Vectrex 3GE Modell - how to adjust DAC Offset

[mastercello]

Few words about myself first - Marcel, 42 yrs. old and from Switzeland (Lucerne) - got in contact with the Vectrex with a friend of mine recently a few years back and loved it from the beginning.

So 2017 - i thought, why not get one and here i am!

 

I got a nice MB Vectrex some weeks ago and already some nice Overlays from Steven and also a Mine Storm one from Mr. Nicolson!

Both did any amazing job - big thumbs up!

 

Also did a Capkit yesterday on it as i thought after 35 years might be a good idea.

 

I have a 3GE Modell which is the Non-buzz if im not mistaken- right?

 

Now there is the problem - few caps are different as the CPU pcb is pretty different from the older modells.

 

For the DAV offset i tried adjusting it like it says in the manual...but the voltage is not changing if i'm turning the knob.

I'm a doing something wrong or is this a completely different task here on the 3GE version?

 

Somebody please enlighten me - thanke folks!

[mastercello]

Anybody?

 

My CPU pcb is this one here:

 

[Zitruskeks]

I have the same issue. Trying to measure the Voltage on Pin 1 of IC304 changes the size of the Text displayed by the vectrex, and though the pot clearly has influence on the vectrex (images shifts diagonally) the measure voltage doesn't change.

[Zitruskeks]

Answering my own question: I inspected the board and compared it with the old vectrex layout. While the old one uses a MC34002 Dual Op-Amp on IC 304, the non buzz versions uses two single op-amp packages MC34001 on locations IC 304 and 306. While on the MCMC34002 the output of one of the op-amps is on Pin1, for the single package MC34001 the output of the Op-Amp is on Pin 6!

 

So I measured Pin6 of IC304 and low and behold: a slight voltage above 0 Volts that reacts to input of R302.