Power Designs TW5005w current overload on side A - Page 1
Forum section: electronics / repair

author: Mixographer
author_rank: Newbie
timestamp: August 28, 2017, 06:10:19 am
content: Hi all,I've obtained a Power Designs power supply and need a little help with where to look for repairing it. One side is working very nicely, but one side overloads and goes into current mode at about 25mA on 5 volts.The manual has some suggestions for troubleshooting the voltage output, but nothing about troubleshooting load promlems. I got the manual on the K04BB site. I don't really know where to start or what to look for. Any advice greatly appreciated,Jimmy

author: Armadillo
author_rank: Super Contributor
timestamp: August 28, 2017, 12:08:04 pm
content: Hi,When you say overload, I suppose you are saying that the voltage swung to the max.?Check voltage across C6. Is it about 12.6 volts?If yes then follow below;Also measure the voltage of this transistor with respect to its emitter Q3 and Q2Q3 ;B-E =C-E =Q2B-E =C-E =then Measure U1;Pin 2 with respect to Pin 11;Pin 3 with respect to Pin 11;

author: Mixographer
author_rank: Newbie
timestamp: August 28, 2017, 02:40:51 pm
content: Sorry, when I say overload I mean that the voltage won't goAny higher and the 'current mode' led comes on. If I watch the meter om mA, it swings all the way up to 500 mA like a short.Thanks!

author: Armadillo
author_rank: Super Contributor
timestamp: August 28, 2017, 02:59:56 pm
content: Quote from: Mixographer on August 28, 2017, 02:40:51 pmThen you should;Check voltage across C6. Is it about 12.6 volts? We need to make sure always the reference is correct.Let make sure R12 potential meter is OK, so you should measure the wiper voltage with respect to the board marked as 12, as you turn the potential meter, the voltage varies on the wiper;Edit: you can also measure the wiper ohms without power on, objective is to check function of potential meter.then Measure U1;Pin 5 with respect to Pin 11;Pin 6 with respect to Pin 11;Edit: Pin 7 with respect to Pin 11;Edit: the silly thing about this design is that it exposes the U1 inverter input Pin 5 to external connection RC unprotected.

author: Mixographer
author_rank: Newbie
timestamp: August 28, 2017, 04:57:46 pm
content: Thanks for thi input. I checked R12 on both A and B sides, they both seem ok and read between 0 and 1K smoothly.On U1A side. 11 to 5. =. 5.7v B side. 6.06v11 to 6. = 5.5v 5.6v11 to 7. = 17.06v. 17.25vThe output caps C7 look good visually. Could a bad out put cap go into a short above a certain current?Thanks!

author: Armadillo
author_rank: Super Contributor
timestamp: August 28, 2017, 05:23:18 pm
content: You can take out C7 temporarily and power up to ascertain whether its shorted or the problem resolved, in that case you confirmed C7 as the problem.Edit: Check Q4 voltages B,C,E with respect to U1 Pin 11; [could be shorted]Edit: Power off, measure R16 the 3W series resistor.

author: Mixographer
author_rank: Newbie
timestamp: August 28, 2017, 06:21:27 pm
content: I took c7 out with no change. I measure q4B 17 (from pin7)E 17 (from R29)E -1 mV (from VR2)All in relation to pin 11 on U1ThanksJimmy

author: Armadillo
author_rank: Super Contributor
timestamp: August 28, 2017, 06:26:26 pm
content: Quote from: Mixographer on August 28, 2017, 06:21:27 pmWhen you measure, the current mode LED isn't light up?how about the Power off, measure R16 the 3W series resistor.

author: Mixographer
author_rank: Newbie
timestamp: August 28, 2017, 06:43:39 pm
content: Correct, the current mode led is off.R16 measures 3.2 ohms on both boards with the power off. I checked q2 with the diode setting and it seems ok, no unexpected short.Thanks again,Jimmy

author: Armadillo
author_rank: Super Contributor
timestamp: August 28, 2017, 07:00:55 pm
content: Quote from: Mixographer on August 28, 2017, 06:43:39 pmSorry, it does take abit of studying cause from the above, I cannot see why the voltage cannot be adjusted up.Can you ascertain C8 Voltage.?Also check F2 fuse.Edit: Measure from the output terminal, adjust the voltage until the current mode LED lights up. What is the voltage.? [Set 500ma Range, R12 to max current]

author: Mixographer
author_rank: Newbie
timestamp: August 28, 2017, 07:34:09 pm
content: I have a little Led load that draws about 30mA at 5v.... at around 7 v the current mode lights up. Maybe S2 is dodgy and in't switching up the range to 500mA? C8 is 78v and the fuse seems good. I'll see what is up with S2.

author: Mixographer
author_rank: Newbie
timestamp: August 28, 2017, 08:08:31 pm
content: S2 was the culprit. I read the giant power designs thread, and should have realized that these old switches would be the most likely culprit. I cleaned S2 again and now the 'current mode doesn't come on unexpectedly.I learned a lot looking at the schematic and taking the readings. Thank you Armadillo for that. As for wasting your time chasing down this issue, I apologize.With much gratitude,Jimmy

author: Armadillo
author_rank: Super Contributor
timestamp: August 28, 2017, 08:16:44 pm
content: Was about to come that way to S2, fortunately you found it first.Great, I enjoyed troubleshooting it with you.Thanks;