Help with quadrapong repair

musicman282

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So I'm working on fixing my newly acquired quadrapong and have a few questions. I think that I've narrowed down the problem to my clock circuit. I've probed different parts of the circuit and have a feeling the clock is the problem. My question is if the clock circuit is supposed to produce a 5V square wave right from the crystal oscillator? Right now the circuit goes through two back to back hex inverter gates and then through a flip flop. When the signal comes to the flip flop it is only reading 2 volts peak. I read the data sheet and it said that the 7404 hex inverters only needed an input of 2V in order to trigger. What I'm finding strange is that they are only outputting 2V also. I'm attaching a picture of the output of the flip flop.
 

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So I'm working on fixing my newly acquired quadrapong and have a few questions. I think that I've narrowed down the problem to my clock circuit. I've probed different parts of the circuit and have a feeling the clock is the problem. My question is if the clock circuit is supposed to produce a 5V square wave right from the crystal oscillator? Right now the circuit goes through two back to back hex inverter gates and then through a flip flop. When the signal comes to the flip flop it is only reading 2 volts peak. I read the data sheet and it said that the 7404 hex inverters only needed an input of 2V in order to trigger. What I'm finding strange is that they are only outputting 2V also. I'm attaching a picture of the output of the flip flop.

I'm attaching a representative schematic of the crystal oscillator.
 

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If it really is 2V then that's probably too low. However it looks like you have a 60Hz hum riding on top of the signal which could mean your scope probe isn't grounded well enough. Try grounding the probe on the IC's ground or try a different scope probe. Also make sure your game board is properly grounded to Earth ground along with the scope. Crank the timebase down so we can see what the clock edges really look like!

Re-check the +5V on the board. Is it low?

As for the 7404, make sure you replace it with a 7404 and not something different like a 74LS04. PONG boards have some race conditions and sometimes replacing an IC with something other than an exact replacement will screw up the timing. Crystal clocking circuits are especially susceptible to this.

EDIT: I meant to say that crystal clocking circuits are a careful balance of drive and load to get them to oscillate, not race condition. But yeah, try to replace components with like components.
 
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If it really is 2V then that's probably too low. However it looks like you have a 60Hz hum riding on top of the signal which could mean your scope probe isn't grounded well enough. Try grounding the probe on the IC's ground or try a different scope probe. Also make sure your game board is properly grounded to Earth ground along with the scope. Crank the timebase down so we can see what the clock edges really look like!

Re-check the +5V on the board. Is it low?

As for the 7404, make sure you replace it with a 7404 and not something different like a 74LS04. PONG boards have some race conditions and sometimes replacing an IC with something other than an exact replacement will screw up the timing. Crystal clocking circuits are especially susceptible to this.
Good call about the bad scope probe. I used channel 2 and my other probe and now it is showing 5v. It still looks like a sine wave, unfortunately the crystal is 14MHZ and my scope is only a 20MHZ so the time base I had it on was the fastest speed it has. I'm actually getting around 5.12V +/- .1 V at all the spots I measured all over the board so I don't think the 5V is the problem. The game 1/2 works. When you coin it up the start button lights up and if you only put 1 coin in, it will start into a 2 player game. The player 1 paddle works fine and responds to the knob, but the player 2 paddle is completely missing. It shows up as a series of dots. It's odd, it looks like it was broken up into a bunch of speckles. Also the ball appears on the display, but it never moves so it is like the ball is loaded up, but the circuit that controls the ball motion is never engaged. I've attached pictures of what is happening. I'm lost now that my clock circuit seems to be ok. That's where I thought my problem was, but now have to look somewhere else. The two player screen shows how the ball is just sitting in the corner and not moving and you should be able to make out the artifact dots of the player 2 paddle. The 4 player game doesn't allow you to see the ball because the paddle is stretched across the screen, which I think is caused by a bad 74161. I appreciate the help you're providing.
 

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I just wanted to add that the monitor/tv looks to have a really nice picture. I finally started putting together an order with Mouser to replace the caps on mine.
 
I just wanted to add that the monitor/tv looks to have a really nice picture. I finally started putting together an order with Mouser to replace the caps on mine.
Does the board work on yours?
I can't remember now. Do you have pics of your machine? I'd like to see it.
 
Does the board work on yours?
I can't remember now. Do you have pics of your machine? I'd like to see it.
Yeah my board works. I wish I had a much better ability to work on and troubleshoot them, but I sent it to bit_slicer for repair. I don't have a ton of pics of mine, but here's a quick shot I took after I got it at auction last year. Mine is in fair shape. It's got a bit of selling in the wood, but still a solid cab. I can't really complain seeing as the cab is hard to find and older than anything else I have.

DSCN0607.jpg
 
Yeah my board works. I wish I had a much better ability to work on and troubleshoot them, but I sent it to bit_slicer for repair. I don't have a ton of pics of mine, but here's a quick shot I took after I got it at auction last year. Mine is in fair shape. It's got a bit of selling in the wood, but still a solid cab. I can't really complain seeing as the cab is hard to find and older than anything else I have.

DSCN0607.jpg

That thing looks great. Your sideart is different than mine. One side has the upper side art and the other has the lower. I wonder if someone reversed the top of yours. Is there sideart on the other side? Good news. I got the board working 100%. I was going to send it to bitslicer but I was too impatient to wait so I broke out my logic comparator and found 2 bad flip flops, a bad counter, a bad hex inverter and a bad 555 timer. Replaced them all and Voila!! The game works perfectly. I think I might need to recap the monitor though because one side of the image, the pixels don't look as tight as the other side. It looks as iff the one side is being stretched a little bit. I'll have to take a picture tomorrow and post it. Do you have a list of the caps that you had to buy to do the monitor on here? I did a brief visual and didn''t see many electrolytic and hesitate to take the board out because the cab wasn't exactly designed for ease of service.
 
That thing looks great. Your sideart is different than mine. One side has the upper side art and the other has the lower. I wonder if someone reversed the top of yours. Is there sideart on the other side? Good news. I got the board working 100%. I was going to send it to bitslicer but I was too impatient to wait so I broke out my logic comparator and found 2 bad flip flops, a bad counter, a bad hex inverter and a bad 555 timer. Replaced them all and Voila!! The game works perfectly. I think I might need to recap the monitor though because one side of the image, the pixels don't look as tight as the other side. It looks as iff the one side is being stretched a little bit. I'll have to take a picture tomorrow and post it. Do you have a list of the caps that you had to buy to do the monitor on here? I did a brief visual and didn''t see many electrolytic and hesitate to take the board out because the cab wasn't exactly designed for ease of service.
Congrats on getting it running. I checked out your Youtube vid and it looks like it's a pretty fun game, especially for 4 people. I've thought about getting a logic comparator before...basically it can check TTL chips on a board with a known good one attached to the comparator, right?

I'm pretty sure mine has art on the other side as well. I haven't placed my order with Mouser just yet, but I've used their shopping cart save feature I don't have to re-enter them again. There are a few oddball value caps on there, especially where it concerns that multi-selection cap. I'm going to skip on replacing it for now and only change the others. Most of them are actually on the bottom side of the chassis. The legs are soldered into those series of posts sticking up on the top side so this isn't using through hole type boards....not sure what the technical term for this would be. This is assuming you have the same Zenith TV as I do, but I'd bet it still has that non-through hole setup.
 
Congrats on getting it running. I checked out your Youtube vid and it looks like it's a pretty fun game, especially for 4 people. I've thought about getting a logic comparator before...basically it can check TTL chips on a board with a known good one attached to the comparator, right?

I'm pretty sure mine has art on the other side as well. I haven't placed my order with Mouser just yet, but I've used their shopping cart save feature I don't have to re-enter them again. There are a few oddball value caps on there, especially where it concerns that multi-selection cap. I'm going to skip on replacing it for now and only change the others. Most of them are actually on the bottom side of the chassis. The legs are soldered into those series of posts sticking up on the top side so this isn't using through hole type boards....not sure what the technical term for this would be. This is assuming you have the same Zenith TV as I do, but I'd bet it still has that non-through hole setup.
The logic compatator just compares the chips on the board to the known good one. It works great. You still need the schematics though in order to make sure the pin actually goes to something and isn't just floating because you'll get a false read. The thing works great for finding bad ttls. I wondered where all the caps were. I haven't pulled the monitor chassis out yet, but plan too because the picture gets a random hula every now and then and this thing produces a TON OF HEAT. So I would assume the electrolytics need replaced. It amazing how the non LS series TTL chips generate so Mich heat.
 
The logic compatator just compares the chips on the board to the known good one. It works great. You still need the schematics though in order to make sure the pin actually goes to something and isn't just floating because you'll get a false read. The thing works great for finding bad ttls. I wondered where all the caps were. I haven't pulled the monitor chassis out yet, but plan too because the picture gets a random hula every now and then and this thing produces a TON OF HEAT. So I would assume the electrolytics need replaced. It amazing how the non LS series TTL chips generate so Mich heat.

Those boards pull 3-4 Amps @ 12V. I have to use a separate bench supply for testing the old B/W games. Be careful, the power resistor used for the +5V regulated supply (the wire-wound that sits offset from the board) gets hot enough to melt solder. Or a finger. Trust me on this. :p

Congrats on getting it going!!! :cool:
 
Be careful, the power resistor used for the +5V regulated supply (the wire-wound that sits offset from the board) gets hot enough to melt solder. Or a finger. Trust me on this. :p

I have a burn on my finger to prove it does. Wish you would've posted this last week ;-).
 
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