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Technics cassette decks and the RDG5772Z gear

Back to the blog overview page | Post date: 2025-03-10

Objective

I got my hands on an old Technics amplifier and double cassette deck a few years ago. During this time it mostly collected dust. After the successful repair of a portable cassette player I (again) found cassettes to be quite enjoyable. The Technics deck turned out to deliver acceptable recording quality using the following combination:

My joy ended after approx. 3 hours of playback when the ongoing tape recording was stopped abruptly. To my surprise, the reverse playback direction did still work. There is the excerpt of the incident, read back using the aforementioned portable cassette player:

While looking for a Service Manual online, I discovered a few forum posts:

As you can see, the search results got more precise iteratively. Thanks to eclectik for mentioning the idler gear! Whereas OP had another issue, it did lead me to a promising path. But where to begin? After half an hour of research, I still found no free Service Manual for my RS-X950 deck. Fear not, the RS-CH950 PDF that I found does describe an almost identical device. An hour later, the cassette unit is now loosely placed on top of the cable mess inside the deck case. With a little trick - which I will get to later - it is possible to fake a cassette such that view to the front gears is not obstructed. Yes, indeed. That gear lacks a tooth.

Which gear?

RS-CH950, page 36, Ref. No. 108, RDG5772ZC

Order placed

I would be surprised if there were any official replacement parts after all these years. Fortunately, few people do sell injection molded RDG5772Z gears on Ebay and Etsy for 7 EUR (à piece). That is a relatively low price considering that injection molds can be quite the investment. One might be tempted to 3D-print the part. However, the precision of the teeth and rigidity are questionable to say the least.

Fast forward - the replacement gear arrived. The missing tooth of the old rubber gear is now solved. Replacing the gear will be tricky, though.

Thanks to SlowPokeGarage for his very pleasant gear replacement tutorial. It did save me headaches trying to understand the order of assembly from the Service Manual.

Replacement and reassembly

With the help of the aforementioned tutorial, replacing the gear (between the two large driver gears [1]) was a straightforward task. Still, it was a time-consuming task.

[1]: If these gears have commonly used name, please let me know.

Part of the reassembly is a metal cover (called "CHASSIS" + "ROD ASS'Y") that needs to be inserted onto the main assembly and slided up. To do so, it might be necessary to push the cassette retention lever upwards, as shown in the image below. The red circle highlights the problematic bolt that initially prevented the metal cover from falling down properly into the mounting position.

Testing

A few screws more and the assembly is ready for some in-circuit tests. With the cables in safe distance, I powered on the deck. The cassette driving logic can be tricked into thinking that a cassette is present by pushing all levers upwards as shown in the image below.

It lives! Forward playback is again working properly. 4 + 4 + 4 + 3 screws and another 15 minutes later the deck is ready for a proper sound test.

Final words

While replacing the idler gear, I applied some high viscosity machine oil to the gears in the front. Retrospectively I found that this was likely a stupid idea. Certain lubricants may chemically react with plastic and the replacement gear is made of Acetal/POM, which is self-lubricating. Bummer.

That is however a problem for another day. I continued recording the cassette where I left off. Sound playback and recording does still work, with both channels, left and right. Mission complete.

I can only hope that Service Manuals make a comeback. Mechanical parts are bound to fail, electronics too. Instructions should not be a luxury but a necessity for long-lasting products.

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