VT101 8085 CPU Fault

[js at cimmeri.com]

On 2/21/2015 1:33 PM, 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.

Then you need a better bench PSU or a 
dual supply that allows you to gang them 
in series (like mine does, 2x30 -> 60V).


> None of them seemed to show any 
> leakage, but my bench PSU will not 
> display currents below 10mA

That's not good enough either.  10Ma 
would be far too much leakage.


> 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%.

Why do you consider +41% to be "worst"?  
Sounds better to me.


> 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.

That was me.  Do you have a photo of the 
actual board you're working on?

As far as the non-electrolytics, mainly 
start by just making sure none of them 
are shorted or open.

- J.