VT101 8085 CPU Fault

Robert Jarratt robert.jarratt at ntlworld.com
Sat Feb 21 12:33:21 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Matt
> Sent: 17 February 2015 01:31
> To: cctalk at classiccmp.org
> Subject: Re: VT101 8085 CPU Fault
> On 16/02/2015 20:38, Robert Jarratt wrote:
> > What other tests would you suggest? I do have a capacitance meter to
> > check values. I would have desolder them of course. Would you test
> > just the electrolytics or all of them? I don't have an insulation
> > tester though. Any other tests?
> >
> > Regards
> >
> > Rob
> >
> Check the leakage current using a bench supply (same setup as reforming)
> - particularly C321. Leakage here may be causing a DC offset to be applied
> the vertical yoke. If you have no luck here then have a look at the
> for the Elston/Digital monitor board in MP00633 or similar. Expected
> waveforms are given for some points in the circuit.
> Using a scope you may be able to work out where the wobble is coming from.
> Matt

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.



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