VT101 8085 CPU Fault

[Robert Jarratt]

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of
> js at cimmeri.com
> Sent: 21 February 2015 18:43
> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic and Off-
> Topic Posts
> Subject: Re: VT101 8085 CPU Fault
> 
> 
> 
> 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).


Longer term I will have to get one, but for now that is all I have. I do
have a variac and a rectifier, so I could get it to 50V that way.


> 
> 
> > 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.

So what sort of value is reasonable for 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.


By "worst" I mean the one that was furthest from its nominal value.


> 
> 
> > 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?

I have posted a picture of it here: http://1drv.ms/1Lo1hWB


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