VT101 8085 CPU Fault

Tothwolf tothwolf at concentric.net
Sun Feb 22 11:14:09 CST 2015

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.

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.

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