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Truetone D724 power xformer replacement? - Printable Version

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Truetone D724 power xformer replacement? - Eric - 10-18-2012

I have a Western Auto Truetone D724 (1938) Riders 11-1 with a bad transformer. I was googling trying to find info and came across a thread in the Antique radio Forum and found a thread on the d724 with a bad xformer. I bought a Hammond replacement xformer a 272DX with a 6.3fil and lamps and 5v for the 80 filament. Now my problem the print shows a winding on the secondary (center winding) going to the 80 plates and a center tap going to the choke and filter cap. Did I get the wrong xformer? What am I missing?

Eric

RE: Truetone D724 power xformer replacement? - morzh - 10-18-2012

No it says "600V CT" which means it has a centertap.
Datasheet has it so Red-Red is 650V and the Red/Yel is the centertap. So both reds go to the 80's plates and Red/Yel. goes to the choke.

I am not chjecking this for the right voltages - I leave it to you.

RE: Truetone D724 power xformer replacement? - Eric - 10-18-2012

So I'm going to put 650vac across the plates of the 80 tube and 325 on the center tap?

RE: Truetone D724 power xformer replacement? - Arran - 10-18-2012

It sounds like this is one of those odd sets that has the speaker coil/filter choke in the B- circuit rather then on the B+ side, so the filter capacitors may have a common positive rather then a common negative. Western Auto did not make their own radios, the chances are that this set was made by Detrola, Sentinel, Wells Gardner, or one of the other outfits that specialized in making private label radios.
Regards
Arran

RE: Truetone D724 power xformer replacement? - Eric - 10-18-2012

I believe it is detrola the print for their Model 147 is the same (rider 8-12) but my question is 650 volts on the plates to much?

RE: Truetone D724 power xformer replacement? - codefox1 - 10-18-2012

A healthy 80 can handle 650 volts Okay. Make sure the voltage divider and capacitors are able to handle the load. Beware of polarity in filter capacitors, CT of high voltage may be negative with respect to chassis to provide bias, and therefore the + side of it's capacitor would go to the chassis. Double check schematic to be sure you have it wired correctly and best use a variac to bring the set up slowly whilst monitoring B+.

RE: Truetone D724 power xformer replacement? - Eric - 10-18-2012

I had recapped it and was running it over a couple of days when I heard a pop and the set died, traced it to a short in the primary winding. The filter (new) caps are rated 450 volts. I have a variac and an amp meter and will slowly increase the voltage when i get it wired in,

Thanks
Eric

RE: Truetone D724 power xformer replacement? - codefox1 - 10-18-2012

Point of failure is uncertain. If transformer and rectifier is still good go slow with the variac and monitor B+. Alternately you can remove rectifier and measure AC from each plate to center tap of HV winding, but be carefull Lots of Volts! They should be almost the same.

RE: Truetone D724 power xformer replacement? - Eric - 10-19-2012

I had disconnected the xformer and fed the primary bringing it up slowly watching the current and with less than 10v I had 2amps draw.

Another question the print shows a center tap ground on the 6.3 filament winding the new xformer doesn't have that should I ground one side of that circuit? If so at what point?

RE: Truetone D724 power xformer replacement? - codefox1 - 10-19-2012

Well, sounds like 2 questions to me.

If you are drawing 2 amps with no load whatsoever, I think the transformer may be toast. Best try to determine if any replacement electrolytics went in backwards (yeah they just might work for a few hours before failing.) Failure of a newly manufactured transformer under normal operating conditions to short out primary is unlikely but not unheard of. It's usually the H.V. secondary that fries. I wouldn't be at all shy about returning the part to the vendor if you think you received a defective piece. Haven't heard about any problems with Hammond Transformers that couldn't be easily resolved, and I've been doing this a long time.

Most sets with center tap filament 6.3 windings will be OK running with a transformer lacking a center tap. Winding must be grounded somehow Original one had center tap to minimize hum. You can rig say 50 or 100 ohm pot between legs and center to ground and adjust to accomplish least hum, or just ground one leg and be done with it.

RE: Truetone D724 power xformer replacement? - morzh - 10-19-2012

yes the primary is toast.

I would follow Codefox's advice though I am not sure why primary and why not the rectifier tube.
Also strange that there was a pop - transformers usually just start smoking if overloaded.

This is why I think it may be a defective transformer in the first place, but do your due diligence in checking everything else before using another one.

RE: Truetone D724 power xformer replacement? - Eric - 10-19-2012

Thanks for the info!
It was the original Xformer that failed not the hammond and the set ran for several days before it failed. Opened it up and it doesn't smell good and I have unstacked the laminations but have not seen anything, might have a go at rewinding the primary. The Xformer should draw next to nothing unloaded just some excitation current. When I get home I will install the new one and probably ground one leg of the 6.3v and see what happens. Thanks again
Eric

Eric

RE: Truetone D724 power xformer replacement? - codefox1 - 10-19-2012

Bring it up really slowly on variac and monitor B+. You have an unknown 'framus' in the set. Just remembered that name from a very long time ago.

RE: Truetone D724 power xformer replacement? - Eric - 10-20-2012

You got me what is a "Framus"??

RE: Truetone D724 power xformer replacement? - codefox1 - 10-20-2012

That which is short circuit or defective part. Might be Yiddish or German in origin but occurs often in old radios. Details long forgotten.