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
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