Atari 7800 only displays games in Black and White

[rou021]

I just got an Atari 7800 and everything seems to work fine--except it displays games (for both the 2600 and 7800) in black and white only. There is no color at all. I tried a couple of different video cables/adapters and nothing seems to fix it. I can't figure out what's wrong with it. Does anyone here have any suggestions?

[ApolloBoy]

What type of TV are you using?

[rou021]

I've used a DLP HDTV and an older CRT SDTV--both of which work fine, and display the proper video signal via RF with other devices.

[gdement]

Are you using a switchbox? If so, try using a direct adapter instead. This is an example:
http://www.radioshack.com/product/index.jsp?productId=2103471&filterName=Cable+type&filterValue=Video+adapters

Also try a different cable.

[rou021]

Like I had stated in my initial post, I tried a couple of cables and switchboxes/adapters to no avail.

[rou021]

I'll try a new power supply and see if that works. Thanks.

[rou021]

Thanks for the help so far; I really appreciate it. Unfortunately, I finally tried a new power supply, but that didn't seem to work. How would I go about testing for, and fixing, the bad ground?

[Mitch]

Have you tried it on both channels? Sometimes the RF modulator drifts a bit out of tune and you lose the color but it may be close enough on the other channel to still receive color.

Mitch

[rou021]

Yep. I've tried it on different channels, with different RF adapters, using different games, on different displays. There seems to be very little that I can do differently. Nothing I've done seems to make any difference. :frustrated: At this point, I'm about to take colored markers to the screen; one way or the other, I'm going to get some color...

[gdement]

Like I had stated in my initial post, I tried a couple of cables and switchboxes/adapters to no avail.
It wasn't clear from your initial post that you weren't using the switchbox type adapters, and that's what people often use. They don't work very well.



If it's an internal problem, then it could be a broken chroma (color) signal. This could be checked if you have a multimeter.
http://www.digitpress.com/forum/attachment.php?attachmentid=2897&stc=1&d=1269309968
This shows where the chroma signals from 2600 (TCOL) and 7800 (MCOL) are mixed together, until they reach the RF modulator. If it's internal then the problem would be somewhere in here, since it's happening in both modes.

R16 and R17 are in the upper left area of the board. To find these resistors, load up this site:
http://saundby.com/atari7800mod/7800_vidmod_construction.html
They're shown as "2600 chroma" and "7800 chroma" if you scroll down about 2/3rds of the way.
The resistors themselves should have a resistance of about 4300ohms (R16) and 4700ohms (R17).
The top side (the output) of each resistor should have continuity to each other. This is where the chroma signals mix and head towards the RF modulator.
Check the resistance between that point and the metal RF shield (ground). It should be around 400-500ohms. If it's much lower then R2 could be bad.

On the schematic, R2, L1, and C8 could all potentially break the chroma before it reaches RF.
L1 is between the RF modulator and the 2 transistors. On my machine it had a couple ohms resistance across it.
C8 is a small capacitor, I don't know any good way to test that with a multimeter.
Make sure the pins going into the RF module are intact.
The RF module itself might be bad, I don't know if those fail in a way that removes the color.



The full 7800 board schematic is here:
http://www.atariage.com/7800/archives/schematics_ntsc/

[raylydiard]

check caps its a 24 year old console


It wasn't clear from your initial post that you weren't using the switchbox type adapters, and that's what people often use. They don't work very well.



If it's an internal problem, then it could be a broken chroma (color) signal. This could be checked if you have a multimeter.
http://www.digitpress.com/forum/attachment.php?attachmentid=2897&stc=1&d=1269309968
This shows where the chroma signals from 2600 (TCOL) and 7800 (MCOL) are mixed together, until they reach the RF modulator. If it's internal then the problem would be somewhere in here, since it's happening in both modes.

R16 and R17 are in the upper left area of the board. To find these resistors, load up this site:
http://saundby.com/atari7800mod/7800_vidmod_construction.html
They're shown as "2600 chroma" and "7800 chroma" if you scroll down about 2/3rds of the way.
The resistors themselves should have a resistance of about 4300ohms (R16) and 4700ohms (R17).
The top side (the output) of each resistor should have continuity to each other. This is where the chroma signals mix and head towards the RF modulator.
Check the resistance between that point and the metal RF shield (ground). It should be around 400-500ohms. If it's much lower then R2 could be bad.

On the schematic, R2, L1, and C8 could all potentially break the chroma before it reaches RF.
L1 is between the RF modulator and the 2 transistors. On my machine it had a couple ohms resistance across it.
C8 is a small capacitor, I don't know any good way to test that with a multimeter.
Make sure the pins going into the RF module are intact.
The RF module itself might be bad, I don't know if those fail in a way that removes the color.



The full 7800 board schematic is here:
http://www.atariage.com/7800/archives/schematics_ntsc/