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

Fri Apr 24 23:50:14 EDT 2020

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

On 25/04/2020 02:11, Ray Albers wrote:
> This thread started as my description of the failure of my Astron RS-20A
> power supply, caused by a bad electrolytic capacitor on the regulator
> circuit board. Several hams posted advice that a 20Amp supply was really
> too small for a K3/100, especially if running full power. This led me to
> order an Astron RS-35 supply.
>
> Chris Hoover, AI6KG, posted the below advice, warning about possible under-
> engineered rectifier diode arrangement in the RS-35A.
>
> Today the big brown truck delivered my RS-35M-AP  (the M and AP signify it
> has meters and Anderson Power Poles on the front panel).  Chris speculated
> that Astron may have made changes more recently. So the first thing I did
> was to open the case to check out the rectifier arrangement.  Here's what I
> found:
>
> My unit bears Serial Number 2019110051.  I am speculating that the leading
> digits 2019 signifies 2019 manufacture.  Interestingly, the schematic that
> shipped with the unit says Rev.1, April 2020.  The schematic shows two
> bridge rectifiers, DB3501. Two diodes are used from each bridge, and sure
> enough the diodes are paralleled.  The DB3501 is spec'd at 35A, so that
> seems to be an improvement from the 25A diodes Chris mentioned.  That's
> what's in the schematic. Inside the power supply, there are indeed  two
> rectifier packages bolted to the floor, with heavy (maybe 14ga) solid wires
> connecting terminals in parallel. I'm unable to confirm what the rating of
> these diode packages is. They are not labeled DB3501. Instead, they say
> "Astron 5001," and one of them also bears some Chinese characters. So
> custom made for Astron?
>
> Well, I have ordered a 50Amp rated diode package, but  am undecided if I
> will replace the diodes in the supply.  35 Amp rated diode bridges is an
> upgrade from the 25 diode bridges that Chris mentioned finding. It does
> puzzle me why, with 50A (and better) diodes being so cheap, would Astron do
> it this way?  I can only think of two reasons:  1) We think having two
> packages bolted to the case will make for better heat dissipation of the
> total heat generated at max current and 2), the ever present, "because
> we've always done it this way!"
>
> Finally, I'll mention that, like my old RS-20A, I found the negative
> terminal bonded to the case. I consider this bad practice so I removed that
> bond.
>
> 73 to all
> Ray K2HYD
>
>
> On Sun, Apr 19, 2020 at 6:07 PM Christopher Hoover <ch at murgatroid.com>
> wrote:
>
>> You might want to replace the bridge rectifier in that new 35A power
>> supply.
>>
>> Yep, right out of the box.
>>
>> At some point, Astron started shipping RS-35A's with a 25A bridge
>> rectifier.   They paralleled two out of four of the 25A diodes twice over
>> to make, supposedly, a pair of 50A diodes for center-tapped full-wave
>> rectification.  That's not good engineering practice as the diodes in each
>> pair will not share current equally because of differences in Vf and
>> tempco.     Once one blow, the other will blow shortly after.
>>
>> After I fixed my own RS-35A with this problem, I've helped several other
>> hams fix this same problem in theirs.  Not a random sample, as this is just
>> folks on my local machine and in local clubs.
>>
>> You can get a 50A bridge in the same package for under $3.
>>
>> Perhaps Astron has fixed the problem since ....
>>
>> 73 de AI6KG  -ch
>>
>>
>>
>>
>> On Thu, Apr 16, 2020 at 10:28 AM Ray Albers <rayalbers at gmail.com> wrote:
>>
>>> Many to all who posted/responded to my recent post about an electrolytic
>>> capacitor failure in my power supply.  Lots of very interesting reading
>>> about peoples' industry experiences - thank you!
>>>
>>> Several have pointed out that using a 20A supply with my K3/100 is pushing
>>> close to (or over!) the limit.  Even though I am measuring just 16A at the
>>> power level I'm running (and not running anything but the K3 on this
>>> supply) I agree that I'm skirting the edges. So even though I've been
>>> getting away with it for a long time, this morning I ordered a 35A supply.
>>> I'll probably sleep better.
>>>
>>> 73
>>> Ray K2HYD
>>>
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