BK Precision 1735a ps, I'm needing serious help with this one! - Page 1
Forum section: electronics / beginners

author: Cody Turner OKC
author_rank: Contributor
timestamp: October 26, 2016, 08:47:37 pm
content: So I bought this power supply (bk 1735a) a week ago, used but virtually not a scratch on it! Well I get home and plug it in and the digital panel ammeter is showing like 1.5a and the CC LeD is on and the volt meter showed 00.00, i could adjust the amps from .090 to 3.6amps but the voltage woulnt adjust for anything so, I started messing around with it looking it up etc and after about 5 hours I realised something really was wrong and It wasn't just me operating it incorrectly, so I opened it up and cleaned it all up and looked around for anything noticeable and I couldn't see anything so far, I used my meter in continuity thinking maybe the output is shorted and it's trying to set to set to hehe current limit which is why I can't do anything with voltage answer sure enough the positive and negative post were a dead short, so I followed the wires from the output to connector P5a (you will see on schematic) and unplugged te connector and to hehe be short on the output was gone so now I have tried to find the short on the big main board starting from that connector obviously but input am so stuck and confused and not even sure if it's supposed to be this way and imy wrong.. but after a day of searching the Internet every image on every site and place I could possibly find and not finding even close to the schematic for the 1735a I emailed bk precision and asked for it thought why not try lol an about 5 minitues after to my surprise they responded back with the full schematic and component values etc but even with this I am still lost.. sorry for my bad typing I'm using my phone and can't hardly see it! Thanks s so much for the feedback guys anything any help at all would be appreciated so much! I'm going to attach pictures of the stuff! Thanks ks again!!

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2016, 09:13:12 pm
content: First thing I would check would be diode D1.. If somebody tried charging car battery or similar, and reversed + an - , D1 would protect from reverse voltage.. and pull some serious current..So if it short circuited, you would see current but no voltage... My 2c

author: Cody Turner OKC
author_rank: Contributor
timestamp: October 26, 2016, 09:19:44 pm
content: Actually right by the current shunt and where the connector attaces to the board there are two diodes and inexpensive if them I got a beep on both sides of the diode either which way I thought it was maybe just both legs were attached to the plane of the short I am looking for or something, so then should I just pull the diode and check it with multimeter or can I do it while it's it's e board?

author: singapol
author_rank: Frequent Contributor
timestamp: October 26, 2016, 09:53:54 pm
content: It's best to understand how a DC power supply works:DC POWER SUPPLY HANDBOOKhttp://exodus.poly.edu/~kurt/manuals/manuals/HP%20Agilent/HP%2090B%20Application%20Note.pdfHow to test bipolar transistor:http://www.vetco.net/blog/?p=184I would zoom in on Q3 and Q4 which are the output to connector P5.Measure the test points (TPxx) if it makes sense to you. Check the associated components in the associatedpath of the output transistors. Sorry if this is not a systematic approach but you can get the idea.

author: 2N3055
author_rank: Super Contributor
timestamp: October 26, 2016, 10:45:01 pm
content: Quote from: Cody Turner OKC on October 26, 2016, 09:19:44 pmYou can lift only one side of the diode and check.. But i looked again and you should check D15 first... If shorted it would short the output... Lift one side and check..

author: Cody Turner OKC
author_rank: Contributor
timestamp: October 27, 2016, 01:20:06 am
content: Okay awesome thank you guys so so much I'm going to try all of that as soon as I get home I will have to try and get that main pcb out which is going to be a pain in the butt but I want to get this fixed asap so I can start experimenting way better I desperately want to learn it all so much I try and study to many different things at once and end up going nowhere with it lol I am not in school right now I am 23 and plan on going in a few years but right now I want to learn these things but man it is challenging! I really hope I can get this supply working!

author: singapol
author_rank: Frequent Contributor
timestamp: October 27, 2016, 04:44:15 am
content: Quote from: Cody Turner OKC on October 27, 2016, 01:20:06 amDon't be too quick to disassemble..test with DMM continuity ,diode mode and measure voltages. It will give you some hints where the fault can be,yes there are tons to learn but get the basics first. Learn what are the circuits commonly used in linear power supply. You can fix this , trust in yourself.http://www.talkingelectronics.com/projects/ThePowerSupply/Page80PowerSupplyP2.html

author: Cody Turner OKC
author_rank: Contributor
timestamp: October 28, 2016, 03:53:11 pm
content: Well after a bunch of testing researching and ideas I think I fixed ut! D15 was a direct short after all so I replaced it and it seems to be good to go now!! But I noticed that the course and fine pots are kind of terrible for 0 to 30 volts like I barely touch it and I move the knob most all the way! EVEN on current 0 to 3 amps it's not as bad but still not what I would like! Is is easy to replace the pots with 10 turn pots or will his mess things up or require that different footprint or anyting?? I really wish I could use a optical encoder to be honest I want to use one!

author: 2N3055
author_rank: Super Contributor
timestamp: October 28, 2016, 05:57:11 pm
content: Quote from: Cody Turner OKC on October 28, 2016, 03:53:11 pmGood news!I think your pots are dirty and/or broken.. Old carbon pots are prone to wearing out.. You can put in 10-turn pots but they can be expensive.. Good ordinary cermet linear pots will serve you well, unless you reeealllyy need such precision.. And going from 1 to 30 V is slooow.. But both is fine, you decide..Take care!