Repairing vintage Quan-Tech Laboratories 173-B PSU - Page 1
Forum section: electronics / testgear

author: Muny
author_rank: Contributor
timestamp: March 05, 2015, 05:46:43 pm
content: Hi all,This will be my first post on the forum, which is surprising because I've been a long-time watcher of the EEVBlog. Anyways, I recently came across an issue with a vintage power supply I had. Due to my inexperience and naivety, a short occurred on the main board of this PSU (A Quan-Tech Laboratories model 173-B) and a resistor released its magic smoke. Now, instead of going through its normal range of 0-30v, the power supply ramps much faster to 50v throughout the rotation of the dial. The color code was unreadable (the resistor had blown apart), so I tried substituting a 1K resistor, however the voltage output does not follow the range it did prior to the mishap; which makes me ponder the possibility of other components on the board being bad. I was unable to find any sort of manual or schematics for it online. Being an electronics noob, any help in diagnosing and fixing any issues this supply may have will be very much appreciated. Here are some pictures of the supply:(^^ those floating leads are where the resistor was)(spoilers don't seem to be a thing, sorry!)If any more pictures/clarifications are needed, let me know. Thanks

author: Fungus
author_rank: Super Contributor
timestamp: March 05, 2015, 06:30:09 pm
content: Quote from: Muny on March 05, 2015, 05:46:43 pmDid you try any other resistor values...? Maybe 1k isn't the right one.

author: Muny
author_rank: Contributor
timestamp: March 05, 2015, 06:36:38 pm
content: Quote from: Fungus on March 05, 2015, 06:30:09 pmWhoops, sorry. I forgot to mention that I tried using a 100K pot, and the lower the value the higher the voltage. However, when the pot was at its lowest value (0 ohms), the output is still 3V lower than it should be. I also tried adjusting the trim pots (whose functions are labeled on the underside of the board), and they had no desirable effect on the voltage output.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 05, 2015, 07:45:31 pm
content: Boy, that's one crusty old power supply. Germanium transistors and cardboard-sleeved electrolytics. Is that a date code of 1-61 on the main capacitor? Wow!Do you have a DMM? Check the transistors that are clustered in the corner where the resistor was. If a resistor exploded, one or more of those transistors is likely gone as well. By the way, did the resistor actually explode, or burn and shatter? Did you look at the pieces to see if you can seeanycolor spots? Any clues as to the original value and physical size would be helpful.It could be a trick of the light, but on the copper side of the board there's at least one joint that could be bad. It's directly above the 'MAX I' label. Any time you touch a unit this old, things can break. Check everything carefully.Other than that, the only way to reverse engineer something like this is to draw out the schematic so you can see what you're doing. By the way, if you get it repaired, you should replace those cardboard-sleeved electrolytics with new ones and give those pots a really good cleaning with contact cleaner!Ed

author: Muny
author_rank: Contributor
timestamp: March 05, 2015, 08:39:01 pm
content: Quote from: edpalmer42 on March 05, 2015, 07:45:31 pmHi Ed,Thanks for the reply.It's more likely that the resistor burned and shattered, as there was a heap of smoke released and when I carefully opened the unit, pieces of the resistor fell out. From the remaining pieces, I can see that it is the same size as the surrounding resistors.Sorry for the poor quality of these pictures, I don't have access to an inspection camera at the moment.Here's one piece. Orange and white.Another. This one is orange and yellow.I'm not quite sure where these pieces were in the original resistor.In regards to that dubious joint, that is a trick of the light. It's solidly connected to the board.The two transistors near the resistor tested bad in-circuit. Should I remove them and test them out of circuit to be sure? I don't want to risk damaging the circuit.

author: Vgkid
author_rank: Super Contributor
timestamp: March 05, 2015, 09:29:16 pm
content: Your pics aren't working.

author: Muny
author_rank: Contributor
timestamp: March 05, 2015, 09:40:27 pm
content: Quote from: Vgkid on March 05, 2015, 09:29:16 pmI don't know what the issue is, they work for me and Ed.Here's an album on Imgur:http://imgur.com/a/2PVWJ

author: xrunner
author_rank: Super Contributor
timestamp: March 05, 2015, 10:58:07 pm
content: Hi Muny,Basically what the others said, check the active components like transistors and diodes. If you can't draw out a schematic then you would want to take them out of the circuit so you aren't testing them with sneak paths that would confuse the readings. All those old caps are probably leaky, so even if you fix the issue you might want to order some new ones.I hope it doesn't cost you too muchMunyMoney.

author: edpalmer42
author_rank: Super Contributor
timestamp: March 05, 2015, 11:08:20 pm
content: Quote from: Muny on March 05, 2015, 08:39:01 pmOrange at the end of the resistor means that's the first digit so you've got 39 which is a standard 10% value. That's consistent with the other resistors. But, I'm puzzled by the orange and yellow piece. The only way that a resistor is going to fail catastrophically like that is with a massive overload. The only way to do that in a small power supply like this is if the resistor is a low value. It would have to be either 39 or maybe 390 ohms. So the other color bands should be black and silver or brown and silver. I guess it could also be 3.9 ohms with missing color bands of gold and gold. Are you sure about those colors?What do you mean by 'tested bad'. Remember that these could be PNP germanium transistors. A diode test will read 0V3 or even less. If they test as anything other than shorted or open you'll have to remove them and test externally, but hold off on that for now.Can you trace out enough of the circuit to show the blown resistor and the components around it - particularly those two transistors?By the way, did you check the pots? If one of them blew open, all kinds of bad things could happen!Ed

author: Muny
author_rank: Contributor
timestamp: March 05, 2015, 11:48:19 pm
content: Quote from: edpalmer42 on March 05, 2015, 11:08:20 pmAfter taking another look at the pieces, it looks like that smaller piece could be yellow/white and red. The reddish-orange color doesn't match the orange on the other piece, and it doesn't match the red of other resistors on the board. I suspect this is due to the discoloration of the paint from heat.Neither of the transistors' lead combinations tests below 0v5, and none of them are shorted or open.What would I be looking for in the pots?I'll attempt to trace the circuit around the resistor and two transistors.Quote from: xrunner on March 05, 2015, 10:58:07 pmHaha, that's not the first time I've heard that joke.Those M's on the capacitor values are for micro, right? I've heard that before they started using ?, they would put MFD for microfarads. That would make sense anyways, as a 1F@50v cap of this vintage would probably be much larger. Most of the diodes test 0V5-0V7 in both polarities, assumedly due to the surrounding components.

author: xrunner
author_rank: Super Contributor
timestamp: March 06, 2015, 12:03:03 am
content: Quote from: Muny on March 05, 2015, 11:48:19 pmYea I know, but I couldn't resist.QuoteThat's right, like they used to say kilocycles and now we say kilohertz.QuoteYep, hard to tell what you are reading without the road map.How did you come to acquire that vintage supply, if I may ask?

author: Muny
author_rank: Contributor
timestamp: March 06, 2015, 12:18:51 am
content: Quote from: xrunner on March 06, 2015, 12:03:03 amNo fun stories to tell, unfortunately. I was scanning through a garage sale of an elder woman whose husband (presumably the one who used it) recently passed. At a measly $10, I couldn't resist purchasing it. I was surprised that it was workingandsomewhat in spec, but that was before the incident...

author: edpalmer42
author_rank: Super Contributor
timestamp: March 06, 2015, 02:51:36 am
content: Quote from: Muny on March 05, 2015, 11:48:19 pmSo then we could be looking at a color code of orange white red (3900 ohms). Nope, still doesn't work. 3900 ohms across the main capacitor (~40V) would only be 0.4 watts. Even if it was a 1/4 watt resistor, it wouldn't shatter - just cook.The value of the pot is stamped on it. Make sure that the end to end resistance is close to that value and that the resistance from the wiper to each end sums to that value. Since resistance measurement is done at a low voltage, it's unlikely that other components will affect the measurement. A capacitor in the area might show a slowly rising value as the capacitor charges. Great precision isn't necessary here. Just ballpark.Ed