Progress!
Work completed:
Replaced parts:
-All electrolytics. I put the two tantalums called out in the SM back to tantalum (they were called out as tants but 'lytics on the board). I also went with the 10uF SM called out caps at C5/6. Thoughts on whether those are critical or if I should try lower values like what was installed before?
-Both trimmers with 47k Bourns cermet pots.
-The vref elements. D4, D5 with EGP20A and the zener with a 1N4737
-Diode D8 for good measure. Replaced with EGP20B. I need to measure the voltage drop across the EGP20A in circuit before I trust them for the snubbers, but my plan is to replace all of the diodes.
To my knowledge, mylars and solid aluminum caps are basically immortal, so I left those alone. I've ordered Vishay Dale 50ppm 1% metal film resistors to put in as well, but I want to do things in stages to avoid causing an issue with a change and then not knowing which thing did it.
Upgrades:
-Cleaned up some of the wiring to the board and added new heat shrink to prevent shorts.
-Replaced 12v zener shunt regulator with TI TL780-12 series linear regulator.
Building the regulator circuit:
.22uF ceramic bypass on the output and .33uF on the input.
Heat shrunk and installed in board with heat sink. Thermal paste on sink as well.
Quick pic of the new pots + diode cascade:
After double checking things I brought it up on the DBT and I managed not to bork anything!
The 12v rail comes up to 12.00v almost instantaneously and over the short amount of time I monitored it (I've been working on this for the last 6 hours and need to eat...) it was very stable.
I am still working on designs for the vref upgrade, and I'm not 100% happy with my zener setup. It's setting at 8.4v instead of 8.8 as the zener is at the low end of it's 5% tolerance and the new diodes drop a bit less per diode than the originals. I chose the diodes after comparing spec sheets and the EGP series seemed like the descendant of the 10D-05/10D-4 series that was in there originally but the EGP10 series is derated to 1.0A from 1.5 in the 10D series so I bumped up to the 2A capable versions. I'm not sure if that affects the forward voltage drop much.
Huge shoutout to
@Pio1980 ,
@tnsilver ,
@restorer-john ,
@maxhifi , and
@totem for their help and encouragement.
Cheers,
Nathan