K7500 colors off

[mclovin]

Hi all, I have a golden tee with a k7500 and I checked all the larger caps and all seems ok but I am having a problem setting up the colors. Hard to get white, black, blue, etc. i have adjusted the knobs but nothing is working right colors are off. What can I check from here?

[ed12]

pictures please

ed

[mclovin]

Yes will do. Looks like my biggest problem is it the white looks yellow.

[big10p]

What knobs have you adjusted? Sounds like the cutoff/gain pots on the neck board need adjusting.

[mclovin]

Yes I can get the white, good but then the blue is black, red is pink etc. ill keep trying to mess with it.

[dssrocks]

You have to stop fixing everyone's golden tees without getting paid

[mclovin]

I work for beer! Here is a picture. I can't seem to get blue without compromising black. I can get blue and white but then my red looks pink and the green looks very light. I can't get purple at all.

[ed12]

man get the emmisions of that picture tube (jub) chked
freq is looking driven

ed

[mclovin]

That sounds like something I probably can't do myself?

[big10p]

Looks like you're missing blue completely, in that pic. Check the blue circuit on the chassis. Could be a shorted blue transistor on the neck board.

[mclovin]

I can get blue by adjusting the neckboar but I seem to lose black. The boot screen is blue. I can keep messing with it.

[lilshawn]

there are 2 adjustments for the guns.... a DRIVE and a CUT-OFF

the drive sets the maximum current flowing out of the guns.

the cut-off adjusts where the flow will start to taper off and eventually stop flowing all together.

your blue DRIVE needs to go UP until the blue pixel dots just starts to show on the black...then back it off a tad. adjust the CUT-OFF up or down to match the red or green (this is best done with a gradiation line for each RGB color)

[mclovin]

Thanks Shawn, where are these adjustments? On the neckboard?

[mclovin]

Looks like you're missing blue completely, in that pic. Check the blue circuit on the chassis. Could be a shorted blue transistor on the neck board.

You may have a point here. Sorry I have no knowledge of this stuff. The extent of my abilities ends with replacing caps. How should I go about checking the blue circuit on chassis and how do I test the transistors on the neck board?

[big10p]

Looks like you're missing blue completely, in that pic. Check the blue circuit on the chassis. Could be a shorted blue transistor on the neck board.

You may have a point here. Sorry I have no knowledge of this stuff. The extent of my abilities ends with replacing caps. How should I go about checking the blue circuit on chassis and how do I test the transistors on the neck board?

The manual/schematic should show you where the blue circuit is. It'll probably be marked on the chassis, also. The RGB transistors are on the neck board, and will probably be marked, too.

[lilshawn]

okay just had a look...

on the 7500 the red and green drive is matched to the blue gun. so there is no adjustment for the blue drive. they also call the cutoff "bias" on the 7500. the blue is run wide open, and the red and green are adjusted up and down to match the output. no big deal.

on the 7500 neckboard the adjustments (a viewed from the rear) from left to right are:

Red drive / Green drive  -----  Red bias / Green bias / Blue bias

the most common issue with the 7400/7500 series is the output transistors heat up and the traces break away from the board. then the traces break off due to vibration. scrape the solder mask off back to good material and solder bridge them back together.

2nd most common is output transistor failure (2SC3782) the heatsinks are garbage, so they can overheat.

3rd most common is the video amp failure (LM1203N) it's input is unprotected (no clamp diodes or nothing) so static and surges get fed directly into the amp and blow out the input.

Ideally you'd want to swap the neck board with a known good one to see if it's the tube.

if you don't find any issue with the output transistors or video amp you may have an issue with the gun.

[ed12]

i stand behind my statement
have the tube chked
common problem with 1 gun full on is exactly what u are going through

ed

[mclovin]

Thanks for the help guys. I don't have a 7500 neckboard to swap, I may convince a friend to let me borrow one. Otherwise I have everything looking better, it impossible for me to achieve purple for some reason but all other colors are reasonable other than the blue is more black.

[ed12]

as blue is always the hard on gun
and u can not get purple (red+blue)
then yes do a neck board swap
if that fails look into the tube a little colser
tell me what happens after 2 hrs of running
can u get purple ?
that will tell the trick of the tail

ed