MS8-26SU vertical compression

[copados33]

Hi, I have recently received this chassis from Chad from arcadecup, and while the picture quality is stunning I have noticed that the image has some vertical range compression at the bottom, I have been messing around with the V-Size, V-Line and V-Pos. pots on the front of the chassis but couldn't fix it, I have also noted that the V-Pos pot seems to be faulty, clockwise it moves the image erratically, it can go up but doesn't move the image down, it is possible that the cause of the problem is a defective potentiometer?



notice how the "credits" letters are compressed:

[ed12]

yes
try to use some contact cleaner on it first
or if have the skill's? just replace it.

ed

[grantspain]

was the original chassis a ms8-26 or a ms8-25?
what is the impedance of the vertical yoke?

if the original is a ms8-26u and you did a direct swap out then there are a couple of components on the ms8-26su that will cause this issue

[copados33]

This is the original chassis, a MS8-26SU, this monitor came installed on an Aero City Sega arcade cab, 26" Mitsubishi tube model A63...something.

Vertical impedance yoke is 81.3 mH

How do I know the model/value in ohms of the pot to replace it if its broken?

[grantspain]

mh is the inductance reading,impedance is ohms-i think it is around 50 ohms on the vertical on ms-26

the pot should have a code on it like 102 or even have the exact value like 5k

[ed12]

right in the 50 ohm range 37-47 resit.


ed

[copados33]

I tried with a multimeter now, vertical winding reads 37 ohms.

[grantspain]

ok thats good,i would look at changing that pot first
does the problem clear after some time,for example after 30 mins

[copados33]

Nope, yesterday I had the cab turned on for the whole day (15+ hours)  and it was looking the same all the time.

[grantspain]

ms8-26su chassis are known for issues on the vertical deflection,there are a couple resistors and diodes that fail
the first sign of this is compression leading to foldover and then finally leading to full collapse

but that vertical shift pot should work irrespective of this so i suggest changing the pot or at least check for bad solder joints around that area

[copados33]

Electronic stores are closed by the weekends around here, so I'll look for a replacement tomorrow and post my results.

[copados33]

Changed the faulty pot and it didn't work, I still see the image compressed at the bottom.

[copados33]

Anyone? is it possible that this monitor has had a MS8-26A instead o an MS8-26SU? I got this cab as "not working" and I'm starting to think that the previous owner swapped the chassis, are 26's and SU's interchangeable?

[grantspain]

i have only seen one model of ms8-26,there is a ms8-25 which is a low impedance chassis but you would get much more extreme vertical issues

my advice is to look at the caps around the deflection transistors plus there are some resistors that break down

[copados33]

This is the list of components that had been replaced and or serviced:

[chriso]

Vertical linearity control