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Power xfmr for AK model 70, L-chassis

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Power xfmr for AK model 70, L-chassis Empty Power xfmr for AK model 70, L-chassis

Post by Dion on Thu Feb 28, 2013 7:02 pm

All, I'm embarrassed to have to ask this question, but here goes. I have an Atwater Kent model 70 with type L chassis. The power xfmr burned up (previous owner said he restored it, but that turned out to not include the 83-year old caps). Schematic does not give the output voltage of the HV winding. The AF outputs are a pair of '45s, and the schematic lists plate voltage of 250 volts for those. Would the correct HV winding have the RMS value (176.75) of a 250 volt peak sine wave? (For a full-wave rectifier, the xfmr would then have a HV winding of 177-0-177?) I would like to paste in part of the schematic, but cannot figure out how to do it. I see that some folks have managed to put image snips in their posts. If anyone can tell me how to do that I would appreciate it. Thank you in advance for any assistance. -Dion Morehouse
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Post by mr_ed01 on Thu Feb 28, 2013 11:38 pm

You need to figure in the approx 100V dropped across the 2 filter chokes in the B+ line and the speaker field coil in the B- line, so B+ measured at the rectifier would be around 350V.

This information from the Thordarson Transformer Encyclopedia:
Their recommended replacement transformer for the AK models 55 through 76, L, L1, L2 is a T-13R04.
Specs are:
350-0-350 @ 100ma
5V @ 3A
2.5V @ 12.5A.

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Post by Dion on Fri Mar 01, 2013 2:12 pm

Mr. Ed: Thank you! I do happen to have a tranformer with 360-0-360 HV tap and a 5V 3A tap. Unfortunately it does not have any 2.5 tap, and I could not find one like that in PTOP's web site. I purchased a separate 2.5v, 40VA xfmr that I think I can use for the tube filaments. I'll try a full electrical resto (all caps and OOT resistors) using these xfmrs and let you know how that works. Previous owner did a fantastic job on the cabinet, but only corrected existing problem; did not actually do an electronic resto on this piece. -Dion
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Post by mr_ed01 on Fri Mar 01, 2013 3:31 pm

The only time you may see the peak voltage at the output of the rectifier is when there is no load on the power supply. Output voltage under load is usually approx equal to the RMS rectifier plate supply voltage

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