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As the battery is old, I recharged it. Trying to do so with my power supply (bloody HAMEG, now Rohde&Schwarz, manual explicitly states battery charging is possible) the PSU went *POOF* Had to use the old style charger from the garage.
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The PSU has blown the pass transistor and vaporized the shunt resistors. Have to order...
Oh, sorry to hear that! What's the exact type/model number, please - just so I can avoid these in the future?
Good luck for your repair.
THDplusN_notsobad
Sure. It is a HM 7042-2 Triple Power Supply. Despite the blown pass transistor, which could be due to a cheap TIP33C from ISCsemi, it has some design flaws (later for more). I got it cheap (€10,-) in defective state, it took me some time due to no available schematics. When I got it the second channel (0-32V, 2A) was displaying silly amps and no output. Problems I found in the device were:
- pre-owner removed the transformer windings from the channel
- pass transistor was blown
- shunt resistors vaporized
Then I got output and the ammeter showed proper results, but it produced nasty spikes on switching the output on/off. I finally found a SMD resistor that is across the DPDT-relay contacts was blown too, this lifts the regulation feedback from the output during switching (break-b4-make) so the regulation will drive the pass transistor like crazy trying to maintain the output voltage during switching.
During burn in tests I loaded it with 2A at 1V and the pass transistor was blown again. I replaced and noted to myself that this is not a foolproof lab power supply. Testing current step response with dynamic loading in CC mode showed some voltage spikes on the output. Did not investigate further (confirmed by another owner of that PSU, at first I thought this was one more fault).
Design flaws that AFAIK still exists in later models (HM7042-5 is different and may behave well):
- channel 2 pass transistor is a TIP33C that will operate out of SOA on maximum load as the voltage after the caps is about 60V unloadad, 50V loaded, so the single TIP33C will (try to) dissipate up to 100W
- dodgy current regulation with very slow response, produces significant overvoltage spikes on step response
- manual states switch mode pre-regulation, but this is only implemented for the first channel (yes, that is quite a surprise that both channels are different)