[Collins] 75A-4 Power Xfmr

[Dr. Gerald N. Johnson]

Be sure to examine the products from Hammond Transformer and others. 
Hammond has a line with 50 volts bias instead of 120 volts. Best I can 
analyze the circuits and available manual data, the 75A-4 needs about 
225 volts each side of center at about 120 ma to a choke input filter 
(which stresses the high voltage winding less than a capacitor input 
filter and about 7 amps of heater current. I don't know which lamps are 
used for pilot lamps the manual only gives a Collins part number but I 
suspect its a type 47 or 44, a common lamp. Ahah, same part number in 
the 32S-1 manual says 150 ma at 6.3, so its a type 47. Makes the total 
heater current 6.75 amps.

A transformer winding doesn't expire with a 10% overload, it just heats 
a little more, eventually charring the paper insulation between winding 
layers (in decades of operation) and it probably has a bit lower output 
voltage because the winding always has inductance and resistance. And 
not all that voltage drop is in the secondary, some of it is in the 
primary and if some other secondary is loaded less the voltage drop 
isn't quite as much on the slightly overloaded winding and the total 
transformer temperature rise is less. Some energy can be saved by using 
a solid state replacement for the 5Y3GT. More energy can be saved by 
running the line at 115 volts instead of the 120+ that modern power 
systems tend to prefer. One line of Hammond transformers comes standard 
with 115 and 125 volt taps for that purpose. The 'A4 was designed in an 
era where 115 was a typical line voltage.

In transformer designs there are conservative designs (for long life and 
lower temperature rise) based on 1000 circular mils per amp, and low 
cost designs based on twice that current density, 500 circular mils per 
amp. The low cost design will operate hotter, but won't burn up. 
Probably won't last as many decades as the conservative designs, but 
fits more applications because there's half the copper and so it can be 
a smaller transformer. These days, there are better insulating materials 
for wire and paper layer insulation to allow safe and long time 
operation at increased temperature and better iron for lower loss at 
higher magnetic field intensities.

Where the bias tap puts out 50 volts instead of the desired 125, I 
figure a voltage doubler can achieve adequate bias voltage. Takes adding 
a 1N4004, anode to the cathode pin of the bias rectifier, cathode to 
ground. Then adding probably a 20 to 50 mf 150 volt electrolytic between 
the transformer wire and the cathode pin of the existing bias rectifier. 
+ of the capacitor to the transformer.

Where there is no bias tap, it should be practical to use a capacitive 
or resistive voltage divider from the 225 volts AC plate winding.

Or it would be practical to use the smallest available 6.3 volt filament 
transformer powered by the main transformer 6.3 volt heater winding 
running backwards to provide an isolated 120 volt AC supply. A practice 
for ham power supplies that has been used all my life.

I see on the Hammond web page that they will do custom designs, $500 
minimum order. Its probably a dream but maybe they could sell 3 or 4 
transformers to fit a 74A4 for that price.

Dave Knepper and I have been discussing these options off list.

It would be nice to have my computed AC plate voltage on the main 
rectifier confirmed.

73, Jerry, Technical Adviser to the Collins Radio Association.

On 4/13/2011 11:58 AM, Chris Kepus wrote:
> Thanks to all who responded to my questions.  You guys are great! :-)
> When I dig in further, you all will be the first to know.
>
> In the meantime, thanks for the leads on tranny rewinding and the
> differences in the A2/A3 trannys and the A4 tranny.
>
> 73 and tnx agn,
> Chris
> W7JPG
>
>
>