Thurlby PL320 (Sinclair) lab power supply - current monitor issue - Page 1
Forum section: electronics / repair

author: xyberlink
author_rank: Contributor
timestamp: April 22, 2018, 06:23:55 pm
content: Hi, I have one of the old brown / cream Thurlby PL320 (30v / 2amp) bench power supply units with a current meter issue.The unit is working, i.e. with the output on, the output voltage is good and it'll take a load ok.The voltage meter is accurate / stable, both with the output on and off.The current meter, however, displays random, varying numbers, and sometimes OFL, changing continually. Essentially it's erratic, and all over the shop. Switching on the output changes the values, but still erratic / random. Values have no resemblance to set (output off) or active current (output on with/without load). It just doesn't sit still or give a reading that makes any sense. Could be 000, 2615, 3954, etc. etc., changing several times a second.With the aid of the service manual, I've checked / tested many components but yet to find anything overly amiss:- Capacitors seem ok (changed a couple that were wide of tolerance).- Resistors seem ok - checked pretty much all in the current control side.- Diodes all seem ok (think I've checked them all)- Transistors seem ok (only checked the 2 or 3 around the current side of things - rest seem to drive the display, hence assumed ok.- R8 (0.5R W/W current sense) seems ok and with a 5v / 80mA load got a v-drop of approx. 0.04v.- VR1 (10k Log Pot for Current control) was iffy - reading 17k and very jumpy, so have swapped that for a 10k linear pot for now.- Also swapped the ADD3701 chips over and issue remained on current side, so both of those appear good.Any ideas on what to check next much appreciated.Thanks.

author: fenugrec
author_rank: Frequent Contributor
timestamp: April 26, 2018, 03:46:08 pm
content: Quote from: xyberlink on April 22, 2018, 06:23:55 pmI'd start there and trace the signal path ; do you have schematics ? That 0.04V probably goes through a differential amplifier before ending up at an ADC.

author: xyberlink
author_rank: Contributor
timestamp: April 28, 2018, 07:05:52 pm
content: I've now done some more work / investigation.While checking through more components, I noticed that (for reasons unknown), the ADD3701 decoupling capacitors (C17/C18) and the resistors (R16/R17) either side of Q4 (turns on decimal points in current limit mode) are spec'd for the PL154, hence I've now changed these to values specified for the PL320. All other components appear to be correct for the PL320.I've removed R8, mainly because somebody's done a bodged replacement in the past which needed tidying up, but it gave me a chance to check it out of circuit and it's in spec.In operation, I've now seen the current limit working, which implies the unit is mostly good, i.e. with the exception of the current monitor.As it stands, with output on or off, the current value increases to 3999 then goes to OFL and stays there. With a load attached, it climbs more slowly, but still ends up at OFL.I think all the IC's are good...IC1 - 5v regulator - is outputting 5v.IC2 - LM358 op-amp - voltage control and current limiter appear operational.IC3 / 5 - ADD3701's - as mentioned in first post, when swapped the issue remains in the current meter side, hence both chips seem good.IC4 - CD4007 - toggles current display with output on/off, which is working.IC6 - CD4011 - controls decimal points on current display, which are working for output on/off and current limitThis appears to leave me working on the comparator area of IC5. vRef is good at 2.49v against a target of 2.45v. I've tested every resistor and capacitor in that section of the circuit and everything seems good, so not sure what my next step is yet.

author: xyberlink
author_rank: Contributor
timestamp: April 29, 2018, 01:04:14 pm
content: When testing the comparator lines - vRef (to ADD3701 pin 14), and vIn (to ADD3701 pin 11), I've noticed that with a multimeter attached between pin 11 and ground, the current display comes broadly into line.vRef is 2 volts.vIn is ~0.34v.Current display is ~680mAGuessing a bit, but I assume a vIn of 2 volts (i.e. equal to vRef) would yield a display of 4000mA (correct for a PL154), and therefore my reading of 0.34v (17% of 2 volts), giving 680mA (17% of 4000) is somewhere near (give or take my readings and the need for calibration). This being a PL320, I'm also assuming vIn shouldn't go above 1 volt, limiting the current to 2000mA.So it would appear that vIn is climbing up to (and possibly beyond) 2 volts, but the tiny draw from a multimeter is enough to arrest this, thereby returning a valid vIn.Question is what would cause vIn to climb beyond the expected value?

author: coromonadalix
author_rank: Super Contributor
timestamp: April 29, 2018, 01:34:09 pm
content: Service manualhttp://www.chriswilson.tv/PL_Series.pdfhttp://k1.dyndns.org/Vintage/Sinclair/OtherInventions/Other Electronics Products/

author: xyberlink
author_rank: Contributor
timestamp: May 06, 2018, 04:03:33 pm
content: Finally traced the problem to IC4 - the CD4007, whose output provides vIn to the comparator.CD4007 "consists of three complementary pairs of N- and P-channel enhancement mode MOS transistors", but also appears to contain numerous diodes. Looks like something within this IC has become leaky, causing the voltage output to be higher than expected, thereby screwing up vIn.Having replaced this IC, it appears to be working properly again and has calibrated ok.