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Gary C
Posts: 5
Joined: Mon Sep 21, 2020 10:35 am


Post by Gary C »

Having a few problems with my MZ-80K PSU

The +5V & -5V are fine, but the 12V regulation is very poor. Open circuit its approx 18V and when connected it falls to about 10V. Pressing play on the tape causes the screen to flicker and you can see the 10V fall on the scope as you press.

Looking at the circuit, it uses a much simpler circuit to the Apple shenanigans with its flyback transformer and has a IC that senses an error between the desired and actual output and increases or decreases the duty cycle driving a Darlington pair. This produces a switched DC supply that is smoothed by a cap.

Now its almost looks as if its not regulating at all and running open loop, but oddly when I adjust the setpoint, the voltage changes so the IC is doing something. The frequency of the oscillator seems fine so the external RC must be ok.

Anyone had any experience trouble shooting the PSU ?

I have an SG3524 on order and am going to replace the electrolytics as one looks a bit suspect.
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