Since there are a lot of different threads now discussing the different ways to upgrade the Lynx’s power regulation circuits, I thought I’d sum them up here.
The Lynx has a built in switching regulation circuit that converts the 9V from batteries or AC into 5V for the system to use. There are few key components that make this circuit work, including a MOSFET (Q12 on Lynx II, Q11 on Lynx I), and a Zener diode (D13 on Lynx II, ZD1 on Lynx I). When the MOSFET fails, typically your Lynx won’t power up. That’s why the headphone ground trick works to bypass it temporarily. When the Zener diode fails, the power supply circuit loses its voltage “reference” and lets 9V go right on through the system, causing other chips to fail.
There was someone in Germany who figured out that you could replace the onboard regulation with a 5V regulator like an LM7805. This involved removing the Zener diode, a transistor, some diodes and modding the PCB. The LM7805 is inefficient and runs hot, so people experimented with the DC-DC converters. I wrote a whole thread on installing a RECOM DC-DC converter in as an upgrade or repair option.
Another option is simply replacing the MOSFET with a new one, and the Zener diode with a new one.
Which method should I choose?
After several different people in the forums (including some bright engineers) examined the original power supply circuit, it was apparent that the Lynx has a efficient switching regulator. See the proof in This Thread! Therefore, by putting in a new regulator, you’re just doubling up circuitry. Also, the 5V regulator mods have the side effect of draining the batteries when the Lynx is off within a day or two. This is because the 5V regulator never truly shuts off.
I’m biased, and even after writing the article on the 5V mod, and doing it on quite a few Lynxes, I just perform the Zener diode and MOSFET replacement. This preserves the original circuitry and power draw of the Lynx. By replacing them with newer parts, they should last another 20+ years as well. Ultimately, we are going to try to come up with an overvoltage protection circuit that can go in the Lynx to shut it down if the Zener ever does fail. This is the best solution.
You can still do the other mod, and some people prefer it, thinking that it may be more reliable in the long run. Of course we may never know for quite some time J
I want to install the new Zener and MOSFET!
All you need to do is order up the latest equivalent for the original MOSFET. Currently a good choice is the 2SK2231. For the Zener diode, a modern equivalent is the 1N5229B. Note that the Zener is through hole, which works great on a Lynx I. For a Lynx II, you’ll just have to chop the leads and mount it. For the Lynx II, the black stripe on the zener should point to the left towards TP19 on the board. DO NOT reverse this! The MOSFET is a simple unsolder, solder a new one in.
If you use digikey, these part numbers are for you!
Q11/Q12 MOSFET: 2SK2231TE16RQCT-ND
ZD1/D13 Zener: 1N5229BFSCT-ND
Here's a picture with the MOSFET and zener replaced:
Close up of the MOSFET:
Close up of the Zener:
Edited by cowdog360, Sun Jul 3, 2016 1:46 PM.