DEC H7822 power supply

[Peter Coghlan]

Maciej W. Rozycki wrote:
> On Thu, 12 May 2022, Peter Coghlan via cctalk wrote:
> 
>>  Anyway, the good news is that I think I have found the source of the
>> problem.  One of the capacitors used to filter the (DC?, pulsed DC?,
>> rippled DC?, biased AC?) supply to the 9V regulator is marked 330uF/25V.
>> It reads 6uF on the capacitance range on my multimeter.  This can't be
>> helping the cause.  It's not showing any signs of leakage but it's got
>> a brown sleeve and the same logo as the nasty, leaky SXF capacitors but
>> it is marked KME.  (I said there was only one capacitor in the filter
>> in a previous posting.  Originally I managed to spot a little 10uF/100V
>> capacitor but somehow failed to notice the chubbier 330uF/25V capacitor
>> completing a PI filter with a small choke...)
>
>  Likely just a general purpose capacitor.  I only have 2 330uF/25V 85°C 
> Nichicon SE parts listed for the H7821 (and I have a note about one PSU of 
> this kind having a 220uF/35V part in place of one of those too).  Also no 
> 100V parts at all, but 4 10uF/35V 105°C parts, either Teapo SE or Daewoo 
> RS.  Teapo SE and Chemi-con KME series are standard general purpose parts 
> and given the low temperature rating Nichicon SE are likely such as well.
> 
>  I guess especially for standard capacitors factories ordered whatever was 
> readily available whereas with the high-ripple low-impedance/ESR parts the 
> choice was much more limited.
> 

Hi Maciej,

I replaced the capacitor in question with a blue Nichicon SE of the same
value scavanged from another H7821 until I order some spares.  I now have
8.7V available on the 9V supply, a little less than I hoped for but it is
enough to make the internal thinwire tranceiver happy.  Now there is just
the seized fan to deal with.

Thanks again to you and Tony for leading me in the direction of the likely
source of this fault.  I would never have thought of looking in the power
supply to find it.

Regards,
Peter.

>
>   Maciej
>