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Electronics => Repair => Topic started by: crt_cowboy on January 17, 2025, 08:02:31 am

Title: Help repairing power supply
Post by: crt_cowboy on January 17, 2025, 08:02:31 am
Hi all,

I am currently trying to repair a power supply.

From the supply voltage (6V), it generates +9V, -9V and +300V

It uses an oscillator consisting of transistors Q3 and Q4 and a transformer T1. This transformer has 3 secondary windings.

The voltages are regulated by a stabilization circuit consisting of a Zener network on the HV of an integrated circuit LM10 Z3 and transistor Q5. The base of Q5 is made more or less conductive by LM 10 depending on the variations in the HV.

Attached the diagram.

Adjusting R34 should adjust the output voltage. In my case changing it value doesn't change I can barely get +-8.4v and 270v by lowering it's value.

Anyone has suggestions on what to look at ?

I checked all resistors and they seems within acceptable range.

Many thanks in advance
Title: Re: Help repairing power supply
Post by: Zero999 on January 17, 2025, 09:28:53 am
What's the voltage on pin 8 of Z4?
Title: Re: Help repairing power supply
Post by: crt_cowboy on January 17, 2025, 09:59:16 am
8.4v and I cannot get it higher adjusting R34
Title: Re: Help repairing power supply
Post by: Zero999 on January 17, 2025, 10:04:42 am
8.4v and I cannot get it higher adjusting R34
That can't be right. The voltage on pin 8 of Z4 (the LM10) can't exceed the power supply voltage to it, which is 6V. The voltage on pin 8 should be 200mV.

Are you sure you've not put one of the meter probes in the wrong place?

EDIT:
Refer to page 17 of the LM10 data sheet.
Pin 8 connects to the inverting input of an op-amp, with a 200mV reference connected to the non-inverting input. If the loop is closed, the input voltages to an op-amp should be equal, so the votlage on pin 8 should be the same as the 200mV reference.
https://www.ti.com/lit/ds/symlink/lm10.pdf (https://www.ti.com/lit/ds/symlink/lm10.pdf)
Title: Re: Help repairing power supply
Post by: crt_cowboy on January 17, 2025, 10:41:45 am
 Oh yes I went too quickly indeed 200mv on pin 8 (my ground probe was not connected to the right place)
Title: Re: Help repairing power supply
Post by: crt_cowboy on January 17, 2025, 12:38:59 pm
Here are some additional measurements:
Pin 1: 380mV
Pin 2: 4.8V
Pin 3: -430mV
Pin 4: 0V
Pin 5: 33mV
Pin 6: 1.7V
Pin 7: 6V
Pin 8: 200mV

Any suggestions based on this ?

Many thanks

EDIT:
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...
Title: Re: Help repairing power supply
Post by: Zero999 on January 17, 2025, 11:21:28 pm
So pins 1, 2, & 3 are now 4.8V? That sounds more reasonable, but what about pin 6? 1.7V doesn't sound right.

The op-amp in the LM10, takes the output voltage of the power supply, compares it to the reference and adjusts the voltage to Q5's bas which in tern alters the supply voltage to the oscillator driving the step-up transformer. It's an error amplifier.

Measurement errors aside, I suspect one of the transistors is faulty. The gain is probably lower than it should be, or it's leaking, so it still oscillates but doesn't reach the required output voltage. Have you tested any of them?
Title: Re: Help repairing power supply
Post by: crt_cowboy on January 18, 2025, 06:41:48 am
I have tested them in circuit and did not find any short.
is there a way to check further Q3 and Q4 ? Oscillation frequency should be around 5KHz. I don't know if I can see more with an oscilloscope?
Q5 seems also fine (no short at least). On the emitter I find 2.6v which sounds a bit low.
Title: Re: Help repairing power supply
Post by: Zero999 on January 18, 2025, 06:19:59 pm
I have tested them in circuit and did not find any short.
is there a way to check further Q3 and Q4 ? Oscillation frequency should be around 5KHz. I don't know if I can see more with an oscilloscope?
Q5 seems also fine (no short at least). On the emitter I find 2.6v which sounds a bit low.
Unfortunately that won't tell you whether the transistors are working properly. One of them might be leaky or low gain and you wouldn't know.

This circuit can be used to test the hFE of a transistor. It will be equal to the ratio of the readings A2 & A2.

hFE = A2/A1
[attachimg=1]
Title: Re: Help repairing power supply
Post by: crt_cowboy on January 19, 2025, 06:52:21 am
Thanks for the reply!
Would a failure of one of those transistors explain the low voltage on pin 6 of the LM10 ? I double checked and it varies between 1.6V and 1.7V.
For Q5, the 2.6V on the emitter seems low no ?


Title: Re: Help repairing power supply
Post by: Zero999 on January 19, 2025, 11:50:46 am
Thanks for the reply!
Would a failure of one of those transistors explain the low voltage on pin 6 of the LM10 ? I double checked and it varies between 1.6V and 1.7V.
For Q5, the 2.6V on the emitter seems low no ?
It depends.

What's the value of the base resistor?

If Q5's gain is low the excessive base current could pull the LM10's output low, assuming the base resistor's value is low enough to allow this.

Failing that, perhaps the LM10 is faulty.
Title: Re: Help repairing power supply
Post by: crt_cowboy on January 19, 2025, 12:38:08 pm
Q5 is a 2N2222 and R38 is 220ohm.

On the base of Q5 I get 3.3 V.

I cleaned a bit more the leg of the LM10 and now I have 3.4V.

Should I assume the LM10 is working fine ? I am puzzled the emitter of Q5 only reads 2.6V
Title: Re: Help repairing power supply
Post by: watchmaker on January 19, 2025, 03:14:11 pm
I have tested them in circuit and did not find any short.
is there a way to check further Q3 and Q4 ? Oscillation frequency should be around 5KHz. I don't know if I can see more with an oscilloscope?
Q5 seems also fine (no short at least). On the emitter I find 2.6v which sounds a bit low.
Unfortunately that won't tell you whether the transistors are working properly. One of them might be leaky or low gain and you wouldn't know.

This circuit can be used to test the hFE of a transistor. It will be equal to the ratio of the readings A2 & A2.

Thank you for that setup.  I just used it to select a transistor to match the gain of a transistor I pulled from an old Japanese radio that I cannot reinstall (it has those very early ribbon type leads).

Indeed, there is a great deal of variation within a transistor number.  Also, for the sake of others, it provides a different value than the Fnirsi.  I trust this method because it is so transparent.
Title: Re: Help repairing power supply
Post by: Zero999 on January 19, 2025, 10:15:52 pm
Q5 is a 2N2222 and R38 is 220ohm.

On the base of Q5 I get 3.3 V.

I cleaned a bit more the leg of the LM10 and now I have 3.4V.

Should I assume the LM10 is working fine ? I am puzzled the emitter of Q5 only reads 2.6V
The emitter voltage is 0.7V below the base voltage, which is to be expected.

Have you tried just powering the oscillator from a variable, current limited bench supply?
Title: Re: Help repairing power supply
Post by: crt_cowboy on January 20, 2025, 05:14:52 am
Thanks for the suggestion. I will try that.
In practice, you mean injecting with the bench power supply somewhere after Q5 emitter to feed the oscillator? Do I need to disconnect anything from the regulation circuit?