Solved Converting an Polo/2 15" to 20"

[Georgian2]

Hello,

I'm working on this conversion. The chassis is an Polo 2 for 15" CRTs. The Neck board had an mini socket so I replaced it with an 25-28" Polo 2 neck board. The chassis is now married to an MTC9000 CRT Samsung 51GGD95X-TC 20". The CRT itself is working great, have been tested before. The chassis has been tested before with original 15" CRT and it worked well too.
The image I get is dimm, maybe a little brighter on top and way out of focus on the sides. Voltage on the middle pins from the yoke connector is 105V (B+?). As I can see from the schematics, there are a lot of jumpers to set in order to set it to different voltages. I have no ideea what to do here. The Flyback is HR7383 or HR 7389 hard to read. I know it is for 110° and the CRT is 90° but it should still work.

So far I changed the folowing:
Neck board: R35 4.7ohm (With Lower values like 1 Ohm or 2.7 Ohm the Image was way to bright)

Chassis: C58 1000uF (was 2200uF),
C97 0.47uF (was 0.33uF),
C99 6.8nF (was 4.7nF).

Nothing else has been changed so far.

If you know what else must be changed or if the values I put in are wrong please do let me know. I apreciate any help. I can check the resistence on the yoke if is important to know.

Thank you.

Schematics PDF

 

[Georgian2]

 

[Georgian2]

Some update. I lowered the R35 from Neckboars to 0.5Ohm and the Image is much brighter. It turns out, the flashing and risig brightness at the begining (posted in another thread) was because of a bad brightness pot on the control board.
There is still a little shaking left and right (whole image) but that should go away with new caps.
Some vertical tiny lines are very little visible but they are there. Again maybe new caps will get rid of them. Other that that I'm happy with it.

Still if you notice anything wrong or missing, please let me know.

I hope to soon call it an successful conversion/repair.

 

[Retroman839]

Thanks for the write up..
I have a couple I would like to enlarge image size on.
This will be helpful thanks

 

[pinballaction]

I have neckboards to sell .

 

[ArcadePCB]

I'm not familiar with this type of chassis - but a common remark: Is it a good idea using a bigger tube combined with a chassis intended for a smaller tube? I don't know. Maybe this chassis is suitable for bigger screens, too. A bigger screen means more energy required for getting the same brightness (higher beam current because of the bigger area the beam has to scan during the same time). This may be an overload for a chassis intended for a smaller tube.

 

[pinballaction]

Hi
saw trying to explain:
chassis 10,14,20 the same circuit as 25,28,33
Differences:
different flybacks, different control boards, absence of east/west board, different source regulation and diodes more strongs, different high voltage capacitors, and the 10 even has a different neckboard than all the others and the control board haven,t two potentiometers.

 

[DaveR]

i also been looking for the differences between boards as they look so similar, sadly only seem to have one schematic

 

[pinballaction]

I have schematic of polo1 and 2 but in paper, more easy buy a board

 

[Georgian2]

I have schematic of polo1 and 2 but in paper, more easy buy a board

I'm not gona throw away a good chassis and buy a working one.

 

[Georgian2]

Quick update. The B+ was 105V, maybe it was enough for the original 17" CRT but too low for the 20" one. I pumped it up to 130V using the RV1 and everything improved. So far didn't make any other changes to the chassis.
Next step is to check if any capacitors are rated for anything lower that it is now or maybe let it run and see which one releases it's soul...
With B+ I mean the voltege on the middle pins from the yoke connector against GND. This thread migh help all of you who bought that nos 17" monitors and what to "resize" them.

 

[pinballaction]

I said buy a neck board not the chassis

 

[pinballaction]

Good to 130v
At night I go see in schematic the value of poliéster capacitors to 10/14/20

 

[Georgian2]

I said buy a neck board not the chassis

Sorry I misunderstood what you wrote. The 25-28" one I got seems to work fine, no need to.

 

[Georgian2]

Replaced the caps on the power supply. Everything is nice and stable (~1 Hour test). Width and height working well, brightness and contrast too. Picture is stable.

Components changed:
Neck board: R35 1R5
Chassis: C58 1000uF (was 2200uF),
C97 0.47uF (was 0.33uF),
C99 6.8nF (was 4.7nF).
C104 4.7nF (was 6.8nF)
C19 100uF (was 47uF)