Safe To Power On? Pong Clone repair, Ground Resistance Question

TokensArcade

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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
 
You will see a decreasing resistance as you add each TTL chip. Any resistance the chip has between VCC and ground will be paralleled up with all the others.

As parallel resistance is the inverse of the sum of the inverses of the resistances, you will continually get a lower resistance with each chip added.

For instance, put ten 10 ohm resistors in parallel and the resistance will be:


1 / (1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10) =
1 / (10/10) = 1 / 1 = 1

1 ohm.

I'm not sure what sort of resistance you should be reading between Vcc and Ground. Hopefully someone else here will have some guidance.
 
You will see a decreasing resistance as you add each TTL chip. Any resistance the chip has between VCC and ground will be paralleled up with all the others.

As parallel resistance is the inverse of the sum of the inverses of the resistances, you will continually get a lower resistance with each chip added.

For instance, put ten 10 ohm resistors in parallel and the resistance will be:


1 / (1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10 + 1/10) =
1 / (10/10) = 1 / 1 = 1

1 ohm.

I'm not sure what sort of resistance you should be reading between Vcc and Ground. Hopefully someone else here will have some guidance.

Thank you for that explanation! I had a feeling it was doing something like that but I didn't know exactly how exactly it worked.
 
Also to anyone who may be reading this, what would potentially happen if I plugged this in and the resistance is too low? I'm pretty sure it would draw more current, but what components would be at risk if I power it I'm this state. I definitely don't want to risk damaging the voltage regulator considering how pricy it was.
 
I have been working on a couple of Pong clones over the lat month.
I had Digital Games 574 (Knockout) that had about 50 ohms and after replacing a few dud 100nF caps it had lifted to around 115 ohms.
I used an ATX (PC) power supply (green wire, Pin16 connected to ground to switch it on) to run the board on the test bench, it won't turn on if there is a short present.
I'd suggest going that way if you have one handy as they only need 5v to run, I was using a reversing camera LCD screen for the monitor which ran on 12v so it was the perfect solution for the test rig.
 
I have been working on a couple of Pong clones over the lat month.
I had Digital Games 574 (Knockout) that had about 50 ohms and after replacing a few dud 100nF caps it had lifted to around 115 ohms.
I used an ATX (PC) power supply (green wire, Pin16 connected to ground to switch it on) to run the board on the test bench, it won't turn on if there is a short present.
I'd suggest going that way if you have one handy as they only need 5v to run, I was using a reversing camera LCD screen for the monitor which ran on 12v so it was the perfect solution for the test rig.

Thank you for this info!

Just to be clear when you say "It won't turn on if a short is present" are you saying the power supply won't turn on or the board won't turn on?

Can you just wire the 5v from the ATX power supply directly to the 5v line after the voltage regulator or would it be wise to remove those components first?

I've replaced the 100nf caps on the 5v rail, but I haven't gotten to the ones elsewhere on the board, so I'll give that a shot.

So in your experience, how many ohms minimum do you think is sufficient to power the board without driving current too high?
 
The boards I worked on had the DC conversion (Mirco Challenge and Digital Games 574) through the Motorola monitor so I didn't have to worry about the AC side of things, personally I'd remove the the regulator from the circuit but maybe someone with more experience can chime in.
The ATX Power Supply wont turn on if it detects a short but I have no idea at what point it cuts out.
I'd check the electrolytics before going any further and theres only a few of them so it may be safer to replace them to be sure or lift one leg at a time while measuring the resistance and look for any increase.
My baords had a 1.5 amp supply and if you apply ohms law you should be able to work out how much curent it will draw. The board I referred to earlier was running fine on around 75 ohms.
 
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