Asteroids Game PCB - No Y output, help needed with the fix

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Asteroids Game PCB - No Y output, help needed with the fix

Hello! After completely rebuilding my 19v2000 deflection board (the journey is/was captured here: http://forums.arcade-museum.com/showthread.php?t=137243) I found that my asteroids PCB was not sending a Y signal. The game worked when I got it, but loose connections on the deflection board were causing the monitor to turn on and off. I left it on for a couple hours after wiggling the deflection wires to get the display on and playing a bit. After returning to it, it was completely blacked out, but playing blind. After my rebuild and cap/transistor/pot kit install I checked the Y signal from the board.... None!

3.jpg


My symptoms:

- Spot Killer on
- Game plays blind with all sounds working
- Thin horizontal line and bright center dot visible, with action visible along it

This seems to jive with the repair encyclopedia here (except for the vertical part, mine is horizontal):
Problem: Display only visible in a very narrow vertical band in centre of screen, some activity can just be seen in the band (revision 04 board).http://www.ionpool.net/arcade/asteroidtech/asteroidsrepair.html#V11

The fix is listed as **Fix: TL082 op amp at C12 had been removed, as well as the two capacitors at C98 and C119 (220 pf radial lead epoxy dipped 100V Mica capacitors). Replaced.**

How do I replace the TL082, and where can I get one? Could anything else be the issue? The voltage regulator at VR2 looks suspect, but how do I test it? Pics:
1.jpg

2.jpg


Thanks!!
 
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You could check to see if you have signals into your Y DAC. If it's good in, but bad out, the DAC could be bad.

Then trace the Y signal back through the board to see where it's missing...
 
You could check to see if you have signals into your Y DAC. If it's good in, but bad out, the DAC could be bad.

Then trace the Y signal back through the board to see where it's missing...

Thanks for the reply! What is the DAC?

and can I do the trace while the power is on without hurting anything?
 
1) DAC = Digital-to-Analog Converter. There are two of them in the silver-boxed area where the adjustment pots are. They should have the letters "DAC" on them. One is for the X and one is for the Y. If you're not sure if they are good or bad, you could swap them and see if the problem moves to the X side.

2) If you have a logic probe, and are careful, there is no reason you can't check the signals while powered up...
 
1) DAC = Digital-to-Analog Converter. There are two of them in the silver-boxed area where the adjustment pots are. They should have the letters "DAC" on them. One is for the X and one is for the Y. If you're not sure if they are good or bad, you could swap them and see if the problem moves to the X side. EDIT - in your first pic they can be seen at B12 and D12. B12 is the Y DAC. Input pins are 1-10. You should see them pulsing on the X and Y DAC's. Pin 15 of both should be pulsing output if the inputs are pulsing. if you have pulsing inputs and no pulsing output, replace the DAC.

2) If you have a logic probe, and are careful, there is no reason you can't check the signals while powered up...

Edited above to add more info...
 
1) DAC = Digital-to-Analog Converter. There are two of them in the silver-boxed area where the adjustment pots are. They should have the letters "DAC" on them. One is for the X and one is for the Y. If you're not sure if they are good or bad, you could swap them and see if the problem moves to the X side.

2) If you have a logic probe, and are careful, there is no reason you can't check the signals while powered up...

Okay, next step, moving chips...! Provided I can't find the logic probe I inherited. =) Or can I use my electric voltmeter?
 
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Your volt meter (on DC) should show a fluctuating reading, but it might fluctuate too fast for your meter to register it properly.

If you swap the chips, put chip sockets in so you can swap them back if necessary - or install a new one easily...
 
Your volt meter (on DC) should show a fluctuating reading, but it might fluctuate too fast for your meter to register it properly.

If you swap the chips, put chip sockets in so you can swap them back if necessary - or install a new one easily...

Great idea on the sockets! I'll post on how it goes.

How about the voltage regulator at VR2? It looks suspect, but how do I test it?
 
A voltage regulator is just basically a bunch of diodes tied together, and can be tested as such if you know the internal workings of the regulator and know which way to test it. Looking up the datasheet for the VR should help.

And if you know the proper voltage inputs and outputs of the regulator, you can test them to see if you are getting what you are supposed to...
 
The TL082 are the small chips above the xgain and y gain print on the vector output section of the game board. Take aclose look at the lettering ont hose chips. Those op-amps are available at Radio shack. Those are prone to fail quite frequently. I would change those first (be sure to use a socket then install the chip) and see if it fixes the problem. The DACs you will have to buy from one of the chip vendors. A little tough to come by lately.

Pat
 
The TL082 are the small chips above the xgain and y gain print on the vector output section of the game board. Take aclose look at the lettering ont hose chips. Those op-amps are available at Radio shack. Those are prone to fail quite frequently. I would change those first (be sure to use a socket then install the chip) and see if it fixes the problem. The DACs you will have to buy from one of the chip vendors. A little tough to come by lately.

Pat

I swapped the DAC chips, same output and lack thereof from the Y. They must be good, so I'll swap the TL082's next, except mine read TL081. I'm guessing that's okay.

One thing I did notice was some flux leaking out of the leads in a chip in the N11 position. The top looks a bit melty too. It is a Motorola MC14016B CP 8025

4.jpg


5.jpg


Is that an easily swapped chip, or does it have a specifically programmed design for this game?



Thanks!
 
I happen to have some IC's at hand, is there anything I can use in this slot besides a 74LS174?

I have a 74LS164....
 
Ordering some from Bob Roberts. Getting a new Big Blue cap just to be thorough! I have some new TO82's from RS I'll install if this new IC doesn't work.
 
The flux you see isn't leaking out of a chip - they don't contain much of anything. It looks like someone simply touched up those solder points. I wouldn't suspect that chip to be bad unless you look it up and determine it is. Its not that hard to.

Re-read what Mod wrote and go from there. If you swapped those chips and nothing happened, is not those, but you need to check the inputs of the chips to see if its getting a signal. If not, start following back from the chip you know does the Y signal.

Just out of curiosity - where did you test said Y signal? I've seen/heard of the molex plugs to the monitors having loose pins that back out and cause things like that to happen ;-)
 
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The flux you see isn't leaking out of a chip - they don't contain much of anything. It looks like someone simply touched up those solder points. I wouldn't suspect that chip to be bad unless you look it up and determine it is. Its not that hard to.

Re-read what Mod wrote and go from there. If you swapped those chips and nothing happened, is not those, but you need to check the inputs of the chips to see if its getting a signal. If not, start following back from the chip you know does the Y signal.

Just out of curiosity - where did you test said Y signal? I've seen/heard of the molex plugs to the monitors having loose pins that back out and cause things like that to happen ;-)

Right, I figured there was enough heat somehow to cause the flux to come out of the solder, or it was tampered with at some point. It's the only visible defect I see on the board, so I figured I'd start there.

I tested the Y signal from the board test point, and also did a continuity test from PCB to PCB, the connectors are good.
 
Replaced the TO82's, and popped in a new 74LS174 in the N11 spot. N11 has to do with Y signal generation so I was hopefull.... and was rewarded with a working Y signal!

Too bad the spot killer is still on, and I have the same problem as when I started messing with the game PCB. Ah well, back to the monitor thread! ;)
 
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