using new technology on old machines

tony duell ard at
Tue Jun 16 23:14:45 CDT 2015

> >
> > Actually, no. That honour goes to the PSU in a Zenith MDA monitor
> > which as I said 'combines the efficiency of a linear with the reliability
> > of a switcher'. The design (if you can call it that) of this PSU is to
> > rectify the mains, feed it into a free-running chopper circuit, then
> > a transformer. The output of that is half-wave (!) rectified giving
> > about 18V DC. Note the chopper free-runs, so there is no regulation
> > applied at this point. That 18V is then fed to a discrete-transistor
> > linear regulator.
> >
> > And that's not the end of the 'curious' design. As you know, a linear
> > regualtor compares the output voltage of the supply with a
> > > reference votlage. That reference voltage is typically produced by
> > a zener diode. Not in this monitor. It uses the drop across the
> > power-on LED. Which means it is important to use a green LED.
> > Another colour, with a different Vf, and the PSU output is wrong.

> I would love to have a copy of that schematic for an Engineering Wall of
> Shame. Seriously, that is the strangest supply design I've ever heard.

I don't know if it was ever published [1], but I should have a reverse-engineered
version somewhere. When I unpack it (after the move...) I will see about getting
it scanned. It really is a crazy design.

[1] There was the standard 'safety component' warning telling you only to use
the parts specified in the service manual on the back cover. But I was totally 
unable to obtain said service manual.


More information about the cctalk mailing list