17 Jun
2009
17 Jun
'09
6:21 p.m.
My memory served me wrong, or I read the post to fast.
Is the symptoms
of a "lost" horizontal hold that the picture will scroll, looping around?
No, that's loss of _vertical_ hold. Loss of horizontal hold wil lcause
the picture to break up into sloping lines across the screen.
This vt220 in front of me has no picture at all. I was using it and
between the blink of my eyes the picture disappeared, no bang, no smoke,
just gone. I think the logic still works, judging by the led indicators
on the keyboard and the satisfying beep it "boots" just fine.
Any ideas where I should start looking?
Do you have a composite video monitor around? If so, connect it to the
BNC socket on the back of the VT220. If you get a display on that
monitor, the VT220 logic is working.
An old book on TV servicing (so old that it doesn't mention transistors,
and even disccussed mains-derrived rather than flyback EHT...) points out
that there are 7 signals applied to a CRT (this applies to monitors and
terminals too)
1) Heater supply
2) Video drive
3) Grid bias
4) First anode voltage and/or focus voltage
5) Horizontal deflection
6) Vertical drfleciton
7) Final anode voltage (EHT)
Problems with any of them can result in a blank screen, although problems
with _just_ the deflection [1] would have to involve steady DC currents
through the coils to deflect the beam off-screen, which is a very
uncommon fault.
[1] Note that in just about every monitor/terminal (and certainly the
VT220), the EHT and other CRT supplies come from the horizontal output
stage. A faut in the horizontal deflection circuit may well remove these
voltages and cause the screen to go blank.
So it's basically a process of finding out which input is wrong and
correcting it. I'd start by checking the CRT heater is glowing (it's easy
to do -- just look at it), and then, assuming you have a suitable EHT
meter, measure the final anode voltage. Expect about 10kV here.
Alas without schematics it's difficult to go much further -- is the VT220
printset on a web site?
-tony