Source for replacement caps in H744 regulators

[Peter Coghlan]

>
>> -----Original Message-----
>> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Christian Corti
>> via cctalk
>> Sent: 06 January 2022 10:02
>> To: General Discussion: On-Topic and Off-Topic Posts
>> <cctalk at classiccmp.org>
>> Subject: Re: Source for replacement caps in H744 regulators
>> 
>> On Thu, 6 Jan 2022, rob at jarratt.me.uk wrote:
>> > If I can't find 10V rated ones, then, generally up to what sort of
>> > voltage rating can I go? Of course, physical size will be a factor,
>> > but electrically can it affect operation of the regulator if the rated
> voltage is
>> too high?
>> 
>> If you need to ask this, are you sure you want to do electronics repair?
>
> I am gradually learning.
>

I'm slowly picking this sort of stuff up as I go along too.

I think that asking questions like this is the right thing to do when we
are not sure about something and is not at all an indication that someone
is somehow not suited to the task.  Quite the reverse in fact.

If we don't do this sort of work ourselves, it's not going to be easy
to get someone else to do it and make sure it is done well for a
reasonable price.   Any contemporary electronics repair outfit, assuming
one can be found, would probably recommend dumping the whole thing and
replacing it with a "modern" power supply.

>
>> And what makes you think that you need to replace these caps at all?
>

I appreciate that there is lots of bogus advice out there which suggests
that all capacitors need to be replaced before even starting to investigate
the cause of problems but I think Rob has demonstrated that he is trying
to figure out what is going on rather than just blindly replacing stuff.

>
> One of the H744 regulators whines and I have been told it could be the ESR
> on these caps. I have measured the ESR on these particular ones (out of
> circuit) and it seems higher than the expected values printed on the meter
> and also the ESR is not stable, it fluctuates randomly. This suggests the
> cap is not in great condition.
>

I guess from that the large capacitance of these units, they are probably
filtering 50Hz or 100Hz ripple and high ESR is probably going to lead to
higher than normal levels of ripple on the output which could possibly
cause some inductive component somewhere to buzz or whine.

I'm not sure if it is easy to measure the ripple on the output or if
it is specified anywhere how much ripple can be tolerated before logic
errors start to become a problem.

Maybe it would be good to replace the capacitors in just the unit which
is whining to begin with and see if it makes a difference?

To address the original question, I think I read somewhere that electrolytic
capacitors that are used for voltages a lot lower than their rating may
fail to maintain their dielectric formation which may in turn result in
increased leakage.

Here's a thought.  Could the whining be coming from an inductive filter
before the capactors because of excessive leakage through the capacitors
rather than because of high ESR? Would they benefit from re-forming?

Regards,
Peter Coghlan.

>
>> 
>> Christian
>