Collins PM-2 - Diode replacement

Hi,

I mentioned that in the capacitor replacement blog I would be changing the diodes from 1N4192 to 1N4007 as they were higher specification with better PIV etc.

At the time of the capacitor blog writing the diodes whilst ordered from RS components hadn't arrived, the cost per component for (50 devices) was just 19.1p each or with VAT just under 23p each not bad for Vishay 1000V 1A diodes ! other suppliers are of course available but when possible I use RS components for all my orders - they are not the cheapest but the service is excellent and the quality of the components is a known quantity.

So a nice straightforward evening job.

Again please take care, ensure that all capacitors are discharged and of course the unit it unplugged from the mains - check both of these again before you start, also don't work on things when your tired. The voltages here can be lethal so here is 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 !

Here is the board in question prior to the work ;

Note the old 'top hat' diodes there are three 'sets' or areas for rectification - the one on the right is for the +800v line, the one on the left is for +275 volt line and then a single diode is for the -50 to -75 bias line.

So after some careful diode removal - I tested them all and they were all acceptable as diodes go however with the increased specification available with the 1N4007 it would be silly not to take advantage of the increased headroom and performance - future proofing it effectively !

Below is the result of a very gentle hours work, still a little 'fettling' to straighten leads etc - but overall very acceptable :)

 I then checked and re-checked the diode orientation to ensure i'd got it right. I then checked against the circuit diagram (i'm always cautious!).Even though it was a very straightforward task I used my variac to bring up the unit. No problems and fully functional as expected.

Below is the circuit diagram for those that are interested - this is for the 6th version and is like mine - there are at least two versions out there so make sure you get the right one !

For the observant amongst you - you will notice that I haven't swapped the dual section capacitor C7 (A & B) - this is multi section capacitor and when I checked it for ESR it was really very good - replacements are available from the US, when I need them I get them from KE9PQ (Mark Olson) who runs Nationwide Radio - gives excellent service. They are quite expensive though when the import tax and vat is added - but they are great quality !

So on balance I decided that I wouldn't replace it 

So that's the end of the PM-2 recap and diode replacement - I was lucky all the other components when I checked them were all well within tolerance and so I left them.  

I'll soak test the unit over the next couple of weeks and then I'm considering trying a circuit to regulate the 275V line - why ? - because the increased line voltages mean that all of these circuits were designed for 110v or 220V and we run close to 244 volts here occasionally and most of the time well in excess of 235V which means that all lines run high the 6.3v runs closer to 6.8v, 275 is close to 290. All of which can cause problems and shorten the life of the tubes.

This is a well known problem and there are several solutions out there for the collins 516F-2 power supply (I've got to update two of these and so will write that up when i do it)  - which involve 'bucking' the mains supply (using spare winding's to effectively drop the mains voltage) but there doesn't seem to be any for the PM2 hence my thinking to regulate the 275 line and examine the possibility of putting together something to reduce the line voltage without simply sticking a high wattage resistor in series (some peoples approach) which will increase the heat generated somewhat !!

73s for now

Chris

M1ABK

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 ! 

TS-790 PA failure repair

Long time no post !

I've been really busy with work and family and so whilst I've been doing work in the shack, carrying out repairs, restorations etc for other amateurs across the country, I've not had any time to write it up in the blog, I usually take some pictures of the work or restorations and so I will try to write them up as some may be useful or interesting to others, or perhaps the techniques used will be useful to someone !

A few things I've completed have been some repairs for other amateurs, including an interesting repair to a TS-790 which was diagnosed as a power block failure in the 144Mhz PA section (M67727). It was indeed a failure, however upon further investigation and very careful disassembly of the hybrid PA 'black block' which involved carefully removing the 'glue' to enable the top cover off - I discovered that the 2nd stage (Base bias ) connector leg was badly discoloured and had a very high resistance to the hybrid circuits substrate. After measuring it the leg actually fell off from the hybrid circuit substrate ! 

Unfortunately I can't find the picture of the failure that I took :( 

I then cleaned the area on the hybrid and the pin itself and successfully  re-soldered the leg to the hybrid circuit and after re assembly surprise surprise the power output returned to normal.

I've no idea why it failed in this manner although I have some thoughts.

It would seem that none of the PA blocks have any heat-sink compound between the hybrid units and the chassis (!) - no idea why but all that I've had through my hands to repair have been the same. So my theory is that the hybrids have hot spots and one of these is around the 2nd voltage feed into the hybrid to 144Mhz and this through heat cycling causes that area to fail either cracking the hybrid substrate or causing the pin to fail its connection.

So if you have problems with power drop off or failure - it might well be worth checking this and if your lucky and haven't - then I'd check of there is any heat sink compound on the PA blocks and if not - add some !!

Certainly with any hi duty cycle use or contest use I would not wish to use it without this update.

So another TS-790 returned to its owner aligned and working very well - result ! 

I wonder how many PA blocks have been changed as faulty (which technically they are!) when a relatively simply fix could have extended their life substantially - also the original blocks are fairly difficult to get hold and substitutes are getting more expensive - so certainly worth checking !

I hope this might be useful to other 790 owners

73s
Chris