[Collins] PM-2 Replacement transformer project

[wabate]

PM-2 Replacement transformer project

 From an aesthetic and compactness standpoint, I prefer the PM-2 power 
supply to the 516F-2.  But the PM-2 has that limitation of intermittent 
operation, whatever that means.  I was not able to find any 
documentation on how intermittent was defined.  Apart from the obvious, 
that RTTY is ruled out, what would be the limitation for audio 
compression?  How about continuous SSB or CW operation in a contest?

This was enough to make me look at transformer design in an effort to 
remove that limitation.  The good news is that wire insulation has 
improved dramatically in the last 50 years or so.  An unexpected (for 
me) advancement was in transformer iron that leads to increased 
efficiency and better magnetic properties.  After much what ifs and 
juggling of parameters, a new transformer design was established that 
met my goals.

So what’s out and what’s in?  Well I dumped he 400 HZ specification. 
Figured I could live without that! :)  I also thought 220 VAC operation 
had limited appeal, so the dual primary got dumped.  What I got was a HV 
primary that was rated for continuous operation.  My PM-2 measured 833 
VDC @ 240 ma.  I can’t figure that out based on the Collins spec of 700 
VDC, even with 120 VAC in a 115 VAC primary.  Well I liked more voltage 
on the plates anyway.  So the new transformer had that voltage spec. 
The low voltage and filaments spec stayed the same.

First I tested the original PM-2 with resistor loads (I was not about to 
rum the KWM-2A at key down as a guinea pig!).  The closest resistor 
values I had put a 240 ma load on the high voltage (833 VDC), 293 VDC at 
210 ma and 6.3 at 12 amps.  This exceeded the original specs slightly. 
I placed a temperature probe inside the windings and on top of the 
laminations and turned on the supply.  After 25 minutes I had a 
temperature inside the transformer of 136 F and the laminations 
temperature was 131 F.  I stopped at 25 minutes as the temperature 
appeared to be taking off.

Next I wired the replacement in and ran the same test.  I had 860 VDC at 
240 ma, 288 VDC at 187 ma and 6.3 VAC at 12 amps.  I could not insert 
the probe in the same winding location so I did not measure winding 
temp, just lamination temperature.  It took 1 hour of operation to get 
to the same 131 F that the original reached in 25 minutes.  So far, so 
good.  After 2 hours and 45 minutes of continuous operation, the 
lamination temperature rose to 167 F.  At that point I felt I was 
reaching equilibrium and felt nothing else was to be proven.  No moke or 
burnt insulation smell either.  The specifications on the new 
transformer wire is a rating of 150 C!  The insulation is rated at 130 
C, so you see we are no where near the ratings even at continuous duty.

Now there was one concession in the design.  I had to accept a slight 
lamination thickness increase.  But the transformer still utilizes the 
same mounting holes.  No Drilling!  I did raise the transformer off the 
chassis just to make sure there would be no interference with the wiring 
grommets.  That’s it!  I’m happy.  I want to tweak the windings a little 
bit to lower the HV but I think that will be it.  Production costs are 
estimated at about $125 plus S&H.  So is there any interest out there? 
I would coordinate the project and handle the paperwork.  All those that 
are interested would have to do is write a check.  What we would get is 
a 516F-2 in a PM-2 package!  And if you let your imagination run, you 
will realize that you can install one of these transformers in a burnt 
out 516F-2, eliminate both rectifier tubes and two chokes and have a 
resurrected 516F-2.  Not too shabby.

If we get enough people to sign on, the cost might lower.  So let me 
know if you are interested. Please email me direct and I’ll get back to 
you.  I’m a few weeks away from solidifying things.

73,

Bill, K3PGB