Bench Power Supply Teardown and Repair - BK Precision 1735

Bench Power Supply intermittent voltage LED display repair.

[@robinsattahip2376]

Good video, well edited, thank you.

[@Electronicle]

Thank you too!

[@konohh]

I always teach my students to clean of any flux residue after a repairjob. First: A bad solderjoint is much better visible without the residue on and all arround it. Secondly, if you spot a not cleaned up solder joint, you immidiatly know someone was in here before who couldn't care less.

[@mtrltoolman]

can you make a video about the output protections of bench power supply ? overload , short-circuit protection . thanks

[@saarike]

Excellent repair! Thank you.

[@jimball816]

great vid. very helpful

[@t1d100]

This is a nice three-part series on the BK Precision 1735 (non-A). Thank you for your effort to share the information. You mentioned that you would not be calibrating the voltage and amperage readings, because they were good enough. Mine are significantly off, so, if you could borrow the unit back from your friend, I would really appreciate a calibration video... Even if you just referred to the schematic and talked about how to do it, that would be helpful... Yes, I know about just using a multimeter, but it bothers me that the on-board meters are not correct/close. Thank you. Special thanks to Cody for the schematic!!! I had trouble finding this information, so I posted your links on an EEVBlog thread, on the subject. www.eevblog.com/forum/repair/bk-precision-1735-power-supply/

[@cjet8905]

Hey, I got a 1735A a while back which was stuck in CC mode, I couldnt find the schematics, so i emailed BK precision on there website and surprisingly they emailed me the schematics for it, mine is the "A" version and only difference i see is that it has a 4 digit v/I display instead of three, all the control circuitry is referenced to the positive output terminal and therfore control ground is riding on top of the main output voltage, the schematic is drawn quite poorly and the lack of a legend makes it very hard to decipher certain parts, i got it fixed and working great no problem but for the last year as time allows ive been trying to understand and redraw the schematic to try and figure out the issue im having, the constant current works, but i can not set it under 96mA, so when i short the output with the course and fine current controls completely turned down it pushes 96mA, instead of (ideally) 0-1mA while the controls are all the way low. it does have a +/- 13 and +/- 5v derived from a extra winding on the secondary(with the center tap referenced to the positive output as stated above) but the configurations of the amplifiers, throw me off, if the inverting and non inverting inputs on the cv amp were switched i would understan it all completely,and thats how i would have done it, but yeah, im lost. so.. could you test, if you still have the unit or your friend can check maybe? if you dont mind, what the minimum constant current can be set to with direct short on the output and the C/F current pots turned low as they can go please? i would so appreciate it and do anything, ive been trying a year to find someone with a similar unit but they information of them seems so scarce online, let me know if the schematics would be helpful to you and i will send them, the schematics are for many models like 1710 1715 1720 1730 and 1735 and theres a table showing any parts/values that differ between the models, thanks for your time.- Cody

[@Electronicle]

Sure. I already gave it back but I may be getting it back to fix. He connected it to a tesla coil kit and it got zapped and won't turn on. Not sure what happened considering the case is grounded but if I get it to fix and once it's fixed I'll check for you.

[@cjet8905]

@@Electronicle Okay great, also if you could please, take a clear picture of the front and back side of the board, im going to post a link to the schematic its in my google drive but its a shareable link thing, that is a easy fix, this schematic is for the 1735 model, i cant find it online anywhere but bk sent it to me, the output has some protection diodes that i bet picked up the transit, if he had the power supply powering the tesla kit then especially. D1 and D15 i think it is, right by the output cant miss them, they are also marked 3A, drive.google.com/open?id=1hPfrbPpVYsPpvWJyk9woXMio2SEN9Y1b thats the link to schematics in my google drive, goodluck and thank you very much for the help! -Cody

[@Electronicle]

Cody Turner hey I’m sorry I didn’t get back to you. I have the power supply here and am working on it so yes I can send u the pictures u requested if still needed. Sorry it’s been so long. Also any chance u could reup the schematic/service manual to ure google drive? Txs again

[@Electronicle]

here are a couple links to images of the board if you still need them: drive.google.com/open?id=1UTZf0lRMxIFOBGM3m2JKutHKFZUPHFZg drive.google.com/open?id=1jZkXvO7JUSCr2tn8F2lbwndxn316j08r

[@cjet8905]

@@Electronicle sure when I get home Monday and thanks again!

[@LiveMusicOntario]

All said and done, what's the opinion on having one of these supplies in 2019? I have been short of spare cash so I passed on getting one of these recently. Three of this exact BK 1735A were available for about Cdn $100 each at a surplus store but they're gone now. I dug out a transformer/regulator chips power supply kit I've left in the original bag for years, i.e. not a switched mode circuit, but by the time I add a large enough case, 2 displays and pots for current and voltage, binding posts, etc. I'll be up to about $100 invested anyway.

[@Electronicle]

Absolutely. I personally use an Eventek bench power supply and paid around $60 for it on Amazon. The DC output is a little noisy, however you get what you pay for and that noise can be cleaned up within the circuit if its a problem. So far I haven't had any problems with it, and its simple enough that if its breaks I can fix it or mod it myself.

[@davidprice2861]

Why would you want a switched mode, they are very noisy (electrically), fine if you just want to charge batteries etc..

[@dpyles9396]

Nice job - THANKS! but c'mon man...clean up your flux. it just isn't a difficult thing to do.

[@AlanLiefting]

why clean off the flux?

[@t1d100]

@ Cody - I have the 1735. At zero volts, my minimum CC setting reads 0.01A, on the on-board display. It reads 77.8uA on my Brymen BM869s DMM, being in series and without any load other than the multimeter's own burden. I am a novice, so if I did not put the multimeter in series correctly, please let me know how to do it and I will re-test for you. My guess is that your 96mA is just caused by a required minimum load for the circuit to operate and is acceptable. The data sheet may provide the exact answer: www.ti.com/lit/ds/symlink/lm723.pdf I am not great at reading these, yet, so, if you do look it up, please post the exact answer. And, if there are any other type of tests that you would like me to do, just ask. My lab is set up permanently, so it would be know problem.

[@cjet8905]

Hey, so the lm733 is only making a 5v reference which is then inverted by the opamp for a +- 5v ref. It has no control over the actual regulation of voltage or current in the power supply it's all controlled by the opamps. The 723 could have been replaced with a 5,v reference Chip but back in the day it was cheaper to use a 723 than get a reference ic accurate enough

[@buddysmith7462]

the fact your wearing gloves tells me your an amateur