Hi guys,
I was checking my linear PSU, just recitifers and caps. My phono preamp runs hot and wanted to check voltage drop the preamp's LM317 has to do. So my DMM probe shorted input supply, made a spark, but still all worked. Haven't seen from were I've shorted I've tried to read voltage again... and spark again! Oh, yes the case is grounded... stupid me! I was in a "OK I'm sleepy and can do it tomorrow, but do it now, I want to know how much of voltage drop..." .
Back on the turntable. It does amplify but now I have some hum. I messed with something seems... one more time... ='(
Do you think the caps are abused and need to be changed? Or even some rectifiers are gone? Short only after smoothing caps, before regulator.
Thanks
Matthieu
I was checking my linear PSU, just recitifers and caps. My phono preamp runs hot and wanted to check voltage drop the preamp's LM317 has to do. So my DMM probe shorted input supply, made a spark, but still all worked. Haven't seen from were I've shorted I've tried to read voltage again... and spark again! Oh, yes the case is grounded... stupid me! I was in a "OK I'm sleepy and can do it tomorrow, but do it now, I want to know how much of voltage drop..." .
Back on the turntable. It does amplify but now I have some hum. I messed with something seems... one more time... ='(
Do you think the caps are abused and need to be changed? Or even some rectifiers are gone? Short only after smoothing caps, before regulator.
Thanks
Matthieu
It's more likely that the regulator was damaged. It's unlikely that the preamp would work at all,
if the rectifier diodes or the input filter capacitor are shorted. Check the regulator output to see
whether the regulated voltage is correct. Do you have reverse diodes across the regulator and
on the adj pin? If not, that could be why the regulator is damaged, if it is. Also check all the
associated parts, including the input rectifiers.
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The regulator and preamp works fine, I can listen to sweet music with it. Just that new humming background noise... BTW, when shorted the reg and preamp only saw 0V DC instead of 24. Still can be damaged? Will see that tomorrow, enough damage for today 🙂
The preamp could still work with the 317 shorted, but the supply DC output voltage
would be higher, with more hum. Does the DC output voltage still measure correctly?
It seems that your DMM probe maneuvering care may be lacking. It can be too easy to short an active part of a circuit to ground somewhere, causing a spark. Maybe, first connect the probes to the desired test points with clips and the unit unplugged from the wall.
Hi Ken,
yes I know, but it was late and quite dark, but wanted to "have a quick look without the need on putting it on the workbench" laziness... lesson learned the hard way.
Anyway, on the bench this time, measured all that on the picture, the RIAA pre is just after (and was powered while PSU caps were shorted). Values may not be the real world one, C5 is more 10.000µF than 1...
Beside the fact the LM317 does not have its 3V headroom, voltages seem OK to me. My DMM can't measure any 50/100Hz AC, but I guess it may be very low level in circuit (under the DMM ability) as it's a tad lower than the music which is low level from a cartridge.
Quite strange, don't know where it's from and where to look at.
yes I know, but it was late and quite dark, but wanted to "have a quick look without the need on putting it on the workbench" laziness... lesson learned the hard way.
Anyway, on the bench this time, measured all that on the picture, the RIAA pre is just after (and was powered while PSU caps were shorted). Values may not be the real world one, C5 is more 10.000µF than 1...
Beside the fact the LM317 does not have its 3V headroom, voltages seem OK to me. My DMM can't measure any 50/100Hz AC, but I guess it may be very low level in circuit (under the DMM ability) as it's a tad lower than the music which is low level from a cartridge.
Quite strange, don't know where it's from and where to look at.
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We don't know much about your application, like how much current does the power supply provide and what package the LM317 is in. So I am guessing that the Q1 transistor is the weakest link in the power chain. Start by measuring the base voltage. If it is not close to 20.2 volts, Q1 is suspect. The collector to emitter voltage of Q1 as you measured is larger than I expected. If Q1 is not bad maybe R1 needs to be reduced to 4.7K or 2.2K. Or maybe the load circuit has a fault and draws too much current.
I agree and I can't answer, my hope was to point an obious faulty part if possible. Prior to the short I made, and the hum I now have, I find it to be very hot, too much to be normal. Will measure Q1 again and maybe get rid of it and start from scratch. Will try to find current used by the fed circuit.
Thanks for helping anyway 🙂
Thanks for helping anyway 🙂
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