Almost There - How to Test a Modulation Transformer?

[W1UJR]

Collins 30K-4 progress report, on the home stretch now.
Ok guys, have the oscillator running well now, cleaned the socket connections, and used a different xtal, runs continuously without crapping out.
Thanks for the suggestions on the xtal question a few weeks back.

Just spent the last week, on and off, checking out the low level audio stages, today finished checking the driver xformer for the 75th tubes, all is ok, so it looks like an issue with the high level audio end of things.

I don't have a spare 30K-4 mod xformer to throw at this rig, the 30K-1 is different. so I'd like to double check this unit before sending it out for repair or replacement.

Is there a good method to test a modulation xformer without a "high pot" tester?
I've read about using 110 VAC on the unit, then measuring voltages, not sure I want to try that until someone else confirms.

I'm getting the xformer out of the transmitter for testing this afternoon, would like to hear your thoughts?

I do have a mod xformer for the Collins 20V, not sure how close that will be.

Thanks!

-Bruce

[KL7OF]

In my experience, a mod transformer of any ratio will work to some extent with most any tube combination...Most of them seem to perform quite well even when the ratio is not near optimum as long as they have enough current capacity for the job.  I am using a 20V mod tranny on 813's modulating 250ths and using a 1:1 RCA BT1 mod tranny on 813's modulating 4-400's .......both transmitters have adequate and excellent audio....I wouldn't be scared to try the 20V iron in your rig...should be just fine...gud luk...Steve

[W1UJR]

Perhaps I should have checked the schematic before I mentioned the 20V, seems it has a different winding config than the 30K-4.
What do you think?

Both below.
http://gallery.mac.com/brucehowes/100091



Tnx!
-Bruce

[WA1GFZ]

Bruce the 20V runs higher power so it should not saturate if you run dc through the secondary as a quick test running lower power. I assume the working voltage is similar but the current will be a lot lower in the 30K.

[W9GT]

Bruce,


The 30K mod iron has an additional screen winding.  If you have a spare mod transformer for the 30K1...it will work just fine.  It is just mechanically mounted different...electrically the same.  As long as you properly ID the connections...it should work fine.  Your audio symptoms were the same as mine when my mod iron went south (or some direction).  I had to replace it with Peter Dahl iron which is really nice, but quite expensive.  Rewinding is certainly an option, but still expensive, especially for that potted transformer.

I tried using a Stancor A-3898 multi-match 300 W transformer and a choke in the screen lead...didn't work well.  Everyone said my audio sounded mushy.  The highest Z setting was apparently not high enough. 

73,  Jack, W9GT

[W9GT]

A few additional comments Bruce.....

Your original question was how do you test mod xfrmrs.  The easiest method is to calculate the turns ratio from the known Z of windings and then apply a low ac voltage to one winding and see if you get a proportional reading out of the other winding(s).  (read the explanation of this relationship in any old ARRL handbook)  I wouldn't use 115-120VAC unless you put a light bulb or some resistance in series to limit current...just use a filament transformer and use 6-12VAC.  The low voltage will work fine for this test.  The problem with this method is that it doesn't tell you if the transformer is breaking down under operating conditions with high voltage applied.  You can use a Hipot tester, but that won't necessarily give you a meaningful check either.  You can use the Hipot to check to see if the transformer is breaking down between a winding and the case or from winding to winding.  The problem is that one of the most common failure modes is windings breaking down and arcing between layers of the same winding and resulting in shorted turns or shorting when high voltage is applied.  (or breaking down on voice peaks)  One good way to check for this is to use a megger.  The megger applies a high voltage and measures breakdown resistance.  It should show up a shorting problem in a winding.  I was fortunate enough to have the original resistance readings of the mod transformer that I took when I first got the rig.  I compared those to the readings I was getting after the transformer blew and I found the the bad winding showed a measurable lower resistance even with no HV applied.  It also showed a breakdown condition existed when I checked it with a megger.  Bottom line..... it is tough to get real good indications and be absolutely certain that a mod transformer has gone bad without having a replacement transformer to substitute.  You can, however, get a pretty good indication with these methods.  Good luck Bruce!

73,  Jack, W9GT

[W1UJR]

Excellent!

Exactly what I needed, thanks W9GT and fellows.
I'm operating on the Gray Hair Net tonight, then the xformer comes right out for testing.
Unless some miracle has occurred, and the guys give me a good report, Bessie's got a bad mod xformer.

As you said Jack, its tough to tell without swapping out xformers, but I'm thinking that I've checked the low level stages pretty well.

This transmitter has been a real learning experience in so many ways. From figuring out the exciter, to the output network loading scheme, building the T/R switching network, to bench testing the speech amp, I've learned a ton from the old girl. I always learn better by reading and then doing, rather than reading alone. This has been quite an education along the way.

Always great info and helpful folks on here, I do appreciate it greatly!
I've not had much experience with defective modulation xformers, so this is a real learning experience.


73 all,
Bruce

[WA1GFZ]

Bruce,
I met Gary the transformer man at Deerfield. He will fix you up if there is a problem. Remember if you swap in the other transformer leave the old one in and connected to the screens or you will need to add a screen dropping resistor off the new transformer.
I was never interested in these old rigs but after seeing a couple they are actually pretty cool.