Re: [amsat-bb] FT-736R power supply

["Colin Hurst" <hurst_cj@xxxxxxxxxx>]

Hello All,
Noted your requests for help on the BB.
My FT736 has also been demonstrating advanced signs of 
deterioration. Geoff VK2AIT addressed this problem some 
years back when he encountered the power supply problem. 
Quite a few VK's have made these mods. 
I have not yet made the time.

To quote Geoff.
When you get the board out, you will notice  a discloured heat 
patch more or less in the centred around the 2 WW resistors 
R17/R18 and up to 3 or 4 inches in diameter.
The resistors are too close to the board, and even worse, the 
pigtail length is too short! This permits the heat to travel directly 
into the PCB at these 4 points.
To prevent a repetition, I recommend replacing these with two 
5 watt units, and standing them on end. 
DO NOT SHORTEN the pigtails. Wind the lower pigtails around
a 1/8" drill or similar. The other pigtails will need to be extended 
anyway. I secured the assembly in position with some neutral 
cure Silicon adhesive.
Depending on the history of your board, some of the capictors 
will have had their characteristics altered ... Impedance & ESR. 
I would be suspicious of  C12 & C9, and possibly C21 & C22. 
If you have access to FARNELLS, they may be carrying the 
Phillips Series 136 capacitor. These feature very low impedance, 
& low ESR, and have an operating life > 4000 hours @ 105 
degrees C!!!  Have a hard look at D6 too.
Finally resolder ALL of the joints within the PCB distress circle. 
You will probably see how crystalline some of these appear.
Give the board a good test before re-installing it. 
Some of them need a load before they work. I gave mine a 5 amp 
run for 15 minutes or so with barely perceptible heat rise on the 
built-in heat sink.

Regards and best wishes.
Colin VK5HI



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