Sony TC-D5M overhaul and repair

When cleaning up my old room in my parents house I discovered a pile of compact cassettes, most recorded by younger me three decades decades ago. Hesitating between tossing them out and keeping them, I felt some kind of nostalgia and decided to keep them. This leads to a new problem: not owning a tape deck anymore. And because I like the style of bouncy needles and black anodized aluminium (and watching a couple of Techmoans videos on youtube) the choice was already clear: it had to be a Sony TC-D5M. Although there are more recent versions of this machine, the TC-D5ProII, this model has XLR microphone inputs and omitted the RCA-jacks for line level input and output which makes it less attractive as a home use player and recorder. So let the hunt begin, it war harder than expected to catch a decent specimen on eBay and the german platform "Kleinanzeigen" (facebook marketplace equivalent) for a reasonable price but in the end I managed to get not one but two machines, one in rather good physical shape but with some electronics issues, one looking like it had a rather hard life but working fine. Both in total a lot cheaper than the "overhauled" snake oil models offered allover the internet...
Initially I intended swapping the big control pcb between the units but once I saw the gazillion thin wires connecting tho both sides of the board (connectors are evil, only directly solder everything to the pcb!) my motivation vanished. Well, I guess a repair might be easier than swapping the pcb...
All of the following is valid for every version of the recorder: The Sony TC-D5, the TC-D5M, the TC-D5pro and the TC-D5proII.

Unit 1 : motor not spinning at all

This is the nicer looking unit. No movement at all when pressing play, ff, rew. Ha, easy fix, it must be the DC/DC-converter stepping up the 3V from the batteries to +/- 9 volts! At least that is what many others throughout the internet discovered. And for sure, there is no output on my converter, too. So I removed it from the pcb, desoldered the metal lid from its base and replaced the electrolytics (sorry, I totally forgot to take any pictures during this surgery...). Long story short, the converter is back to live but the motor is still dead. Bummer!
So it is time to switch to a more methodical approach and powering up the oscilloscope to take a look at the servo control integrated circuit, s Sony CX069. Good luck finding any information on this one, not even think about a datasheet. No chance! Well, I have the not-so-nice unit, which works, so I can at least compare between both of them.

Unit 2 : takeup reel torque too low

At first glance this unit appears to be technically in good shape but once I started using it a bit more, there was one issue with some tapes: the takeup reel torque is really low. On a couple of tapes it isn't strong enough and the takeup reel stalls. At first I was thinking it is due to some deteriorated belts but then I found a great forum thread showing how to adjust the torque of the slip clutch by shortening a spring by three turns. This approach proved to be a success!
The spring is quite easy to reach from the top of the machine. The first step to get access to the spring is to remove the top cover of the unit, including the tape lid. Then, remove the metal plate on top of the mechanism:

This reveals the mechanism including the spring which engages the slip-clutch:

Removing the spring can be a bit tricky but using tweezers in one hand and an exacto-knife in the other hand it was quite easy. I had to shorten the spring by removing three turns. I've done that by forcing the blade of the exacto knive in between the spring turns and using it as a hard surface to bend the spring by 90 degrees. I used side clippers to remove the excess spring material. Reinstalling was done the same way as removing the spring with tweezers and the exacto knife as a pointy tool.
After reassembling the machine the final result using an old torque meter test tape:

I would call this a success, before restauration, the indicator was showing zero, now it shows round about 5. The torque meter cassette certainly is out of calibration but it is still working great for comparative measurements.

--- WORK IN PROGRESS, TO BE CONTINUED... ---