DEC H7822 power supply

Maciej W. Rozycki macro at
Thu May 12 11:12:33 CDT 2022

On Thu, 12 May 2022, Peter Coghlan via cctalk wrote:

> 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.

 Tolerance is 5% on this line, so you're well within.

> 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.

 You are welcome!  I wish I had the skills to actually diagnose a faulty 
PSU myself like you do.

 Experience has taught me to look into the PSU first for all kinds of odd 
phenomena.  For example I had a WiFi+5-port 100BASE-TX Ethernet bridge 
which stopped talking to one particular device, a 10BASE-T to 10BASE2 
media converter/repeater regardless of the port it was wired to; IIRC 
frame reception worked, but transmission did not make it through.

 It cost me a lot of hair pulling to sort it, including actually buying a 
brand new identical media converter, the first suspect, as it was the only 
device that caused the bridge to exhibit its odd behaviour.  Which did not 
help of course and the new device triggered exactly the same symptoms.

 Eventually it has turned out to be the bridge's PSU being on its way out. 
It was one of those nasty plug top PSUs that are not worth even looking 
inside, and just replacing it with a new one (which of course cost more 
than the bridge combined with its PSU originally did!) cured the problem.  
Out of 4 such bridges I bought ~15 years ago 3 are still in service, but 
all their PSUs had to be replaced.  At least they had a standard connector 
for the power plug (the newer replacement devices don't anymore, sigh).

 The 4th bridge, the same that caused problems, started having more issues 
with its wired ports later on if more than 2 devices were plugged IIRC, 
with no clear reason except for suspision of a power deficit, so I put it 
aside with a plan to recap it and see whether it helps (this discussion 
just reminded me about it, so maybe I'll try doing it over the coming 
weekend as I have since got the parts required).

 And it was just one of many incidents I had with PSUs playing tricks, so 
the PSU is now my first and primary suspect if a device misbehaves in a 
way that cannot be clearly attributed to a software bug (which is by far 
the most common case nowadays, a clear sign of a generation change to me).


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