Transistor replacement, Power Designs 2005? - Page 1
Forum section: electronics / repair

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: March 01, 2018, 06:32:15 am
content: A friend of mine gave me a Power Designs 2005 a few weeks ago for my birthday and it was in excellent condition (the panel meter had a little crack thats about it) and it was only about +/-2mV out of cal depending on the range setting. I tested it on a 100uV scope and the AC offset was 100uV as stated in the manual, but its offset would jump to 400uV every few seconds and then jump back, and the 10v set lamp was acting flaky. These were all easy enough problems to fix.A few days ago I was trickle charging a batch of niCad cells and all the sudden the supply just went out, maybe it had something to do with hitting the current set button (which I did not know shorted the output) to many times with the battery plugged in to the psu. Right now I turn it on and the supply and setting the voltage does nothing but turning the current set knob quickly set the voltage to ~35v (this is a 20v supply), and as soon as I apply even a small load to the DC output jacks the voltage goes back down to 0.I have checked all the caps with a meser-100 esr meter and replaced any that were suspect and changed any that needed changing. Ive checked all the voltage points in the trouble shooting schematic and most are right, including the oven and reference voltages, but the transistors on the amp board all have close to the correct base to emitter voltage but are a few volts off on the collector to emitter voltages. I figured I would just start with the main pass transistor and work from there, to my surprise all the transistors in this thing use in house numbers or something, I cant find any datasheet on them at all, and I cant find any equivalent on sites like alltransistors etc... here is the readings I got pulling them and throwing them on a cheap AVR mega328 tester,q3/q5/q9 are in oven and probably not relevant, at least not atmq1 RC1700 NPN/hFE 22q2 ms1028A PNP/forgot to write down hFEq4 ms1028A PNP/hFE 84q6 ms2270u NPN/hFE 74q7 ms2270u NPN/hFE 100q8 ms2270u NPN/forgot to write hFEq10 ms1028a PNP/hFE 73* From what I can tell my supply was made in 65, all ms1028a transistors were actually replaced with GI-696PD6612, which I also can find no data onThe simplified schematic for my supply ishere, and the full schematic with troubleshooting voltages ishere. I know there are a few Power Designs buffs on the forums and one of them is selling original working transistors etc. I don't want to go that way though, I want to use modern parts that aren't next to impossible to source while replacing things inside (Ive even thought about putting a modern reference in the oven), For now I just want to get the thing working so can I just use any jellybean small signal NPN/PNPs as replacements and a 2n3055 as a replacement for the RC1700? If so can someone recommend ultra quiet low drift equivalents (form factor isn't to important here ill make them physically fit)? If not does anyone have the specs on these transistors? I understand wanting original parts when it comes to cosmetics, but not for components, specs have come a long way on actives and passives in 55 years.