Fix vertical hold on a monochrome monitor

[Renate]

Yes, the MDA signals are TTL which never goes to 5V, only > 2.8V

The only IC is a 74LS86 - a quad XOR gate.

This is probably doing sync detection for H and maybe V.
You should be able to find two input pins that have nice V signals on them (also two with H).
Using two channels on the scope should show you what's what as you play with V hold control.

[vk6zgo]

the very low displayed signal level may be a bit problematical.

According to the MDA spec, high voltage levels are 2.4V to 3.5V. See http://minuszerodegrees.net/oa/OA%20-%20IBM%20Monochrome%20Display%20and%20Printer%20Adapter.pdf, page 9.

I'm surprised. I would've guessed 5V. But apparently not.

It still shouldn't bother your 'scope, or the monitor, PAL syncs are 400mv, & even with noticeably lower levels monitors stay locked.

My reasoning with dropping the volts/div on the 'scope (giving more vertical divisions of the display for the same input signal) was not to do with concern about the actual sync level, but because, at least, in all the Oscilloscopes I have used, the triggering is taken after the input gain & attenuator stages.
If a larger signal appears at the trigger circuitry, it will almost universally be easier to trigger on.

'Scope triggering is also more likely to present problems at quite long time/div settings, like 10ms/div, than at, say, 2ms/div.

Have you compared the video signal out of the source with the corresponding syncs, using two 'scope channels?
Obviously, they should be in "sync".

Does the monitor pix roll vertically "as a whole", or does the vertical blanking drift w.r.t. the picture?

[vk6zgo]

Yes, the MDA signals are TTL which never goes to 5V, only > 2.8V

The only IC is a 74LS86 - a quad XOR gate.

This is probably doing sync detection for H and maybe V.
You should be able to find two input pins that have nice V signals on them (also two with H).
Using two channels on the scope should show you what's what as you play with V hold control.

In any case, checking the pins will show you what the IC is used for.
It could be something to do with the front controls, for instance, & all the sync stuff done using discrete devices.

[ambrosia heart]

Hi,

I have a monochrome monitor with a beautiful amber display, but I can't get the vertical hold to stay put.

It's a Magnavox 7BM623 074T. I can carefully find a sweet spot for the V Hold to stop the screen from rolling, but if I turn the computer and monitor off then on, it loses the hold and starts rolling slowly.

I've replaced all the electrolytic caps and the V Hold potentiometer. The problem remains.

Anything else I can try?

Thank you,
   Bob

Search for word黑白電視故障檢修 and垂直不同步 at
https://translate.google.com/     
translate them to English.  Choose left hand side third one .  Right hand side
second one. Copy 黑白電視故障檢修 and垂直不同步 to left hand side third one to translate.

This is an old book bought in 80s. Schematics are National TV.
In those day called National NOT panasonic.
I know you are unable to read chinese characters. I translate it
to you. In your cases, there are 2 situations I or II. Read it carefully.
In the top right hand corner written 黑白電視故障檢修
Comparing those waveforms printed in this book. Despite different
brands of black and white TVs, the working principle inovlved is
basically same.


1. Even rotate V.Hold, screen is still unable to be still. Horizontal
sync. is of course normal.

2.Reason:(a) integration circuit is faulty, making vertical sync.
signal unable to trigger oscillator. (b)vertical oscillation circuit
and the oscillation freq. are abnormal. (c)sync. circuit is faulty.
In spite of getting sync. the sync. signal is very weak.

3.prepare to adjust.
Rotate V. HOLD to see if

(a)after adjust., the screen is still rotating upwards or downwards.
(b)Still unable to make the screen be still at one point. This implies
the oscillation freq. is of about 60Hz, but no sync. signal inputted.
This explains why screen is rotating.

[II]
(a)Screen is rotating either upwards or downwards in one direction only.
This implies the oscillation freq is highly distorted. In spite of the
presence of sync. signal, rotation still occurs.

4.Solution
For
When vertical oscillation circuit without sync. signal, this is due to
either integration circuit faulty or sync. pulse not inputted.
In fig. 218 either R421 open or (積分電路) resistors of integration
ciruit open or cap. shorted.  (垂直振盪):vertical oscillation
In fig. 219 is integration circuit consisting of R421-423, integraiton
caps. C421-422 and coupling diode D421. They are fautly.They all connect
to vertical oscillation transformer. If transformer,T421, primary side opens,
problem appears. Finding these words in schematics 積分電路 = integration
circuit, 垂直振盪 vertical oscillation

For [II]
This is a great distortion in vertical oscillation.
(a)screen rotates downwards, vertical osc freq. > 60Hz.
When freq. > 60Hz.
In fig. 219
(y)feedback resistor R425 and VR471 either open or greater in value.
If open, adjust V HOLD, no any change.
(z)time constant cap. C423, sawtooth charging cap C425 capacitance
decrease.

(b)screen rotates upwards, vertical osc freq. < 60Hz.
In fig.219
(v)bias feedback resistor R424 value becomes greater. Emitter time
constant resistor R426 value becomes greater. The transistor 2N3461
is bad due to increase in Icbo.

In fig.220
(w)vertical osc freq. is high due to R312 open. C301-303 capacitance
decrease. R314 and VR31 in high value or C321 capacitance decrease.
(x)vertical osc freq. is low due to R311 is greater or R305, R312,
R321,R323-324 also high in value.
(y)vertical oscillation transistor 2SC828A is bad.