Great, my VT52 is shot.

[Brent Hilpert]

On 2020-Apr-19, at 3:09 PM, Chris Zach via cctalk wrote:

>> As I wrote earlier in the thread I think it is a good idea to check the V
>> sync and H sync signals to check that they are right in pulse length and
>> shape.
> 
> Good starting point. From the schematics would that be scope probe to J1 pin 5 for horizontal and j1 pin 9 for vertical? Also which is J1, and how are they numbered :-) Also is there a good ground reference point?
> 
> Also is there a document that links the part numbers on the boards with what's on the schematics?
> 
>> There are indeed electrolytic capacitors in the vertical deflection circuit
>> but I am not sure if that would make the beam move much faster since for a
>> faster move of the beam also requires a higher voltage over the deflection
>> coil to create a faster ramp up of the current trough it. But it is
>> definitely worth checking. Especially since the scan lines are sloping a
>> bit and not straight as one would expect.
> 
> I would believe a capacitor failure due to heat, any ones in particular I should check?
> 
>> So do I. And now we need Chris to do some measurements on it to get further.
> 
> More than happy, just haven't worked on TV sets before and would rather not blow off my hand. I'm guessing the really dangerous voltage is the one on the bottom left that comes off the step up transformer over to the odd plug going into the VT52 rear bulkhead (HV to display)


Using the schematic and maintenance manual from bitsavers:
	http://bitsavers.org/pdf/dec/terminal/vt52/MP00035_VT52schem.pdf
	http://bitsavers.org/pdf/dec/terminal/vt52/EK-VT52-MM-002_maint_Jul78.pdf

Monitor schematic: MP00035_VT52schem pdf-page 10
Monitor board component layout: MP00035_VT52schem pdf-page 8

There are *two* horizontal signals from the logic to the monitor (plus the vertical & video).

There are some waveform diagrams for what to expect around the monitor board presented in the maint manual: 
	EK-VT52-MM-002_maint_Jul78 pdf-page 92

Something else that might help with diagnosis is taking pictures of the screen with known, simple elements on the screen.
e.g.:
	- clear the screen
	- take pic if it doesn't clear
	- type a simple character like a "-"or "1"
	- pic
	- type a half line or full line of characters
	- pic
	- type different characters on two different lines
	- pic
	
The idea being to find out, from simple known patterns, where the pixels end up being displayed,
so it may be possible to figure out what's happening with the scan, whether it's getting stretched, folding over, etc.

Yes, you want to watch out for HV around the flyback transformer.
The very HV is reasonably insulated, but there's also a few-hundred volts around the open componentry around the focus and intensity controls, the stuff feeding the neck end of the CRT.