| Electronics > Repair |
| SOLVED: help on repairing a faulty 80s style stabilized DC power supply |
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| Axel1973:
UPDATE: Problem is SOLVED in the meanwhile. See www.awerner.homeip.net for the latest analysis and FIX. Hi there electronic Gurus! i lately dropped one of my old 80s style stabilized dc power supplies and it seems it started to work faulty now. the internal PCB and circuit seemed to me small and easy to give it a try on repairing instead of replacing. but since im out of electronics now for more than 20yrs i could need some advice from more experienced peoples. WHY DONT JUST REPLACE THE OLD DARN THING? Well, i want to come back into electronics some more and therefor i thought it would be a nice little challenge/training/re-learning for me to start by repairing this little faulty PS. So im asking for advice/tips/explanations that helps me understand what is going on in my old lil power supply and how i can track down the source of the problem. i yet reverse engineered the schematics and started some measurements. im at a point where i think i found something. but im not sure since im not very experienced in classic analog transistor circuits and voltage regulators. Therefor i put all Schematics, Measurements n stuff up on my webpage. You can look and download anything from here: www.awerner.homeip.net All Discussion can he held here on the EEV Forum or by email. Thanks for any Help and Assistance in my lil kickstart project. best regards Axel Werner |
| Kiriakos-GR:
13.8V/3A fixed from CONRAD Electronic, Germany.. I love to help people, but for a such tiny and simplistic PSU, asking for help in the EEV sounds crazy to me. ;D |
| jimmc:
Kiriakos, that's a bit harsh we've all got to start somewhere and it is posted in the beginners section. Diagram looks about right, but BD137 is not a TO3 device (javascript:openreq('http://www.datasheetcatalog.org/datasheet/philips/BD139-16.pdf') Check R2, a 1K resistor base to emitter of T2 would be more usual. (Google "darlington transistor') Simple answer, T1 has a collector emitter short The circuit is not overload protected and and drawing excess current from the output will cause T1, T2 or both to overheat and fail. When a transistor fails it is quite possible for a collector - emitter short to occur and yet both the base - emitter and collector diodes appear as normal. Remove the device again and check between collector and emitter, it should read open circuit in both directions, but I bet it won't be. As to a replacement for for T1, I would look for a medium power NPN device with Vceo >40v, Icmax >500mA and HFE >40 at 300mA. That should give a fair safety margin. This page (http://english.electronica-pt.com/db/cross-reference.php?ref=SF826) suggests BC337, BC635, 7 or 9 as replacements. (BC338 with a max Vceo of 25v is not a good idea.) Check pinout, it may not match the original. Jim |
| Rufus:
If you found a Motorola datasheet on the BD317 the first page is confusing but later above the graphs it states BD315 and BD317 are NPN. I go along with jimmc's analysis of T1 being dead. Also R2 looking a bit strange, but, not impossible. It might be to limit the current transient in T1 should the output be short circuited. If you are interested the circuit is just a crude zener diode voltage regulator feeding an emitter follower made from T1 and T2 as a darlington pair. That might give you some terms to search for. The diodes in series with the zener are probably intended compensate for temperature drift in T1 and T2 base emitter diodes, however, drift in the 13v zener is in the same direction and probably larger. |
| Axel1973:
jimmc + Rufus , THANKS A LOT for your Replys! Thats what i hoped to start/find here. A nice discussion/explaination about what i got here so me and others can learn something. THANKS! @jimmc : my version of T2 seems to be a TO3, a large 2 pin + case thingy. ill see if i can make photos of the whole thing later. about overload protection im sure you are right. as far i remember it once said that this PS is not capable to survive short circuits n overloading. it reads 13.8V 3A (5A) on the front. Indicating that it can handle up to 5A "peak currents" for a short amount of time. About T1, i think you are right too. i cant remember meassuring CE and EC conducting yet. will try that. R2 is labeled 1ohm. So i guess its supposed to be "kinda" a darlington transistor with some kind of protection base R. But ill go n test this R to be sure. @Rufus : yes. about the zeners i figgured that the only thing that keeps a stable voltage in this circuitry is the zener series. in fact, those realy seem to be 3 zeners. 2 with 1V breakthrough but used in FORWARD direction, 1 at 13V breakthrough V (or breakdown?). with all the tollerances resulting in a 15.3V stabilized level. My guess for that is, that they wanted to lift the 13V with the two other zeners (in forward direction) to "add" two silicon forward breakthrough voltages to it. Maybe to compensate the the following two "base-emiter" stages (also round about 2x Uf 0.7v )?! but have no clue why they used zeners instead of plain diodes in forward direction. price? temperature drift? idk. So thanks again guys for your participation! you already helped me and pushed me even further to hopefully solve this lil puzzle. more test results, photos n reports coming up soon. c ya l8r best regards Axel |
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