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Sony wm D6 repair: muting module, speed control module, belt replacement, and recap

Just score a Sony wm-D6 in a good condition. The seller sold it as junk and untested. Maybe the seller (or whoever the previous owner is) actually tried and it did not work. I pulled the trigger anyway because the price (43000 yen) was not bad even for a non-working one. When it arrived earlier today, I tried it out and I was surprised that it actually played. Of course, there are some small issues, which I found and fixed later on.
Muting Module
The first problem I noticed was that the sound was crackling and a bit distorted. Later on it became intermittent (on and off). That happened even when it was in record monitor mode. I thought it was due to issues at the play-record switch, but no, cleaning the switch did not help. A quick search on tapeheads forum pointed me to the muting module. And indeed, removing the muting module made it play beautifully again (w&f about 0.05%rms). Time to buy a new muting module from scribbler_92 on ebay.
Update (12/15/2022): the mute module from ebay sounds like crap. It sounds much better without the module (I ended up removing it). I will try to restore the old muting module.
Speed Control Module (12/3/2022)
The second issue is also very well known among D6 fan: the speed control module. According to the tapeheads forum, the aging resin of the module can ruin the chip. Once it is completely broken, there will be no replacement (unlike D6C). In my case, it worked fine for a few weeks until one day the walkman suddenly played very fast. It also drained the battery when turned off. The only way to fix it is to remove the resin with something like acetone before the chip is completely ruined, and that was what I did. Acetone is hard to find here in Okinawa. I used nail polish remover from one hundred yen store. It was not very good. I had to soak the module in the solution for more than a week before the resin started to disintegrate. I removed bits of the resin and soaked it in the solution again. Repeat until all resin was removed. Finally, I cleaned up the module with deoxit and then denatured alcohol. It took a long time, but luckily, it worked out just fine. The walkman was working normal again.
Sony D6 speed module resinSony D6 speed module resinSony D6 speed module removedSony D6 speed module resin removed
Belt Replacement (1/3/2023)
The walkman liked to eat tape. The reason was that the take up pully has weak torque. The pinch roller probably moved more tape than the take up pully can pull - they had to work in sync for the mechanism to work. For a two-capstan machine like this one, all we need is a stronger take up torque (I think) (double-capstan machine is more complicated). Anyway, to fix this, I ordered the complete belt set from fixyouraduio, which contained five belts: 2 belts, 2 idler tires, 1 capstan ring. While replacing the belt, I also took this opportunity to perform a partial overhaul (took gears out; clean the old oil; apply new oil (CRC lubricant)). After that, the walkman worked great. I think there would be no major issues for some years unless I do something stupid.

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7/8/2023 update: recap. Replace the old capacitor with audio grade nichicon capacitors.
Old caps with labels.
New cap




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