[MRCA] ART-13 Transformer Update

[mac]

ART-13s seem to get rediscovered periodically.  Compiled and forwarded  
the information below to several interested parties 5 - 6 years ago.

Dennis D. W7QHO
Glendale, CA

1. The original dynamotor power supply had two high voltage windings,  
one that put out 400 vdc and another that produced 750 volts.  The 400  
volt output provided power to all the low voltage circuits in the  
transmitter.  To get the 1150 volts for the PA and modulators the 750  
volt winding was connected in series with the 400 volt output.  Above  
10,000 ft., however, a relay “unseriesed” the 750 volt section and the  
PA and modulators ran at 750 volts.  This was done to reduce corona  
effects that can occur at high altitudes.  The  AC supply that I built  
for my ART-13 (20 years ago) is also designed this way except that I  
left out the voltage reducing relay (never planned to run on a  
mountain that high :^).  Just worked out that I had the right  
transformer in my junk box to do it that way.  Most  ham AC  
conversions use separate, conventional 400 and 1100 - 1500 volt  
supplies.

2.  Metering for the PA and modulator plate current is done by  
measuring the voltage drop across a resistor in series with the 1150  
volt supply. In the original dynamotor, a tapped resistor was used in  
the negative side of the 750 volt winding which resulted in the PA  
plate  metering circuit riding at a +400 volt potential under normal  
conditions and at ground potential above 10K ft.

3. Now for the tricky part.  The meter on the front of the transmitter  
has both a 0 - 200 scale and also blocked off zones (ranges) for CW  
(white), Phone (red) and MCW (white).  As the set was originally  
designed the numerical scale DOES NOT NECESSARILY CORRESPOND TO THE  
ACTUAL PLATE CURRENT IN MILLIAMPERES.  In operation, the set was tuned  
and loaded to put the meter needle anywhere in the CW zone.  The  
middle of this zone is also mid-scale on the meter or 100 on the  
numerical scale but actually corresponds to approximately 150 ma.  
plate current.   In Phone (and MCW) both the plate and modulator  
currents are being measured together, and so under modulation, the  
needle will properly kick up into the red “Phone” zone on voice peaks.  
Above 10K ft. altitude the PA plate voltage was reduced as described  
above and so the plate current would reduce also, and to keep the  
meter reading in the right zone the value of the series metering  
resistor was increased to continue to give a mid-scale reading (now  
approx. 100 ma.).  All this was done to make life easier for the  
operators, i.e., don’t worry about the numbers, just keep the needle  
in the right zone.  To do all this, the dynamotor had a 20.1 ohm  
metering resistor tapped at 13.4 ohms and the taps to the meter were  
switched appropriately by the altitude sensing circuit.  For ham use,  
however, just use a 25 ohm pot in the negative lead of the HV supply  
and adjust to make the meter read whatever you want.

*******************************

On May 9, 2010, at 11:37 AM, WA5CAB at cs.com wrote:

> I thought about sending this the other night after Jeep's last  
> transformer
> update.  But forgot until now.  Some time ago, while helping another
> collector sort out correct numerous wiring errors in a home-brew  
> supply that he
> bought supposedly working, I in effect did up a rather generic  
> schematic
> diagram of what the supply should look like.  It was adapted from  
> the schematic of
> the AC supply for the TCZ-2 (one of the shipboard versions of the  
> ATC).  It
> has has separate MV and HV transformers and shows gas rectifiers but  
> using
> it to wire up a supply using one transformer with two windings and  
> silicon
> rectifiers would be a no-brainer.  What it will show is how properly  
> to
> connect it to the 52286 or T-47 such that all the controls work  
> properly and the
> metering in the transmitter works.  If all you have to go on is a  
> dynamotor
> schematic, it's far from obvious what goes where, due in part to the
> pressure altitude switch and the way in which the HV is derived from  
> two or one
> winding.
>
> If anyone wants a copy, I can email a PDF or mail a hard copy for  
> costs.
> The drawing fits a B-Size (Ledger or 11x17) sheet.
>
> Robert Downs - Houston
> wa5cab dot com (Web Store)
> MVPA 9480
> ______________________________________________________________
> MRCA mailing list
> Home: http://mailman.qth.net/mailman/listinfo/mrca
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:MRCA at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html