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Apr 08, 2009, 02:00 PM
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Caltek PSM2/5A PSM3/2A Lab Bench Supplies Calibration


Well. So I bought three of these in auction as faulty... Two with display problems. Two with no output. Two which would fire a 'deadly' surge to the channels when powered down...

All repaired now! The purpose of this thread is to find a home for the information which I picked up along the way, namely which pots do what in terms of calibration.

The killer surge was caused by the Zener's in the power/frequency stage being the WRONG WAY ROUND. That was on one of them. On the other the EMF relay diodes had died, such that in it's final breath whatever was left in the transformer/caps was dumped straight to the channels.

The displays were caused by dry SMD joints on the 7-Segment LED boards/modules. Relatively straight forward.

The dead channels were caused on one by a broken wire, and on the other the bridge rectifier (1n4007) network which powered the main output/frequency stage, were shot.

Quality control appears to be an issue, a MAJOR issue. Where products are shipped to distributors with faults that are potentially dangerous, certainly to your target circuits anyway. On one of them I've taken the front panel switches apart and removed the metal contacts.. That was was firing 60v out per channel due to an internal short. The miniature contacts in the switches were bent rendering them useless as they're all machine pressed parts of only a few mm. I've hard-wired the switches to independent channel operation on that one.

Operationally these supplies are quite straight-forward, with each channel essentially having a mirror circuit to the other. That at least makes fault diagnosis easier. They're certainly independent of one another.

Would I buy one? Well probably yes. The main reason being that provided that the main toroidal transformer (custom) doesn't go, then you could keep on patching it up indefinitely.



With that calibration diagram which shows what pots do what it's possible to crank the output voltage up to some 80-volts! Current can also be increased somewhat too.

The 2-amp models are 2-amps per channel, thus giving 4-amps of output. They're perhaps underselling themselves a bit there. The 5-amp models can kick out in excess of 10-amps. Again, underselling themselves when marketed purely as a 5-amp supply.

All in all it's been a fun couple of days. Oh, they do get through fuses quite easily, and certainly not as a result of any malfunction or short. I've replaced the 3.15A 250VAC fast blows with 4A anti surge 250VAC ones. These are also multi voltage too, be it 110V/220V.
Last edited by Alison F; Apr 08, 2009 at 02:06 PM.
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Apr 09, 2009, 03:56 AM
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Nice work Alison F..

How did you come by these nice bits of kit?
Apr 09, 2009, 04:55 PM
Thread OP
Hi Chippie,

I picked them up off of fleabay as faulty. Bit more than I expected to pay, but it was local pickup so not as expensive as they could have been.

Still looking over them now and burning them in. As said earlier, providing that the main transformer doesn't go then they're otherwise patchable indefinitely. Almost all of the parts are essentially low-tech and standard.
Feb 16, 2011, 08:27 AM
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Please someone have the service manual for these?? I need it to repair it!!!
Dec 15, 2012, 12:03 PM
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I know this is an old thread but did you ever get a manual.? I am looking for one also.
Oct 01, 2013, 10:49 AM
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soemtron220's Avatar
Originally Posted by OnlyMe
I know this is an old thread but did you ever get a manual.? I am looking for one also.

I have a circuit diagram (as a pdf) for the PSM2/5A supply reversed out of a unit I have. Email ([email protected]) or PM me for a copy, It is unwarranted but should be good for fault finding.

I have found several anomalies on the PCB's which are listed on the circuit. It will be revised in the future to add some circuit measurements and misc info.


Last edited by soemtron220; Oct 01, 2013 at 10:58 AM. Reason: small text edits
May 02, 2016, 02:19 PM
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PSM3/2A output stage shortcircuit fix

Hi! I have got also one of these PSM3/2A lab supplies. Problem with my supply was, that the channel CH1 was internally short circuited. Current meter was able to measure current, but voltage didn't rose.

I opened the supply, and found a short circuited 1N5408 diode very next to the output terminal CH1- (please see screenshot).

I replaced it with BY329-1200, that I happened to have in my spare part box. Now both channels are working just fine.
Jul 25, 2017, 04:54 AM
Registered User
Hi did anyone have a schematic for these ?

I am attempting a repair on a unit where the voltage is fluctuating by a volt on one channel

Feb 10, 2019, 10:58 PM
Registered User

PSM 2/5A Wiring Diagram

Link to wiring diagram

There are some revisions marked on it, so be careful.
Sep 10, 2019, 07:40 AM
Registered User

5A supply repair and modifications

I have the 5A version of this bought from the Farnell trade counter.
The fault was missing digit on channel1 display.
Changed both current display modules for Rohm parts from RS Components.

The wires connecting the main board to the display were rubbish. They had used IDC cables and put crimps on them. The crimps were not holding the copper..not a good idea. They would just pull out and some fell out without doing anything!
Replaced with 28AWG ribbon cable directly soldered to both boards with some UV glue to strengthen the joints.
Next problems are design.
The displays vary in intensity depending of the number of segments lit. It always did this even with the old green current display module.
Analysis of the design showed that the 27 ohm resistor that feeds the displays is a design error. This is replaced with two series IN4007 diodes.
Typically the 7107 sinks 8mA per segment, so the maximum current it needs (when displaying 2.88 amps) 8mA x 20 segments, so 160mA. BUT 27ohms x 0.16 is 4.32volts, so it's clear the display circuit will run out of volts as you only have 5V to start with!! I decided on: 5V supply minus 1.6V for the two 1N4007 diodes, minus 2.2V for the LEDs (ROHM spec) leaving 1.2V on the 7107 pin which is sufficient to sinkaround 6mA. Why fit any diodes at all? To reduce the power loss in the 7107 chips. This is strongly advised in 7107 datasheets.
Next problem is the current sense resistors. Not only did they smoke when running at 5 amps, they are in the wrong place. I will be moving then to the other side of the board rather than having them trapped in between the display and main board. At full current this is an 8 watt source of heat!!!
The display driver chips (7107) are prone to variation with temperature so moving the resistors is a good idea. I am also considering fitting a slow moving fan just to cool them. To create room for this, the main transformer may be moved over an inch or so.

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