Lambda LPT-7202-FM Triple Output Power Supply - Page 1
Forum section: electronics / testgear

author: Chuy
author_rank: Newbie
timestamp: October 23, 2022, 10:49:57 pm
content: Hello everyone.I just got this used Lambda LPT-7202 but it doesn't deliver enough current on all 3 outputs.I calibrated all 3 voltages and was able to bring the voltage to the maximum value but on the Current calibration, I was short 1 amp even though I rotated the R102. R202 and R302 to the maximum.The problem is when I connect a load even a small motor or light bulb, the voltage drops down to 3 volts and the motor bearly rotates or the lights barely light up, this happens on all 3 outputs.I also put a jumper to the V+ and V- to adjust the current to the maximum and it does the same thing.For example;I turn on the power supply without load and adjust the voltage to 10 volts with the maximum current then I connect a car light bulb (12 v) immediately the voltage drops to 3 volts and the light bulb bearly lights up.Gif attached.I can turn the voltage knob to a higher voltage but it doesn't go up, stays on at 3 volts.Happens on all 3 outputs.Any suggestions/advice on what to do?

author: edavid
author_rank: Super Contributor
timestamp: October 24, 2022, 12:01:37 am
content: When you connect the lamp and the voltage drops to 3V, what does the current meter read?

author: Chuy
author_rank: Newbie
timestamp: October 24, 2022, 01:08:27 am
content: About 10maPhoto attached.

author: Chuy
author_rank: Newbie
timestamp: October 24, 2022, 01:12:38 am
content: 100maCorrection

author: alm
author_rank: Super Contributor
timestamp: October 24, 2022, 09:06:57 am
content: Can you measure the voltage going into the pass transistors (at the collector side) with a scope? I wonder if the filter caps are dry and the regulator is dropping out due to excessive ripple. Also measure the voltage rails for the regulation circuit while you're at it.

author: Chuy
author_rank: Newbie
timestamp: October 24, 2022, 12:58:47 pm
content: Quote from: edavid on October 24, 2022, 04:13:04 amYes, I was thinking the same thing, how can all 3 circuits fail the same way? Maybe the problem is before those circuits affecting all 3.

author: Chuy
author_rank: Newbie
timestamp: October 24, 2022, 12:59:55 pm
content: Quote from: alm on October 24, 2022, 09:06:57 amI will, thanks.

author: MarkL
author_rank: Unknown
timestamp: October 24, 2022, 02:29:38 pm
content: Check to make sure all the default jumpers on the back terminal blocks are in place for each supply. There should be three jumpers on each terminal block in the positions shown by the printing on the back panel. Maybe someone rewired your unit for a non-standalone setup.Otherwise, I agree that it's strange behavior since the supplies are independent. The only thing in common is the transformer primary and line side wiring. I think I would start by picking one of the supplies and monitor the two AC secondaries that drive that supply while you put that output under load. The expected AC voltages are shown in the schematic.Maybe it's a 240V transformer? Although, I don't see that such an option ever existed for the LPT-7202-FM.There's also an outside chance someone blew up each output in the same way. "Hmmm, my circuit doesn't work here. I'll try the next output..."Does the front panel current limiter do anything (like adjust gradually from 0 to 100mA)? Or does it get pinned early in the rotation to 100mA?

author: Chuy
author_rank: Newbie
timestamp: October 24, 2022, 03:15:35 pm
content: Rotating the current knob to the left goes gradually to zero and the voltage goes to zero as well without moving the voltage knob. And rotating the current knob to the right, it goes gradually from zero to 100ma and the voltage goes gradually up to 3 volts.I'm going to check the jumpers on the back, Thanks!

author: Chuy
author_rank: Newbie
timestamp: November 01, 2022, 05:51:23 am
content: We got it Alm!!!All 3 Capacitors dry (C101, C201, C301), replaced and we got good voltage and current, I can burn the light bulb if I want to LOL.All 3 Outputs are good now, I recalibrated them as well.Photos attached.Thank you so much Alm and everybody else who collaborated on this post, I really like this power supply because it shows the voltage and current at the same time on all 3 outputs.Awesomeness!!!

author: alm
author_rank: Super Contributor
timestamp: November 01, 2022, 08:59:00 am
content: Congratulations on the fix! These power supplies are indeed very solid.