Yet Another Power Query

[displaced]

I've read through all the power-related threads I can, but would like to double-check my conclusions before wielding my soldering iron...

 

My Lynx Mk. I works normally on batteries, but with a 9V PSU, the display is too bright even with the brightness pot turned all the way down. I've measured 7.6V on the board with my multimeter.

 

From reading through here, I think this is correct:

 

- The Q11 MOSFET is likely fine -- the Lynx powers on/off perfectly OK.

- The ZD1 Zener diode is likely failing, allowing >5V to pass.

 

So, on the face of it, I should replace the failing Zener.

 

But I'd read that when ZD1 fails, it'll dump all 9V to the board. Is it possible for ZD1 to partially fail, leading to >5V but <9V to reach the board? Or is there possibly something else that needs looking at? I measured the 7.6V on the output of ZD1, so if there is something else to replace, it'd be upstream of the diode.

 

[EDIT: One quick thought... during testing when I couldn't lay my hands on any batteries, I fed 5V to the battery +/- terminals, and it worked fine. Would it be crazy to add a micro-USB socket to the Lynx, with the VDC/GND pins connected to the pads for the battery? Hmmmm.]

 

Thanks for any help,

 

Chris

Edited September 1, 2018 by displaced

[Orange-Phantom]

Please replace the FET too. It is not just an on off switch, it is high frequency switched which is what regulates the power. It is this that predominatly fails after years of being high frequency switched. Replace the Zener too as a precaution.

[displaced]

Cool, will do.

 

I couldnt find the original MOSFET or the recommended replacement. Farnell recommends this alternative:

 

https://uk.farnell.com/on-semiconductor/rfd14n05l/mosfet-n-logic-i-pak/dp/1017789

 

Reckon itll do the trick?

[+Stephen Moss]

I measured the 7.6V on the output of ZD1, so if there is something else to replace, it'd be upstream of the diode.

If you were trying to check the 5V from where were you taking the measurement?

You have to measure in from the +ve (9/5V) rail to the bottom of the Zenner, not relative to 0V.

[displaced]

If you were trying to check the 5V from where were you taking the measurement?

You have to measure in from the +ve (9/5V) rail to the bottom of the Zenner, not relative to 0V.

 

 

I can't quite remember where it was from, but I'm following the McWill installation guide, so re-checked from the +/GND points exposed after removing the backlight power module. The guide says this must read ~5V in order to not break the new LCD.

 

Anyway, the old MOSFET and Zener have been removed. I've got a replacement MOSFET (https://uk.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&langId=44&urlRequestType=Base&partNumber=1013486&storeId=10151)

 

I'm waiting for the correct Zener to arrive from mouser.co.uk -- it's shipping over from Texas, but should have it on Friday.

[displaced]

Zener and MOSFET replaced fine and that got me 5V as expected!

 

McWill screen has been fitted and works great!

 

So my Lynx is now happy internally. It could do with a whole new shell though. Gonna keep an eye out for a nice-looking parts and spares sale on eBay.