Gentlemen,
Thank you for your feedback.
I arrived at the office this morning and flipped on the swatch at back. I never booted the machine. The case-back was off, of course, but it can't be helped. There is no air movement in the room in which I am testing the machine. Ambient was almost exactly like last night, right at 30°C. It did not reach 130°C like it did last night, as you can see below. But I think it reached 130°C last night in part because the machine was one for quite some time and more heated overall. Then when the machine is Shutdown, DL22, for whatever reason experiences a temperature RISE.
Note that what I call DL21 in my previous posts is really the same Zener soldered into DL22 pads, not DL21 pads. I should have written DL22 in those earlier images, but I'm not going back to fix them. Please just take note of this minor correction.
I then press the keyboard's power key to boot the machine, and only seconds later it had booted to the Desktop and DL22 had dropped 17.2°C — a rather enormous temperature drop! But that's what I confirmed last night too.
And after waiting 42 minutes for RL22 to heat up, what once was a wicked-hot 170°C component with the tiny stock resistor is now a much cooler-operating 115.7°C. And, if I had the case-back on and fan running, it probably would be cooler than that.
And 43 minutes after booting, RL22 had heated up a bit, but still much cooler than in the Shutdown condition:
But only a minute after doing a Shutdown (power SW at back still ON), that dirty rat DL22 was back at a whopping 130.5°C again, just like I reported last night. For reasons I do not understand, immediately after Shutdown is the hottest time for that 5W Zener at DL22. It stays that way a long time (more than a half hour), but it will eventually drop down to the 122°C to 123°C range and stabilize there. But again, that's with the case-back off, and with it on Shutdown, it's probably a tad hotter. It can operate at up to 150°C, but for a margin of safety, we wouldn't want it to go beyond 140°C. And when you consider the stock Zeners were at about 105°C to 110°C (at least, that's what I measured back in 2023), that's about a 20°C temperature increase only because we switched to a single 5W Zener. Better electrically, to be sure, but super hot too (when Shutdown).
And here are the nearby capacitor temperatures (lower today as compared to last night when DL22 was 130°C)...
I am furious at myself for having forgotten to measure Voltage at the cathode of the stock DL21/DL22 zeners prior to my component swap! But I absolutely will NOT remove my current part only to swap in that original part and measure because I would need to do 3 applications of heat to get it back to where it is now, and with the board pads already compromised due to heat, I don't want to do that.
What I can say again is what I said last night. With the
1N5355B 5W
18V (Izt=65mA), I am measuring between 19.1V and 19.3V. That tells me it's getting less than Izt (less than 65mA). The stock parts were 18V 1.25W, Izt=20.8mA 1N5931B. But there were two of them, so you'd need double that Izt and maybe a tad more, so 20.8x2=41.6mA. The circuit may have fed it 50mA or so. But if true, 50mA is less than the 65mA Izt of our replacement, which would explain why I see 19.1 rather than something closer to 18V. In our limited testing, that 1v difference (assuming there is one because again, I never tested the voltage using the stock Zeners) probably doesn't matter much.
But if anyone reading this thread has their Analog Board stock, please hook up your meter as shown below (hooking to that one end of RL62 is easier than hooking to the Zeners), and report your voltage measurement with the power switch on but shutdown...
