My 7800 composite video mod fail

[yorxs]

Finally composite video modded my 7800. I've modded several consoles and satellite receivers in the past so I figured this would be routine, but I can't figure out where I failed.

 

Here is the symptom, the video and audio will be clear for a few seconds before the video starts to flicker. After 30 secs the video starts flickering so bad I can't see the screen, the audio will start to get very quiet shortly after.

 

I have checked and tripple checked the wiring and I am positive its correctly wired an instructed, I cannot find any damage done to the board either.

 

I'm following the guide here: http://www.vintagegamingandmore.com/installation-guide-7800/

 

Eveything is very straight forward but the guide neglects to mention the transistor installation. I installed it as pictured, the is the only grey area. Could this be my problem.

 

Anyone know what causes my symptom?

[RevEng]

Symptoms that change with time are often the result of a poor connection somewhere. E.g. A cold solder joint, cracked resistor, lifted trace ...

 

Try flowing your solder joints again and see if that helps. If not, try running it with the case off and observe output changes as you put a small bit of pressure on various components.

[iesposta]

This thread, and specifically these pictures helped me with my mod:

http://atariage.com/forums/topic/127926-easier-7800-composite-video-mod/page__st__75#entry1796143

[mimo]

Are you using a crt tv or something more modern. Some LCD and plasma TVs can't handle the out of spec video output from the 7800

[yorxs]

I am using a CRT, but it is an HDTV, Sony VEGA. Re-wired from scratch and still the same symptom.

 

Picked up another 7800 fro $10 that I will try again on, as soon as another kit comes in the mail.

[Jinks]

If it aint broke....

[iesposta]

If it aint broke....

 

They may not be broken, but my experience is that compared to Heavy Sixer 2600 RF, 7800 output is quite lousy.

[mimo]

I guess the Sony runs at 100hz? Do you have another tv or 2 that you could test with before writing off the mod as not working. It could well be the tv that is the problem

[Trebor]

yorxs,

 

Please try it on a CRT before driving yourself nuts. The difference and results from a modern flat panel HDTV and a CRT is night and day. Here's a comparison of the same 7800, same Composite Mod. The captures are from a Panasonic LCD (Results are similar on a Panasonic Plasma, Insigma LED and a LG LCD) and a Commodore 1702 CRT - Both have their user settings @default:

 

http://atariage.com/...25#entry2699592

 

Granted the pictures from the LCD don't look as bad as what you are describing, but I just wouldn't trust any results of the 7800 on a flat panel, I've only seen and experienced 'ok' to lousy results in one form or another.

 

It's not just 7800 specific either, as I have a Woody 4-switch 2600 go blindingly white on the flat panels as well, while a CRT keeps consistent gorgeous brightness, contrasts and color levels.

[robertski]

I am familiar with this circuit, If the video starts getting worse and worse, check the soldering

on the board where the two resistors are. They are like a voltade divider and a cold solder

on either one will cause this INvariably lowered voltage to dwindle off..

I think. or one of those resistors may be bad..

Hope this helps.