I have been working on this DC2 for months off and on. Its been fully gone through mechanically, I replaced the center gear, capacitors, cleaned and reapplied all lubricants (watch oil and silicon grease where appropriate), and I even re-etched the tip of the capstan shaft with FeCI3. After all that work, it functioned properly for maybe an hour and then it started playing slow. I measured the speed and its consistently 2800Khz on a 3150Khz tape. I saw in the service manual that you can check the tape speed without quartz lock by unsoldering the PLL pad ("quartz tap") near the speed potentiometer. However the DC2 behaves strangely when quartz lock is unsoldered... it will only engage play mode for about 3 seconds and then the power light shuts off and the motor stops - and that's with no tape in it. When I put a tape in, it won't even turn the tape before shutting the power off. I discovered that if I help turn the reel myself for a couple seconds after hitting play, it will continue playing fine unassisted without shutting off. And again, if I resolder the PLL quartz tap, it will start up on its own without assistance and play a tape (albeit slowly) - no shutoff problems. Surely this is not expected behavior? Maybe it is a symptom of why I am having speed problems? Does anyone have any suggestions on what to check next? Thanks. EDIT: It seems if I touch RV601 (the speed pot) with anything conductive, even ever so slightly, the motor wakes up and runs full speed. I've tried cleaning the pot with deoxit already so I'm assuming it needs to be replaced. Looking at it under a microscope it looks like it has lots of green microscopic flakes on it. I also think the top piece of it was broken off already in a previous life - there's no notch for me to turn it with, I have to push it from the edge with a tool to get it to move at all. Looks like in the service manual its a 22K pot - I don't have any this physically small but I do have some I could use for testing at least - if that's the part I need then I'm sure I could source a smaller one to fit in here. EDIT 2: I replaced the pot and the replacement behaves identically. Grazing the wiper, even ever so slightly, with anything conductive boosts the speed past 3000Hz no problem. When I let go it falls back to something stupidly slow.... I even tried touching a pair of metal tweezers to the center leg of the pot (the wiper) and that also boosts the speed full blast. WTF? Is this causing some kind of extra capacitance that's fixing some other problem here? There's a tantalum filter capacitor directly nearby in the circuit (C607) - my ESR meter shows very high ESR, it fails the electrolytic chart... which is especially surprising for a tantalum... hmm...
i was gonna say pretty much the same thing. spinning the reel lessens the impact on C607 a little, and keeps it running steadily for long enough that C607 can reach equilibrium and keep itself charged or somethin like that. most caps have their capacitance/charge rate go to stink with age. worth checking out the tant one atleast might be wrong and its one of the ~2-3 electrolytics around there. they look like bypass/decoupling ones so you might try testing those if the tantalum checks out after all.
Looks like it. However these tantalum capacitors have higher ESR than electrolytics, that alone doesn't tell much. The unfortunate part is the symptoms point towards a faulty CX20084. Symptoms are very similar to what is described here https://stereo2go.com/forums/threads/sony-wm-dd33-speed-issues-and-more.9456/ and these do exist on units using CX20084 also.
Thanks for that link @Valentin it was extremely helpful. I owe that person a beer for buying an entire DD22 just to confirm waveforms... I compared all 16 pins of my CX20084 to the waveforms they provided on page 2 and was able to confirm my IC seems to be fine. I realized that I hadn't properly resoldered the PLL back in after replacing the RV601 speed potentiometer and now everything appears to be perfect. The speed is locked in and W%F is less than 0.05WRMS... knock on wood.... For anyone else that needs to replace RV601 its a standard 3mm surface mount size. I ordered this replacement, its not exactly 22k but the extra range isn't going to hurt anything (except for maybe adjustment precision, but 30K is close enough to maintain precision adjustment IMO): https://www.digikey.com/en/products/detail/bourns-inc/TC33X-2-303E/2566895 Notice how the replacement clearly has a piece attached on its face to allow it to be turned by a screwdriver - mine (and some others I've seen online) have this piece missing and its nearly impossible to turn. Someone must have manhandled these pots back in the day. Here's what my broken pot looks like, for reference:
