Changing Pots to multiturn  in a 30V/3A  PSU - Page 1
Forum section: electronics / projects

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 27, 2015, 07:25:33 pm
content: Hi All,I have 30 V 3A dual bench power supply Selectronic SL-1731SB, which exists on many other namesas the Velleman PS23023, and many other ones.See my earlier posthttps://www.eevblog.com/forum/repair/tracking-problem-in-a-dual-bench-power-supply/msg529140/#msg529140and the schematics below.I would like to change all the 4 front pots (6.8k RP4,RP5,RP6,RP7) to multiturn ones in order to getmuch better precision on the tuning of the PSU.I have several 10k Bourns 10 turns pots which I could put there.(Edit : In fact I made a mistake, I had only one. )DO you see any problem that could occur from changing the 6.8 k pots to 10k ones ?Thanks

author: LaurenceW
author_rank: Frequent Contributor
timestamp: January 27, 2015, 08:03:45 pm
content: I'd say that you'd get away with that. The four pots are used as a mix of variable taps between different voltages (ground and Vref, for example), so a slighter higher values resitance will have little impact, there. You are still "tapping off" 50% of the same voltage, when the pot is centred, whether the pot is 6K8, 10K or even a little more. Yes, somebody will claim that it means the source impedance offered to the circuit is higher, but I really doubt this will have a great impact. You could do worse that strap a 15K resistor between each end of the track on the variable resistor (usually labled pins 1 & 3), As this reduces the 10K pot // 15K to around 6K. But you may even get away without those.Try it!Not sure I'd even BOTHER with 10 turn pots for the current limit, though. The occasions when you actually NEED to set current limits to a value whose precision you could not achieve with a single turn pot are few and far between, in practice. Remember current limitting is mainly there to save you from FRYING a circuit which developed a short fault, or something similar.Oh, and one other thing. Ten-turn pots are a PAIN IN THE @RSE when you find that the voltage you want next is at the OTHER END of the ten turns! What you would have achieved in a single twist of the wrist now looks set to give you RSI. 2-3 turn pots might actually make for a more USEABLE instrument, and again allow the setting of the output voltage to "close enough" (for all real world apps). It's easy to get carried away with precision for the wrong reasons.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 27, 2015, 08:27:43 pm
content: Thanks a lot for the advice, LaurenceW,I will give it a try.As it is a dual PSU, I may put 10 turns only on the Master side.I put 10 turns because I have them on hand. I do not have 2-3 turns pots.For the Current, I understand your point, but the difficulty I have at present is for very smallcurrents, (and same thing for small voltage), where the the present pots are quite unusable.

author: erikj
author_rank: Contributor
timestamp: January 27, 2015, 08:35:44 pm
content: I think you can change the pots yes, but you will likely need to change the value of some resistors as well.R15 and R16 might need to be increased or you will get the wrong ranges.

author: MrAl
author_rank: Super Contributor
timestamp: January 29, 2015, 09:40:45 am
content: Hi,Are you saying that the original had 6.8k and you want to use 10k?If that is true, then the rest of the circuit will see a slight difference in series impedance, but luckily the remedy is really simple.The fix is to parallel the new pots outer terminals with a resistor that makes the total feed through resistance equal to 6.8k again. This works out to exactly 21.25k, but you can probably get by with 21k or maybe even 20k.We can not do this if the resistance ratio is too large, but since yours is only 10k to 6.8k (about 70 percent difference) it will work very well. The times when it doesnt work is when we have to go from say 10k to 1k, in which case the adjustment will turn out to be very non linear and so will not 'feel' right when the user adjusts the voltage, but for a small change the non linearity is very very slight, so it seems natural.You are very lucky in that your power supply uses 6.8k resistors for all the adjustments. Many of them use much lower values like 500 ohms.One catch here though is that the new pots may be wire wound. This may make the adjustment a little 'bumpy' anyway. It would be best to wire ONE pot in as a test to see if you like the way it works before going through the trouble of doing all the pots and then finding out maybe you dont like the way it works.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 29, 2015, 11:36:55 am
content: Thanks MrAI,This is somewhat what I thought was the way to go after reading the hint of LaurenceW and other stuff on the Web.In fact, on the web, you find plenty of discussions on more complex settings of resistances in parallel to thevarious pins of the pot, but not much on the most obvious one which is a resistor R3 in parallel on the whole resistor 1-3 (R= R1+R2).From elementary Ohm's law, I would figure that the output voltage in pin 2 is not change at all :If V is on pin 1, GND on pin 3, one should have V x R2/(R1+R2) at pin 2 with or without R3.Why do you say that there will be a change in the linearity of the pot ?

author: splin
author_rank: Frequent Contributor
timestamp: January 29, 2015, 12:26:18 pm
content: Anyone else spot the mistake in the schematic? Transistor V18 wouldn't do much if it really was a pnp device - it's actually npn.I wonder if (published) schematics ever have deliberate mistakes to make it a bit harder to copy? Like map makers who deliberately include errors to catch out copyright thieves.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 29, 2015, 05:47:38 pm
content: Quote from: LaurenceW on January 27, 2015, 08:03:45 pmMotorize it like studio pots used to be. Maybe if you're in a hurry use a motor like thishttp://www.hobbyking.com/hobbyking/store/__31677__Turnigy_AquaStar_4084_1050KV_Water_Cooled_Brushless_Motor.html

author: LaurenceW
author_rank: Frequent Contributor
timestamp: January 29, 2015, 07:20:06 pm
content: The secret for a current limit pot is to use a standard LOG LAW (as opposed to liner law) one. This basically expands the resolution at low currents (i.e makes it easy to set 10, 20, 50 mA) while sacrificing resolution at the higher end. But then who cares if you can only set the current limit to 2.5A +/- 200mA? Inpractice, it most likely wont matter.

author: ruffy91
author_rank: Regular Contributor
timestamp: January 29, 2015, 07:47:43 pm
content: If you just parallel a resistor over the whole pot you wont change the source impedance on the wiper. Just think about it.

author: nctnico
author_rank: Super Contributor
timestamp: January 30, 2015, 01:14:09 am
content: A resistor parallel across the ends of a pot is the best way. I did that to get an odd potmeter value. I did some calculations with Excel and a resistor across the ends of a pot gives the flattest response.I agree about the current setting. On a cheap PSU I only changed the voltage setting potmeter for a 10 turns one. For me the current adjust is just to prevent (too much) smoke.

author: MrAl
author_rank: Super Contributor
timestamp: January 30, 2015, 05:07:24 pm
content: Quote from: JacquesBBB on January 29, 2015, 11:36:55 amHi,There are a couple ways to use a pot.One is with the arm shorted to one end. With this arrangement and a parallel resistor, the pot resistance no longer changes linearly with shaft rotation angle.The other is as a voltage divider. I think you are using this arrangement. When used like this, the voltage output of the arm (as you say) is linear with shaft rotation, but that is only true for a driven impedance that is infinite or very high relative to the total pot resistance. With any reasonable loading, the output voltage will not follow the rule Vs*(R2/(R1+R2)) because there is more loading effect near the center of travel than at the ends because the ends will be lower impedance than the center. Luckily it may not be that much different than it is now though, because it must be loading a least a little anyway even with 6.8k, so with 10k there should be only a small difference. You may not even notice it.The time when these things really matter is when the resistance change has to be substantial, or for some kind of process control where accuracy is very important.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 22, 2015, 01:43:33 pm
content: I finally did the change.I put a 10k 10 turn pot with a 21k resistor in parallel.I also added a 10 uF ceramic capacitor at each output.It works perfectly. I have also the impression of a better stabilityof the voltage output with the 10k (bourns) pot.I must say that this is not overkill. When dealing with low voltage (0-5V),the 10 turn pot is very nice to use, and I would not like less.Going from one end to the other is quite fast as well, as the pot is very smooth, andI put a large knob that I can turn with one finger.I may even do it on other places if I get hold of additional 10k 10 turn pots(this one was scavenged from discarded equipment).

author: OldNeurons
author_rank: Regular Contributor
timestamp: March 22, 2015, 04:45:18 pm
content: Quote from: JacquesBBB on March 22, 2015, 01:43:33 pmJust another idea...Instead of the 10 turns, why not using dual concentric potentiometers?You will get both speed and accuracy adjustment.http://www.bourns.com/ProAudio.aspx?name=pammhttp://www.bourns.com/data/global/pdfs/PTH90.pdf

author: m100
author_rank: Regular Contributor
timestamp: March 23, 2015, 05:38:43 pm
content: Quote from: OldNeurons on March 22, 2015, 04:45:18 pmAren't those actually dual gang pots where both gangs operate together using one knob, and a rotary switch on a concentric knob thus giving no independent operation of each pot for a fine adjustment?

author: paulie
author_rank: Frequent Contributor
timestamp: March 23, 2015, 06:21:49 pm
content: Quote from: m100 on March 23, 2015, 05:38:43 pmThat's true of the first link which is probably a typo. The PDF is correct type with independent control. Much better for this app IMO. I had expensive multi-turn in my first PSU but got tired of twisting knobs so replaced with concentric. Huge improvement. Not only CHEAPER but more ergonomic and capable of better precision!

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: March 23, 2015, 06:41:59 pm
content: For me it was not an option, as I got the 10 turns pots for free as a salvaged componentfrom dumpster. By the way, it was also the case for the PSU, which I had to fix a little bit.The nice thing about salvaged equipment, is that you do not hesitate to hack it,searching for some improvement. Right now, I am quite satisfied with the new pot,but I have another of these PSU (from the same dumpster) that I may try to hack in a different way,keeping this one fully functional.

author: OldNeurons
author_rank: Regular Contributor
timestamp: March 23, 2015, 10:18:19 pm
content: Quote from: m100 on March 23, 2015, 05:38:43 pmNo. Each gang is controlled by a separate knob. They are totally independent.To my knowledge, this kind of potentiometer is used mainly on certain guitars models.The difficulty for the application above is to find adequate knobs.

author: TimFox
author_rank: Super Contributor
timestamp: March 23, 2015, 10:30:57 pm
content: I have several older -hp- power supplies with concentric independent potentiometers for coarse and fine adjustment.They can be adjusted quickly, but it is easy to go too far on the coarse, try to bring it back with fine, curse, re-adjust coarse, try to bring it back with fine, curse again, etc. The best technique is to set fine to the center of its range, adjust coarse, then re-adjust fine and hopefully obtain the value.A ten-turn pot is generally easier.Note that wire-wound 1-turn pots and all 10-turn pots (except for very expensive "hybritron" units with plastic coating on the wire) have a finite resolution, and one can note the "click-click-click" as the pot is adjusted. However, good wire-wound units are more stable than other types.

author: ym58
author_rank: Regular Contributor
timestamp: July 14, 2015, 06:14:35 am
content: Hi Jacques,I too have anSL-1731SBdual 30V/3A P/S which I am happy with but I definitely want to change its tension pot (master, slave or both) with a multiturn pot.I have foundthis rather cheap modelon eBay but before I open the P/S case I was wondering if there's enough room in there to fit the quite long depth of this pot ?What do you think ?

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 14, 2015, 06:23:34 am
content: No,It is much too large. It will not fit.I will post later on some more details.

author: ym58
author_rank: Regular Contributor
timestamp: July 14, 2015, 05:34:03 pm
content: Do you thinkthis onewould be more appropriate ?Thks.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 14, 2015, 06:59:15 pm
content: Yes this one is similar to the one I put, maybe a little bit smaller.So it will be OK.But beware, you will have to enlarge the hole for the pot handle. It is larger than theprevious one. This is what I did. I changed also the knob.As the pot cannot be hold on the PCB, now it is hold by the front panel. Seemy previous pictures.Good luck.

author: nctnico
author_rank: Super Contributor
timestamp: July 14, 2015, 08:51:16 pm
content: I wouldn't buy these kind of potmeters from Ebay. My experience so far has been that their range does not extend to zero. A 10k pot may have a minimum of 100 Ohms or so.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 14, 2015, 09:45:36 pm
content: I used a Bourns 10 turn pot that goes to zero.The one mentioned above looks like, but if it is not certified to be a bourns pot, it is probably not.

author: ym58
author_rank: Regular Contributor
timestamp: July 15, 2015, 03:23:29 am
content: Quote from: nctnico on July 14, 2015, 08:51:16 pmYou bet !Once I receive them, they'll be showing markings like"Boornes"or"Burns"!!!Quote from: JacquesBBB on July 14, 2015, 09:45:36 pmDo you still have one of these 10-turn pots that you once salvaged to mod your PSU ?I can buy one from you ... better trust yours than theirs !

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 15, 2015, 08:23:04 am
content: Quote from: ym58 on July 15, 2015, 03:23:29 amSorry, I do not have spare 10k 10 turn pot. Maybe you can give a try to the chinese one, but at your own risks (I buy many parts on ebay, but you have to be ready for the worst).Otherwisethis is a good price for a genuine onehttp://www.mouser.fr/ProductDetail/Bourns/3590S-2-103L/?qs=sGAEpiMZZMvygUB3GLcD7uCOAMig3YAGWQdFc%252blENJE%3d

author: ym58
author_rank: Regular Contributor
timestamp: July 15, 2015, 09:00:01 am
content: Quote from: JacquesBBB on July 15, 2015, 08:23:04 am

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 15, 2015, 09:34:17 am
content: On ebay, they do not charge you for small items as the postage from China is per gram and very cheap. This is why they remove all packaging. Postage takes from 3 to 8 weeks to deliver.If you order larger items, the postage can be very expensive.Mouser sends from the US (not France) or other places by FedEX, and you receive your items in two days from the order.Moreover, the postage is free for orders above 65 € . I always order with invoice over 65 € HT.I usually have a backlog list of thing to order, so when I need something urgently, I fill the order with other stuff that are in my backlog list. Like that, postage is free and very fast.

author: ym58
author_rank: Regular Contributor
timestamp: July 15, 2015, 11:00:07 am
content: Yes, you're right (concerning shipments from Asia).I place orders for electronic parts nearly everyday and up to now I never"really"had to complain about quality (except sporadic resistor inversions on PCBs) nor delivery times (mostly 3-4 weeks from China mainland and 2-3 weeks from HK).So, my guess is that I don't take much risks to order from there a pair of"Burn's"pots given their price (less than 4 USD/each, free shipping).Actually, my asking you if you still had a spare pot handy was just to have a chance to make (small) business with another salvager frog!

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 15, 2015, 01:18:18 pm
content: Quote from: ym58 on July 15, 2015, 11:00:07 amNo problem. If you are missing any parts from your PSU, do not hesitate to ask. I have several of them. Some that I fixed, some others that I keep for parts.

author: ym58
author_rank: Regular Contributor
timestamp: July 15, 2015, 02:05:52 pm
content: Good to know, thank you for proposingI just purchased mine 63€ on Price Minister (almost new, a good deal, I think).What I really miss on it though, is:(a)fine tuning pots (this topic's subject)(b)backlit LCD (I wish I could fit a few under-volted LEDs inside, but not quite surewhereandhow... ?)(c)4 or 5-digit LCD especially for the ammeters (like those LCD described and testedby paulie here)Any enlightenment ?

author: ym58
author_rank: Regular Contributor
timestamp: July 16, 2015, 04:07:36 am
content: @JacquesBBB,Just to keep you updated :Apart from that, do you have any suggestion regarding points (b) and (c) mentioned in my previous post (backlit LCD and 4-5 digit LCD) ?Thanks.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 16, 2015, 04:27:57 am
content: Quote from: ym58 on July 16, 2015, 04:07:36 amI would not recommend to do b) before c).Both are possible. You can take the power at the 5V PSU or at the display which I think is 9V.You can try. This is more simple than changing the pots as more accessible.

author: ym58
author_rank: Regular Contributor
timestamp: July 16, 2015, 04:39:12 am
content: do you thinkpaulie's LCDwill fit inside the PSU and workstraight onwithout having to mod any electronic (especially for the probe lines) ?I am asking you so because you wrote that you have a few of these PSUs broken apart and I think that you can assess better than me the feasibility of such a LCD swap ...

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 17, 2015, 09:44:50 am
content: Is this what you want ?For the Voltmeter, I just put it in parallel with the previous one (who do not appreciate that much. Should be better to remove it ).For the ammeter, its more complex, but should be possible.I am not sure its worth it on this PSU, though .

author: ym58
author_rank: Regular Contributor
timestamp: July 17, 2015, 11:05:02 am
content: Actually, I'd rather have theammetersto be5-digitthan the voltmeters, the reason being that constant current circuits generally needs to be more precise (see Lithium revitalize or charge cycles !) than constant voltage circuits (1/100 V accuracy is way sufficient).Have you ever measured theinternal resistanceof the original Selectronic ammeter ?I'll try to PMpaulieto check what his are ...

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 17, 2015, 11:43:16 am
content: I just wanted to show you what it would look like.I just made a temporary lousy solder to put the new display in parallel to the old one.I confirm that is is 9V that is there for the alimentation of the display. Works fine.I have already removed it. Just let it for the picture.So Yes, it is possible to fit these voltmeters (these are the same as Paulie's).I have done the study long ago for the ammeters, but do not remember. Will have to dig out, but no time now.

author: ym58
author_rank: Regular Contributor
timestamp: July 17, 2015, 12:00:25 pm
content: Thanks for the info.Concerning the ammeters : ain't no rush ... any time you get a hand on your previous studies, just let me know !

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 19, 2015, 01:38:35 pm
content: As it is, 10 mV corresponds to 0.16 A at the ammeter.

author: ym58
author_rank: Regular Contributor
timestamp: July 19, 2015, 01:48:52 pm
content: Quote from: JacquesBBB on July 19, 2015, 01:38:35 pmthat is a mere 62.5mR impedance ... meaning that at full current capacity (3A), the voltage drop due to the serial ammeter would be around 200mV.am I right ?

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 19, 2015, 02:52:32 pm
content: I just made a direct measure of the shunt resistor and found 65 mR ( with 5 mR uncertainty)So this is it.

author: ym58
author_rank: Regular Contributor
timestamp: July 19, 2015, 03:08:03 pm
content: thank you for your commitment and so precise info concerning this 30V/3A PSU !---all that remains for me to do now is to obtain as precise specs as yours from eBay suppliers for I guess some of them (not all, fortunately) don't even know what an impedance isselling ammeters, swimming suits and plastic wedding rings all together does not require to be skilled in all fields ...

author: ym58
author_rank: Regular Contributor
timestamp: July 20, 2015, 12:48:52 am
content: JacquesBBB,FYI, the discussion related to PSUammeterscontinueshere... you're warmly welcome to join and contribute.

author: FrankT
author_rank: Regular Contributor
timestamp: July 22, 2015, 08:15:06 am
content: I have a slightly different model, a Sinometer HY3005D-3, 30 volts, 5 amps.It had the same 6.8k pots, so I replaced them with 10k multi turn. Voltage still goes up to 30v, but the current is now limited to 4.4A.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: July 22, 2015, 08:54:06 am
content: Quote from: FrankT on July 22, 2015, 08:15:06 amI presume that if you put a 21k resistor in parallel to the pots, you will retrieve exactly the previous setting with thesame limits .

author: ym58
author_rank: Regular Contributor
timestamp: July 22, 2015, 11:41:23 am
content: Quote from: FrankT on July 22, 2015, 08:15:06 amFrankT,Which pots did you change with multiturn-10K's :Current,Voltageorboth?Coz JacquesBB's tweak to parallel then with a 21K resistor to drop the impedance down to 6.8K should normally affect (improve) the pots' linearity only.I am a bit surprised it has now limited the current output ...

author: FrankT
author_rank: Regular Contributor
timestamp: July 24, 2015, 09:11:12 am
content: Quote from: ym58 on July 22, 2015, 11:41:23 amI changed both.I didn't notice the change to the maximum current until after I had reassembled the unit. Then it was a case of "meh, it will do for now". I will try a parallel resistor if I ever take it apart again.

author: ym58
author_rank: Regular Contributor
timestamp: July 24, 2015, 11:30:38 am
content: Quote from: FrankT on July 24, 2015, 09:11:12 amMeh, I can understand...The real need to have a 5A PSU and not a limited 4.4A is not that crucial ...HOWEVER, the point is to be sure that the current indication is reliable since you changed the pots !Did you crosscheck it with another ammeter (one outside of the PSU box) ?

author: FrankT
author_rank: Regular Contributor
timestamp: July 25, 2015, 03:21:13 am
content: Quote from: ym58 on July 24, 2015, 11:30:38 amGood point. I just checked it and the indicated current is within 20mA.

author: ym58
author_rank: Regular Contributor
timestamp: July 25, 2015, 06:53:22 am
content: Quote from: FrankT on July 25, 2015, 03:21:13 amwell, this tends to proove that replacing the 6.8K potSTRAIGHTwith a 10K alters the current capacity ...by chance, let us know if you re-open the PSU case and try to parallel the "CURRENT" multiturn pot with a 22K resistor coz I'm pretty concerned about it too (I am still awaiting the multiturns that I ordered last week from HK).

author: ym58
author_rank: Regular Contributor
timestamp: July 31, 2015, 06:39:44 am
content: I have finally received my two 8.00USD multi-turn Bournes pots from HK (actually, they were sent from S'pore).Did the swap in the PSU (only the voltage control as for now ...).Had to unsolder and change physical location of a thick coiled resistance (used by the ammeter, I guess) that was hindering the new pot from fitting in (the Bournes pot is thicker and longer than the old one) but it finally worked.Had also to enlarge slightly the hole in the front panel since the Bournes pot has also got a wider thread !I did not solder a 22K resistor to lower the pot value since it seems to be working fine ... I double checked the output voltage with a multimeter to be sure.The onlyASTOUNDINGpoint is that I now have a 48V PSU instead of a 30V one ... at least on channel #2 (it's a dual PSU).I am not quite sure that was not the caseBEFOREI changed the pot since it's a second-hand PSU purchased a month ago, maybe the previous owner had already changed something (transformer ?) in the PSU ... but that was quite of a (good) surprise !However, don't take it for granted if you also hack this PSU on your side ... !!!

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: August 03, 2015, 11:16:20 am
content: Quote from: ym58 on July 31, 2015, 06:39:44 amThis is probably because of the change from 6.8 k to 10 k for the total resistor of the pot.30/6.8*10 =44It is like if you were changing the RP2 220 ohms tension Max spanning pot into a 3 k pot.I would not recommend to leave it like that. All the components of this PSU are barely enough for the existing max value of the voltage. Increasing it to 48 V will be an additional source of breakdown, in particular for the capacitors.If you need 48V, it is better to pair the two outputs.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2015, 11:49:03 am
content: The 48 volts are likely only available without load and enough voltage in the grid. Under load and with 10 % less voltage from the grid the available volatage will be lower by something like 20-30%. So the higher voltage are not really usefull, but up to about 40 V might work.Wether a hhigher output voltage is more stress to the components depends on the circuit. In a simple linear regulator its essentially only the output caps seeing the higher voltage. So this usually is not problem.A higher volatage can be a problem if a preregulator is used. In this case one should not increase the voltage by much.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: August 03, 2015, 12:33:05 pm
content: The output capacitors are rated 50 V, but like all components, not of the best quality.

author: ym58
author_rank: Regular Contributor
timestamp: August 03, 2015, 12:45:33 pm
content: Quote from: JacquesBBB on August 03, 2015, 11:16:20 amI understand your perspective, but it does not explain why I should get 48V instead of, say, 44V !The resistor/potentiometer tolerances are way below this 44V versus 48V discrepancy !Moreover, I am not quite sure that changing the6.8K(yellow in the following diagram) with a10Kpot would necessarily lead to a higher output voltage !Honestly, I feel very lazyto reopen it (you've done it so you know what I mean : this PSU is just so messy inside !) to ascertain that either the pot value should be reduced to a proper one***OR***(and that is an option) the previous owner has already changed the (red, see below) transformer to a higher range one (>50V secondary).What d'you think ?

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: August 03, 2015, 01:20:38 pm
content: The pot is not the right one. I think that the voltage pot is the RP5 pot, which is in series with RP2.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2015, 03:40:10 pm
content: The set voltage is derectly proportional to the pot (RP5). The is also a trimmer PR2 to adjust the maximum voltage. This may have been set to a little more than 30 V before.Especially with a higher voltage transformer the maximium current should not be set higher than original. Even a lower setting may be needed.

author: ym58
author_rank: Regular Contributor
timestamp: August 03, 2015, 05:07:33 pm
content: Quote from: Kleinstein on August 03, 2015, 03:40:10 pmQuote from: JacquesBBB on August 03, 2015, 01:20:38 pmGot you both, but still ... remember whatFrankTonce said earlier in this topic :Quote from: FrankT on July 22, 2015, 08:15:06 amI am not sure his PSU design's quite the same as mine, but his point deserves some clarification ... which I will definitely do as soon as I open the box again.... and this shouldn't be very long since I am expecting my 4-digit ammeter/voltmeter any time from now!

author: Bsupport
author_rank: Newbie
timestamp: June 07, 2024, 02:00:02 pm
content: So I had this LAB PSU and also found the voltage tuning too sensitive. I didn’t want to use the 10turn because the endless turning when setting voltages so I thought of a way to improve that by adding a ‘Fine’ pot meter with a low value in series with the current pot as many othe PSUs have. I checked several diagrams from the net but they all seemed a little different from mine, I guess I have a newer version, the PSU I have has a 10K pot for the voltage setting.The solution would be to add a 1K pot in series with the 10K pot. But I didn’t want the extra holes, there wouldn’t be any space anyway, so I went for a stacked pot with 10K and 1K: no chance of course! As a test I soldered a 1K resistor in series with the pot to see the effect: you can’t have the full scale anymore since the total will be 11K. So be it, I would cut off at the end, giving me 29V.So I ended up with the following: Bought two 10K stacked pot’s and two 1k pot’s from the ALPS type from our Chinese supplier.When the pots came in I checked the values just for the interest.Although the stacked pots were marked 10K both didn’t go any further than 9.1K and the 1K pots also didn’t really get any further than a 9.8K.This turned out to be perfect since in the combination I still had my 10K resistor there!I drilled out the rivets and removed the lower decks.Did the same on the 1K pots.Then assembled the 1K deck on the stacked pot together with the wiper.Added the cover from the stacked pot and put the rivets back in again.This really worked! I can now fine-tune my voltage within 10% of the set value.Added some pictures of the stacked pot assembly and the fitting in the PSU.I guess you can do the same with the Amps knobs as well.Hope this helps other to improve this otherwise fine PSU!