Lynx II Voltage issue - Too high

[Blitter]

I was going to put a new McWill LCD in my Lynx II and wanted to swap out the basic stuff before doing so.

 

New Mosfet, zener, and cap kit. Started it up and Q8 blew. Measured voltage at the VCC and gnd as in step 3 of McWill's manual and got 8.9 I believe. Thought it was gone.

 

I put in a new transistor at Q8 and it starts up and I hear sound. Not sure if I have video as I took out the components not needed for the new LCD.

 

Remeasured voltage and I get 5.85V. I'm not sure at this point how to remedy that.

 

Rechecked all the caps. Even put back the old zener and mosfet to see if that was an issue and it's still 5.85.

 

Any ideas?

 

Thanks for any help.

[per]

Alternatively you can run off a 5v adapter if you modify the DC jack so it connects to the regulated side of the power-supply. It should work fine since only the old screen backlight needs voltage on the unregulated side.

[+Stephen Moss]

I can't see why Q8 would have blown as a result of changing those parts except perhaps for inserting the zenner the wrong way around causing too much current to flow through it.

 

Assuming you have inserted the everything the correct way around it could be that the specs of the replacement components are not quite what they should be resulting in the extra voltage, however it sounds like you are running it without the LDC attached/running. If that is the case the extra voltage could simply be a result of a less than designed for current draw on the supply not creating the necessary voltage drop and that when you attach a working LDC the voltage will then be correct.

Edited May 15, 2018 by Stephen Moss

[samson7point1]

I can't see why Q8 would have blown as a result of changing those parts except perhaps for inserting the zenner the wrong way around causing too much current to flow through it.

 

Assuming you have inserted the everything the correct way around it could be that the specs of the replacement components are not quite what they should be resulting in the extra voltage, however it sounds like you are running it without the LDC attached/running. If that is the case the extra voltage could simply be a result of a less than designed for current draw on the supply not creating the necessary voltage drop and that when you attach a working LDC the voltage will then be correct.

I realize this is a bit of an old thread, but in my testing, the voltage difference between the LCD connected and not was never more than a couple mV.

 

As unlikely as it is, it's possible a bad R74 is causing the voltage to be too high. At any rate, I believe that replacing R74 with a higher-rated resistor should allow you to drop the supply voltage, though it's beyond my skill to calculate the exact rating you'd need. I'd probably start by soldering a trim pot in the 150Ohm range in place of R74 and trying to dial it in from there.

[+Stephen Moss]

I'd probably start by soldering a trim pot in the 150Ohm range in place of R74 and trying to dial it in from there.

 

If you are going to put trim pots in I would recommend putting it in series with a fixed resistor of say 80% of the original value, that way you cannot create a short as the result of turning the trim pot down to 0. I would also try to use a trim pot of around half the value of the original resistor (put a resistor in parallel with it if you have to) as the smaller the value the finer the adjustment will be.