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VECTREX WITH DESPERATION

Desperado

Desperado

Veteran Member
Joined
Nov 25, 2017
Messages
6,869
Hello guys, i'm here again because i need your precious help!
I bought an old Vectrex console, when i turn on the console i can see only a white dot, more or less in the middle of the screen..
I can hear sound ad music....sometimes this dot collapse in a corner...
Anyone know what it could be?
Thanks so much!
 

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Back on the Vectrex machines now again :)...

Is there anything coming out if the digital to analogue converter (DAC) first?

Dave
 
daver2 said:
Back on the Vectrex machines now again :)...

Is there anything coming out if the digital to analogue converter (DAC) first?

Dave
Click to expand...
Need i found first manual ;)
It is possible check DAC outputs with scope?
 
You obviously need the schematic for the machine (and the PCB layout)...

The next thing is to check that all of the DC voltage rails are present and correct.

You have a diagnostic cartridge don't you? Are you using it? This should give you known signals from the machine.

Yes, you need to use an oscilloscope to measure the output from the DAC. It is analogue after all!

With the spot being in the middle of the screen indicates to me no deflection (X and Y). This could be due to missing power supplies, problems on the logic board, or problems in the monitor deflection circuitry. We need to use the oscilloscope to see where the X and Y signals dissapear...

This "should" be a simple fix...

Dave
 
Yes Dave i inserted diagnostic cart but i have always same dot... I can hear the sound of diagnostic cart... Please can you help me to see where video signal disappear? Thanks
 
Yes,

But do the power supply checks first (as always with all repairs).

Then look at the output from the DAC next. This is the first link in our chain...

Dave
 
Are these the Dac outputs?
 

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On the Vectrex schematics I have, IC301 is the MC-1408 DAC. However, this DAC outputs a current on pins 2 and 4.

You can't directly measure a current using your oscilloscope. Oscilloscopes measure voltages.

However, IC304 is configured as a current to voltage converter. As a result, you can measure the voltage on IC304 pin 1 as the DAC output.

Of course, your particular Vectrex may not correspond to my schematics...

Dave
 
IC304 pin 1 (LF353) - but that is according to MY schematics. You may have a different revision of the Vectrex than I have the schematics for...

Dave
 
Unfortunately in my logic board ic304 is this:
 

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So, if you do a bit of googling for the two part numbers (LF353 and MC34002) you should observe that they are the same parts...

At some point in time either:

a) The manufacturer used a different source for these parts.
b) The OP AMP has been replaced at some point with an equivalent.

Dave
 
Ok, i inserted the test cart and i measured pin 1 with scope but i have a negative tension...more or less -5V....
 
So, have you checked the power supply rails?

Check IC304 pins 4 and 8 relative to 0V/GND (e.g. IC304 pin 3).

Dave
 
Pin 3 should be -13 Volts.

So you have committed the cardinal sin and not checked all of power supply rails to start with as I have asked you to do...

Back to the power supply to find out where the -13 Volts has gone to...

Dave
 
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