Found this interesting YT comment from someone who did Warranty Service for Sony on the TPS-L2

I basically found it because a TPS-L2 teardown video came into my recommended because I used the same channel to help me replace my belts when I bought my WM 3 off Ebay then I saw this comment by Paul G which looked too interesting to not share

" In the late 70's through to the mid 80's, I was working part time for contracted service center that was doing warranty service. They were servicing Sony, Panasonic, Niko, and some others. The managers of that service center wanted me to work for them because of my capability and knowledge. My main job was working in a major TV station servicing broadcast monitors, switchers, tape machines, routers, and cameras. I also worked a number of years in consumer TV and audio servicing.

I used to service that model of Walkman. The man doing the video knows a fair amount, but he lacks some in debt knowledge of this particular model. This is normal unless having the proper training or a lot of experience with it. The pot that he pointed at, and I also saw it in the schematic he was showing later on, is the servo speed adjust for the transport motor during the play mode. How this pot is adjusted is we used a 1 kHz test tape. The earphone output was connected to a frequency counter, a scope, and a wow-and-flutter meter. The wow-and-flutter had to be less than 1% for the human ear to not hear instability in the tape motion. The frequency counter had to be measure exactly 1 kHz to know that the tape is moving at the proper rate of velocity or speed. The best I have seen on these was about +-10 to 15 Hz to the 1000 Hz reference. The batteries must be new to do this adjustment. The thermo resistor was to compensate for temperature change in the servo amp area to give more accurate speed with temperature change.

Wow-and-flutter is usually fixed by replacing worn rubber parts, making sure lubrication is proper, changing warn mechanical parts, and the belts are in proper specs.

We used specific frequency response tapes with tones to adjust the head height and azimuth. We connected the output of the player to a scope and a VU meter. The tape started at 20 Hz and had successive tones going up the scale to 22 kHz. That particular tape player if I remember correctly, with the tone boost set to default, it was able to reproduce from about 40 Hz to about 16 kHz with about +-1.5 to +-2 Db when working very well. It was very surprising. There were test tones to also verify for stereo separation cross channel coming from bad alignment of the head, or possible problems in the circuits. When working properly there was more than 47 Db of channel separation. The signal to noise at best was about 60 to 70 Db. The newer versions of the player technology had Dolby noise reduction. The newer ones were able to have a signal to noise of about 80 Db if I remember right. The sound quality was excellent for a small cassette player. With all of this it is important to verify the left to right phase. This was verified mainly with the 1 kHz test tape to start with. The total phase test had to be verified through the complete range using the main frequency response tape.

The head angle adjustments would effect more the channel separation. The height would effect more the low frequency adjustment. The azimuth adjustment effected more the high frequencies. These all interacted with each other, and therefore it was important to keep repeating all the three adjustments until there cannot be any more improvement. The head angle with the azimuth adjustments also effected the phase. On this level of player it is more important to verify the frequency response and channel separation than the phase. The human ear would not hear a phase error in the sound unless it was more than about 20 degrees out, and that is a lot.

If you look at the top cover of that player you would see a small hole located on the top of the playback head. This is for touching up the azimuth adjuster. To do the main head alignment the top cover is removed. Then the test tape is inserted. Once all the tone frequencies are verified, the test tape is removed and the cover is put back. Once the cover is put back, the playback response is to be tested again. The azimuth adjustment is extremely critical compared to the head angle and head height adjustments. Using a 10 kHz tone tape the azimuth is re-verified. This is the range for the high frequency audio where the human ear is most sensitive for loss of highs in music.

The man did not know what the transformer was. This is a toroidal high frequency choke. The motor in that unit had to be well shielded. This motor is a DC brush type motor. The frequency choke was there to subdue motor brush noise from coming back out the motor to the circuit board.

In the newer model they added a low noise transistor between the head and the amplifier processor IC for the sound. This was to allow for less amplification inside of the chip to reduce chip noise. The amplifier IC's back then were not as good as they were in later years. They compensated this with the added preamp transistor for the head. The newer version of that chip was able to be biased to have higher output and lower impedance output than the previous. Emitter output is low impedance compared to collector output. This is why they were able to no longer require the emitter follower driving the output amp.

The man doing the review knew a lot more than the average, but he lacked some very important knowledge about that player. I am sure I serviced a number of hundreds of these and similar ones over a number of years. It is obvious I cannot remember most of the details I knew back then.

Many of the problems were user caused and mostly mechanical. Normal problems were dirt in the mechanics of the player, worn belts, worn pinch roller, and cleaning and demagnetizing the head. I remember a number of them when new the heads became defective and had to be replaced. There were some where the users played so many tapes they actually caused some wear on the head surface and the head had to be replaced to bring it back to specs. This took over two years with a lot of use. When the head surface is worn it is more difficult to get the frequency response and channel phase to be in proper spec range.

The circuitry was very well designed, and there were very few actual circuit faults unless the user did something very stupid. I remember seeing some where the output amp chip was blown. The users tried to connect the unit to big sound system amplifier and had electrical grounding issues. There were some people who tried custom things with these players and damaged them, because they had no clue to what they were doing. After warranty this unit was not cheap to service. If abusing of this unit the warranty was void."

 

This is super interesting and one of the reasons I'm glad I found this forum. I certainly didn't know or appreciate this.