VT101 8085 CPU Fault

Tothwolf tothwolf at concentric.net
Sun Feb 22 19:43:23 CST 2015

On Sun, 22 Feb 2015, Robert Jarratt wrote:
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Tothwolf
>> Is C441 really 10uF 25V? That is what both the parts list and schematic 
>> show, but even as old as a VT101 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.

Ah. That explains it then. I was beginning to wonder if the printset had a 
typo and it was supposed to be a 250V rated part. I didn't see any 
indication in in the parts manifest that it was a NP part either, but 
perhaps I overlooked it. Are any of the others NP parts?

>> 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 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.

Well...I tend to look at replacing aluminum electrolytic capacitors as 
more of a maintenance or preventive maintenance thing vs a repair issue. 
If they are old and/or you have ripple, replace them and get that 
potential cause out of the way first and be done with it.

Were you able to find the correct axial mount parts for the 22uF, 4.7uF, 
and 75uF capacitors? I did a quick search through some of my past projects 
for compatible parts and managed to work up a short list of part numbers.

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