Radio Shack 4P Video Repair

I own a vintage Radio Shack Model 4P computer - the "P" stands for portable. This little Z80 system is about the size of a sewing machine when it is in its case. With 128k of RAM (only 64k of which is available for programs, the rest is ramdisk) this system plays a key role in my wife's business, Nine Lives. With the proper software, it makes a perfect terminal to the UNIX system which runs the store. Amazingly Radio Shack once sold a terminal emulator that is 100% compatible with UNIX, and that is the software that makes this system useful.

The 4P sees daily use in the store. Not bad for a computer manufactured in 1983. One thing has always bothered me about this computer. Ever since it was new, the video has had problems. The monitor is built-in, so simply replacing the video part of the computer is not possible. The symptoms of the problem were sudden "blooming" of the screen, with the retrace lines becoming very bright, and jitter on the edges of the screen. These symptoms would continue for about 5 seconds, then go away for hours or even days. Then in early 1998 the worst happened. The screen began "piling up" on the left. Characters would simply be superimposed on top of each other.

I am somewhat familiar with electronics, having worked as a computer repair center tech and manager at Radio Shack during the time this computer was manufactured. I opened the case up and went to work.

*** WARNING: Working around CRT's is EXTREMELY DANGEROUS! DO NOT try this unless you know what you are doing! ***

Thinking the problems were caused by faulty capacitors (a theory reinforced by lots of research on capacitors and video on the web), I replaced all but one of the capacitors on the video board. I just could not reach the last one without severe disassembly, so I did all the rest and closed everything back up. I was delighted to have the "piled characters" problem disappear. Sadly, the "blooming" continued.

I thought all was well for another 15 years, until July 1998, about four months after I had done the repair. The video collapsed to a vertical line exactly in the middle of the screen. I immediately suspected a capacitor in the horizontal sync circuit. Back into the machine I went. When I rebuilt the video board, I had used the best capacitors I could get. I used 105c instead of 85c (standard temperature) electrolytic capacitors. I replaced small electrolytic capacitors with solid tantalum, because they are more stable and their lifetime is almost unlimited. Unfortunately, I made one mistake. In the horizontal output stage, there is a 10mfd 25v bipolar capacitor. I replaced it with a similarly rated capacitor from Radio Shack. I was a little worried because the replacement was much smaller than the original. Well, this little capacitor was "cooked." The body of the capacitor was covered in black vinyl, and the vinyl had split at both ends. The original application of the capacitor was for speaker crossover use, at low frequencies (under 10khz). The higher frequencies of the horizontal sweep circuit had simply broken the capacitor.

I realized this was my chance to replace the last original electrolytic capacitor as well, a 22mfd 100v "can" right in the middle of the circuit board. When I unsoldered it, there was a telltale crystalline deposit on the board under the capacitor, most likely some electrolyte that had leaked out of the capacitor over time. Could this finally be the bad capacitor that was causing the "blooms?" I soldered together three 35v 22mfd tantalum capacitors (in series) as a replacement, and went shopping for a "real" nonpolarized capacitor. Sure enough, at HSC Electronic Supply I found a 10mfd, 50v nonpolarized electrolytic capacitor marked "high frequency." It's usually OK if a capacitor is rated a little over the voltage you are planning on using it for, so the higher voltage rating was not a problem. I finished replacing both capacitors and closed everything back up. I won't keep you in suspense any longer, the computer came up with the best video it has ever had. With the higher quality capacitors in the video board, the system is "better-than-new" and ready for another 15 years of service.

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