It can take a while for the internal memory backup batter to charge which can lead to the machine not booting. Sometimes the contrast pot can get dirty and after playing with it a while it will work better.
The really old (up to early 1970s) electrolytics caps can be put through a ‘reforming’ process to improve their performance after prolonged storage. More modern electrolytics, as are used on our old computers, are made differently though. It is an interesting subject. I found this page which compares the leakage on the two types, and they found that the more modern capacitors have only 10% the leakage (after the first 5 min) compared to the older type. Though if the old caps are properly reformed, they have similar performance to the newer type. http://www.electrojumble.org/reforming.htm Jeff Birt From: M100 <[email protected]> On Behalf Of [email protected] Sent: Tuesday, May 11, 2021 5:30 PM To: [email protected] Subject: Re: [M100] NEC PC-8201(A) Recapping Help I was reading with interest that problems you have been experiencing. I have a NEC PC-8201A that I have had since the early 80s. I used it extensively and wrote lots of programs on it. Unfortunately, none of them survived. I had retired the NEC about a decade or more ago and it lived in a closet when we lived in Houston. I have since retired from my job and moved to Illinois. The poor 8201A then spent almost a year in a non-climate controlled storage where it probably cooked a bit from the summer heat. When I got it from storage, I fired it up and it appeared to work. I then took the battery compartment out and replaced the corroded NiCads with NiMH. When I put the batteries back in, the display came to life but only for a second or so and then went out. This was how it was for a week or so. I still left it plugged into the AC adapter hoping it would get better. To my surprise, it did! Every now and then I would try it and turn it on. Usually there was no joy but once when it was on, I was playing with the contrast and noticed the display backlight was changing slightly. Suddenly it came to life, but then the contrast would flicker a bit then go out and stay out. When I would try it again a few days later the same result. The symptoms you were describing sounded very similar to what I was experiencing and I thought I would have to start replacing capacitors. I was in no hurry to start and a good thing too, because it seems the longer this has been left plugged in, the better the display behaves. In fact, right now it seems to be back to totally functional and has no longer been exhibiting any of the bad behavior I described earlier. My theory is that somehow the electrolytic capacitors were healing themselves. The thought was the oxide layers within the caps was being repaired by the steady application of working voltage. Embedded within a Wiki article it states: For antique radio equipment or for electrolytic capacitors built in the 1970s or earlier, "pre-conditioning" may be appropriate. For this purpose, the rated voltage is applied to the capacitor via a series resistance of approximately 1 kΩ for one hour. Applying a voltage via a safety resistor repairs the oxide layer by self-healing. Capacitors that fail leakage current requirements after preconditioning, may have experienced mechanical damage. I was not doing any careful application of rated voltage so I ‘m not sure if this is what got mine working again, but working it is and I’m happy. I hope you have good luck with yours. I have lots of plans for mine. :) Lloyd Johnson From: M100 <[email protected] <mailto:[email protected]> > On Behalf Of ExPLIT | Pawel Radomychelski Sent: Wednesday, May 5, 2021 4:45 PM To: [email protected] <mailto:[email protected]> Subject: Re: [M100] NEC PC-8201(A) Recapping Help Thanks guys. LCD unit is working fine on another NEC. So maybe some chips. Need to find a guru (who doesnt fear the word osciloscope) 🙂 here in Europe.... -- Kind regards / Mit freundlichen Grüßen ExPLIT IT Solutions Pawel Radomychelski -----Original Message----- From: Jeffrey Birt <[email protected] <mailto:jeffrey%20birt%20%[email protected]%3e> > Reply-To: [email protected] <mailto:[email protected]> To: [email protected] <mailto:[email protected]> Subject: Re: [M100] NEC PC-8201(A) Recapping Help Date: Wed, 5 May 2021 16:30:23 -0500 If I recall correctly that was keeping your computer from booting up. The display dimming out over time but the voltages being OK is an odd one. The contrast input to the LCD is on pin 4, labeled VR1. If that voltage does not lower as the display grows dim, then I might suspect the LCD unit itself. The LCD is built like the Model 100. The contrast voltage input is used to create v2, v3, v4, v5, v6 ON THE lcd. Those can be checked to see if they change as LCD dims. One of the HD 44103 driver chips might be going bad as well. Jeff Birt From: M100 <[email protected] <mailto:[email protected]> > On Behalf Of Robert J. Hutchins Sent: Wednesday, May 5, 2021 4:04 PM To: [email protected] <mailto:[email protected]> Subject: Re: [M100] NEC PC-8201(A) Recapping Help Jeff, Maybe it has the problem that you fixed on mine? The bad buffer chip? Robert From: M100 [mailto:[email protected]] On Behalf Of ExPLIT | Pawel Radomychelski Sent: Wednesday, May 05, 2021 1:28 PM To: [email protected] <mailto:[email protected]> Subject: Re: [M100] NEC PC-8201(A) Recapping Help Thanks for the Info, Jeff. No, i have no leaking caps on my PC-8201, but i think something is wrong with couple of my units. Sometimes, they switched, sometimes display became unreadable, sometimes contrast can't be changed. +5 and -5 are ok, so maybe some of the chips need to be replaced... -- Kind regards / Mit freundlichen Grüßen ExPLIT IT Solutions Pawel Radomychelski -----Original Message----- From: Jeffrey Birt <[email protected] <mailto:jeffrey%20birt%20%[email protected]%3e> > Reply-To: [email protected] <mailto:[email protected]> To: [email protected] <mailto:[email protected]> Subject: Re: [M100] NEC PC-8201(A) Recapping Help Date: Wed, 5 May 2021 14:38:42 -0500 I have only worked on a few PC-8201/A computers. I have not seen one that needed recapped. The TRS-80 Model 100 and Olivetti M10 are a different story, they all need recapped. Do your capacitors appear to be leaking or do you suspect they are faulty? Jeff Birt From: M100 <[email protected] <mailto:[email protected]> > On Behalf Of ExPLIT | Pawel Radomychelski Sent: Wednesday, May 5, 2021 7:06 AM To: [email protected] <mailto:[email protected]> Subject: [M100] NEC PC-8201(A) Recapping Help Hello Friends! Have someone already did a recapping job on the NEC PC-8201 or PC-8201A ? I am searching for a starting point, or simple table sheet which capacitors / voltage / capacity should be used ? Any help is welcome. Thanks -- Kind regards / Mit freundlichen Grüßen ExPLIT IT Solutions Pawel Radomychelski
