Vectrex with really hair pulling strange -13v problem..

[kbjunkie]

Hi miata,

I've checked that previously, and all was fine throughout the circuit. I will however, check it again as it was some time ago and I have replaced a couple of diodes since then. I'll get back when I've had time to do it.

Many thanks,

Mark.

 

[daver2]

So, we still know that everything is OK up to C121-.

Yes, the value of R106 is fine...

The next thing to do will be to unplug P204 and check the voltage again on C121-.

I would also check the voltage on C122- as well.

With P204 removed, we are removing the possibility of a short circuit on the Vectrex logic board itself.

What we would be left with now is a short circuit (or reversed) DZ102 or a short circuit C122 or a short circuit somewhere on the PCB tracking around DZ102 and C122.

Dave

 

[kbjunkie]

Hi miata,

I've checked that previously, and all was fine throughout the circuit. I will however, check it again as it was some time ago and I have replaced a couple of diodes since then. I'll get back when I've had time to do it.

Many thanks,

Mark.

Hi miata,

I checked all around the -13v circuit this morning and there's no shorts and all components are connected correctly with good continuity between everything.

Thanks,

Mark.

 

[kbjunkie]

So, we still know that everything is OK up to C121-.

Yes, the value of R106 is fine...

The next thing to do will be to unplug P204 and check the voltage again on C121-.

I would also check the voltage on C122- as well.

With P204 removed, we are removing the possibility of a short circuit on the Vectrex logic board itself.

What we would be left with now is a short circuit (or reversed) DZ102 or a short circuit C122 or a short circuit somewhere on the PCB tracking around DZ102 and C122.

Dave

Hi Dave,

With J204 unplugged, the voltage on C121- 17.3VDC. On C122- it's 13.3VDC

I've checked all of the components in the -13v circuit and all are in the correct way around. I cannot find any shorts at all and DZ102 is in the correct way around (PCB silkscreen denoting the cathode and the diode placed in that way with the black band on the diode as per the silkscreen) - The rest of the diodes are the same. C120/121/122 are brand new caps from RS and are soldered top and bottom of the board (as are the diodes).

It's strange, as I said in my first post. If I turn on the Vectrex when it's been off for an hour or so (cold start), I get no picture but I do get audio. There is no -13v present. If I leave the power switch on and remove and reinsert J204, the picture comes back and I get the -13v back too. The Vectrex works normally. If there was a short on the logic board or an issue with DZ102/C122, surely that wouldn't happen?

I replaced the whole of J204 - cables, plug and socket on the logic board as the old one was corroded. I've check to make sure the socket is soldered fine - continuity from each of the pins is getting to where it should go on the board.

It's almost like the -13v circuit only starts to produce voltage when it's sort of 'kickstarted' by the action of plugging in J204 when the power is already on - but I understand that's a little weird..

Many thanks,

Mark.

 

[daver2]

>>> With J204 unplugged, the voltage on C121- 17.3VDC. On C122- it's 13.3VDC

By 17.3VDC and 13.3VDC I assume you really mean -17.3VDC and -13.3VDC. The voltage should be negative.

These are the correct voltages, so all of the components within the -13V DC voltage doubler are working correctly.

>>> It's almost like the -13v circuit only starts to produce voltage when it's sort of 'kickstarted' by the action of plugging in J204 when the power is already on - but I understand that's a little weird..

This is a most unlikely situation. I suspect you have an intermittent short circuit somewhere. Disturbing J204 causes the problem to go.

I would do a few experiments over the next few days. When there is no picture (and the fault is present) measure the voltage between 0V/GND and:

1. C120+ (AC).
2. C121- (DC).
3. C122- (DC).

If 1. is present, 2. is low and 3 is none existent, you most likely have a short circuit (either in J204 or afterwards).

I would then power OFF the Vectrex. Wait for a short period of time to let the capacitors discharge. Read the resistance (using a multimeter) across C122. You should observe a very low resistance that starts to increase as C122 charges up. If you observe a permanently low resistance, then there is a short circuit after C122.

We have been here numerous times before... I would also suggest posting a high resolution photograph of the PCB to see if we can identify any dubious looking solder joints or other issues with the PCB.

Dave

 

[MikeSynapse]

Take a plastic or wooden rod (a chopstick works well) and push and prod the PCB and different parts to see if there is any change.