09 October 2016

Project "Audio Technica AT770 Mister Disc Portable Turntable"

It's been a really, really long time since I created a post here.. but here's a recent turntable I've fixed up: the AT770 Audio Technica Mister Disc by Audio Technica made in the 1980s, also known as the AT727 Sound Burger depending on the region they were sold in. The Sound Burger and the Mister Disc are exactly identical (as far as I know) and I used the service manual for the Sound Burger from VinylEngine to service mine.

Won this pretty thing from an eBay bidding!
I've been looking for this turntable for quite a long time but they go for pretty high prices on eBay for working copies.. and the non-working copies I found were in bad shape until this one came along.. cosmetically it was perfect and the seller mentioned that the motor spins but no sound comes out of the line or headphone outputs.. so it was right up my alley for a good restoration!

So this little thing plays both 33rpm and 45rpm discs and runs on 3 x C sized batteries. It has a single line-level output (red & white RCA), together with 2 x headphone outputs (3.5mm) which I assume allowed couples to bring this on picnics and connect two different headphones to it and listen to music without disturbing the peace, so it is a very nice touch. Nowadays this turntable is most useful at vinyl fairs where collectors can test the vinyl on the spot.. all in all still a very useful and unique turntable to own nowadays!

Powers on okay!

No sound out of the line output, nor either one of the headphone outputs
 I knew beforehand that the outputs were not working.. and true enough when I powered it on and spun a record on it, I heard a half second burst of sound from the player's line out before it went silent.. and this indicated to me a clear case of faulty electrolytic capacitors.

Time to start taking it apart!
The service manual is available on http://www.vinylengine.com/ and includes clear instructions on how to strip the turntable down.

Top cover comes off first

Tonearm comes right off too
The tonearm connectors are pretty small and have to be aligned perfectly before they can be slotted back in

The motor with belt pulley, and the hand soldered circuit board

The motor with belt pulley, and the hand soldered circuit board

The belts seemed fine and didn't need to be replaced

The motor is able to spin at 2 speeds to allow 33rpm and 45rpm operation

Top side of the circuit board with motor control and audio outputs all on board

You can see the dual headphone outputs on the left, and the DC input jack on the right

The turntable also accepts a 4.5V DC input which you can use instead of batteries. But ensure that the polarity of your DC jack is correct before plugging one in!



First step of capacitor replacement: knowing which capacitors to buy
As with all of my restoration projects, the very first thing I would do is to replace all of the electrolytic capacitors as they're usually the first components to fail after a few decades of service.

Koba Electronics, where I usually get my audio grade capacitors
I made a quick trip down to Koba Electronics in Chinatown (Singapore, if you're wondering) where they sell shelves upon shelves of Nichicon audio grade capacitors (my personal favorite) and replaced every capacitor there was to replace.

Nichicon Fine Gold and Elna capacitors as replacements!
Well there's your problem.. a blown capacitor!
A good amount of unmarked wires!
There are a lot of wires connected to pins on the top of the circuit board so it would be a good idea to stick some labels to the wires to make sure you don't get lost trying to put them back together later!

Japanese capacitors in a Japanese machine

Cleaned the volume potentiometer thoroughly for good measure!




I put the turntable back together after replacing all of the electrolytic capacitors and put a record under the needle and IT CAME ALIVE!! But..... for some reason the headphone outputs did not work.. I could hear a very very faint whisper of the the music that was playing. It bugged me for a very long time and I replaced just about nearly every component there was to be replaced.. the tantalum capacitors, ceramic capacitors.. and even spent a day in Sim Lim Tower searching for the replacement line out and headphone out amplifier chips (TA7330p and TA7331p) which I was pretty impressed that some vintage radio shops there still have stock on.


Replacing just about every single component that can be de-soldered and replaced!

Replacing just about every single component that can be de-soldered and replaced!
After all that effort the headphone outputs still refused to come alive.. so I measured the output voltage of the transistor (TR1) and found that instead of outputting a voltage of 3.8V, the voltage was merely 2V! So for some reason the transistor had partially failed but that low voltage was enough to power the line out amplifier ICs (2 x TA7330p) but not the headphone out ICs (2 x TA7331p).

I found an equivalent transistor TIP41 which was quite a lot bigger than the 2SD471 but it could fit into the case with no problems so in it went!
TIP41 replacement transistor.. a lot bigger than the 2SD471

Large.. but still able to fit just fine..
Replacement needle ATN-103
Lastly for even more good measure I bought a replacement needle (also from eBay) ATN-103 and installed it on the tonearm.. powered it up.. connected some headphones to the outputs and heard wonderful music!


7 comments:

  1. This is really helpful. I'm in the process of repairing mine now. Great timing.

    Thanks!

    ReplyDelete
    Replies
    1. i'm glad that it would help you!

      Delete
  2. Great post.

    See our GPS-TS RS485 Time Source Module: https://www.andrewlabs.com/time-source-modules/17-gps-rs485-time-source-module.html

    ReplyDelete
  3. Thanks for this info. I just finished replacing the d471 transistor on my non-working at770 thanks to your help! Otherwise I would never have been able to find the problem. Whoever owned my unit must have attempted to fix the problem because it looks like they thought the potentiometers were screws and broke them both. So I replaced them both with 10k pots and now it's working well. Thank you for the detailed information!

    ReplyDelete
    Replies
    1. Thanks for your kind words, I'm glad that my post helped you!

      Delete
  4. Can someone repair my AT770 Mister Disc? Please let me know. Thank you

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  5. I'm looking to repair my 33 rpm speed adjustment pot because it appears the adjusting screw has fallen off the pot. The 45 rpm is just fine but the other pot needs to be replaced. Does anyone know how to replace this part?

    ReplyDelete