Electronics > Repair
Help repairing power supply
crt_cowboy:
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
Zero999:
What's the voltage on pin 8 of Z4?
crt_cowboy:
8.4v and I cannot get it higher adjusting R34
Zero999:
--- Quote from: crt_cowboy on January 17, 2025, 09:59:16 am ---8.4v and I cannot get it higher adjusting R34
--- End quote ---
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
crt_cowboy:
Oh yes I went too quickly indeed 200mv on pin 8 (my ground probe was not connected to the right place)
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