TokensArcade
Member
Safe To Power On? Pong Clone repair, Ground Resistance Question
Hello Klov:
I am currently working on repairing a Chicago Coin TV Ping Pong machine, which uses an atari pong clone board. The board was shorting between 5v and Ground, and so I went and desoldered every chip from the board and found one 7400 IC was shorted completely with others showing around 70-120 ohms of resistance between VCC and ground.
Since then I have replaced a bad Diode as well as the voltage regulator as I found out that was completely shot. I tested it again with no chips in it and found I had a steady 5 volts. However, despite replacing every 7400n chip with new ones, I found I still had some other chips that were lowering the resistance between 5v and ground (I tested the ohms after I re-inserted each chip). I also made note of over a dozen chips that seemed to be really dropping the ohms.
With all chips reinserted into the board I have around 19 ohms between power and ground. I'm also finding around 30 ohms of continuity across one of the diodes in circuit as well as about 30 ohms reading across each side of some resistors connected to the 5v line. Just for comparison I looked at some of my working boards (an Asteroids and Vs. Tennis) and found that they read out around 50 ohms of resistance between 5v and ground.
So here are my questions:
-Is there an acceptable margin of error for how much resistance can be between VCC and ground in a TTL board (or any PCB for that matter)? If so, what is it?
-Do you think its safe for me to try powering on the Pong Clone, given the information I have given?
-Should I try replace some more TTL chips until I get the resistance up to a "safe" level?
-Should there be any reading AT ALL between VCC and ground? Or should it be completely OL?
Please respond with any advice you may have. I am open to anything
Hello Klov:
I am currently working on repairing a Chicago Coin TV Ping Pong machine, which uses an atari pong clone board. The board was shorting between 5v and Ground, and so I went and desoldered every chip from the board and found one 7400 IC was shorted completely with others showing around 70-120 ohms of resistance between VCC and ground.
Since then I have replaced a bad Diode as well as the voltage regulator as I found out that was completely shot. I tested it again with no chips in it and found I had a steady 5 volts. However, despite replacing every 7400n chip with new ones, I found I still had some other chips that were lowering the resistance between 5v and ground (I tested the ohms after I re-inserted each chip). I also made note of over a dozen chips that seemed to be really dropping the ohms.
With all chips reinserted into the board I have around 19 ohms between power and ground. I'm also finding around 30 ohms of continuity across one of the diodes in circuit as well as about 30 ohms reading across each side of some resistors connected to the 5v line. Just for comparison I looked at some of my working boards (an Asteroids and Vs. Tennis) and found that they read out around 50 ohms of resistance between 5v and ground.
So here are my questions:
-Is there an acceptable margin of error for how much resistance can be between VCC and ground in a TTL board (or any PCB for that matter)? If so, what is it?
-Do you think its safe for me to try powering on the Pong Clone, given the information I have given?
-Should I try replace some more TTL chips until I get the resistance up to a "safe" level?
-Should there be any reading AT ALL between VCC and ground? Or should it be completely OL?
Please respond with any advice you may have. I am open to anything
