shopping day maybe? — Remove most moisture it’s accumulated over the years.

Then as suggested, I’d check the windings for gross shorts and opens with low voltages for starters, noting that the respective voltage ratios correspond to its high voltage rating. Then try out the HV at half ratings with the series bulb in line as already suggested and with say a 100k ohm, 100watt across the secondary for a minute or so. 

Are the terminals showing previous use? NOS maybe, etc. ? That will have a lot of bearing on its shape and your or buyer’s confidence.

1/2 of primary should not vary at all from the other side of primary. If it does, its no good. 10 vac is not enough to see if there is a turn to turn short.
For example:
Lets take a modulation transformer that has 2000 turns in the primary. 1/2 of that would be 1000 turns. At 10 vac your volts per turn is .01. At 100 vac your VPT is .1 so, if you have, lets say 5 turns shorted, with 10 vac applied your only seeing .05 vac. At 100 vac your seeing 1/2 volt. Digital meter readings mite jump around a little thus hiding the fact there is a ,05 differance between the center tap and outside of the primary. Analog meters wont see it at all. Withh 100 vac applied you WILL see the differance between the center tap and each side of the primary. 100 volts applied to 1/2 of primary with a 1000 turns will yield very little current draw. In other wirds, using the past 25 years experience as a transformer rewinder, 10 vac is a waste of effort and time.
Regards,
TRS
Gary

Oh, sorry.

I read incompletely. You're using the primary as sort of an autotransformer.
Very nice way to test the primary.

I was thinking of primary TO secondary.

don't forget to also test ALL ceramic feed thrus at HV .... it has surprised me over the years how many have failed, especially n the humid south

Note that in case of any special test that passes DC though a winding that suddenly interrupting it may cause arcing as with any high inductance.

Yep, so very true. I always bring my hi-pot tester up slowly and bring it down slowly. And, ALWAYS short out the windings when done.
Regards,
TRS
Gary

It checked out OK and has gone to a new owner this past Friday at NEARFEST in Deerfield, NH.  Hopefully it will bark again.

--Brad