Help fixing POLO 28" dull colors - also contrast issue I think

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keropi
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Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

Hello!

once again with some chassis questions :lolno:
I was not satisfied with my POLO 28" colors - I just can't seem to find the right balance. They look dull, if you raise the pots on the neckboard you lose black purity, brightness crashes blacks and contrast has little adjustment space - if you go too much it oversaturates the picture , makes it blur and distorts it. I just can't find the correct balance and I think there is more to it that what the service manual says about black/white levels:

Image

I did what the above says to my best ability but I still can't get nice vivid colors. :oops:

Image

Image

the pics aren't the best (or the tube, it's just a test one with some screen burn and some magnetization on the edge) but the colors are dull. Raise brightness a little? It crashes black. Raise contrast? It kills the picture and at max setting it looks like this:

Image

Image

Image

it's like blooming, blurriness and distortion all together. With dull-color-settings (TM) there is no distortion, things look sharp and nice if you don't mind washed-out colors :shifty:

What am I missing here? Maybe some voltage on the main chassis is wrong? The chassis has a new lopt, all electrolytic caps are new as well as 90% of the poly ones.
I don't have any other issues with the chassis, just the color balances that are not optimal.
I feel like I need to raise contrast to fix this but raising it just blurs the image - it loses sharpness significantly and it's a no-go.

Any help/tips will be greatly appreciated.
Thanks!
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by nem »

I don't think there's anything you can do to the chassis, as it's most likely the tube itself. You could try rejuvenating it if you have the means.
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by grantspain »

yeah it does sound like weak tube
what is the value of the resistor at R35 and is R235 reading correctly 220k
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

thanks for the replies
that tube I use in the pics is just a test one I kept in storage - it has some screen-burn from a beat-em-up . I also have another test tube that has tetris burned to it BUT the tube on my cabinet is fine. What are the chances all 3 are spent the same way and in all 3 colors and in all 3 tubes? :(
Is it normal behavior for the CONTRAST pot to work like this? it's only usable at ~1/5th of it's full range and only near the beginning. You can't go past 40~50% and it produces a crap picture.
My knowledge in this is limited so maybe you are both correct but my guts tell me it's something on the chassis, drifted settings or something that became more apparent with the recap.
I will check the components grantspain mentions and report back tomorrow, thanks for the tips!
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

grantspain wrote:[...]
what is the value of the resistor at R35 and is R235 reading correctly 220k
- R35 is a 0.68ohm resistor (that reads 0.85ohm out of circuit)
- R235 is a 220K ohm resistos (that reads 245.3K out of circuit)

The tubes in my cab and the test one are both A66EAS13x01 28" ones , I have a feeling R35 should be another value? (or is this only valid for the POLO2 chassis? :eh: )
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by grantspain »

r35 is used to match tube heaters, the value is correct- sometimes you find these resistors have been changed
r235 is in the pre brightness circuit, when this goes bad it causes picture quality issues

as you have changed virtually everything and checked the two components I suggested I am at a loss to think what else could be at fault
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

I see... I guess I will try the POLO2 chassis I also have and see if there is anything different with the tube - should definitely indicate the culprit.
I'll report back in a little while
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by grantspain »

have you tested b+, it should be 145vdc
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

grantspain wrote:have you tested b+, it should be 145vdc
that I have not :oops:
is there a test point for this?

I put the POLO2 chassis and it is way better - but it can't stay on for long - that R35 on the neckboard gets EXTREMELY hot and it just shuts down I think. I am using a quality 4.7Ω /7W one - like a white cement brick.
We've talked about this in the past grantspain and you approved of the manufacturer 8-)
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by grantspain »

b+ test point is flyback heatshield for ground and either of the centre two pins of the deflection yoke connector on the chassis( pins 3 or 4)

that polo 2 should be fine, r35 should be 5r6 5w and it will get hot
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

grantspain wrote:b+ test point is flyback heatshield for ground and either of the centre two pins of the deflection yoke connector on the chassis( pins 3 or 4)

that polo 2 should be fine, r35 should be 5r6 5w and it will get hot
5.6Ω? I am using a 4.7 one just like the service notes mention - could this be the reason that after a while I hear the tube turning off and once things get cool again it will display a picture once more?
This is what the POLO2 chassis looks like on the same tube - the pics don't do it justice everything is vibrant and nice

Image

and this is with full contrast, no blooming or distortions here:

Image

I will try and check the B+ on both chassis just in case. Just to be sure, is the "flyback heatshield" the aluminum heatsink? and the center pins you mention are these marked with pink dots?

Image

I just want to make proper measurements :)
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by grantspain »

yes you are correct about where to read the b+

I do not why your chassis is shutting down, I use 5r6 on polo 2 chassis on your tube type and have had no issues, 12 ohms is used on other tube numbers
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

grantspain wrote:yes you are correct about where to read the b+

I do not why your chassis is shutting down, I use 5r6 on polo 2 chassis on your tube type and have had no issues, 12 ohms is used on other tube numbers
Measured B+ as per instructions:

POLO : ~144.7v
POLO2: ~109v

about that polo2/r35 value my neckboard has this label at the back:

Image

No idea if 5.6/5w and 4.7/7w have the same result - I asked you last year about that because I don't know about such things :oops: (I added that tube model with the blue ink)
I can easily order a 5.6Ω one and replace R35 - does it need to have any special specs?
Here is the resistor I currently use: http://uk.farnell.com/welwyn/sqp7s-4r7j ... dp/1292527
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by grantspain »

the 4r7 should be ok, I just 5r6
I have no idea why your chassis is cutting out, might not be related to that resistor and you only think it does because it gets hot
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

I think I'll leave the polo2 chassis ON and see what happens
No other idea what to check on the polo1 ? I understand this is "remote troubleshooting" but could there be anything else related to contrast I can verify locally?
I am seriously thinking sending the chassis over for a check-up :)
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Re: Help fixing POLO 28" dull colors - also contrast issue I think

Post by keropi »

did extensive test - polo2 does not shutdown it must have been a bad connection - this was not exactly a steady test setup
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