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sideburn

No X-axis - vertical line down center - need a vectrexpert!

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Posted (edited)

Hey all, I’ve been repairing this old board for a while now and I am down to the last issue and I’m currently stumped!

 

my x axis signal is extremely weak and I only a single pixel image down the center. 
 

I’ve replaced most chips and socketed them. 
Looks like I’m getting a good signal on ic304 pin 1, and ic302 pin 13 looks good. 
 

i just don’t quite understand how the system controls x and y or switches control between them. 
 

as you can hear in the video clip here, the sounds is wonky and a game won’t start without hitting a button on the player 2 controller so i think the ay sound chip is back but I don’t think that would be related to the video collapsing issue would it?

 

 

I’ve recapped the board and every chip you see that’s socketed is new. New sockets new chips. 

 

60E182DD-2B30-4B04-A8B1-83D0104DE62E.jpeg

 

Edited by sideburn
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Posted (edited)

Not an expert, but I can read a schematic :)

 

The CD4052 is the analog switcher between X and Y.  Looks like when IC304 is outputting the Y signal, the ramp controls to the IC305 analog switches will be off.  Instead, the 4052 will be sending IC304's output to the sample and hold IC303 for the Y axis.  Then when it switches to X, it turns on IC305 to allow the Y sample-and-hold input to the Y deflection and IC304 will be controlling the X deflection directly.  Note that both X and Y deflection final outputs are integrators.  That means when there's a positive voltage supplied, they will creep towards negative deflection, and when the input voltage is negative they will creep towards positive deflection.  With neutral input they will hold their position.  So the system seems to be stepping them around instead of trying to control them directly.  It turns on the "Zero" to move the deflection back to center.

 

So to find where the signal stopped, you could check IC303 pin 14 for attempts to control the horizontal.  If it can't, then maybe the Outer Limits are in control.  I mean, check that there is input signal on 13.  If not, try 305 pin 9.  It's almost a given that there's signal on pin 8, otherwise you wouldn't ever see Y deflection.

 

If you have good signal up to IC303, check IC305 pin 10 and 11.  Well, it's hard to check them without removing the IC and putting it on a breadboard.  But if this particular switch is stuck in "on", you will likely see very slight left/right variation.

 

Another note:

 

One of CD4052's outputs mixes with audio output.  The 4 analog inputs in channel 2 are piped to the joystick comparator in sequence through channel 2 out.  Notice that this chip has 2 things in common with your failure condition.  So you might also need to check it out.

Edited by ChildOfCv

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4 hours ago, ChildOfCv said:

Not an expert, but I can read a schematic :)

 

The CD4052 is the analog switcher between X and Y.  Looks like when IC304 is outputting the Y signal, the ramp controls to the IC305 analog switches will be off.  Instead, the 4052 will be sending IC304's output to the sample and hold IC303 for the Y axis.  Then when it switches to X, it turns on IC305 to allow the Y sample-and-hold input to the Y deflection and IC304 will be controlling the X deflection directly.  Note that both X and Y deflection final outputs are integrators.  That means when there's a positive voltage supplied, they will creep towards negative deflection, and when the input voltage is negative they will creep towards positive deflection.  With neutral input they will hold their position.  So the system seems to be stepping them around instead of trying to control them directly.  It turns on the "Zero" to move the deflection back to center.

 

So to find where the signal stopped, you could check IC303 pin 14 for attempts to control the horizontal.  If it can't, then maybe the Outer Limits are in control.  I mean, check that there is input signal on 13.  If not, try 305 pin 9.  It's almost a given that there's signal on pin 8, otherwise you wouldn't ever see Y deflection.

 

If you have good signal up to IC303, check IC305 pin 10 and 11.  Well, it's hard to check them without removing the IC and putting it on a breadboard.  But if this particular switch is stuck in "on", you will likely see very slight left/right variation.

 

Another note:

 

One of CD4052's outputs mixes with audio output.  The 4 analog inputs in channel 2 are piped to the joystick comparator in sequence through channel 2 out.  Notice that this chip has 2 things in common with your failure condition.  So you might also need to check it out.

Do you think ic208, the sound / joystick input controller could have anything to do with it? The sound is wonky and the game won’t start unless I pull pin 7 on that chip low which is not normal so I think that chip is going bad. Could not somehow be affecting the switcher for x?! 

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Looks like my previous reply through email didn’t go through but this is what I said:

 

Oh my gosh thank you so much. You can definitely read one better than I can! I don’t know how the mux and switchers work so it was boggling my mind how the x signal is working!  I was thinking the 4052 had nothing to do with X and it was controlled by the 1408 d/a converter! 😆 🤦‍♂️ 

 

and I here’s what I am getting off the scope:

 
305 pin 9 has a strong signal 
305 pin 10 has a weak signal but when I turn the volts/div up I can see data
305 pin 11 has no signal
 
303 pin 14 has a weak signal as well but I can see data with higher voltage/div
 
303 pin 13 - none 
 
With 303 removed from the board:
 
303 pin 14 - strong signal
303 pin 13 - strong signal
303 pin 9 - strong
 
305 pin 9 strong

 

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I'm going to swap out the sound chip. I think it might be causing 304 to block the x output. The fact  that i have to muck with the player 2 controller to get a game to start is a clue and the horrible sound output. I dont understand why the 4 analog joystick pots are going to the CD4052 and IC304 in the first place though. What is the comparator doing there?

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I don't see a way for the sound chip to interfere with the XY switching, but it still might be bad on the sound output side.

 

I'm more concerned that 303(14) has a weak signal.  Well, it could be possible if it is not being told to deflect over any distance. Does 305(9) look like a dancing signal, or does it tend to hold a shape?

 

Also, here's a deflection test for you:  There's a centering pot for X position (R335).  If you adjust that, does your vertical line move left or right?  Just make a small adjustment and see if the line moves at all.  Don't bother trying to move it to either extreme.

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2 hours ago, sideburn said:

I was thinking the 4052 had nothing to do with X and it was controlled by the 1408 d/a converter!

Well, so the steps are:

 

D/A outputs through IC304(1) to the analog switch that controls X, and to the input of channel 1 for the 4052(13).

 

To set Y, 4052 connects (13) to (12) as if to jump a wire between the two pins.  It does this when (6), (9), and (10) are set appropriately.  At that point, it can change the input voltage on IC303(5).  After holding that for some fraction of a millisecond, it disconnects (13) and (12) of the 4052, and the capacitor on IC303(5) holds the voltage in place.

 

Then the D/A outputs X, which goes through to IC305(8).  It also goes to the 4052, but it is currently not connecting pins (12) to (13).  Next the controller activates RAMP, which turns on transistor Q301.  This turns on analog switches IC305(5) and (6), which allow both Y and X to affect their deflection drivers.

 

After drawing everything in the field, it likely activates ZERO, which turns on IC305(12) and (13) and forces IC303(1) and (14) back to center.

 

So IC305(5) and (6) need to be turning off and on at regular intervals (I suspect more time off than on) in order to make sure that the D/A output only reaches the X deflection amp when it is outputting X data.  If this switch never turns off, all data output by the D/A will be going through X, whether it be Y data, X data, joystick threshold levels, or brightness data.

 

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I just swapped out the sound chip. No fox on X but the sound is fixed and the game starts like it should. Could it be one the R335 POT?

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It’s pin 11 on ic305! I poked around with my finger and it started working. It’s a bad socket. I’m swapping it out now. 

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Er... so the Vectrex multiplexes the X and Y (and more) through a 4052 and 4066?

 

A few years ago I got one that (IIRC) had no horizontal, it was just a vertical line. Except it was a pretty straight vertical.

 

I checked and I have quite a few spare 4052 and 4066 chips. Hopefully it won't be the amplifier. (Did anyone ever come up with a good solution for that long-obsolete chip?)

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On 4/20/2022 at 11:08 AM, Bruce Tomlin said:

Er... so the Vectrex multiplexes the X and Y (and more) through a 4052 and 4066?

 

A few years ago I got one that (IIRC) had no horizontal, it was just a vertical line. Except it was a pretty straight vertical.

 

I checked and I have quite a few spare 4052 and 4066 chips. Hopefully it won't be the amplifier. (Did anyone ever come up with a good solution for that long-obsolete chip?)

It's more likely the 4066 than the 4052 if you have vertical deflection.  X comes directly from the D/A and amplifier.  Y goes through the multiplexer.  So if you have Y, then you have the multiplexer.

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