Information on vintage Thordarson Power Transformer

[K8JLY]

I am seeking specifications for a vintage (1930's) Thordarson power transformer.  The number stamped in the case is: T-6878.  I have access to Thordarson cagalogs from the 1930's, but do not see this number listed.  Specification as near as I can document: Primary - 115v, Secondary:  1280 VCT @ 150ma, 5 v @ 3a, 2.5 vct @ 10a, and 7.5 vct @ 2.5a.  I believe this transformer with the T-6878 was specifically made for a certain manufacturer.  I am using this transformer to power a modulator in a vintage homebrew transmitter.  I have had two fail. Both failures occurred in the primary winding.  It does not appear that the transformer is overloaded.  When in stand-by the CT of the HV winding is open and only filament current is drawn.  I guess it is possible that due to the age of these transformers the insulation could be weak.  I have a DC meggar and tested the windings to ground (case) and other windings with readings near infinity.  Any suggestions are appreciated.  Please note that I have gone through my old catalogs and checked in Stancor and Thordarson replacement parts sections. Neither show this (T6878) part number.   
tnx...
Cid
ARS K8JLY

[N2DTS]

The center tap is opened above ground and no current flows, but you still have high voltage (no load) on the secondary, maybe very high voltage...maybe that is a problem?

[KA2DZT]

Problem with Thordarson is they made thousands of xfmrs.  The one you have is probably from about 1937-38.  Every time they added one turn on a winding they gave it another part number.  Around this period Thordarson began adding letters to the four digit part numbers.  So, a T-6878 would have become a T-68R78 if it is a power xfmr.  If it is a plate xfmr it would have become T-68P78.  I checked about 15 Thordarson catalogs I have and came up empty.  Only a very few 68 series xfmrs shown anywhere.

Fred

[KA2DZT]

I just located your xfmr T-6878 in a 1937 Newark catalog.  The specs are 1200vct at 200ma, 2.5v at 10amps, 5v at 3amps, 7.5v at 3amps.  It doesn't show what the primary voltage should be, but, for sure not more than 115v and maybe even 110v.  Keep the line voltage at no more than this level, could be the cause of the failures.

This type xfmr was made right to the end of xfmr manufacturing.  Some of the later ones were Thordarson 22R36 and Stancor PC-8414.  The later ones only had 6.3v and 5v filament windings.

Brent may be right about opening the CT.  This allows high voltage to appear on the CT.  Very often the high voltage winding is first on the core with the primary over it then the filaments last.  The CT may be close to the lamination causing it to arc and short out.

Fred

[K8JLY]

Fred,
Thanks for the replies.  That's the first time anyone has come up with the actual part number.  It must have been fairly common.  I was able to purchase 2 from Gary Schneider at Old Radio Parts, located in near by Medina, Ohio.  Gary had no actual data and listed the voltage ratings by a no load bench test.  The transformer is used in the modulator power supply of what I describe as a T-50 Transmitter circa - 1935.  The T-50 was the second tube that Eimac manufactured and was only in production a couple of years before being superseded with the 100T series.  This supplies plate voltage to the speech amp string:  57 - 56 - 56 - (2) 45s PP through a voltage divider and Plate voltage to a pair of TZ-20s.  The 200ma rating of the HV winding should be within my operating parameters.  The original power supply used a choke input filter which I changed to a capacitor input to boost the output voltage to 600 volts under load.  The choke input delivered about 500 volts under load.  The first transformer failed in this configuration.  During operation the transformer was not heating excessively.  I did a series of test this evening with both spent transformers.  I powered up the first failed core & coil and in a short time it began to smoke at the base of the core (assumed it was the primary or HV winding).  I took readings with my A/R meggar and found the primary faulted to the core and well as the HV winding.  What you describe makes sense.  Wonder if the later transformers would have a better type of enamel insulation on the windings.   

Thank you for taking the time to help me out .... should have it back on the air tomorrow.  Also will post a photo of the rig on QRZ..

tnx... Cid
ARS K8JLY

[KA2DZT]

You mentioned something very interesting that most folks miss.  When you use a cap input filter instead of a choke input, you must reduce the current load by about 30%.  Thordarson doesn't indicate which filter type the xfmr was designed for (with the stated current rating).  Since the common filter design of the period was usually a LCLC type you can assume the current rating was for choke input.  That 1200vct xfmr was typically used to produce a 450-500 volt supply (choke input). Remember, it's the overall Volt-Amp load on the primary that counts as for the xfmr overheating.

The slight increase in high voltage on the xfmr should not have caused both to fail (as for arcing).  Don't use that megger tester if it's putting high voltages on the windings, that could also help to break down the insulation.  Something else was the underlying cause.  Possible both xfmrs may have had too much moisture in them.  You don't know the history of your two xfmrs, they may have been under water at some time.  Folks get water in their basements, which usually is where xfmrs are keep (I have well over 1000 xfmrs and chokes in my basement).

Fred