[Elecraft] About my capacitor failure (etc.) issue:

[Bob McGraw K4TAX]

I've always put a bit of heat sink compound on the rectifier unit before 
mounting it to the bottom plate.    I've found several power supplies 
where the bridge rectifier assembly failed and upon removal, there was 
no heat sink compound.   Add a new bridge assembly and some heat sink 
compound resolved the issues.   Never had one come back.

73

Bob, K4TAX



On 4/24/2020 10:50 PM, Martin Sole wrote:
> I can add perhaps another data point on the rectifier matter. Years 
> back my employer, one of the largest ground to air radio manufacturers 
> was having failures with their mainstay transmitter, a 50w carrier AM 
> unit. It had a large toroidal  transformer and a packaged rectifier. 
> The rectifier was bolted to the steel sub chassis and all was fine in 
> regular intermittent service but  in ATIS service the transmitter runs 
> essentially continuous, so about 160-180 PEP, and rectifiers were 
> failing.
>
> Investigation showed that the steel chassis was not 100% flat and the 
> contact surface of the rectifier was compromised. The fix was to 
> insert a (really) flat aluminium bar under the rectifier and bolt that 
> to the chassis. The bar did much of the heat sinking and had many more 
> contact patches with the steel sub chassis. Even paint ridges can be a 
> source of reduced contact leading to overheating of the rectifier 
> package and those things really do need to shift some heat. A good way 
> to think of it is like you would a PA transistor in a 100 watt 
> amplifier, we appreciate how well they need to be bolted to a heat 
> sink and your rectifier package wants very similar assessment.
>
> It's not so much the current that causes failure rather the heat is 
> not being dissipated properly from the smaller package.
>
> Martin, HS0ZED
>