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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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9th Apr 2018, 2:18 pm | #1 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
Posts: 1,483
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Repairing a Time electronics 2003S & 202 PSU
This was unit that had been sitting around on my pile for a while so I decided to take a look. The mains lead had been cut off and when opening it up I found the old NiCad’s had leaked badly and were US.
Starting with the PSU I replaced the mains lead along with a new plug and 3A fuse, dismantled and cleaned off the boards and had a rummage through my stock of batteries and found 11 1000mA NiCad’s but they had no tags, so I carefully cleaned up the ends fluxed and soldered short wires working quickly with a powerful hot iron so as not to warm the batteries up too much and used heat-shrink to form them into a battery pack with flying leads with a Molex connector on the end. Also I replaced the main cap C1 1000uf @ 35V. I reassembled the unit set VR1 to give 16.3V and measured the charging current which was around 55mA which seemed abut right, and I was getting the correct voltage at the 2 terminals of around 14.5V. On to the main board calibration. The module zeroed correctly and I was able to set the 1V & 10V settings but when it came to 100mV and 10mV they were both out and the pre-sets would not bring them back in range. The ranges are set by precision 0.1% resistors which are in a ratio for each range. R14, R18, R16. On checking R16 should be 619k but had risen to 632K and R18 has to be chosen to give total close to 629K . So I ordered some 619K, 9K, 10K, 0.1% resistors from RS. Once they had arrived I stripped out the existing resistors R18 being made up of 8.2K and another which I could not read in parallel and I replaced R16 619K and a R18 10K, which turned out just a little high so I added another 1M resistor in parallel which brought them so I could adjust VR5 500R to set 100mV. Now on to 10mV, which is set by VR3 again I did not have the range to adjust it and it was apparent that the combination of R8,R9 and VR3 need to go lower to bring it in range so I tried a spare 619K to be select on test (SOT) for R7 which had not been fitted, this also allowed VR3 to adjust. Whilst I had the iron out I also replaced the Cap C1 220uf @10V. I then put the units back together and let it run for 1hour to stabilise and went through the full calibration procedure and all seems well. I then left it on charge for 24hours to give full charge and tried it on batteries and mains and all the voltages are in specification and stable. A few observations the PSU seems to be unstable without the batteries in place with the voltage going high at the terminals. There are 2 sets of manuals going around July 80 with original hand drawn diagrams from 1975 and a redrawn one with what looks like CAD drawing of the same dates. However, the later drawing has slightly less info about the ratios and has reversed the values of VR3 and VR5. The power supply unit has been used in a few items like the 202 Null meter so I also used this opportunity to repair the PSU in that, which was missing its batteries and had D5 lifted to keep the voltage to 18V. So again I replaced C1 1000uf 35V and refitted a new In4005 diode for D5. I then set about rebuilding the battery pack but had run out of replacement NiCad’s so I bought 12 NiMH 800mA AAA whilst much higher values are available for only a little more using them would have extended the charge time to many days. Again I used 11 batteries building them up the same as above into a pack and testing and reassembling. On test the 202 null meter performed as before and seems a little more stable than as mains only which is too be expected. |
11th Apr 2018, 12:01 am | #2 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,748
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Re: Repairing a Time electronics 2003S & 202 PSU
Nice work, Chris. I enjoy reading about test gear restorations. Many thanks.
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Phil Optimist [n]: One who is not in possession of the full facts |
11th Apr 2018, 4:24 pm | #3 |
Nonode
Join Date: Mar 2008
Location: Virginia Water, Surrey, UK.
Posts: 2,877
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Re: Repairing a Time electronics 2003S & 202 PSU
Good job Chris, those are very useful portable cal devices.
When replacing NiCd battery packs (with NiMh usually works), I find that people like Strikalite will do custom battery packs exactly to your spec all welded, shrink wrapped and with correct lead outs and plug for not much more than the cost of the batteries. Worth a try if you are stuck.
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Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk |
13th Apr 2018, 12:00 pm | #4 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
Posts: 1,483
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Re: Repairing a Time electronics 2003S & 202 PSU
Phil, thanks for your kind words, Jeremy that is useful info I will keep it in mind for the next battery rebuild.
Chris |