Source for replacement caps in H744 regulators

[Rob Jarratt]

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Peter Coghlan
> via cctalk
> Sent: 06 January 2022 12:16
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: RE: Source for replacement caps in H744 regulators
> 
> 
> >
> >> -----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?

I will try this by using the caps from other H744s that don't whine.

> 
> 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?

I did re-form the capacitors first. They didn't seem to need much as I ran
them at 10V with zero leakage current (i.e. not measurable). Maybe I need to
run the reforming process a bit longer anyway?

> 
> Regards,
> Peter Coghlan.
> 
> >
> >>
> >> Christian
> >