Author Topic: Help repairing power supply  (Read 3661 times)

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Online Pfriemler

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Re: Help repairing power supply
« Reply #25 on: January 23, 2025, 01:42:24 pm »
OK. pin8 voltage should not vary in a wide range of R34, as @Zero999 mentioned earlier, as the reference OPV in LM10 should push it via R35. Only if pin1 is saturated at +6V, you may pull it further downwards.
But I asked for the pin1 variety. If it varies up to 6V (lower end is defined by the ratio of R34 and R35), then this part is ok.
Now: As long as pin3 is only a bit higher than pin2, LM10s ouput 6 should drive up to 6V. But it does not. You mentioned a 0.1v difference on R38, that's 0.5 mA base current to Q5. LM10 should deliver it.
Please report how pin1, pin2 and pin3 changes if you turn R34 into both directions.
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Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #26 on: January 24, 2025, 08:11:36 am »
Thanks for the reply. Here is what I observe:

R34 low (0 ohm):
- pin 1: 3.9V
- pin 2: 3.9V
- pin 3: 3.9V

R34 High (1kohm):
- pin 1: 568mV
- pin 2: 568mV
- pin 3: 568mV
 

Online Pfriemler

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Re: Help repairing power supply
« Reply #27 on: January 24, 2025, 01:09:40 pm »
No difference between 1,2 and 3 - as it should if the LM10 works fine.

But I wonder why only 3.9V is reached now:

I have redone the measures and for pin 1, 2 and 3 I now get 4.8v. I am not sure what is going on. Same voltmeter and same measurement procedure...
There's no reason why 4.8v and more are not possible, except of 0 ohm isn't 0 Ohm. For R34=1k and 200mV at this point, R35 calculates to ~1,9k. Given this, the low end of R34 rather comes to ~105 Ohms. If you can read them, pls give us a few more values of R28,R29, R35..R37

Ah, I forgot: You put Q5 back in its place? You definitely should see an output voltage variation on the 300v. If you can reach, pickup the span on C17, too. If you got the 3.9 / 0.568mV without the inverter working, repeat measuring them, pls.
What are thes curious CR11-CR13?. Are C14-C16 ok (no leakage currents)? It's difficult to measure, I know, but if you can, check the voltages in this divider, too, and report. Still my idea is that the feedback voltage to the LM10 is too high.
And/or if the sliding contact inside R34 is poor.

Weird.


« Last Edit: January 24, 2025, 01:14:39 pm by Pfriemler »
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Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #28 on: January 24, 2025, 02:17:19 pm »
Ok so yes Q5 was in place for the measures I provided.

Here are the resistors values:
R28: 100K
R29: 47K
R35: 2K (your guess was correct)
R36: 47
R37: 3.9M

I checked and R34 varies between 0 and 1kohm.

C17 when R34 is 0 measures 3.9V and when R34 is 1K measures 0.5V

CR11-CR13 are Zener diodes.

CR11 : 275V
CR12 : 184V
CR13: 92V
(Measurements made on cathode)

Sliding of R34 seems ok. I checked at different values between the min and max.
« Last Edit: January 24, 2025, 02:48:56 pm by crt_cowboy »
 

Online Pfriemler

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Re: Help repairing power supply
« Reply #29 on: January 24, 2025, 08:26:26 pm »
Good and normal values. We can forget the theory that Q5 is feeded bypassing the LM10 through R37 - not with 3.9M.
No other suspects.

So, let's go to the basics.
First: just for sure: The 300V output is bufferd by C7. Am I right that you do not have any oscilloscope equipment to check the 300V for any ripple? If not, try to measure the 300V ouput with a DMM in pure AC mode. You should not get a value of more than a few volts. If you have a ripple of 20-50v, there's maybe an issue with C7, C17 should buffer the (by Zeners) downscaled 300v output peaks, so the oscillator is regulated not to exceed 3xx volts. But with a significant ripple you will meausre a lower DC average on your DMM, and the output lowers more with any load than usual.
Note: When measuring in AC mode ... If you have an idea which frequency the oscillator works ... this could lead to false interpretations of AC readings at this point. I just checked my DMMs for that and got weired results with older (non rms) DMM, whereas two new OWON HDS120 and HDS242s are blind with frequencies >10 kHz. If you have any sine oscillator (AWG etc), check your meter for accuracy with various frequencys (if not done already, it's always good to know).

Second: The Zener voltages are 91v, 92v and something between 88 and 91.5v, depending on the R34 position. Given you measured while R34 was at the low end (and C17 voltage 3.9v) all three Zeners are equal, otherwise there may be a problem with CR13. But I do not think so.

Third: Did you find any output voltage variation when turning R34 from 0 to 1k, and if, what variations of Q5 base or emitter voltage you see? If you find the output variation of let's say 20v and 0.X v at Q5, this may be normal.

Fourth: If you can vary the output with R34 and your are shure that C7 is ok and there's no significant ripple on the 300v output (a few volts are ok), and only then!, try to lower R28 a bit with a bypass resistor (start with ~ 5MegOhm in parallel). 3.9 volts on 100k result in a 0.039 mA current through them, resulting in ~12 mW total power dissipating, if I do not fuck the maths. With a 1MOhm in parallel, current and power dissipating rises by 10%, should be safe. 0.034 mA (or 4 mW) are not much for a Zener (rather 100mW are), and in this area they have a rather soft U/I characteristics, with only a bit more current you should get up to 100 volts per Zener and thus more output voltage. Do we have a spec for the Zeners? I do not think they are 91volts, rather 100 oder 110 volts.
Start the circuit with R34 on the 1k side and try to rise the output voltage.
Good luck!
once you do it right, it works :-)
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #30 on: January 25, 2025, 01:24:33 pm »
Thanks for the very detailed reply.

1 - Yes I do have an oscilloscope. On the 300 V line I see very little ripple. Around 1.2 VPP.

2 - Zener diodes are BZX46C 100V

3 - R34 = 1K then HV measures 281V, Q5 base 3.1V, emitter 2.4V.
    R34 = 0 ohm the HV measures 285V, Q5 base 3.4V, emitter 2.7V

4 - as there is not so much change on the 300V line when changing R34 I am not sure I should proceed.
   
 

Online Pfriemler

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Re: Help repairing power supply
« Reply #31 on: January 25, 2025, 01:54:39 pm »
You’re right, the voltage variation of 4 Volts is a bit narrow. But: It still looks to me that the Zeners are not loaded enough. I would have a closer look to R28 again. Is it 100k, really?
Try to vary it, as I mentioned in the ost before.
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Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #32 on: January 25, 2025, 02:11:58 pm »
Yes R28 measures 100k.
I will try as suggested and report the results
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #33 on: January 25, 2025, 03:51:14 pm »
Just did a quick test of the R28 bypass with resistors from 5M to 1M. Strangely it made no difference in the output voltage.
 

Online Pfriemler

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Re: Help repairing power supply
« Reply #34 on: January 25, 2025, 10:08:10 pm »
A fault in C17 should rather lead to higher than lower voltages as intended.
Damned. I hate to say it, but I have no more ideas. Sorry!
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Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #35 on: January 26, 2025, 07:28:54 am »
No problem thanks for all the advices.
There are not so many components involved but as we are facing a partial failure it is hard to find unfortunately.
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #36 on: January 26, 2025, 08:38:41 am »
Here is what I observe on the base of Q3. Shouldn't it be 5KHz as this is the frequency it is supposed to run on ?
« Last Edit: January 26, 2025, 08:40:23 am by crt_cowboy »
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #37 on: January 26, 2025, 04:09:30 pm »
Can my problem be related to the fact it doesn't oscillate at the right frequency? It is supposed to have 5KHz and I observe 4KHz. Thanks
 

Online Zero999

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Re: Help repairing power supply
« Reply #38 on: January 26, 2025, 05:51:25 pm »
How do you know what frequency it's supposed to work at? 4kHz is a little low, but it wouldn't surprise me. My experience of this type of circuit is the frequency varies with the load.
 

Online Pfriemler

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Re: Help repairing power supply
« Reply #39 on: January 26, 2025, 06:32:07 pm »
Can my problem be related to the fact it doesn't oscillate at the right frequency? It is supposed to have 5KHz and I observe 4KHz. Thanks
Normally not. Lower frequency may result in a lower energy transport (simply spoken), but the feedback circuitry is DC and this should result in a higher supply voltage to the oscillator via Q5. Still I am wondering why changing the R34 has such a low effect to the output voltage and why it is not possible to get >5V in the feedback regulation. Any ideas to this, Zero999?
once you do it right, it works :-)
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #40 on: January 27, 2025, 05:22:44 am »
How do you know what frequency it's supposed to work at?
It is written in the manufacturer specifications. So we can assume that 4KHz is indeed too low.
 

Online Zero999

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Re: Help repairing power supply
« Reply #41 on: January 27, 2025, 11:48:09 am »
How do you know what frequency it's supposed to work at?
It is written in the manufacturer specifications. So we can assume that 4KHz is indeed too low.
Not really. The typical frequency is 5kHz which will vary depending on the load and the tolerance of some components, such as capacitors, can be as much as 20%, I wouldn't say that it being 4kHz means it's definitely faulty. It's on the low side, but not necessarily a cause for concern.

To make things easier, annotate the schematic with component values and the measured voltages. We should then be able to work out what the voltages should be.
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #42 on: January 27, 2025, 12:52:14 pm »
Here is the annotated circuit diagram. Measures taken when R34 equals 0 ohm
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #43 on: January 27, 2025, 01:11:04 pm »
And here is he version when R34 equals 1K.
« Last Edit: January 27, 2025, 01:15:24 pm by crt_cowboy »
 

Online Pfriemler

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Re: Help repairing power supply
« Reply #44 on: January 27, 2025, 04:03:55 pm »
That's interesting news and different values!

R29: 47K
Now 47 Ohm? what's right?

Quote
C17 when R34 is 0 measures 3.9V and when R34 is 1K measures 0.5V

That's completely different to your pics.
If in R34=0 C17 has 3.97v and pin2@LM10 3.9v, that may be ok with a R29=47k(!) but not with 47 ohms, and with R34=1k the voltage @CR13 is now only 88v instead of 92v.
Regarding the feedback voltage "divider", 3.97/279v and 3.46/275v sounds reasonable, but the voltages @CR11 and @CR12 should be the same as @CR13.

And neither with R29=47R nor R29=47k the right part of LM10 with its feedback of R37=3.9M should be able to lower the voltage on pin2 down to 590mV.

If you are sure you got good connections to the measuring points, check all connections around R29.



once you do it right, it works :-)
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #45 on: January 27, 2025, 04:13:10 pm »
My bad for R29. Sorry. I confirm 47 ohm is correct.

EDIT: I quickly checked between C17 and the pin 2 of LM10 and I measure 47 ohm.

I will take check again this area and re-do measurements to confirm.
« Last Edit: January 27, 2025, 05:14:56 pm by crt_cowboy »
 

Online Pfriemler

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Re: Help repairing power supply
« Reply #46 on: January 27, 2025, 05:20:45 pm »
Rather good: For the 1k-R34: 3.46 -> R29 -> 0.59V would be a current of 62 mA, if the connections are good. No such current can come from the CR11..CR13 (they would be blown up from heat), nor there's a sink at the LM10 - only if the LM10 input pin2 is defective, rather unlikely if the other R34's end with 3.9v works.
Heat dissipation of R29 should then be 180 mW, that would make it considerably warm.
There must be a failure. Solder joint, hairline crack. Check for continuity.

EDIT: ah, you did. Makes no sense until now. We'll see..
« Last Edit: January 27, 2025, 05:25:23 pm by Pfriemler »
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Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #47 on: January 27, 2025, 06:05:28 pm »
R28 measures 47K in circuit and not 100K. I will desolder it tomorrow to confirm. Probably a measurement issue with the in-circuit measure.
 

Offline Smile2016

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Re: Help repairing power supply
« Reply #48 on: January 27, 2025, 06:34:25 pm »
Are CR11, CR12, and CR13 identical in value? If they are, and the measured voltages are accurate, CR13 seems a bit lower at around 88.5V. Can you check the Zener values or markings on them? Considering they are biased at only a few tenths of microamperes, the circuit is sensitive to small leakages, CR13?. I would replace CR13, [&the capacitor C17, and Q5]. Standard values are 91V...and next up is 100V. 91V won't get you to 300V output...Also check if battery is still 6V when Pot is at 0 Ohms...

I just noticed you mentioned, "2 - Zener diodes are BZX46C 100V." Probably these diodes were pre-selected to have a Zener voltage (Vz) of around 100V at ~ 40µA, they have an initial spread of 94–106V according to the datasheet. Probably your PSU works just fine except other than Vz is not tailored to give you exactly 300V. again... make sure battery holds 6V
« Last Edit: January 28, 2025, 04:31:14 am by Smile2016 »
 

Offline crt_cowboyTopic starter

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Re: Help repairing power supply
« Reply #49 on: January 28, 2025, 12:19:02 pm »
Yes but as the 300v line is low the two other 9v lines are also low as the regulation relies on the 300v line.

Removed R28 and it has the right value when measured alone.

 


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