Heathkit H89 Video Question

From: Patrick Rigney <patrick_at_evocative.com>
Date: Mon Sep 1 13:24:00 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.

> > > After tracing the (good) horizontal output signal from the
> terminal logic
> > > board into various part of the video board, and checking
> voltages where I
> > > felt I could reach safely in a hot set, I discovered that if I shot
> > > horizontal output transistor Q217 with cold spray I'd get
> video back right
> > > away.
>
> 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.

> > 2) The load is too heavy. This could be the flyback (but I
> doubt it -- a
> > shorted turn in the flyback is going to have more serious effects that
> > this), it could be something powered from the flyback (again unlikely)

Again, I have very little experience in this area, but given the voltages
and general temperment of the circuit, it didn't seem likely to me that
flyback failure would normally be this subtle.

> Check the diodes (D207-D211) on the output side of the flyback, and the
> associated caps.

Will do...

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

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
Received on Mon Sep 01 2003 - 13:24:00 BST