Heathkit H89 Video Question

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Sep 1 16:09:00 2003

> > >From the schematic, the CRT filament runs off the flyback (2 brown
> > wires), so it's not suprising it goes out when the horizontal output
> > stage fails.
> >
> > I also assume that the +53V supply (emitter of Q204) is fine when the
> > thing fails.
>
> Yes, no problem there.

Good... As I said, 'alwyas check the power supplies' :-)

> > One problem -- on my schematic the HOT is Q214. There is no Q217.
>
> I'll bet the schematic you are looking at it is for a later reduced-emission
> version. I don't have that schematic, but the one I do have matches the
> machine I'm working on... an upgraded H88.

Quite possibly. It's the schematic that came with my Z90 (which has
always AFAIK been a Z90, it wasn't upgraded from anything), and it
matches the machine I have.

> > > 3) The drive waveform is wrong. I don't have the H89 schematics
> > to hand,
> > > but there's often a low-value electrolytic capacitor (a few uF) in the
> >
> > There is,. It's C226 (10uf) on my schematic. Also check R254 (1.2R), R255
> > (100R) and L202 (a small coil).
>
> I don't have my schematic at hand at this moment, but those are all familiar
> parts/values... If the drive waveform was disrupted, are there any other

The component references are probably different. Look at the base circuit
of the HOT. Typically you'll find it's driven by the secondary of a small
transformer (the horizontal drive transformer) with some R's and C's
around. It's those you need to check.


> symptoms typical? When the display is operating, it appears pretty much
> perfect, until just a few seconds before the HOT shuts down.

Electtolyic caps can do some very strange things!. The capacitor may be
OK when cold, and then fail when it warms up....

>
> Thanks for the tips... I have enough spare parts that I could just send the
> whole video section to the graveyard, but it seems a minor problem and a
> good education to figure it out and fix it. I appreciate the
> elp. --Patrick

What's the fun in board swapping :-)

-tony
Received on Mon Sep 01 2003 - 16:09:00 BST