SOLVED: help on repairing a faulty 80s style stabilized DC power supply

[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. 

[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 H_FE >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

[BravoV]

I love to help people, but for a such tiny and simplistic PSU, asking for help in the EEV sounds crazy to me. 

Love to help people ? If you don't have anything nice to say or "contribute" to this thread (which I doubt about it), you should not insult and provoke people like that, that is a legitimate beginner question and a legitimate place to ask.

Tiny & simplistic ? With your track record around here, I doubt you even have any idea what he is talking about, or understand how that circuit works at all.

Btw, who the fuck do you think you are that you can decide what can be posted here or not.


Axel,

Apologize for this brief out of topic discussion, just don't let this kind of junk post demotivated you for asking, I'm sure there are lots of nice forum members here that are known to be very helpful & kind such like above gentlemen. That dude is well known asshole around here for a long time, just ignore it.

[Uncle Vernon]

I love to help people, but for a such tiny and simplistic PSU, asking for help in the EEV sounds crazy to me. 

Love to help people ? If you don't have anything nice to say or "contribute" to this thread (which I doubt about it), you should not insult and provoke people like that, that is a legitimate beginner question and a legitimate place to ask.

Tiny & simplistic ? With your track record around here, I doubt you even have any idea what he is talking about, or understand how that circuit works at all.

Btw, who the fuck do you think you are that you can decide what can be posted here or not.

The self appointed welcoming committee. Great way to treat a newbie to the forum.
 It's been suggested more than once if Captain Athenia wants to run everything his own way he could start his own website. But he's tried that already so he lurks about here where there are actual people to annoy with the idle bragging and postal updates.

Axel,

Apologize for this brief out of topic discussion, just don't let this kind of junk post demotivated you for asking, I'm sure there are lots of nice forum members here that are known to be very helpful & kind such like above gentlemen. That dude is well known asshole around here for a long time, just ignore it.

Agree wholeheartedly. Welcome to the forum Axel, apologies for the idiot's response to you.
Your questions were entirely reasonable and were located in the beginners section, all of which is entirely reasonable. I'm sure you will see many people here will take the time to help with reasonably posted questions like yours above.

[EEVblog]

Good to hear you got it fixed.
Awesome work on the reverse engineering and generating the schematic and notes, that's the way you learn a lot!
Now that it's fixed, and you've learned how it works, but not regulating that well, how about simply replacing the main pass transistor with an LM317?

Dave.

[Axel1973]

Hello all Helpers!
Hello Dave,

I wanted to give a little update my tiny little project.

THE PROBLEM IS SOLVED!

With the Tips you gave me here i was able to locate the DEAD T1 and replaced it with an old desoldered BC338 i had lying around for decades of no use. Yeah sorry, i didnt had a BC337 And the internet said a BC338 would replace a BC337, so i tried and it worked. The more detailed process can be read on my project page at www.awerner.homeip.net.


THANKS FOR ANY HELP/ADVICE!
THANKS TO ANY MAKER WHO FREELY SHARES KNOWLEDGE, VIDEOS, TUTORIALS AND OTHER FREE LEARNING MATERIAL ON THE INTERNET!
Information wants to be free! Keep information free for everyone!

best regards
Axel

@DAVE: gotcha! MANY little experiments and basic learning circuits are coming up on me. Ive got to re-learn so much after 20 years doing nuthin in electronics. Covering basic 555 and 47xx oscilators, buildin basic ESR meter to test electrolyt caps, integrated voltage regulators to "improve" the old CONRAD PSU with a much better highly linear high current regulator, basic op-amp circuits to adapt voltage ranges of sensors or to sense current flows through shunt resistors, variable electronic DC dummy load to test small PSUs or regulator circuits, also very interesting to me are simple switch mode PS like step-up/step-down converters, fly-back transformers etc.

And somewhere in between ill have to repair my old portable Standard C528 HAM Radio which seems to have died in the last 12 months or so (problem: permanently transmitting). I already googled  the problem. It seems its one of many common problems todays with the C528. Most often the reason are the SMD electrolyte caps and tantals that are dying slowly over the last 15yrs or so. That will be a dirty job coming up, replacing all the SMD caps in there. But just buying a new radio would not be that much fun and interesting

73 et 55 de Axel (DG2GWE)

[jimmc]

Congratulations Axel, I'm pleased to see you've fixed it and impressed by your write-up.

To my mind a desire to understand and repair a basic piece of equipment is to be encouraged.
Never be afraid to ask.

Jim