Commandeered the Wife's craft table for my set up.
To remind all because it's been like a year, the task was replacement of a dual n-FET (the "mixer" FET103) since it was 3 ohms drain to source and presumably shorted.
Acquired Signetics 40673 on the recommendation of dr*audio. Did final check (is this really the right transistor? where am I in danger of screwing something up in the tight space?) and then desoldered the original 3SK-39, trying hard not to break anything else. It was non-trivial... some of the coil wires, particularly, are really tiny. This was right after I desoldered the drain...
The old FET had a plastic jacket, likely to isolate the can from other circuitry (the can is and is supposed to be shorted to the source, near as I can tell). I removed it and fitted it to the new FET... probably didn't matter, but why not?
Followed instructions to attach source first, then drain, then the gates (plot twist: the schematic showed gate 1 attached to what was in the actual stereo the gate 2 circuitry... I followed the actual stereo connectivity, obviously--based on what I could discern it shouldn't matter, but it would actually have been a more complicated deal to slavishly try to match the schematic, so why do so?).
The soldering went okay. One thing that went wrong near the end was on reattaching the L103 coil wire: can't tell if it broke or if the insulation melted and flowed over the end of the wire, but I had to strip the insulation back in a space where no wire strippers could get. Ended up cutting it with my fingernails and pulling the insulation snippet out using two pairs of pliers [shudder]. I think it's shorter now, but what're ya gonna do?
Best I could do. Made sure I checked all solder joints with my ohmmeter to verify they were getting connectivity on both sides. Checked drain-to-source resistance: 115 ohms. Much better than 3. Checked overall tuner power supply to ground resistance: 143 ohms. Better than 83 ohms (previous value).
Time to add power... I plugged the 5000X in, set to AM. Verified the expected 14V at AM incoming, then grimaced as I turned the Selector to FM, half expecting popping noises and to see the magic smoke released. Nothing. So far so good. Checked incoming power to the tuner. 14V... if you'll recall, before the FET replacement, it was like 6V. Yet another small victory.
Turning the tuning dial gave me very little on the signal dial... but wait, was that a fluctuation? Eff it, speaker time... and lo and behold, real actual sound. Plugged in an antenna (okay, so it was just a couple of alligator-clipped wires), and... tears of joy...
She works, folks. It's a lovely thing, and I have you guys to thank for the inspiration and guidance.
I still have some stuff to do... while the FM STEREO indicator fluctuates in and out, pressing the FM STEREO ONLY button in basically kills any station, even if it's at Signal 4 or above. Would love some advice there; suspect it's either some kind of alignment issue or something in the demux circuitry...
But overall, I am proud to flip the middle finger at the service shop guys.
"Unserviceable" my ass. Unserviceable no more.
