Question for the pro's: vertical position resistor values.

[Level42]

OK, one disadvantage of using a TV with SCART (RGB input) as arcade monitor is that there are fewer adjustments SOMETIMES and mostly only on the OLDER TV's.

So I've got this TV (Philips) in my Centipede cab now, and I need to adjust the vertical position. There is a standard pot for the vertical SIZE, but not for the position. I've got the service manual, and here's the schematics part of the vertical deflection:


Now, the service manual says that if you want to adjust the vertical position, you have to "insert" resistors 3401 or 3408. Now that's not very clear.
There is no mention of a 19" in the table, but what I found was that 3401=2k4 and 3408=680 ohms.

So I've experimented with a pot in position 3401. Nothing happened with the vertical position when I turned the pot....
Then I put back the original 2k4 at 3401 and increased 3408 to 1k2 and next to 3k9. No change on screen.....

So I'm puzzled about this. Any suggestions from the pro's here ?

[qrz]

that is not a common adjustment .  newer sets use electronic potentiometers that are adjustable via service menu
what this circuit does is inject a dc bias voltage onto the v yoke return. this voltage will change the position slightly .
since 1st attempt resulted in no change , suggest  check 26 V source on 3401.
the other question is the " ** " . this may be an optional part or of other significance....?

assuming the other parts marked ** are present and have 26v source , suggest connecting one leg of a 1.5k res to 26v side of "3401", the other to the outside leg of a 1k pot , wiper of the pot to where 3401 and 3408 meet on pcb,
remaining leg to a 470 ohm res, the remaining leg of the res to gnd side of 3408.

this "should" provide a dc range of 3.9 to 8.2v at the wiper .       

would be helpful to see entire print   

qrz

[Level42]

Thanks for that great info qrz. Indeed, this is a rather old TV without electronic (on-screen) set-up...regretfully....
The 26V source shows as 25V on my Fluke. The other end of 3401 (and also the top end of 3408 of course) shows a 11,5 Voltage.

The **'s refer to a table which is just cut-off of that picture I put up....I've just e-mailed you a link where you can download the entire Service Manual.

The table shows the various values for the parts when used with a smaller or larger CRT. 19" is not listed on that chart.

Searched everywhere but I don't have a 1K pot at hand....will report back later....thanks again !

[Kevin Mullins]

The table shows the various values for the parts when used with a smaller or larger CRT. 19" is not listed on that chart.

Couldn't you just compare values of the other sizes shown and mathematically come to an approximate value for the 19" though?
I guess it would be a matter of seeing what the resistor values are in relationship to crt size.
For instance - If the resistor value drops when the crt size is bigger then you should be able to determine that you need a higher value resistor. (or vice versa)

Just throwing this out there......

[Level42]

There are already resistors in those positions (at the values I mentioned, 3401=2k4, 3408=680 Ohms), they are simply not listed in the table on the schematics.

So they did figure out the correct values for the 19" CRT. I just need something to alter them I will try QRZ's suggestion.

Here's the missing table of value's:

[Level42]

assuming the other parts marked ** are present and have 26v source , suggest connecting one leg of a 1.5k res to 26v side of "3401", the other to the outside leg of a 1k pot , wiper of the pot to where 3401 and 3408 meet on pcb,
remaining leg to a 470 ohm res, the remaining leg of the res to gnd side of 3408.

this "should" provide a dc range of 3.9 to 8.2v at the wiper .       

Just tried this....amazingly the voltage across 3408 stays stable at 11,7V whatever I turn the pot to....