Thurlby PL320QMT Later type power supply – Adjustment after repair? - Page 1
Forum section: electronics / repair

author: Chris56000
author_rank: Frequent Contributor
timestamp: April 18, 2018, 03:08:25 am
content: HiThe following relates to the "Phase 2" Thurlby PL320QMT Three Channel PSU in use as standard on all our work benches at my employer's!My unit had developed a fault where the middle (is that the "Slave" section?) 30V 2A section bogged out at 130mA and point–blank refused to supply anything above that, and the setup adjustment was useless!I decided to investigate (grumpy manager on holiday this week!!!!), but nothing obviously amiss came to light, so decided to replace the LM324 (IC7, current control/sense amp), the 4052 analogue switch that selects between the current sense o/p and the front panel current limit control, the electrolytics and Q6 (BC559C) – this restored the full rated 2.2A output, but I can't get the adjustment of VR7, the one to set the PSU for 10mA output with the current front panel control set for 10mA – the pot VR7 doesn't do anything!Can anyone suggest what I might have done wrong? My employer only had BC557Bs in stock for Q6 – is it important to use the correct BC559C here?Also, has anyone got the circuit diagram for the 5V 4A board in these? I used the 49 page manual from K04BB and the 5V 4A bit appears to be missing!Chris WilliamsPS!Nearly all of these units have a Driver board with a W02 bridge rectifier and two 2A pico–fuses in lieu of the four separate 1N4002 diodes given on all the diagrams in the manual – I've not found a revised page detailing this design change – has anyone got info from TTi on it for completeness?

author: wraper
author_rank: Unknown
timestamp: April 18, 2018, 10:35:17 am
content: QuoteNot repairing it completely?Often more than one component fails in anything that is power related.

author: Chris56000
author_rank: Frequent Contributor
timestamp: April 18, 2018, 12:45:39 pm
content: Hi!I replaced IC7 (LM324), IC3 (4052B), the TIP120 on the driver PCB, tested all the 1R0 small current–sharing 2N3055 emitter–resistors which were all perfect, all four of the 2N3055 pass transistors which also tested all perfect, checked the current–sense resistor R51 on the control PCB, and the two short wire loops provided by TTi to derive a 4–terminal connection to same, and the presets on the control PCB themselves – I doubt I would have missed anything obviously amiss from that lot, there was no sign whatsoever of overheating anywhere, and the PSU now gives 2.2A out at up to max voltage – it's just that I can't set the initial 10mA current point by VR7 as detailed in the manual – turning VR7 from one end to the other gives no current whatsoever – I WAS able to carry out this adjustment with whatever faulty component(s) were causing the 130mA hard current–limit fault!Chris Williams

author: Kevin.D
author_rank: Frequent Contributor
timestamp: April 19, 2018, 09:55:06 pm
content: Not sure which pcb version you are referring to by 'Phase 2' ?, but the full service manualfor the pl series covers pcb issue 2 to 4 is available somewhere in a thread on eevblog andalso elsewhere online .In regards your other questions :-Q6 is not critical it's only job is to pull 'ilimit' high to indicate CC limiting is active.Regarding the 5V/4A module, The manual states it is very similar to the 5V/7A module , theschematic of that is included in the service manual mentioned above.Vr7 is not used for what you think it is .VR7 is used to trim the current sense resistor (r51) so that the internal meter reading isaccurate when compared to an external ammeter reading .VR8 is used to trim real CC value (output enabled) to match preset Value (output disabled) this is best set at 10mA.Vr4 sets absolute maximum current limit.Extract from PL service manual :-Current calibrationSet output switch to OFFSet the current limit control for a reading of 10mASet output switch to ON ON and short the output terminalsAdjust VR8 for a reading of 10mARemove the short and connect an ammeter between the output terminals.Set current limit to approximately:900mA - PL3101800mA - PL3202700mA - PL3303600mA - PL154and adjust VR7 so that the internal and external ammeters read the same.Regards

author: Chris56000
author_rank: Frequent Contributor
timestamp: April 25, 2018, 06:59:04 pm
content: Hi!Thanks for your assistance – I've just remembered I was trying to set it up with VR7 instead of VR8!I'll have another go at it next time my grumpy manager's out of the way!Chris Williams