Collins PM-2 Capacitor Replacement

For those that I don't know I am an avid collector / repairer of Collins equipment.

I recently acquired a KWM-2a and a PM-2 to add to my other Collins equipment collection, I collected it and as per the advert both were in good condition and working quite well. Both were well looked after and the seller thought that they were both ex MOD units. Certainly the PM-2 unit had an MOD mod plate on it and so its likely that these were ex RAF. I wonder what service they did ?

After a thorough check over I decided that the PM-2 could do with capacitor and diode replacement, whilst it was working well and so no signs of misuse whatsoever to ensure a long and stable further life a change would be needed.

Now the warning - if you choose to do this sort of work - you do so entirely at your own risk - the voltages here can be lethal and if you are not absolutely sure what your doing - don't - get someone else to do it who has the experience !

For those that don't know - the PM-2 is a "lightweight" (!!) power supply that fixes to the rear of a Collins KWM2 / 2a transceiver, elements of it are very similar to the classic 516F-2 power supply but has solid state rectifiers not valve and in is not designed for high duty cycles - although in amateur use (other than RTTY etc) they can give really good service.

Capacitors are mounted on the top and bottom of the main chassis with the diodes on a point to point Bakelite/Phenolic board where diodes and other components are mounted (including two 10uF electrolytics on the rear of it)

So after some dismantling here are the electrolytics on the top of the chassis (they are under the speaker) - they were not in bad condition but clearly were original (1966 date code) - they are 100uF (450V) - modern electrolytics are quite a bit smaller and so I decided to replace with modern axial equivalents 



So after some careful work two new 100uF 450V capacitors were in put in place, with careful reuse of original insulation and where not possible micro silicone sleeving as used originally in later Collins equipment. As can be seen they are considerably smaller but have better specification such as ripple current etc.  


 Now onto the underside of the chassis - the two 100uf 450V capacitors are under the Phenolic board - the board is held by four screws two short ones goes into insulated standoffs and the longer ones go into the metal standoffs (don't get them mixed up as the long ones will damage or crack the insulated stand offs!) - you can see them in the picture left - as per the other capacitors whilst not in bad condition they were the originals and so well over 50 years old !



After some more careful work - new capacitors were in place again using the original insulation where possible and where not using silicone sleeving as per Collins later spec, as previously they are considerably smaller but with a better specification, this time I used two small cable ties to ensure that the capacitors don't move around at all (unlikely I know)

So onto the two 10uF 100V capacitors that are under the board (don't forget them!) - these were replaced with two 10Uf 450V capacitors - again these are substantially smaller with a higher spec - however please note - make sure that they are wired correctly (I know you always check like I do!) but as they are used in the negative bias supply they are 'back to front' with regard to ground - ie the +ve terminal goes to ground - follow the original connections very carefully and you'll be fine :)

And here are the new ones, smaller and certainly cleaner !I ordered the diode replacements for the 1N4192 diodes but they hadn't arrived at the time of doing this so I'll put together a short blog entry when they arrive, these will be replaced with 1N4007 diodes which I do with all Collins equipment power supplies - I've done this since reading an article in the Collins Collector Associations magazine (Signal) on the 30L1  amplifier by Bill Carns, N7OTQ which details the advantages.


Diode                                Forward Current    PIV           Rating Reverse leakage Package 
1N1492 (Original)                   750 mA         600 Vdc               300 microA max       Metal 
1N4007                                 1000 mA       1000 Vdc               50 microA                 Plastic

As you can see its a no brainer !

Some tips now - the bias pot collar should be removed to allow the pot to move (and be removed) as otherwise you may damage the cable connections to it (they are almost certainly brittle) - I did this but I still had to repair all three connections to the pot as they were virtually hanging off the control !

Another tip - when you complete each stage - check recheck and then check again you've done it right observing polarity of capacitors etc very carefully. I do this and then have a cup of coffee take a break and then check again. I usually will put it to one side and check the next day - even though I'm experienced I still like to take care - at best you'll destroy the components - at worst you'll damage something else and your confidence will take a bad knock - these things go bang quite nicely (and loudly!) - we've all done it - but lets avoid it if at all possible!

When all assembled and double checked (as per above) I then used a variac to bring up the unit - sometime this isn't viable due to the way they are designed - but usually it is.

Result - no smoke and correct voltages - the KWM2a worked nicely and is now waiting for the diodes to be replaced when they arrive !