VT101 8085 CPU Fault

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Tothwolf
> Sent: 22 February 2015 17:14
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: RE: VT101 8085 CPU Fault
> 
> On Sat, 21 Feb 2015, Robert Jarratt wrote:
> 
> > I have been desoldering and checking all the electrolytics. I have set
> > each one up on my bench PSU with the current limiter set, and taking
> > them up to their rated voltage, or as near as I could because some are
> > rated 50V and my bench PSU only reaches 30V. None of them seemed to
> > show any leakage, but my bench PSU will not display currents below
> > 10mA, I am not sure if that is enough to show leakage that matters. I
> > also tested their capacitance values, a few are well above nominal
> > value, but within the tolerance shown on the printset. The worst one
> > is +41% when the range is +50-10%.
> >
> > I started checking some of the tantalum capacitors. They all looked
> > fine, but I don't know what a sensible ESR value is for these. One of
> > them (C315) is rated 1uF 35V and has an ESR of 3.4. Is that high for a
> > tantalum?
> >
> > One respondent asked for a picture, the printset is here,
> > http://manx.classiccmp.org/details.php/1,5422 and the relevant drawing
> > is on page 58.
> 
> Is C441 really 10uF 25V? That is what both the parts list and schematic
show,
> but even as old as a VT100 is, I wouldn't think a standard 10uF 25V part
would
> be in an 18mm diameter can.


Yes it is. I have it right in front of me. It is a non-polarised capacitor
that looks like an electrolytic.


> 
> I've had this discussion with several people recently, but why bother even
> attempting to reform the capacitors on this particular board? If you are
going
> to desolder them anyway, why not just replace them and be done with it?
From
> the high resolution photo, the original axial mount parts at C439 and C437
have
> already been replaced and almost all of these parts are cheap and very
easy to
> obtain. The only exception I can see is C437, which is a 75uF 6V part.
Even with
> that one, you can still get 75uF axial mount parts from Vishay in 25V and
50V
> which would probably fit the pad layout just fine.
> 
> I dunno...maybe I just have a totally different way of rebuilding older
gear. I'd
> rather replace any 20-30 year old aluminum electrolytics wholesale with
known
> good modern parts (which as long as you stay away from the knock-offs and
> counterfeits on eBay, are likely to far outlast the originals) and then
move on to
> testing and troubleshooting other stuff and not have to later return to
> troubleshooting a power supply or something else because of intermittent
> issues caused by old aluminum electrolytics. This is definitely the way
things
> are done in the arcade and coin-op world (no point in troubleshooting
faulty
> logic chips until you clean up any power supply ripple), and also the way
things
> are usually done with vintage TVs and radios (vacuum tubes/valves), but
I've
> seen pushback from some people in the vintage computing community to
> wholesale replacement of aluminum electrolytics which are long past their
life
> expectancy and I just don't get it.
> 
> In the vintage audio communities, there are of course a handful of
"purists"
> who would rather have 40+ year old leaky (as in voltage) dried out paper
caps
> in there audio gear than modern poly film parts, but why the reluctance to
> changing out aluminum electrolytics in things like DC power supplies? It's
not
> like these parts wouldn't have been replaced already had a vintage
computer
> still been in a production environment.



I suppose I could do that, but I do like to try to find the actual problem
if I possibly can, and keep as much original as possible, and it helps me to
learn about the circuits too. Perhaps I should just replace and be done with
it, which saves me storing up trouble for the future. I have the parts ready
but just want to see if I can find the problem first.

Thanks

Rob