Old Thurlby PL320 TGP (Sinclair) Dual channel lab power supply - Page 1
Forum section: electronics / repair

author: DoricLoon
author_rank: Contributor
timestamp: September 21, 2017, 08:47:11 pm
content: Hello, recently purchased an old Brown/Cream two channel 2A 30V power supply with GPIB interface. When it arrived it worked fine in local (a little out of calibration but not too far off) However when I plugged it into the GPIB bus not only was it undiscoverable but all my other instruments couldn't be found either. The talk LED on the TGP board was occasionally giving a wink.Removed the GPIB controller from the power supply and had a look over it. Found broken connections on the ribbon cable connecting the two PCBs of the controller together. (There are only a small number of plug and socket connections on this power supply. The majority of the interconnections are soldered directly board to board, which makes removal a bit of a pain)Also found that every one of the electrolytic capacitors barring the large input filter cap were high ESR and hardly any capacitance. Decided to change all caps on the GPIB controller and both of the power supply channel control PCB's.Refitted the GPIB controller and happily the power supply now speaks GPIB.As an aside almost all of the capacitors on both of the channel control PCB's were also completely dried out!I have now followed the service manual and calibrated the local controls but the GPIB controller PCB has a bank about 20 trimmers on top of it and the voltages by GPIB are a little off. Still plenty useable for my needs but it would be nice to calibrate this section also.I did send AIM TTI an email with a request for the information. To no avail, They did however send the GPIB controller user manual (Which just explains the syntax and addressing).Does anyone know how to calibrate the digital to analog GPIB controller on one of these supplies?I also have a 15V 4A model which works perfectly but after finding all those duff caps I think it's time they were all replaced too.

author: DoricLoon
author_rank: Contributor
timestamp: September 21, 2017, 08:48:53 pm
content: Service Manual for PL320 old version attached below.

author: DoricLoon
author_rank: Contributor
timestamp: September 21, 2017, 09:19:03 pm
content: GPIB Controller commands attached. Sorry it's from a scan and I had to split the scan into three pages to be accepted here.

author: DoricLoon
author_rank: Contributor
timestamp: September 21, 2017, 09:22:52 pm
content: Sorry I thought that I had attached all three pages. Here's page 2

author: DoricLoon
author_rank: Contributor
timestamp: September 21, 2017, 09:23:53 pm
content: And to complete the reverse orderpage 1

author: DoricLoon
author_rank: Contributor
timestamp: September 21, 2017, 09:32:35 pm
content: EPROM Binary just incase someone need it.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 22, 2017, 12:25:27 am
content: So the GPIB-board contains the D/A-converters as well, if I read that correctly. And the circuit diagram is missing along with the calibration instruction. Does it reuse data from the display A/D-converters or does it have separate ones?Maybe looking at typical circuits for the components can solve this?

author: DoricLoon
author_rank: Contributor
timestamp: September 22, 2017, 09:31:38 am
content: I'm not sure about DAC on the GPIB side? When I had the PCB's out and checked them over I didn't identify a DAC. I didn't look up each of the components on the second PCB, as the fault I was having appeared to be a communications issue. That all seemed to come from the top processor card, which didn't have a DAC.I just assumed that since the setting of the power supply is analog while in local control. The GPIB controller would have to convert the data to analog output somehow?There are two rows of trimmers on one of the GPIB cards which I assume are for each channel. Because of the sheer number of trimmers I wonder if the DAC is done discretely and these are trimming a resistor based DAC?I'm an electrician with a desire to learn. Desire and ability are not the same, unfortunately

author: bd139
author_rank: Super Contributor
timestamp: September 22, 2017, 09:57:08 am
content: Just a heads up and I'm sorry if this sounds extreme but get rid of that power supply. They regularly suffer from pot failures (which are expensive to replace on the coarse controls), ADC/metering problems and the current damping sometimes doesn't work properly and gets stuck down at zero. They are also really poorly assembled. And as you say the caps are probably shot. I couldn't get one of them to stay calibrated as well and couldn't trace the fault.The later grey ones and the TS series (without the 5v rail output) are absolutely fine. I've got four in total of those. The black/cream/Racal branded ones are nasty though. Nothing but trouble from them over the years.

author: DoricLoon
author_rank: Contributor
timestamp: September 22, 2017, 04:17:08 pm
content: Quote from: bd139 on September 22, 2017, 09:57:08 amThat sounds a bit extreme.I see where you are coming from regards assembly. That said it's not something that I am using for extended periods of time and it will be static in the workshop, so don't think the humble interconnections should be under too much stress. Now that I've re-capped it hopefully be good for another thirty yearsI also have a triple outlet TS series with the limited use 5V rail. Recently purchased a PL320 quad mode, PL320 TGP and PL154 TGP (PL series all the brown/cream versions) OK so I'll have to recap the other two PL series units but for what I paid for them I think they are good enough value. Even if I do have to stamp up for a pot or two.I am a bit dissapointed with the current limit resolution by GPIB though (only 10mA steps). And the read back current method is a bit naff. The current is stepped down in 10mA increments until current limit is found!

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 22, 2017, 09:16:20 pm
content: Well, if no one finds the missing diagram, we have to proceed along this way.The basic device as shown in the circuit diagram is strictly analogue.So 4 setting values (2x voltage, 2x current) have to come out of the GPIB module.As the description of the 'current measurement' function, which is indeed a bit unconventional, tells us,there is no real measurement function and because of this, no ADCs.'About' 20 trimpots is a bit unspecific. But the identification of two clearly discernible rows does probably correspond to the X and Y channel. So looking at the components would be the next thing to do. And you should identify the points in the circuit diagram of the basic unit to which the GPIB-module interfaces. Maybe do some measurements on those signals in order to find out how they behave. I am a bit irritated that the circuit diagram does not even hint of a connector which would carry those signals. Are they wired singly into various points of the regulator PCBs?

author: DoricLoon
author_rank: Contributor
timestamp: September 23, 2017, 11:19:01 pm
content: QuoteI opened it up again and there are two staggered rows of nine trimmers:Channel X? | Channel Y?|25k 100k 25k 2k2 |100k 4k7 25k 4k7 10k100k 4k7 25k 4k7 10k | 25k 100k 25k 2k2They aren't laid out as two identical rows, and I think they are more like a bank of pots at one end are for channel X and the other end for the Y channel. There is also a bank of 50 resistors directly below the trimmers with four reference zenners below that. The zenners are spaced evenly along the bank of resistors.4-4016 (Quad analog switches)2-4066 - (Quad bilateral analog switches)6-4015 (Dual 4 bit shift registers)So I'm thinking that yes it does look like some form of DAC resistor network using a shift register. Then I look up the last unknown part on the board:Ferranti ZN429E (8 bit R2R DAC) There are four of these.So I think it must use a combination of shift register to set a coarse level with the DAC as fine control?I did start tracing the circuit but it would take me a considerable time to try to ascertain what function each of the trimmers has? I could just tweek a bit and see what happens, but I'm apt to make it worse than it is.Schematic or calibration instructions would be the easy way. Someone must have a copy?I may well spend an evening tracing it out (probably two at my speed) now that I see the split at least I only have to figure out nine of the pots........The biggest pain are the vias hidden under the row of resistors.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 24, 2017, 12:03:20 am
content: Quote from: DoricLoon on September 23, 2017, 11:19:01 pmYou seem to grasp the circuit and the issue, if you are willing to go through it.Quote from: DoricLoon on September 23, 2017, 11:19:01 pmIndeed, as you have to be certain that the binary step you are tweaking is included in the values you try.Quote from: DoricLoon on September 23, 2017, 11:19:01 pmBeen there, done that, got too much t-shirts. If someone describes retracing a PCB as fun, I suppose it was tiny (in the order of 1 8-pin or a few transistors, or that he didn't finish it!

author: DoricLoon
author_rank: Contributor
timestamp: September 26, 2017, 08:34:58 am
content: So I traced for the best bit of an eveing and decided stuff this for a game of soldiers.I could see the vaguest idea of how things were working. Fine control by the 8bit DAC and four coarse bits via shift register, analog switch and resistor network. But nailing down the full schematic was proving too tedious.So I decided to step the supply in 10mV (smallest step) intervals and log the readings. I then created a chart with deviation values with the X axis scaled at 8bit intervals. Repeated for the second channel.Then started tweaking(Marked the pots first, so more like a devil with wings) At first I couldn't see any movement regardless of which pot I adjusted. First mistake, never work on assumptions. The first bank of pots is channel Y not X!Found what seemed most likely to be the zero pot did bring things down to zero. Then tried to step the supply in 8bit steps and found the pot that would adjust that step. I could see that I was chasing my tail a bit and that one adjustment would muck up another but my voltages were closer so ran the deviation test again.QuoteThe attached Word document shows the chart with X & Y deviation then Y deviation after the first bit of twidling. I can now see the span on the DAC is too low. Bit one and two of the coarse are a little out but close. Bit three is little low and bit four the worst offender is relatively high.So what I plan to do is set 2.55V and find the only pot that adjusts it. (DAC Ref). Then set 2.56V should be able to find only one pot that affects it also. Then 10.24V for bit 3 and finally 2048 for the fourth bit. Each of those steps should be identifiable. From my earlier tweeking It looked like the pots may be in bit order? (yet to be confirmed) Zero, DAC, Bit1, Bit2........Will give it a go tonight. Once I have identified the function of each of the pots I'll post a my findings with details for anyone else who may find them helpful.

author: MX123
author_rank: Newbie
timestamp: January 25, 2018, 11:45:11 pm
content: So I finally got around to repairing this PL320 TGP.After cleaning off some corrosion on the green Molex connectors on the TGP board (The one with all of the presets at the top) and another fault on the damping switch contacts (open circuit), the current meter now stops ramping up until it reached O.F.L. All good.The only issue is that I inadvertently knocked off the single wire to the TGP - see photo. Does anyone know which pin it goes to? I'm sure it was in the centre!Alternatively, please may someone take a look at their unit for me? or is there a service manual available for the TGP.Glad to see that Thurlby upgraded these rubbish connections in the next versions of the unit, but of no use to me

author: DoricLoon
author_rank: Contributor
timestamp: January 26, 2018, 12:16:17 am
content: Just opened up my PL154 TGP and the wire is connected to the fifth pin from the right. Don't know if it is different on the PL320 TGP but looking at you photo the fifth pin from the right also looks marked as opposed to the other pins. Sorry should have opened the PL320 just that it is buiried deeper and the PL154 needs some attention anyway. If need be I can open up the other supply?Jim

author: DoricLoon
author_rank: Contributor
timestamp: January 26, 2018, 12:33:12 am
content: I never did get around to posting the funcion of the pots. The attached PDF shows the function of the first channel pots.

author: MX123
author_rank: Newbie
timestamp: January 26, 2018, 10:12:04 pm
content: Thank you so much. Now looking at it pin 5 from the right does look correctI have also been trying to find the replacement metal links that short to the sense terminal posts when the remote sense feature are not being used.I don't know what they are called? and cannot find anything similar on Google or ebay. Just need the right search term? LOL

author: MX123
author_rank: Newbie
timestamp: January 26, 2018, 10:17:54 pm
content: I have also been trying to find the replacement metal links that short to the sense terminal posts when the remote sense feature are not being used.I don't know what they are called? and cannot find anything similar on Google or ebay. Just need the right search term? LOL

author: DoricLoon
author_rank: Contributor
timestamp: January 26, 2018, 10:48:29 pm
content: I used one from another supply as a template and cut a set from copper strap that I had lying around. I have attached the dimmensions of an original if you want to make a set?Posted this before I noticed your image, you already have a template.

author: texaspyro
author_rank: Super Contributor
timestamp: January 27, 2018, 02:34:26 am
content: Quote from: MX123 on January 26, 2018, 10:17:54 pmI did a circuit board for these. You can order them from OSHPARK. Use their 0.8 mm thickness / 2oz copper option.https://oshpark.com/shared_projects/QBeVyK9Xhttps://oshpark.com/shared_projects/j4Htt3KM

author: Pete2
author_rank: Contributor
timestamp: March 22, 2018, 06:40:58 am
content: Quote from: DoricLoon on September 21, 2017, 08:48:53 pmHello and thanks for providing the manual. Do you happen to have better scans of the schematics? I have an older PL320 Triple that I'm restoring. It has a K2 module and a PL320 module.I'm definitely replacing all the caps and trimmers that are carbon film open-type. Cannot get them stable enough to calibrate the unit.I need the scematics to figure out if I can replace some of the trimmers with different values: PR4 on K2 module is marked as 4k7, replacing it with 5k and PR103 on PL320 module marked as 25k, 20k is easier to get. Depending on the model of course but I want to get cermet ones for stability.Other than that the unit seems to be working fine but some of the selection switches on the front panel have bad contact. And they are expensive switches! 10-20 eur each. Model is C&K 7201, not sure about the exact variant but it's a DPDT, ON-ON, with black lever.

author: DoricLoon
author_rank: Contributor
timestamp: March 22, 2018, 07:34:10 am
content: Sorry the manual attached further up the thread is the best I have.I had no trouble with the trimmers.Little bit of trouble with the front pots which are easy enough to take apart and clean, which cured them.If you haven't already changed the caps? Do that first they were the biggest culprit for stability by far!

author: Pete2
author_rank: Contributor
timestamp: March 27, 2018, 04:43:56 am
content: Do you mean the wire-wound pots? How did you clean them? The last time I used the unit they seemed fine but we'll see after I've replaced all the electrolytics. I was going to replace the regular carbon pots though.By the way if someone needs to replace the switches, their original type is C&K 7201, type M2022TJW01-GA, you can maybe get other pin variants, W01 is with solder lugs. They are quite expensive so found a few replacements:1. NKK M2022TJW01-GA-1A (https://www.digikey.fi/product-detail/en/nkk-switches/M2022TJW01-GA-1A/360-2218-ND/1165803)2. TE Connectivity A201J61ZQ0004 (http://export.rsdelivers.com/product/te-connectivity/a201j61zq0004/te-connectivity-dpdt-on-on-rocker-switch-panel/7109375)The TE one is the cheapest and will do just fine.Also, I'm thinking of replacing the voltage references. Just for fun, they seem to work but are very old and maybe not that accurate although of course what matters is the stability as this unit is calibrated. Their type is ZN404, which is a 2.45 V reference, without getting an adjustable version a 2.5 V will do. Any one know what spec actually matters in the reference? Drift (ppm)?

author: DoricLoon
author_rank: Contributor
timestamp: March 27, 2018, 05:10:07 am
content: Quote from: Pete2 on March 27, 2018, 04:43:56 amThe back is crimped on with two small lugs, bend the lugs back and you can pop the back off. They bend back just fine.

author: Pete2
author_rank: Contributor
timestamp: March 30, 2018, 03:42:50 pm
content: OK I have finished replacing all the caps (el and polyester) and trimmers (TME supplied me with 2 pcs of 200k pots and one 1k when I ordered 3 pcs 1k, no wonder the cal was so touchy!) + changed the awful sockets for the voltage meter ics to precision ones. The PL320 side is working just fine, have to get a more accurate meter to calibrate it properly.The other side (K2) which supplies 4-6 V @ 7 A and 0-30 V @ 1 A is not so good. It doesn't have a voltage display, only current and without load it displays 0.14 A. I can load it and it displays the current but doesn't supply over 2 A or so. The voltage is bang on though. Calibration doesn't help, max. current is the 2 A or so.So looks like it's a repair after allLuckily I got the other ics from TME while I got the caps. Not much there, an LM324 and an LM358. And tons of discrete BJTs. Sadly the K2 schematic is the worse one so could pretty hit and miss to repair.

author: Pete2
author_rank: Contributor
timestamp: March 31, 2018, 04:16:24 pm
content: Figured out the meter issue, was a broken track due to me replacing the metering ic socket. But the current issue (pun) still remains, outputs only 3.45 A when should get 7+ A. The adjustment just doesn't go far enough. Checked the output stage transistors, they're fine (2 x 2N3055) and the one driving them looks like OK but I don't have a replacement at the moment. Replacing the opamp did not help either.It's a rather complex circuit with several different ground points and voltages.

author: bd139
author_rank: Super Contributor
timestamp: March 31, 2018, 04:18:30 pm
content: Being old thurlby, check for dry joints

author: Pete2
author_rank: Contributor
timestamp: April 02, 2018, 09:23:07 am
content: Well, I think it's working now - and has been the whole time. The 4-6 V side is actually smarter with its current limiting as it's not supplying the entire 7 A when the output is shorted. I had a doubt about this but didn't try it until now.The calibration instructions say "load the unit to 7 A" and that means not short the output. Not sure what the limit is for short detection but after I constructed a suitable resistor I was able to load it to just over 7 A and set the current limit to 7.35 A as per the instructions. If it detects a short the output current is limited to about 2.65 A.Now I can finish upgrading some of the wiring (seems a bit thin for 7 A). I already replaced the mains wiring and added a protective earth point. Also I want to add a discharge resistor for the K2 module 30 V / 1 A output. When the unit is switched off, the main cap doesn't discharge at all and that holds 55 V. Not that it's dangerous but when servicing it's nice it discharges eventually.