If the heads are all clean on the under-performing decks, I'd personally dump them, and find something else to your liking. You might try searching for direct-drive decks, which most times do not employ belts that can stretch and break over time. These decks are a bit more expensive, but they'll also last much longer.
If I really wanted to get into cassette again, I would purchase a 3 head Nakamichi deck that has been entirely gone through. They were the best out there. I still have a Sony El Cassette I bought through Sony as a “salesman accommodation”. Stupidly, I only have 3 tapes for it and haven’t tried it since the early 80’s.
I cleaned the heads of course.
CT-F1250 is a 3-head direct drive, considered as one of the best decks made by Pioneer and in general.
I did not try to record; this makes no sense if it does not play well. Not many decks have azimuth control for recording, and a very few of them have also for playback. How do you use your azimuth tape, would not any well recorded tape will serve the same purpose?
I tried a well recorded tape on CT-F1250, interestingly, if i stop the playback and immediately resume it, just for two seconds or so the mids and hights appear and then instantaneously disappear. A technician may know if such problem comes from heads, or some other (easier arrangeable) reason may exist.
It’s not the heads if you stop and start it again and the miss and highs come back for a few seconds. It sounds like transistor issues and/or possibly failure of electrolytic capacitors ( remember how old this deck is and both transistors and caps fail or degrade over time). Find a tech in your area who can service vintage gear. It’s not gonna be cheap that’s for sure. Unless you have the skill set to fault find yourself. All the service manuals are freely available now online. Good luck
I also suggested that it should not be the heads, in fact they look quite good. I cleaned once again all the interior parts that i could access without disassembling the deck and tried to calibrate bias, level and equalizer according to the manual. Bias and level calibration worked well, i was not able to calibrate equalizer. I have no test type. I don't know what precisely helped but the deck now works almost well. Sometimes I need to stop and play again and mids and highs return. Even though it is not functioning 100%, i can already hear a superior type of reproduction it gives. In fact, it is difficult for me to say whether I like more a CD or cassette reproduction with this deck, CD is cleaner but the cassettes give somewhat more analog realist sound. Given this and that this is one of the most good looking (if not the most good looking) decks, i decided to keep it.
@garryfreeman , thanks for your post. I have now a better idea what things need be arranged.