Repair, upgrade, or part out DIY Multi-Tap, Wound, transformer-based PS - Page 1
Forum section: electronics / beginners

author: itsme_Bernie
author_rank: Newbie
timestamp: February 03, 2023, 11:47:19 pm
content: Hi EEV FolksI am new to posting here.I have a DIY Bench Power Supply I acquired, and now I’m finally getting to building my little electronics lab. I would like some help deciding whether to fix or part this out to make other bench supplies with its components, I posted another thread about the other power supplies I am working on.I acquired this from the guy who built it (I was told), but he has since passed away. I was planning to customize/update/refresh/upgrade components to my needs as I learned what needed to be updated, and what is best to upgrade.I am building several other power supplies (of different arrangements) anyway, so now is the time for me to decide to part this out or not. I have plenty of room to add components to this if I can and it makes sense.ANOTHER FACTOR is it started acting up the other day, but now the voltage gage just slowly dropped to zero. It may not be necessary to fix this if I should modify it or part it out anyway, as long as I can test the transformer etc.-There is a multi-tap transformer which has numbers I looked up and googled and could not identify. I plug this unit into US 115v AC, so I assume that is the input voltage, but cannot verify unless I take this Power Supply apart.The only info I can see see printed on it without removing it (yet) are:“CDCC 30460579-1001TTI 20844”Then there are numbers printed around on the actual windings, and seem to identify the taps: “11 12 13 14 15 17”There are more, but I can't read them without removing the transformer. I cannot find out specs on it, and don’t know enough about multi-tap transformers to figure it out.-There is an Electro Switch Corp Series 31 selector switch which switched between three voltage range settings (it appears there are mechanical limits set on the back)-There is a potentiometer that would vary the voltage in each of three settings of the Electro Switch.. the Electro Switch would give voltage ranges from 0-16, 0-36, and 0-46 volts.-One of the uses for the PS was to power a power feed on a VERY small Unimat lathe. There is a custom plug/output for this.-There is a board in the PS that might be to manage the variable power to the lathe feed?-I guess it is hard to know the possibilities the Electro Switch can give me without knowing more about the Transformer? I believe it can handle power pretty well, and maybe multiple poles, and at least this company still exists!-There is a voltage and an amperage meter on the front panel of the housing.I want to figure out if I can build a decent linear power supply with the components of this DIY PS?** It ALSO seemed to be working fine until the other day, but sometimes the voltage just slowly drops, sometimes all the way. I am suspecting the potentiometer.It may not be necessary to fix this if I should modify it anyway, as long as I can test the transformer etc and reuse them.I was using this only lightly, and now it needs some troubleshooting. The voltage just started dropping slowly before my eyes (according to the analog voltage gage on the front panel).I was using this only lightly, and now it needs some troubleshooting. The voltage just started dropping slowly before my eyes (according to the analog voltage gage on the front panel).

author: donlisms
author_rank: Frequent Contributor
timestamp: February 04, 2023, 08:34:45 am
content: What a nice looking box! Kudos to the maker, Rest In Peace.It appears to be an old-school design. I would suggest reverse-engineering it to see what makes it tick, and evaluate it from there. Draw the schematic. It will require much effort, and you will acquire deep understandings.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: February 04, 2023, 08:59:40 am
content: QuoteTry some contact cleaner in the pot. At worst, a new pot should be easy enough to find something to keep the whole thing going.Squirt the selector switch while you're at it.

author: itsme_Bernie
author_rank: Newbie
timestamp: February 04, 2023, 02:50:05 pm
content: Do you guys know how to identify this transformer, or is it the same as others you have seen? I don't know how to determine what the tapped voltages would be without cutting the wires, as they are totally insulated.Bernie

author: ledtester
author_rank: Super Contributor
timestamp: February 04, 2023, 10:15:13 pm
content: Quote from: itsme_Bernie on February 04, 2023, 02:50:05 pmAfter you draw a schematic it might be clear how you can probe the voltages without modifying the circuit.Heat shrink tubing doesn't bond to wire - it simply grips it. To remove it just score it lengthwise with a box cutter/exacto blade until you cut through it. You can then just peel it off.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 04, 2023, 10:42:00 pm
content: Good old time design.When it drops volts does the big cap or the big resistor heat up ?Likely suspects. If the cap "LEAKS" it will drop your available voltage. A common occurrence.If the resistor is not physically large enough to dissipate heat,it may change its value quite a bit.It could be the pot but I dont think so usually old dirty pots just are very uneven or go open at some points because of dirt....but maybe.Usually transformers burn up in failure mode, this one looks nice.You can check the resistor for ohms in circuit when cold and then when warm (after pulling the plug) but that resistor looks OK.But my money is on a leaky cap.If you disconnect it you will have an unsmoothed output but you can do this just to check if the volts go down or not on the meter when the cap is disconnected, just unsolder one lead.Maybe you have a capacitor checker that checks for leaks. (not ESR)Or better yet, just replace it if you have one.There are color codes for some transformers for the leads, but you would have to find them on the manufactures site.

author: itsme_Bernie
author_rank: Newbie
timestamp: February 05, 2023, 08:37:16 pm
content: Thanks Don!So My first homework is to attempt to draw a schematic.Do any of the numbers on the transformer look familiar, or like they may give away anything? I guess they could just be sequentially numbered, which would be meaningless without a schematic or readings.Bernie

author: itsme_Bernie
author_rank: Newbie
timestamp: February 06, 2023, 12:26:10 am
content: Hi WallaceThere is a sizable capacitor fastened to the top of the enclosure. I also suspect this. I believe it let a little smoke out, and the PS started working again.. usually things go the opposite way! Hah. I have to be careful, but I will try to check this or replace it.

author: inse
author_rank: Super Contributor
timestamp: February 06, 2023, 05:54:59 am
content: I would keep the beautiful housing and implement a new control circuit.The minimalistic electronics are not sufficient for a decent lab supply.It looks like the regulation transistor is screwed to the housing, I doubt whether this provides sufficient cooling.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 06, 2023, 05:12:19 pm
content: The big cap was probably partially shorted i.e. "Leaky" and heated up and really shorted then went open. No More LEAK!But you probably now have a pretty rough DC output!. If you have a scope with 10X probes you could check it.A new cap would be in order. Get one with highest volt rating you can within reason.If this thing meets you present needs, as I suspect it would, use it until you find out it's limitations, then you will have a good idea of what you need next.These linear (not switching) PS are nice in that they do not have a lot of noise. this is very necessary at times.The analog meters are not famously accurate, It might be nice to put little sockets on the unit so you can plug in a multimeter for better volt control, Or just use stacking banana plugs that include your digital voltmeter, I do this all the time.You can do all sorts of things like using multiple big mosfets in parallel for voltage control, without much other changes in the unit. You can spread the fets out over an entire side or the back of the case to distribute any heat. You can get surplus mosfets for pennies at hamfests and the like. But if the big BJT does not heat up much, why bother?The ranging is done with the different secondary leads of the transformer, so the regulation within each range does not have to deal with as much voltage change, so the regulation control does not dissipate much energy as heat. That would be expensive to make today as the cost of these transformers has gone way up. Higher power rotary switches are also expensive.Anyway, if it works, use it!