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39-6 power tranny question - Printable Version

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39-6 power tranny question - Shaleradio - 11-14-2017

KThanks for help on this!
Here are the facts so far...at some point, electrolytics were replaced (although not to spec on all).  She passes the dim bulb test ( power consumption is 30 w and fired up with 40 w bulb).  It powers up but weak and has low volume.  Good news is I have a 38-12 in working order which has identical tubes to swap.  My concern is with some initial testing.  Attached is a picture of the Rectifier and tranny connections from the schematics.  My primary winding has continuity as do the filament pins.  When I read from the Rectifier plates I should get 1050 ohms.  BUT, I only get 745.  Checking from the green center tap to pin  2 is 380 and pin 3 is 360.  This seems too low but looking for your expert advise.
Mark

RE: 39-6 power tranny question - Radioroslyn - 11-14-2017

Hi Mark,
>at some point, electrolytics were replaced
So point is a bit vague post a pic to give us a better idea of the time frame. Values aren't very critical and manufacturers try to use the smallest value possible as it's cheaper. I would probably use a pair of 10mf @ 450v just for convenience. With that being said on older ac sets from the late 20's and early 30's that use small value caps like 1 and 2mf caps are more critical and should stick to the original values.

>When I read from the Rectifier plates I should get 1050 ohms.
Would lose any sleep over it. Check to see if the AC voltage from each plate to the chassis, should be fairly even within a few volts. Generally I look for signs of over heating like wax under the transformer on the cabinet or the transformer get hot w/o the rectifier tube plugged in after 5 or so minuets.

> Checking from the green center tap to pin  2 is 380 and pin 3 is 360.
To have the same number of turns from center to each end the inside winding is going to be shorter than the outside as it is wrapped around the inside. Longer winding = higher resistance.

> but looking for your expert advise.
[Image: http://philcoradio.com/phorum/images/smilies/icon_lol.gif]  Just a hobbyist.

RE: 39-6 power tranny question - Shaleradio - 11-14-2017

Thanks Terry,
Could you elaborate more on the plate AC test?  I haven't done that yet. 
Thanks for the help.

RE: 39-6 power tranny question - Radioroslyn - 11-15-2017

Just measure the ac voltage from the each of the rectifier plate to chassis gnd or the power transformer center tap w/ the rectifier tube out. We're looking the see if the two voltages are fairly equal. Like what you did with the ohm measurement from ct to each end just we're looking at voltage not resistance.

If the voltages are unequal by 10% or more it may indicate shorted turns in the hv winding. Same w/ transformer heating in a relatively short time.

RE: 39-6 power tranny question - Shaleradio - 11-15-2017

Thanks! I'll try it out and check back!

RE: 39-6 power tranny question - Shaleradio - 11-15-2017

[attachment=14748]Looks like my AC tests are good!  I guess I can move forward.  Hopefully my picture attachment made it across, if not, let me know.  Terry requested pix of the capacitor replacement, they are not brand new and out of spec ( the large one upper left is a 40mfd and although I'm waiting for my enlarged schematics from Mr Swark, I see nothing close).  I'll replace all caps and check resistors and reach out if I get stalled.
Thanks for all the incredible help on this site and any further thoughts, please let me know.
Now comes a Miller Time,
Mark

RE: 39-6 power tranny question - Radioroslyn - 11-15-2017

The ecaps are form the '60s, time for replacements. Also not seeing much rubber wire just the ac cord and on the if transformer.