Products > Test Equipment
Test Equipment Anonymous (TEA) group therapy thread
Robert763:
So the 8920A PSU didn't work. The OVP crowbar is triggering. Now this is a issue because its hard to tell if the crowbar is faulty or the regulation loop. The unit is densly packed so hard to work on. And of course it's mains powered so can bite. These power supplies are unusal in that they use a transformer to isolate the feedback. It's T2 in the picture.
A small ferrite toriod with 3 windings. Two on the mains side, one on the low voltage side. One mains winding drive the gate of the MOSFET. Not totally clear how it works but looks like the LV side causes the core to saturate and terninate the drive to MOSFET. Anyway, one of the primary windings was open circuit. Hard to find in circuit because it is shunted by low value resistors.
Fortunately I was able to dig the fused end of the winding out and repair it.
PSU fixed :)
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Don't worry about the failure message, the input module isn't connected. I was not going to put it all back together for a PSU test. Not putting it back together tonight. It's been a long day and I don't want to malke a mistake.
Robert.
tautech:
--- Quote from: Robert763 on June 02, 2023, 07:39:09 pm ---Anyway, one of the primary windings was open circuit.
--- End quote ---
Had the glue corroded the winding ?
factory:
--- Quote from: Robert763 on June 02, 2023, 04:53:03 pm ---HP 8920A Radio test set PSU repair update.
On the mains side we have literally blown up IRF 840 main switch MOSFET, ZXT650 driver bipolar, 0.1R 1W, 4.7R 1/4W resistors and 47nF RIFA. Not obvious is 9.1V 1.3W zener short circuit. Replacing those got it running but output voltages ar a bit off and a couple of output electrolytics getting warm ::)
Most of the output electrolytics were starting to leak physically and electrically. So replaced all those too. Just having a cup of coffee before a final check and putting the PSU back together and plugging in...
Robert.
--- End quote ---
Having read your repair, I'm glad I replaced the RIFA madness snubber cap, in the HP 54615B* scope when I acquired it, seems to also use a Computer Products PSU, but not the same one.
Are you going to name & shame the peeing capacitors?
*Seems Scottrade has a basket case 54615B, described as missing the power socket, nope missing the whole PSU more like. :-BROKE eBay auction: #325678748332
David
Robert763:
--- Quote from: tautech on June 02, 2023, 07:48:26 pm ---
--- Quote from: Robert763 on June 02, 2023, 07:39:09 pm ---Anyway, one of the primary windings was open circuit.
--- End quote ---
Had the glue corroded the winding ?
--- End quote ---
No. When the MOSFET when short due to the snubber failure the main (300V DC) filter capacitor discharge path included the winding and it fused.
Robert763:
--- Quote from: factory on June 02, 2023, 08:10:13 pm ---
--- Quote from: Robert763 on June 02, 2023, 04:53:03 pm ---HP 8920A Radio test set PSU repair update.
On the mains side we have literally blown up IRF 840 main switch MOSFET, ZXT650 driver bipolar, 0.1R 1W, 4.7R 1/4W resistors and 47nF RIFA. Not obvious is 9.1V 1.3W zener short circuit. Replacing those got it running but output voltages ar a bit off and a couple of output electrolytics getting warm ::)
Most of the output electrolytics were starting to leak physically and electrically. So replaced all those too. Just having a cup of coffee before a final check and putting the PSU back together and plugging in...
Robert.
--- End quote ---
Having read your repair, I'm glad I replaced the RIFA madness snubber cap, in the HP 54615B* scope when I acquired it, seems to also use a Computer Products PSU, but not the same one.
Are you going to name & shame the peeing capacitors?
*Seems Scottrade has a basket case 54615B, described as missing the power socket, nope missing the whole PSU more like. :-BROKE eBay auction: #325678748332
David
--- End quote ---
The Failed parts. Not much left of the ZTX650. he fuse just did it's job.
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