Okay, so I have done some digging and came across this website: https://www.turntableneedles.com/

They sell needles, but more importantly for us - belts of all types and sizes! They have a section where you can select your cassette player by part number. This would be great, but no listed player part number matches any of the varieties Pioneer calls this deck (DEH-K4041ZM, DEH-K4141ZM, DEX-K4141ZM-91). So, we have to go to the by size description, which they caption as not for the faint of heart. They list the tutorial for how they measure the belts, thickness and length (or circumference since these are circles). They unfortunately use Imperial freedom units instead of metric.

Given you have said this was a 12mm diameter belt, that makes it's circumference approximately 37.7mm or 1.48 inches (circumference = pi*d or 3.14159*12 or 37.7mm or (37.7/25.4) inches). So we are going to need a belt that is around 1.5 inches in diameter. For square belts they list several sizes - reel, orbicular, small, transitional, medium, large and extra large.

https://www.turntableneedles.com/fin...by_description

We could seemingly limit our search to reel (0.7mm thick), orbicular (1.0mm), or small (1.2mm). It should be noted the 0.7mm thick may be closer to 0.74 based on their inch size listing.

In the reel section, they list a 1.55 inch belt (39.4mm,so 1.7mm too long). Promising. In the orbicular section, they list a 1.6 inch belt. This is seemingly getting a bit too large. In the small section, they also list a 1.6 inch belt.

Also, the large belt is extremely easy. The Pioneer Service manual gives the part number CNT-111. Google it: Pioneer Electronics (USA) Inc. Parts & Accessories Online - Part Number: CNT-111 Information

Pioneer lists it as out of stock, but gives substitute CNT1046. Google that and you end up right back at turntableneedles.com and this: https://www.turntableneedles.com/pio...ioneer-cnt1046, which you can purchase here: https://www.turntableneedles.com/sbm...-av-components

So, we have a definite for the big belt and a maybe for the small belts.

BUT WAIT! Why not search the part number for the small belt dummy? I did google it and got nothing. But then I went back to the pioneerelectornics link and typed in CNT1010, and it gives the alternate of CNT1047, which turntableneedles.com lists as....the 1.55 inch long 0.7mm square belt.
https://www.turntableneedles.com/pio...ioneer-ctn1047

The total will be $12 plus shipping but it won't let me add a second of the small belts to my cart, I'm going to send them a message.

Hopefully this helps further the knowledge of how to get these things operational again.

 

ETA 2-4 weeks on additional SBR1.55. He said the best thing to do was keep checking the website.

 

Heck yeah buddy!

Good sleuthing! We'll have these things in tip top shape one of these days. Thanks for tracking that down. I just finished teaching my university class just ended for the semester, and I am not doing MLB this summer, so I'm hoping to get back on these rx7 projects this weekend possibly. Gotta get the 3d printer back to working. It forgot all the calibration settings when I went to use it the other day.

 

I got the belts and they have been installed.



Here are the belts from turntableneedles.com. I bought two sets to repair this one again in the future or send off elsewhere.



First step is to remove the old belt remnants. This is a good picture of what is stuck on the first two pulleys in the foreground, and the absolute gunk stuck on the motor pulley in the background (out of focus). Swabs, picks, and copious amounts of isopropyl alcohol are needed. You can also use acetone, but don't let that touch any plastic and there is lots of plastic around.



I am gathering quite the pile of gunk here.



If you remove a wire guide, this piece can be flipped over to gain access to the two smaller belts. These were much easier to clean up - note one of the pulleys for each is plastic - no acetone there.



Installed the smaller belts.





Then the large belts were installed and my pile of used up swabs and mini swabs grew. I put it all back together and....

...it works! Sort of. The tape plays and all of the functions work. But, the sound quality is atrocious. I tried Def Leppard Hysteria and REO Speedwagon Hi Infidelity. They all sounded like they were using helium. I don't want to take it all apart, so my wife brought some much longer swabs home from work that I can stick in there to clean the head and capstan and hopefully fix some of the sound quality issue.

 

Awesome update. Looks like a good and clean install. As far as cleaning the tape head goes, if the tape deck is working mechanically, you might try getting a cleaning cassette off amazon. They're about 8 bucks.

 

That's a good call, I think I will give that a shot too.