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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE! |
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2nd Feb 2016, 7:48 am | #1 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,340
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Bearcat scanner back in the land of the living
I've had an old UBC175XL scanner in a box for years. Don't remember where it came from, only that it didn't work.
In one of those rare "I should clean out the junk moments", I decided to have a look at it to see if it was repairable. Symptons were: as deaf as a post and the LCD display was very dim. The usual voltage checks didn't show anything obviously out of the ordinary, but with no service information available, that was not conclusive. As I was putting it back together, I happened to touch something that was way hotter than it should of been, namely an electrolytic cap. Pulling the cap (100uF 16v) showed it to be almost shorted, but replacing it didn't make any difference, other than nothing was getting hot now. Next step was to draw up by hand the power supply/voltage rails part of the set. This proved to be the light bulb moment, part of the power supply is a voltage doubler that uses an LM386 (in an oscillator configuration) and a whole series of 4.7uF caps and 1N4148 diodes. Measuring the voltage out of this showed the voltage taking a very long time to get above the nominal 12v input voltage. Replacing all the caps in the doubler proved to be the problem. Almost instant 22V out of the doubler. So, to be on the safe side, all the electrolytic caps in the set were replaced. All of them were the old grey style electrolytics and 90% of them were 10Uf or less. It is now back to full operation. As to why the voltage doubler was having such an effect on the rest of the set, I'm not sure, as, as far as I can tell, this voltage is only used for the Varactor tuning. Terry VK5TM |
9th Feb 2016, 6:17 pm | #2 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,748
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Re: Bearcat scanner back in the land of the living
Well done, Terry! It's particularly rewarding when you manage to get something working again in the absence of any service data. Knowing the sort of components to suspect, and their potential failure modes, is one of the most vital skills and one that is largely learned through experience.
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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 9th Feb 2016 at 6:18 pm. Reason: Duplication |