90s Vintage Boxed Dick Smith Power Supply Kit (K-3206) - Page 1
Forum section: electronics / projects

author: Webserfer
author_rank: Newbie
timestamp: January 30, 2016, 10:35:25 am
content: So, I happened to get lucky on eBay and found this Unopened 90s dick smith power supply kit for just $49!I thought some people might want to see the un boxing and maybe progress, would anyone be interested in that?Also the dude currently still has several so if you want one be quick and snap one upeBay auction: #http://www.ebay.com.au/itm/272112421537?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT(he added the $10 shipping after I bought it so $59 for anyone now)Anyway Im putting up the unboxing photos, if people want to see progress photos or anymore just tell me.Now I was too young to have any kits like this in the 90s but I will say it was fun unboxing all this and I wish more kits like this were around (maybe im blind?)I'll just post one photo on here and link to the rest in a imgur photo album to avoid it being to largeRest of the unboxing- (currently uploading)And for anyone who's had experience with this, anything I should look out for?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: January 30, 2016, 11:31:01 am
content: I fixed up one of those for a flatmate quite a long time ago. I think he had reverse connected a 12v battery to it.Obviously had to change the output transistors. Cant remember any other failed components.So watch out for that, maybe look at some sort of reverse polarity protection or a fuse.I for myself made a simpler version, without transformer taps, as I write it is still in use, currently powering my telephone base station, due to it being electrically quieter than the supplied wall wart.Though it's probably not very economical.

author: peteb2
author_rank: Regular Contributor
timestamp: May 31, 2016, 12:10:03 am
content: Just for the record i too have recently being made a 'victim' of this kitset. A friend came to me having picked one up off an auction site and asked if i could do a favour and assemble it. Once i finally had some downtime i had it finished in a couple of nights. Then the fun started....I spent an age going through trying to make it work like the line-up instructions said it would. I became a bit obsessed over why i couldn't resolve the units inability to output much more the 1.5A at 40V with a big fat high wattage 13.5R load connected... (it kept dropping out of regulation all too easily). About all i could safetly get was 20V into 10R for 1A... The reason my friend had bought this old kit up was because he has an application around 40V at around 2.5A (variable range)....Then i stupidly went off on a tangent and plugged everything into the iCircuit simulation App on my Mac and proved to myself just what was meant to happen. It's an interesting design and proves ok under simulation...Back to actual kit and with a cleared out mind of all previous thoughts and finally i had a look at the transformer where i found it's terminal voltages were sagging way, way too soon as the output voltage is adjusted with a load connected. In short it's not up to doing the job. A junkbox bigger VA xfmr 16-0-16 temporarily substituted, things worked fine. Only issue is it will not fit the case so it looks like a whole new case is needed...

author: rob.manderson
author_rank: Regular Contributor
timestamp: May 31, 2016, 12:35:52 am
content: So is it the little dick or the big dick?

author: Circlotron
author_rank: Super Contributor
timestamp: May 31, 2016, 12:58:35 am
content: ^^ Yep, real names for two different size supplies sold by DSE back in the day.

author: rob.manderson
author_rank: Regular Contributor
timestamp: May 31, 2016, 01:10:37 am
content: Yep, remember em well.(my location may be Phoenix these days but back when DSE was a place worth visiting (before Woolies) I lived in Melbourne).

author: Unknown
author_rank: Unknown
timestamp: May 31, 2016, 01:38:11 am
content: IIRC, that's the one where the EA article shows the transformer secondary wired incorrectly to the board. From vague memory, the wiring layout diagram swaps the CT & one side of the secondary swapped, so you can imagine how well that works.I spent a few months trying to convince them that the article was wrong before I eventually gave up. I think they published an errata about a year later, and I think the instructions in the later kits were corrected, but check your kit - if the secondary connections to the board don't make sense & contradict the schematic, trust the schematic!The whole design is problematic - it's from that era of EA when you suspect they built prototypes to photograph rather than test. For a start, all the op-amp sections are borderline unstable and ring/oscillate at the slightest provocation; you need to add/correct compensation around them. The DSE kit department also swapped the recommended EI transformer for a similar-but-different toroid (edit: or was it the other way around?), with the result that the kit drops out of regulation near its V or I limits and the pass transistors overheat too much at low levels.Best thing you could do with that is send it to Dave so he can critique the design, construct it, then show the practical issues with its many flaws.

author: GK
author_rank: Super Contributor
timestamp: June 01, 2016, 10:06:02 am
content: I also have one of these, built ~ 20 years ago when I was not long out of high school. This thread has given me cause to finally pull it out of storage. Ineveractually used this supply. As already mentioned, the thing is prone to oscillation. Mine buzzed away on initial test/calibration and promptly poofed itself. Somewhat annoyed I put the thing aside temporarily in disgust but in the meanwhile acquired other power supply units which satisfied by requirements, so this thing was never looked at again.............. until this evening.Yes the power transformer is hugely underrated for the specified current output of 3A. The transformer is only rated at 2A! Best case from a full-wave rectified + filtered output you can only draw ~0.63 times that DC (= ~1.3A) So how exactly how the designer thought the 40VDC/3A specification was valid is beyond me. 40VDC is also highly optimistic from a 30V transformer secondary, given regulator drop-out, ripple, etc. However I don't see anything wrong with the secondary connections either in the schematic or the overlay/wiring diagram. The transformer has a single 30VAC secondary winding with taps at 12V, 17.5V, 20V, 24V and 27.5V. The winding ends are labeled "0V" and "30V", so the "15V" tap is the secondary windings effective center tap. This is exactly how it is shown in the wiring diagram - - "15V" goes to the circuits ground at the junction of the main filter capacitors while "0V" and "30V" go to the "AC" terminals of the bridge rectifier.I've attached schematic with modifications I'm about to make to my unit to rectify the main issues. I think it's about time that I get this thing operational. The cause of the instability is obvious. The bypassing of the feedback resistors for the op-amps of the regulators ensure that 100% feedback is provided at HF. So we essentially have a voltage follower with the series pass transistors inside the feedback loop. The TL072 is a 3MHz GBWP opamp. With 100% feedback the closed loop bandwidth is 3MHz and the phase margin is at its minimal value. The phase margin might be in the vicinity of 45-60 degrees and there is no way a pedestrian 2955/3055 pass transistor wrapped up inside the feedback loop is going to reliably contribute less than that much additional phase-shift out to 3MHz, especially with its seriously current-starved driver transistor. The only reason this thing may be stable in limited operational usage is by some "convenient" fluke ofconditionalstability. I'm entirely unsurprised that mine oscillated itself to death or to read reports of stability issues from others.Anyway, the most obvious cure for the stability issue with minimal modification to the original design is to re-wire the (what the original EA artical calls "bandwidth limiting") capacitors as I've shown in the attached modified schematic. The caps now act as frequency compensation, bypassing the pass transistors at HF. The capacitors corner with the feedback resistances at approx 6 kHz, which should be a low enough regulator bandwidth to ensure stability with wide phase margin in all by the largest capacitive/low-esr loads. I've also added 680 ohm resistors between the B-E leads of the 3055/2955 to establish a decent quiescent current in the driver transistors and added protection diodes to prevent damage to the pass/driver transistors if the PSU output is accidentally connected in parallel with an external source of power.