[vcf-midatlantic] Repairs to a Heath WH19 Terminal

Sun Mar 26 16:51:30 EDT 2017

I noticed the schematic I linked in the first post doesn't match this
terminal exactly. This one does:
https://stacks.stanford.edu/file/druid:vg695gp5537/vg695gp5537.pdf

A picture of the video PSU section we're looking at here:
https://goo.gl/photos/Lu8YTcNhE6NpyztB9

Before the regulator I measure 66 volts DC w/ .25v of ripple at 120hz.

After the regulator I measure 61.5 volts DC w/ .35v to .41v of ripple at
60hz. - sounds like it's not regulating much

Reading the manual more, it doesn't use an integrated voltage regulator
(page 33):

"The unregulated 65-volts DC from the power supply circuit board enters the
video circuit board at plug P202, pins 2 and 3.
Assume that the Terminal has just been turned on and the output of the
+53-volt regulator is at zero volts.
The base current of Q209 is supplied through resistors R201,R202, and R203.
The collector current of Q209 causes Q211 to turn on and supply current to
the base of Q213. As the output voltage at the emitter of Q213 rises, D204
begins to supply current to zener diode D203 through resistor R202.
D203 stabilizes at 12.8 volts and provides a reference for the output
voltage. The divider formed by resistors R207 and R208 samples the output
voltage as it continues to rise, and applies a fraction of the voltage to
the emitter of Q209. When the emitter voltage of Q209 reaches 12.15volts
(12.8 -.65), its collector current is reduced to a value that keeps the
output voltage stabilized at +53 volts."

Q209's emitter measures 13.71V.
D203's cathode measures 12.8V.

So the PSU is regulating some, but not enough? Any suggestions where I
should check next?

- - - -

Now that the terminal is running the next problem shows itself... after
it's been on for ~10 minutes the keyboard stops working. All the supply
rails on the logic board are fine when this happens (no change between
working and not). Power cycling the terminal will get the keyboard working
again for a short while. The reset key combo (right shift + reset) always
works.

-J

On Sun, Mar 26, 2017 at 1:48 PM, Ian Primus via vcf-midatlantic <
vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:

> Check the actual voltage of the B+ rail. It may be a bit too high. If
> so, then suspect your voltage regulator transistor/transistors. It's
> possible to have a bad voltage regulator, but still have things almost
> working, but there will be too much ripple and you won't be able to
> get rid of it with capacitors.
>
> -Ian
>
> On Sun, Mar 26, 2017 at 12:41 PM, Jason Perkins via vcf-midatlantic
> <vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
> > So I had some more time to look into this.
> >
> > I swapped in some caps I removed from some computer power supplies as a
> > test... no change. I then added the original cap in parallel... it made
> > some improvement: down to .3v AC ripple on the monitor supply rail and a
> > less distorted display.
> >
> > When I was trying to find the short circuit I fried 2 of the bridge
> diodes,
> > and replaced them with some modern computer PSU takeouts.I swapped all 4
> > out with new 1N4004's, no change.
> >
> > This is such a simple circuit I'm having trouble understanding where the
> > ripple could be coming from. If I use a light bulb as a load on the
> circuit
> > the ripple is the same (the monitor draws about half an amp, and a 70w
> 120v
> > light bulb works as a good stand in).
> >
> > Any other suggestions?
> >
> > Thanks,
> >
> > -J
> >
> > On Thu, Mar 16, 2017 at 12:07 AM, Matt Patoray via vcf-midatlantic <
> > vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
> >
> >> Jason,
> >>
> >> That is great news! That is also too much AC on a DC rail, I would
> suspect
> >> those caps and get replacements, 1500 uF would be hard to find but a
> 2200
> >> would be ok in this case, remember those older electrolytics may have a
> >> tolerance of -20% +85% and modern caps are all +- 20% or better.
> >>
> >> Sent from my iPhone
> >>
> >> > On Mar 15, 2017, at 10:17 PM, Jason Perkins via vcf-midatlantic <
> >> vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
> >> >
> >> > Replacing the shorted cap and blown current limiting resistors
> results in
> >> > signs of life!
> >> >
> >> > https://goo.gl/photos/nk5kvxuCT2VGztgf6
> >> >
> >> > Good: The terminal works! https://goo.gl/photos/TPoPQ1c8QajthkZM7
> (yes,
> >> I
> >> > tested with a loopback adapter, then with a usb to RS232 adapter
> before
> >> > hooking it up to the Lisa :) ) The terminal draws 51 watts running.
> >> Nothing
> >> > inside seems to be getting unusually hot.
> >> >
> >> > Problems: The display has a major wave.
> >> > https://goo.gl/photos/wXKydMr5syKU1SWd7 Guessing there are more bad
> >> caps in
> >> > the video section. The 65V input, and 53V rail in the monitor have
> .5v of
> >> > ac ripple. Perhaps the big 1500uf electrolytic is going bad...
> >> >
> >> > The keyboard didn't work very well at first, but after typing on it
> for a
> >> > bit it now works perfectly.
> >> >
> >> > -J
> >> >
> >> > On Mon, Mar 13, 2017 at 12:10 PM, Jason Perkins <
> perkins.jason at gmail.com
> >> >
> >> > wrote:
> >> >
> >> >> Man what a small world... My other big not-work time investment is
> >> >> TDIClub, looks like the OP of that article is into TDIs a bit as
> well!
> >> >>
> >> >>> On Mon, Mar 13, 2017 at 11:56 AM, Neil Cherry <ncherry at linuxha.com>
> >> wrote:
> >> >>>
> >> >>>> On 03/12/2017 11:25 PM, Jason Perkins via vcf-midatlantic wrote:
> >> >>>>
> >> >>>> You were right, the big .22 600v cap C232 was shorted. I've placed
> an
> >> >>>> order
> >> >>>> from DigiKey, hopefully the impending snow doesn't delay things too
> >> >>>> badly.
> >> >>>>
> >> >>>> -J
> >> >>>>
> >> >>>> On Sun, Mar 12, 2017 at 6:55 PM, David Gesswein via
> vcf-midatlantic <
> >> >>>> vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
> >> >>>>
> >> >>>> On Sun, Mar 12, 2017 at 05:43:22PM -0400, Jason Perkins via
> >> >>>>> vcf-midatlantic wrote:
> >> >>>>>
> >> >>>>>>
> >> >>>>>> There aren't many things on that ground return from the flyback,
> >> I'll
> >> >>>>>>
> >> >>>>> take
> >> >>>>>
> >> >>>>>> them out one at a time till I find the shorted part.
> >> >>>>>>
> >> >>>>>> The caps to ground look like the most obvious path to create a
> >> short.
> >> >>>>>
> >> >>>>
> >> >>> Queue eerie Twilight Zone Music:
> >> >>>
> >> >>> http://hackaday.com/2017/03/13/raspberry-pi-hitches-ride-ins
> >> >>> ide-vintage-terminal/
> >> >>>
> >> >>> ... but he quickly determined that a bad cap was shorting out the
> >> -12VDC
> >> >>> rail ...
> >> >>>
> >> >>> hehe
> >> >>>
> >> >>> --
> >> >>> Linux Home Automation         Neil Cherry       ncherry at linuxha.com
> >> >>> http://www.linuxha.com/                         Main site
> >> >>> http://linuxha.blogspot.com/                    My HA Blog
> >> >>> Author of:      Linux Smart Homes For Dummies
> >> >>>
> >> >>
> >> >>
> >> >>
> >> >> --
> >> >> Jason Perkins
> >> >> 313 355 0085 <(313)%20355-0085>
> >> >>
> >> >
> >> >
> >> >
> >> > --
> >> > Jason Perkins
> >> > 313 355 0085
> >>
> >
> >
> >
> > --
> > Jason Perkins
> > 313 355 0085
>

-- 
Jason Perkins
313 355 0085