Repair: Thurlby Thandar Instruments (TTI) PL330 current limiting problem - Page 1
Forum section: electronics / repair

author: DC73
author_rank: Newbie
timestamp: March 30, 2018, 07:56:21 pm
content: Hi, I purchased a TTI PL330 linear power supply from a merchant on eBay.The transformer is labelled with a date in 1998.It essentially works, and is able to deliver power at the designated voltage, however the current limiting isn't working correctly. I'll explain further below.With the output power turned off, the unit consumes around 10W/40VA. I can set the current, however the voltage setting remains at zero until I have cranked up the current to around 864mA, then the output voltage can be set in the usual manner. If I set the voltage to, for example, 5V and then turn the current limit back down to 500mA, then the voltage will drop back down to zero again.With the output power turned on, the output voltage measured at the terminals with my Fluke DMM generally agrees with the indicated output to within around 20mV, right across the scale from 0V to 32V, as long as the current limit is set to 864mA or above. The indicated current will waver around this figure. If I attempt to reduce the current limit to less than ~864mA then the current limiting dots flash and the output voltage drops to zero.If I set the maximum limits of 32V and 3.1A and then turn on the output power, the ammeter will indicate 864mA at 32v, suggesting a power delivery of over 27W. The mains power meter still indicates a power consumption of only 10W / 40VA.I have inspected the driver and control boards, and I haven't observed any obvious damage to components or the boards.I am wondering if somebody more experienced than myself with repairing these units might have a clue as to what might cause the above behaviour?I think my next step would be to remove the power transistors and test individually in isolation, as well the filter capacitors on the driver board. Does this seem reasonable?Cheers!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: March 30, 2018, 09:22:06 pm
content: Measure the drop across the current sense resistor (R51 on the PL330Q)) directly with a meter to determine if there is actually the indicated current through it or it's a sensing circuit fault .If it's sensing circuit fault then there are some presets for CC mode trimming that can go intermittent over the years, I would check these first.(VR8,VR7 and VR4 referring to pl330 quad).CAUTION If it's like the dual PL330 then BE VERY VERY CAREFUL when your twiddling those presets in situ because the back of your hand will be less than a few centimeters from the exposed LIVE and NEUTRAL lugs on top of the transformer.!!!!Regards

author: DC73
author_rank: Newbie
timestamp: April 26, 2018, 12:02:31 pm
content: Thanks Kevin.Yes, the mains transformer in this PL330 also shares the exposed L & N lugs feature found in the other PL3x0 PSUs!I had the idea to verify that the ZN404 voltage reference wasn't going bad, so had to partially dismantle the display to check that it was still creating a reference of 2.45V. In the process of jury-rigging it so that I could do this, and thus unplugging and re-seating the three cables between the digital, driver and output boards it suddenly started working in the desired manner. I therefore carefully reassembled the unit before I could undo whatever I did, and it has been working fine ever since.