Don't trash your Zyxel,
...possibly it only needs a full CAPS replacement.
This page describes a couple of successfull repairs concerning a Zyxel Prestige 660-HW ADSL2 router.
After a summer storm I have found my Zyxel 660Hw out of order. It has received a voltage surge from the ADSL line.
Reconstructing PCB lines behind the ADSL socket fixed the problem.
Unstable LAN connection.
With age, my Zyxel 660-HW started to be instable on all 4 LAN connections. All other functions were fine.
Symptoms were: poor LAN performances and continually loosing all LAN connections and soon reestablishing them (every 15 seconds or so).
The cause was the electrolytic CAP highlighted red in the aside picture.
Replacing that cap cured the problem.
Most of the electrolytic CAPs measured bad ESR values. In particular a couple in the power supply (marked in yellow in the previous picture).
Shouldn't you have a chance to measure ESR, my suggestion is to replace all electrolytic CAPs.
This old CD player came to me from a flee market for 15 EU. Not so much for a CDM2/10 transport and a TDA1541 (non-A) DAC.
It is in good shape but it randomly failed to read CDs.
After a lot of testing on the transport controller and power suppliers electronics, laser power adjustment, etc. ..my attention was driven toward the CDM2 transport itself. You know, it is a beautiful galvanometric transport (the arm swings like needles in galvanometers..) ..but it is prone to failures.
I cleaned the lens and double checked the foiled PCB. Nothing.
Playing with the transport (remove-check-reinstall-test cycles) I got random results. I was not able to reproduce a predictable sequence of behaviours... BUT.... I eventually observed that the lens movement was not always free to move along its full range. Sometimes it got stuck requiring manual intervention to unlock it. So I suspected some dirt inside the pickup it self and I was very nervous about how to proceed.
I eventually decided for a pickup surgery. Opening it up I discovered the internal lens, supposed to be joint with the frame, was instead floating around, having the glue keeping it in place dried up and cracked. After sticking it again on the frame the player worked perfectly.