Putting this here in case it helps someone out in future [emoji28]
The issue with the unit I got was that FM tuning would be okay for a while, and then would start jumping wildly about (creating a horrendous noise in the process). It would sometimes settle down again for a while then go nuts when you least expect. It would also sometimes take a long time to settle on a station.
The to220 power transistor on the power supply board was originally a Darlington but had been swapped for a normal transistor. I ended up replacing this with a better transistor (albeit not a Darlington) as the one in place had low gain and as a result the rail wasn't very stable. I also replaced the associated zener after I accidentally shorted the pins while probing it to check the voltages (oops).
I replaced all the electrolytic caps on the power supply board, as I could see obvious ripple on some of the rails. This seemed to help a little but didn't resolve the problem.
I poked around to see if moving or tapping any components caused it, seemingly not. I also tried heat / cold and no luck there.
I found the service manual and followed the alignment procedure (as much as possible without an FM signal gen). I thought it was good, but half an hour later it was playing up again. So not that.
I probed more with an oscilloscope and found the voltage jumping about on the varicap diodes eg. D5. But I still couldn't figure out if cause or symptom. So I started there anyway to see what related to that node, replacing any suspect parts.
Any way, found a decoupling capacitor on that node which was the wrong value (1u - specified 2u2) - and probably bad too. Replacing this seems to have solved the problem (more burn in time is needed to be 100% sure). It was C23 - I also replaced C25 and all the other electros in this section. I'll probably go ahead and replace the remainder in the tuner.
I also found 1u bipolar caps where 100p ceramic were specified...decoupling for a 4011 chip used for latching. I don't think these were causing a problem, but I cannot for the life of me figure out why someone would replace them with completely different parts.
Perhaps I should have shotgunned the electros as a first step, as some are wont to do, but I don't usually do this unless they're obviously bad, or once I've solved any other problems (in case it's some unobtanium part to blame and it's going to get parked). Well, this time I got caught, lesson learned!
Annoyingly, none of the components are labelled on the board (although the polarity and component symbols are marked) which didn't help in locating the right components from the circuit diagram on the board, especially when someone's changed them for the wrong value. As usual there's a layout in the manual but it's basically unreadable in the copy.
The issue with the unit I got was that FM tuning would be okay for a while, and then would start jumping wildly about (creating a horrendous noise in the process). It would sometimes settle down again for a while then go nuts when you least expect. It would also sometimes take a long time to settle on a station.
The to220 power transistor on the power supply board was originally a Darlington but had been swapped for a normal transistor. I ended up replacing this with a better transistor (albeit not a Darlington) as the one in place had low gain and as a result the rail wasn't very stable. I also replaced the associated zener after I accidentally shorted the pins while probing it to check the voltages (oops).
I replaced all the electrolytic caps on the power supply board, as I could see obvious ripple on some of the rails. This seemed to help a little but didn't resolve the problem.
I poked around to see if moving or tapping any components caused it, seemingly not. I also tried heat / cold and no luck there.
I found the service manual and followed the alignment procedure (as much as possible without an FM signal gen). I thought it was good, but half an hour later it was playing up again. So not that.
I probed more with an oscilloscope and found the voltage jumping about on the varicap diodes eg. D5. But I still couldn't figure out if cause or symptom. So I started there anyway to see what related to that node, replacing any suspect parts.
Any way, found a decoupling capacitor on that node which was the wrong value (1u - specified 2u2) - and probably bad too. Replacing this seems to have solved the problem (more burn in time is needed to be 100% sure). It was C23 - I also replaced C25 and all the other electros in this section. I'll probably go ahead and replace the remainder in the tuner.
I also found 1u bipolar caps where 100p ceramic were specified...decoupling for a 4011 chip used for latching. I don't think these were causing a problem, but I cannot for the life of me figure out why someone would replace them with completely different parts.
Perhaps I should have shotgunned the electros as a first step, as some are wont to do, but I don't usually do this unless they're obviously bad, or once I've solved any other problems (in case it's some unobtanium part to blame and it's going to get parked). Well, this time I got caught, lesson learned!
Annoyingly, none of the components are labelled on the board (although the polarity and component symbols are marked) which didn't help in locating the right components from the circuit diagram on the board, especially when someone's changed them for the wrong value. As usual there's a layout in the manual but it's basically unreadable in the copy.
Determined, after a few more red herrings I finally tracked the source to D2, one of the varactor diodes. Take a look at the scope when on the cathode. That's on 200mV/div scale! Something very wrong there. Glad I found the fault at last, but now I need a replacement. The tuner actually works without it, but with reduced signal strength.
I'm planning to replace the 1sv68 with a BBY40 (on an adapter board). Both data sheets are in this post, it appears to be an equivalent but in SOT-23-3 package. Now, we wait.
https://www.diyaudio.com/forums/showthread.php?p=6223195
Adapter boards: #Aliexpress NZ$ 1.50 5%OFF | 20PCS SOT23 SOT23-3 turn SIP3 DIP 0.95MM Pitch Zener Diode MOS Transistor IC adapter Socket / Adapter plate PCB
20PCS SOT23 SOT23 3 turn SIP3 DIP 0.95MM Pitch Zener Diode MOS Transistor IC adapter Socket / Adapter plate PCB|pcb|pcb printpcb spindle - AliExpress
BBY40: BBY40,215 | NXP BBY40,215 Varactor, 26pF min, 5:1 Tuning Ratio, 30V, 3-Pin SOT-23 (TO-236AB) | RS Components
https://www.diyaudio.com/forums/showthread.php?p=6223195
Adapter boards: #Aliexpress NZ$ 1.50 5%OFF | 20PCS SOT23 SOT23-3 turn SIP3 DIP 0.95MM Pitch Zener Diode MOS Transistor IC adapter Socket / Adapter plate PCB
20PCS SOT23 SOT23 3 turn SIP3 DIP 0.95MM Pitch Zener Diode MOS Transistor IC adapter Socket / Adapter plate PCB|pcb|pcb printpcb spindle - AliExpress
BBY40: BBY40,215 | NXP BBY40,215 Varactor, 26pF min, 5:1 Tuning Ratio, 30V, 3-Pin SOT-23 (TO-236AB) | RS Components
Varicaps are real oddball devices but do occasionally fail... geez it must be 25+ years since I last changed one. TV tuners back in the day 
things were pretty desperate when we resorted to going that deep inside tuner cans.- Status
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