Tips for a relative newbie trying to repair a SMPS? - Page 1
Forum section: electronics / beginners

author: LightTangent
author_rank: Newbie
timestamp: March 10, 2022, 07:35:07 pm
content: As per title -I have an old Commodore SX-64 which was in working condition until a couple of years ago. Was using it to copy some disks from an SD2IEC SD-Card device to real floppies and at some point a large bang and blown fuse. Deadski! I believe using the drive is particularly heavy on the PSU. Anyway, would love to get it back into working condition.I did try briefly repairing it shortly afterwards, but never got it working, and so am looking for tips as to where to start?http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/sx-64/power_l.gifhttp://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/sx-64/power_r.gifThe PSU seems to have a "standard" ish topology - i.e. AC -> Bridge Rectifier -> Large Electrolytic -> Transformer through a 2SC2792 NPN power transistor. The base of the switching transistor is driven through another small transformer controller by an MB3759 driver IC (which I understand is compatible with a TL494) which gets its power from a linear transformer/bridge rectifier setup.The output from all of this is supposed to be 12 volts, and that's then fed into a second stage which then uses another MB3759 board to drop that to 5VIf I stuff 12V in from my bench PSU where it's meant to be (with AC disconnected of course), then the 5v DC-DC circuit does appear to be working as intended, so the issue here is the first stage clearly isn't generating the 12V and that's what I want to try and solve.The only info I can share is:1. The fuse did indeed blow when it failed - and HARD!2. I can't remember whether the Bridge rectifier - an S3WB60 - was dead (I guess it may have been as I had a Chinese colleague get me a replacement from TaoBao as they're unusually laid out and unobtainium outside of China - the AC and DC are on opposite sides instead of the more conventional opposite corners) but it's replaced anyway.3. The power tranny was DEFINITELY a short-to-ground on at least one side (I can't remember if it was both sides, but I think not) - the replacement I've fitted (Again Chinese-sourced NOS, also an unobtainium part that I would have had to cross-match otherwise) checks out in the diode mode as .501 Ohms (Base-Collector) and .555 Ohms (Base-Emitter).4. The primary bridge rectifier definitely works, and I'm getting around 350-400v across the primary cap as expected.5. Powering it back up after replacing parts didn't blow the fuse any more, but no 12v output still.6. All other components "seem" fine - I removed and checked each transistor, bridge rectifier and dual-diode and again they check fine in diode mode.7. All electrolytics (removed) measure correctly from a capacitance perspective (but I don't have an ESR meter sadly) - and are genuine Nippon Chemi-Con jobs, no signs of bursting or leaking.Since the last time I worked on it, I've now obtained an isolation transformer and variac. I do have a decent scope too, but no HV differential probe set. Any clues what I might want to check and any tips for any measurements I might want to safely make? I guess I should be particularly interested to know if the "12v board" (which contains the MB2759 PWM controller which I've replaced with a TL494 just to see if that made a difference) is actually generating the HF switching and if so, why not?Thanks in advance for any guidance/suggestions.Tony

author: Ian.M
author_rank: Super Contributor
timestamp: March 10, 2022, 09:04:37 pm
content: Its rather an odd-ball circuit, as IC1 which controls the primary side switching transistor Q1, driving it via pulse transformer T3, is powered from the line frequency auxiliary transformer T2 which also provides 9V AC to the SX-64, (IIRC for peripherals).TheMB3759 datasheetshould give you some idea what to check around IC1, and as long as you stay away from the mains side of T3 and Q1 + associated components, its all low voltage signals you can probe with reasonable safety. However as you've got an isolating transformer, use it as it will *probably* save you and your scope if you probe in the wrong place.For general SMPS repair advice, see'Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies'by 'Silicon Sam' Goldwasser.There are various low cost ESR testers you can build to check its electrolytic caps, some of which use a scope for display and others an analog meter or DMM. Ask if you need help finding one to build.Sorry I cant be more help, as I have no experience of this specific power supply.

author: MathWizard
author_rank: Super Contributor
timestamp: March 11, 2022, 02:41:19 am
content: My mom threw out "my"/her C64 that she bought for us when I was a kid, that could have been avoided.So how common were SMPS back then? A few years ago when I could barely remember how to light an LED, some Ahole sold me a broken computer PSU. And I had a DMM, and actually an Oscope (so not just an EE lurker , but they look like sci-fi LOL)So I did manage to find a bad driver BJT for the PFC iirc, on that PCB, and got it working again. That was my 1st intro to SMPS, inductors besides transformers, maybe reactance.Now any time I look at big SMPS, like in a TV or computer, I check for rectified mains, then primary Aux and secondary standby pwr, then on from there. 1 computer PSU I had, just un-powered, working in from the plug with a DMM, I found a wire broke off the 1st or 2nd, primary side coil, of the aux/stby pwr inductors, and fixed it and sold it.I have a few sitting in a box that I picked at years ago, if my day job ever slows down, I can't wait to look at them now.