Power transformer... Going south?

[W4AMV]

Good day forum,

Have a small 6DQ6 exciter. Runs about 40 W CW out.
Using it last night noted the output power suddenly dropped off
about 10W. Checked the secondary power xmfr Vac
which when loaded should be 600V. It is now 515V.
Measured the secondary winding R and its just over 400 ohms.
Seems quite HIGH. A measurement on similar xmfrs show about 140 ohms. The transformer case runs HOT. even in standby mode. Nominal Plate V and I when operating is 500 V at 120 mA. I suspect this power xmfr needs rebuilt. Its 60's vintage. Comments? Thanks!, Alan

[flintstone mop]

Before the poor thing gives up the ghost, consider checking the rectifier and the electrolytics in the PS. One of these devices might be sucking the transformer down to its death!! Most likely the electrolytics leaking DC volts.

Fred

[KA2DZT]

Pull out the rectifier and measure the secondary DC resistance again,  end to end measurement.  400 ohms seems high even for a low grade xfmr.  Run the set with the rectifier out to see how warm the xfmr gets.  There should be some heating even with just the filament winding loaded.  It should not feel hot.

I think you said your power went from 40 watts down to 10 watts.  You said the voltage dropped to about 515 volts.  Power should not have dropped that much with only about 10-15% drop in plate voltage.  I would do what Fred said,  look for a possible short or bad electrolytics.

Reread your comments,  I think you're saying the power dropped from 40 watts down to about 30 watts.  That would make more sense with the amount of voltage drop.

Fred

It should be noted, you have two Freds helping you and Freds are usually never wrong.  

[W4AMV]

Yes! You are both correct! And my error on wording. Indeed the Pout dropped about 10W... So we are looking at 30 W Pout down from about 40 W. Last evening, I pulled the rectifier and the oscillator... buffer and the final. So all the filament dissipation demands are gone as well as Plate Supply requirements. Applied power and monitored the xmfr heating touching both the bell side as well the lamination's. After about 5 minutes the xmfr is WARM, not boiling hot but cozy warm. Huh... there is no power being required. The unit should be near stone cold. The real clue and this is interesting. AFTER TURNING THE UNIT OFF, THE XMFR CONTINUES TO HEAT UP! After about 3 minutes the power xmfr is REALLY HOT! So I suspect the I^2R losses and power dissipation is occuring in the "core" and probably quite hot there. However, the heat is being dissipated. Once off, the heat continues to travel to the surface and simply wait awhile and the entire unit lamination's as well as bell get bloody hot. I guess another PS (power supply) is in the works. Tnx!

[KA2DZT]

Your analysis of the heating is probably spot on.  Most likely a shorted turn or turns on the HV winding.  Most of those xfmrs are wound with the HV secondary on the core first, then the primary is next.  The filament windings are at the outer layers over the primary.

Fred #1

[W3RSW]

How would shorted turns make the secondary have higher resistance?
Partially conducting windings with melted-away wire cross section?
Windings May be shorted to ground also.

Anyway the latent rise in heating after power is off is a great demonstration of heat dissapation hysteresis.  Oops, said same thing twice. 

[Opcom]

Good day forum,

Have a small 6DQ6 exciter. Runs about 40 W CW out.
Using it last night noted the output power suddenly dropped off
about 10W. Checked the secondary power xmfr Vac
which when loaded should be 600V. It is now 515V.
Measured the secondary winding R and its just over 400 ohms.
Seems quite HIGH. A measurement on similar xmfrs show about 140 ohms. The transformer case runs HOT. even in standby mode. Nominal Plate V and I when operating is 500 V at 120 mA. I suspect this power xmfr needs rebuilt. Its 60's vintage. Comments? Thanks!, Alan

How would shorted turns make the secondary have higher resistance?
Partially conducting windings with melted-away wire cross section?
Windings May be shorted to ground also.

Anyway the latent rise in heating after power is off is a great demonstration of heat dissapation hysteresis.  Oops, said same thing twice. 

Perhaps the resistance is a fluke, or maybe the poor winding has been so charred that it has nearly decomposed.

@W4AMV If you have not yet found a replacement, PM me the dimensions, mounting, allowable dimensions, and specs (maybe a photo?) and I will look through my collection of combination plate-fil transformers. I have about 100 transformers in the size range.

[W4AMV]

Great Patrick. Will do! Currently my approach is building an external supply and routing the HV and filament via the rear access octal socket. I would leave the internal xmfr in place but disconnect it from the 120V line. Alan

[Opcom]

I got your e-mail. That 1200VDC in such a small unit could be difficult but I'll see tomorrow. The external supply is a good idea and not messing up the unit. I believe a couple of the members here rewind transformers commercially so maybe that would help.

[KA2DZT]

Thordarson 22R36 and Stancor P-8414 are both 1200Vac CT power xfmrs.  About 200ma rating.

[KD6VXI]

I'll throw it out there,  as I've purchased a few things from him.

Gary,  at TRS in Maine.   I have a fil xformer I bought from him,  completely happy.

Send him your old one,  a rewound one comes back.

He's also clearing stock for retirement.   He may have one.

--Shane
KD6VXI

[Opcom]

I have no suitable transformer, sorry about that. 1200VCT in a small form factor is the thing!  Also heard only good things about Gary but never had a job done myself.

[W4AMV]

Thanks to all! No issue. An outboard supply arrangement is reasonable and provides some motivation to add other items to the same chassis. Yes, I spoke via mail to Gary, good work but price prohibitive for this small project. 

[WD5JKO]

Perhaps look at using a control transformer.

Look on Ebay for this item:

"Square D 9070T150D1 Industrial Control Transformer"

This is 120/240 or 120/480 100va rated. I also see 150va rating, not sure which is true.

These will step 120v to 480vac. Follow that with a FWB rectifier and a big cap --- the result will be around 600vdc with load.

I see one seller that has them for $30 each.

The downside is you will need a separate filament transformer to provide 6.3v

Jim
Wd5JKO

[W4AMV]

Hi Jim. Yes, a good approach. I used the same idea for a pair of 811A's, voltage doubled though to 1200 V and under load, sags about 200 V. This was a 500 VA unit, was hoping it was 1kV as there was no label. It weighed in about 20 pounds. Thanks for the head up on that idea. Alan