More progress on pretty, pretty, keeper final updates.
1. I ordered and assembled the Elcon EL-31 display replacement. I calibrated the display using a precision freq counter and sig gen on the bench. This is easy, just apply 5vdc and have it it. After the simple install all is well, beautiful modern 7 segment displays. I bought 6 spares from Mouser, cheap $1.80.
2. I ordered and assembled the Elcon EL-33 VFO stabilizer. After the somewhat difficult install I was hoping for the most excellent? <20Hz drift in 2 hours like my first FT-301D. Of course, something is wrong with this one >120Hz drift and it never stabilizes. This had a major rabbit hole chase to find the root cause. After considerable debug effort here is what I found:
a. The EL-31 is not drifting and is performing perfectly.
b. The EL-33 is performing as designed to stabilize the VFO frequency. The fake news is coming from the display system.
c. The front panel CALIB pot voltage is what is drifting causing my fake news.This voltage is developed from the reg 5v and controls a varactor diode in the Counter board to vary the display indicated frequency.
5. I changed the 8v and 5v 3 legged regulators to modern TI linear parts to improve reg 5v. This made reg 5v perfect BUT IS NOT THE STINKING ROOT CAUSE of the display drift.
6. The D2211 varactor tuning diode in the Counter Mix card is hyper temperature sensitive. I could blow on it and and it changed the load. I found 10 original NEC parts in TAIWAN and I uploaded the data sheet to Files. My first FT-301D is super stable so this is just a bad part problem.
Hopefully the end is near, I plan some minor calibration touch ups then putting covers on.
