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A trimpot is basically a resistor with a center tap to vary the ratio of both halves like a fixed voltage divider
In this particular circuit thought, the power supply, why do we need adjustment instead of a fixed value resistor ? Is this something that needs to be adjusted from time to time in a power supply or it's only adjusted when the PSU is built (or when we changed other components in it ?). -
A trimpot is basically a resistor with a center tap to vary the ratio of both halves like a fixed voltage divider
In this particular circuit thought, the power supply, why do we need adjustment instead of a fixed value resistor ? Is this something that needs to be adjusted from time to time in a power supply or it's only adjusted when the PSU is built (or when we changed other components in it ?).
Real world components like resistor's resistance will drift with temperature and time/aging so you need
to recalculate the new resistors to put in but with a potentiometer all you need is just turn the pot until the
value/resistance corelate to the required voltage. Although overall they will remain in tolerance over the years. It's just convenient for maintenance. -
I tried the trimpot and the voltages goes from -8.5V to about -10V .
I also looked for cracked joints, busted caps, residue on the board, etc... but didn't find anything.
I guess my next step is to replace all the caps like others have suggested. -
If an active component in a switch mode power supply fails, the output is zero. As another poster mentioned, they operate as an oscillator and without a load to drag the voltage down, they won't generate the upward swing. So you'll see no output without a load, and that load could be internal to the power supply, or with a failed active component (as well as inactive) but I point this out since you have output. This means that though it has an incorrect level, it is unlikely to be an active component which points at capacitors, most likely, or a resistor that changed value. The resistors would be easy to check, but save yourself some time and swap out the caps.
If you can put a constant load on the PS that would eliminate the downstream electronics. If you are worried about putting more money into the scope, then wire up some resistors and load the supply. -
If an active component in a switch mode power supply fails, the output is zero. As another poster mentioned, they operate as an oscillator and without a load to drag the voltage down, they won't generate the upward swing. So you'll see no output without a load, and that load could be internal to the power supply, or with a failed active component (as well as inactive) but I point this out since you have output. This means that though it has an incorrect level, it is unlikely to be an active component which points at capacitors, most likely, or a resistor that changed value. The resistors would be easy to check, but save yourself some time and swap out the caps.
If you can put a constant load on the PS that would eliminate the downstream electronics. If you are worried about putting more money into the scope, then wire up some resistors and load the supply.
Quite a few switchmode supplies will run without a load,notably,those on Sony CRT TV sets.
The old Philips ones will hiccup without a load,& PC supplies don't regulate properly without one. -
If you can put a constant load on the PS that would eliminate the downstream electronics. If you are worried about putting more money into the scope, then wire up some resistors and load the supply.
The service guide mentioned using a 2 ohms 25Watts resistor as a load. I ordered one so I will try it when it arrives. If the voltages are correct using the resistor as a load, but incorrect when plugged in the main board, it would mean the problem is not in the power supply but somewhere else in the device, right ? -
most likely, yes, if the resistor loads the supply correctly and the voltages look good, look somewhere downstream. I suspect you will find the voltages are still off with the resistive load.
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I read somewhere that an ESR meter could be used to test for bad capacitors without removing them from the board, but I also saw comments about the fact the it can not be used when there is multiple capacitors close together.
Could that help with that power supply ? I don't have an ESR meter so I would need to buy one. I was a lot of cheap ones and some expensive ones. Would something simple like that work http://www.ebay.ca/itm/2015-newest-version-of-inductor-capacitor-ESR-meter-DIY-MG328-multifunction-test-/111521859338?hash=item19f738870a ? -
I read somewhere that an ESR meter could be used to test for bad capacitors without removing them from the board, but I also saw comments about the fact the it can not be used when there is multiple capacitors close together.
Could that help with that power supply ? I don't have an ESR meter so I would need to buy one. I was a lot of cheap ones and some expensive ones. Would something simple like that work http://www.ebay.ca/itm/2015-newest-version-of-inductor-capacitor-ESR-meter-DIY-MG328-multifunction-test-/111521859338?hash=item19f738870a ?
Pennywise pound foolish. MESR100 V2 is the one to get ( only esr not multi purpose like diy), it has probe resistance zeroing feature. Get the esr/cap meter combo if you can afford it. Although if you hunt around there are cap meters that also do esr and some more but this cheapy test at 100Khz. good for switchmode power supply.Those diy ones may not test at 100Khz. and may be lower frequency like 10Khz.
Or alternatively get UT139C multimeter that can test caps up to 100K uf. but not esr.
PS: MESR100 V2 can test in circuit but you have to discharge the caps first with a resistor, a ceramic 1K ohm 5 watt or larger and double checked with DMM if it discharge to about 1V or less it should be safe ( big caps takes longer time,you can use smaller values as the discharge go down to speed up the time) . It does not have auto discharge like the Peak esr meter. Good practice to check cap voltage before using esr meter.
http://www.ebay.com/bhp/mesr-100 -
I checked the power supply using the service manual posted by singapol at http://cp.literature.agilent.com/litweb/pdf/54510-90903.pdf .
Using page 6-21 (page 111 of the PDF) I did the first test "Supply Loaded". The test point that is supposed to be -12V is around -9V. The others are ok (except when the unit reboots on it's own, the power drops and comes back up again to the correct value). Is that test alone enough to know the power supply if faulty or do I need to do the "Supply isolated" test also. I guess I need to do it but since I don't have a 2-ohms 25 watt resistor I figured I'll ask first.
I also checked the solder joints and caps and nothing seems wrong. I will check more thoroughly if the problem is not with the power supply.
You have already found a -12V descrepancy, unplug W1 to main assembly ( mains power switch off) turn on power switch and check -12V again. Now that it's unloaded it should measure -12V or higher. If it's still -9V the problem is in the switchmode supply.
singapol has hit the nail on the head----if everything is OK except for -12volts,that is where your problem is.
If the linked "reverse engineered" schematic is correct,it isn't one of those SMPS that won't work without a load.
Indeed,you have already disconnected it,& shown that is so,so don't get yourself off on a tangent.
ignore all the rest of the device except the -12volt supply .
This is quite a simple circuit,consisting of CR25,a flter capacitor,& the regulator,A8.
In a switchmode,CR25 is probably a "fast-recovery" type,& the cap should be low ESR.
If the cap dries out,the output of the circuit can look like a series of pulses,with low total energy.
A DMM will often see this as a lower value of dc,as some of them don't know the difference.
Sometimes "fast recovery" diodes go "a bit funny" & lose their normal characteristics,so need replacement.
Other times,you will find someone has been there before you,& has fitted an ordinary power diode without the correct characteristics. -
vk6zgo
You took the time to read and understand. Was wondering why OP did not
comprehend my post.
All he needs to do is check the -12V circuit. Betting on LM350 but the caps could be bad too.
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I removed the LM350 on the -12V . I'll test it tonight when I get home. I don't know how to test it yet so I'll need to do a bit of research.
Thank you again for taking the time to help and give advices, it's really appreciated. If I seem to ignore some of the things you say it's simply because I have so much to learn that I sometimes miss important things you said. I have the intention to re-read every post and look up everything until I understand every word though, it might just take a really long time
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There's no need to reread every post when in essence you found the problem by yourself. We just gave a
helping hand.
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Hello,
I had the exact same problem -- the power supply would oscillate with a short "on" pulse every second or so.
The root cause was leaked electrolyte from a capacitor. The leaked liquid is quickly evaporated by the airflow, but it leaves a dry, conductive residue on the board. In my case, the residue was on the voltage-setting resistors of the LM350, which kicked the LM350 output voltage out of limits and triggered the crowbar. Then the power supply would quickly restart and shot down again.
The quick fix is to clean the residue with running water. When dry, the power supply works again!
Of course, you still have to replace the leaking capacitor(s).
I hope this helps!
Joel_s -
Thanks for your reply. I did not find any sign of leak on mine, but I ordered an ESR meter so I will be able to test each capacitors.
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You have been at this for a month.
You could save a lot of time & money by just replacing the capacitor in the -12v supply"on spec".
The same goes for the LM350, probably the diode,& maybe the pot.
If you are going to do this sort of thing for a living "down the track",you have to learn when it is more economical to use the "scattergun" approach. -
Yea I know. I found an other oscilloscope that uses the same power supply for 100 CAD ( about $75 USD) so I just bought it for the PSU. I still want to fix it for the learning experience though. I'm doing this as a hobby when I have some spare time.
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Just for future reference, what model is that?
Also it might be a good idea to go ahead and try to troubleshoot the old PSU when you have time since failures of these are common, there's a good chance that your "new" could eventually fail too. And having a working one to compare to should make troubleshooting much quicker. -
Just for future reference, what model is that?
It's an HP 54501A. The NVRAM battery is dead but other than that, it seems to be working.Also it might be a good idea to go ahead and try to troubleshoot the old PSU when you have time since failures of these are common, there's a good chance that your "new" could eventually fail too. And having a working one to compare to should make troubleshooting much quicker.
Yes that's a good point. It will also give me a backup PSU so I will be able to use the oscilloscope even if I did not fix the newly failed PSU. -
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Yes that's a good point. It will also give me a backup PSU so I will be able to use the oscilloscope even if I did not fix the newly failed PSU.
You can never too many oscilloscopes.
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Sorry for being greedy here, arab blood kicking in. O0 O0 O0
My wife designs MEAN power supplies (its her hobby). So, if you end up with a scope sans PSU which is headed to the dustbin, think about us. We would gladly pay shipping... -
Sorry TAMHAN, I'll keep it.
I ended up changing all the 220uF and it's now working
For the record, it DOES need a load to have a stable voltage. I testing with both PSU and the voltage was varying a lot for the 12V and -12V if I didn't put a load on it.
Thanks everyone for your help and suggestions, it's really appreciated. -
Hello,
if you managed to fix it, thats great! We mainly wanted to save it from the scrap heap
(We already have four scopes between the two of us. LeCroy 9354AM, Tek754D, and two Iwatsus. Plus, if I can twist the arm of my former school, a Gould OS4000).
Tam