[Collins] PM-2 Replacement transformer project

[Dr. Gerald N. Johnson]

On 4/26/2010 10:42 AM, wabate wrote:
> 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?

Voice and CW duty cycle even in a contest is not like RTTY key down. But 
you can get the same effect on RTTY by running lower power, just keep 
the plate current down to 125 ma instead of 250 ma.
>
> 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.

You can get further improvement in core losses by going to a tape wound 
(and cut) grain oriented steel. New in about 1960, there was a good 
article in QST back then. My dad and I built a couple compact 
transformers then.

The insulation of the conductors is less a limit than the temperature 
tolerance of the paper used to make flat layers and the varnish in 
conventional construction. Though many a transformer today is wound 
randomly in bobbins without the time consuming paper layers.

Thermal cycling is another limit that leads to cracked varnish and 
loosened turns that vibrate and wear through the insulation. And that is 
tough duty but common duty for a transceiver power transformer.
>
> 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.

400Hz for lowest loss takes thinner laminations, but isn't that hard to 
tolerate increase core looses for the few occasions you will need to 
work with 400 Hz power. A transformer design is a HP filter, not a low 
pass and it takes more material to handle 50 Hz than 60 Hz.

220 can be handy outside the US but often is 50 Hz power. Often its 
easier than 120 simply by allowing the use of two smaller wires in 
parallel for 120 and series for 220, the added space is only for the 
extra two terminations. Sometimes two smaller wires wind more compactly 
than one larger wire. But 50 Hz requires more copper and wire.

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.

Collins spec'd minimum output voltages and the capacitor input filters 
tended to cause higher voltages, especially unloaded. You need a higher 
current rating to account for the pulsing of the current into the filter 
capacitors only near voltage peaks and for the filter capacitor bleeder 
resistors. That bleed current has to be significant to keep the no load 
voltage from soaring all the way to 1.4 times the RMS. Not testing with 
the bleed current means your test load is less than the radio load 
unless you tested in the supply.
>
> 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.

The laminations will probably heat more unloaded than loaded. You can 
measure winding resistance to get temperature rise without having to 
stick a probe inside the windings. Some feel that the resistance 
measurement is a better sample of temperature than a temperature probe 
unless the probe is wound into the motor or transformer when its made.

The procedure goes like this: Measure the cold resistance of a winding 
(or several if you wish) after the part has rested in a constant 
temperature environment for 12 to 24 hours. Note that temperature. 
Operate the transformer, and measure the hot resistance ASAP after 
taking the load off.

T = -234.5 + RT ( 234.5 + t) / Rt where RT is the hot resistance, Rt is 
the cold resistance and t is the cold temperature. I think the 234.5 
comes from the temperature coefficient for copper resistance.
>
> 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.

I've put out feelers in the past about the need for 516F-2 supply 
replacements, but the responses have been too small (1, 2, or 3) to 
consider working out the logistics of the metal work and the multi 
conductor captive cable from power supply to radio.
>
> 73,
>
> Bill, K3PGB
>
>
73, Jerry, K0CQ, Technical adviser to the Collins Radio Association.