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!

23 January 2014

Project "Dictogrand Dictograph R-3 Horn Speaker" (2/2)

I managed to fix this speaker up the way I wanted it, and I have to say it works beautifully!

I replaced the cardboard mini horn with a metal one with nearly the exact dimensions. I basically went into Daiso one fine day and went around with the cardboard horn/funnel and tried to find something as close and possible and I'd say I found something very close!

Not bad for $2 eh?

It is a measuring cup meant for bartenders to measure out shots for drinks and it is made of stainless steel.. perfect for my intended use! Just cut it at the joint and make a hole at the end and it is now a mini metal horn!

Like so!

Using a metal cone/horn gives much more clarity in sound over the cardboard one, which to me sounded very bloated and muffled. I read up on the basics of using horns to amplify sounds and the design of the Dictogrand's horn amplifies the middle to high frequencies while attenuating the lower frequencies. In other words: you won't be hearing much bass coming out of the horn, which is okay because I didn't intend to use the horn for dance music anyway! Initial testing showed me that the horn speaker shines when playing acoustic music with uncomplicated arrangements, i.e: simple chill-out tones that do not have an overwhelming amount of instruments.

Moving on, I kept in mind that everything I planned to put into the box of the speaker should be kept replaceable in the event something better comes along in the future. Thus no epoxy or nails should be used or at least kept to a minimum. I attached this metal horn to the horn entrance plate by using hot glue, which can be easily peeled or dissolved with alcohol in the future if needed but strong enough to hold things together for a long time.

The last piece of the puzzle was extending the microUSB charging port from the X-Mini to the outside of the box. I couldn't find any proper microUSB breakout boards that could be mounted vertically and thus had to ask my brother who very graciously provided me with a solution by making one (or two) from scratch. 


I ended up only using 2 pins out of the 5 shown there for +5V and GND which are used for charging. It is possible to use the data pins on the X-Mini for audio playback but I intended to use the 3.5mm jack for that so I kept those pins unconnected. In the process of hooking this USB board up to the speaker I absolutely forgot to take a picture of it wired up before sticking it in. I basically got a standard microUSB cable (also from Daiso by the way!) and cut off one end. I soldered the +5V and GND wires to this breakout board and connected the other end of the cable to the microUSB port on the X-Mini.

All there was left to do was to drill the holes required and fine tune them with a file. I got my father to help with his experienced drilling skills and I fine tuned the holes ever so slightly for a nice fit.

As the interfaces were already extended out as shown previously in part 1 of this post, all I had to do now was put in the switches and jacks and wire them all up. The X-Mini is secured snugly in place by the isolating foam below and behind it, which is forcing it up against the metal horn. I tried shaking the box repeatedly and saw that the speaker did not budge at all so I decided against using any glue or other measures to hold it down.. all in the name of semi-temporal-permanent-ism! You never know if a year from now they come out with the X-Mini Uno EXTREME and I'd be kicking myself while I'm tearing out all the permanent fixtures I made previously to this current speaker. It isn't pretty now I'd admit that, but it will work just fine.

Power switch in place, ready to be wired up

You can see the microUSB port in the picture below on the right below the 3.5mm jack. I applied a generous amount of hot glue to the parts that would be enduring a lot of pushing and pulling.

The volume potentiometer was glued on to a horizontal balsa wood support (also from Daiso! Daiso has everything!) and aligned with the Dictogrand's volume knob, which I forgot to mention, was cut down to a much more manageable length as shown below so that it could turn the X-Mini's volume potentiometer with ease.

Hot glue; it ain't pretty, but it gets the job done!

Finally, I put another piece of isolating foam on the top of everything so that when I close the lid of the box it would sandwich the speaker in place very tightly.

The LED lights up blue when the speaker is turned on, and it changes to red when the USB charging cable is plugged in. The battery in the X-Mini is only 550mAh and is easily replaceable with another battery of a bigger capacity, but that would be a side-project for another time. Also, it is possible to power the speaker solely by USB power alone, so if the battery ever dies one day then the speaker would still be usable by plugging in a microUSB cable.

To celebrate the completion of this speaker, I made myself a 'commemorative' 3.5mm interconnect cable to always be used by this speaker. Just plug one end into the speaker and the other end to any compatible device like an iPhone or a MP3 player and it is good to go!

And that.. is the story of how I got myself a snazzy horn speaker after all!

It works wonderfully with jazz and easy-listening music, or period-correct music, if you will (tunes from the 1920s onwards to the 1970s). What the horn does is add a touch of reverb to every sound, so you can hear an echo that lingers after every note for a few milliseconds which is completely natural due to the acoustical design of the horn.

(Link to part 1/2)

12 January 2014

Project "Dictogrand Dictograph R-3 Horn Speaker" (1/2)

I won this obnoxiously big horn speaker on eBay and it finally arrived here on New Year's Eve all the way from the US of A. This is a Dictogrand R-3 Speaker made by the Dictograph company in 1923 according to the information on this renowned website (http://www.radiomuseum.org/r/dictograph_dictogrand_r_3.html) which makes this speaker about 90 years old. The horn is made of brass and is connected to a (cast iron?) elbow that links it to the speaker driver located in the rectangular box made of wood. It has a volume control knob on the front of the box where the brand is stamped.

The funny reason why I bought this obnoxiously big horn speaker is as follows:

I was out in town some weeks ago and I saw a shop in Singapore selling this "Gramophone for iPhone and iPad" which really caught my eye because I am such a sucker for vintage audio stuff as this blog can attest for. I thought it was a wired dock which you can plug your iPhone into and it'll play it through the speaker but it is solely an acoustic amplifier with no electronics or active speakers whatsoever.. kind of steep considering it was selling for $400sgd++ (!!!) if I saw the pricetag correctly.. it looks beautiful though as a decoration piece but I feel it isn't exactly a 'functional' speaker. Also it isn't exactly 'real' vintage unless the horn was taken from an actual antique speaker but I couldn't verify if that was the case.

Image from Restoration Hardware and for your reference only, click to go to their website

I also saw this in another shop for a whopping $700sgd++ (!!!!!??) by enandis (en & is), which is also a really nice decor piece but also has no electronics and depends solely on the iPhone's built in speaker for sound before being amplified by the ceramic horn. Another nice curio for your home if you can afford it..

Image from enandis and for your reference only, click to go to their website

Then finally there's this brilliantly designed project on Kickstarter: the Gramovox, which uses an active speaker to play sounds through the functional horn and even has bluetooth capability to stream music. It costs slightly more than $400sgd++ with shipping included (pricey but at least it has some functionality over the other two) which means taxes are applicable if I do purchase it and that would send the cost even higher.. so no snazzy retro horn speaker for me..

Image from Gramovox and for your reference only, click to go to their website

At least until I found this gem on eBay and won the bid a few days before Christmas for $68USD! It isn't exactly 'cheap' as well but compared to the examples above it is very affordable.. and I would believe it is increasingly rare as well which justifies a little more cost. It is ideal for a modification project because it has a good sized box below the horn where the driver (speaker) sits and I would be able to replace it with whatever I wanted to. As mentioned earlier, it also comes with a volume knob on the front that I should be able to link to a volume potentiometer as well.

The horn is fixed to the back of the box by two easily removable thumbscrews

Once the thumbscrews are removed, the horn slides right off the two screws and will be freed.

It is a good sized horn, no?

The bottom of the box can be removed as well and is held together by two fastening screws on either sides of the box shown here.

Once the screws are removed then the bottom cover comes right off as well.

The bottom of the box carries the now faded and torn instructions on how to use the speaker. There are readable examples available online but the instructions contain nothing useful in terms of modern day technology.

It says: "Best results will probably be obtained by using about 67.5 volts (!!!!!!) of "B" battery on the amplification unit

The antique speaker driver has an impedance of over 1kOhm which compared to modern speakers which are anywhere from 4-16 ohms is a great deal more and would not be compatible with modern day amplifiers because the sound would probably be too soft to be heard, assuming the driver works at all after 90 years! You could use a impedance matching transformer to hook up your modern day amplifier to this speaker but I believe the sound would come out very distorted.

The gears you see connected from the volume knob to the speaker actually adjusts the distance of the speaker diaphragm to the horn entrance. When the volume is turned lower, the gear turns the mechanism inside the speaker that adjusts the diaphragm further away from the horn entrance at the back and it does the opposite when the volume is turned up. I do not have a powerful enough amplifier to hook this horn speaker up as it is and decided right from the beginning that it is time to remove this speaker driver and let modern electronics take over.

The volume knob is held by a retaining pin that can be simply pulled out and the whole knob can be taken out of the box. The speaker driver can then be removed by lightly (or violently, depending on how warped the wood box is after 90 years) tapping on the two screws at the back.

It is time for you to retire!

The back of the speaker driver holds the two (irremovable) screws that can still be used because they are needed to hold the horn in place to the box. The screws have been rusted shut, but a little machine oil left to seep in for 5 minutes helped make the task a lot easier. 

So that is a complete teardown of the speaker. I intend to make this into a self-powered speaker with a rechargeable battery similar to the Gramovox above, but without the fancy bluetooth feature (perhaps in the future). I did my research and found many possible components I can fit inside from sites such as Adafruit where they have a Class D amplifier breakout board together with a USB/DC battery charger that I can hook up together.

Image from Adafruit and for your reference only, click to go to their website
Image from Adafruit and for your reference only, click to go to their website
But then I realized, there is only 1 horn, thus 2 channels for stereo is overkill.. and I couldn't find an acceptable mono amplifier. The other big problem was sourcing for a good speaker: it had to be small, but able to play audio at a good volume with no distortion. The small speakers available for DIY projects (also available on Adafruit) are the common ones with paper cones and are designed for simple tones and buzzes and not built to handle music playback.

Image from Adafruit and for your reference only, click to go to their website
So what then? Well, thankfully, the best small speakers (according to several reviews on the internet) is made right here in Singapore and it is the X-Mini Capsule speaker and it is readily available in all the shops here.

Okay.. readily available except in the black color that I wanted it but heck.. 

The X-Mini is a very common portable speaker that has been around for quite some time now. It has an expandable accordion baffle in the middle that gives the speaker a lot more air volume to work with so the relatively small 40mm driver sound a lot better and louder than it should and so I felt it was very ideal for this current project. The best part is that it has a complete circuit built in, so there's no need for me to design a battery charging circuit with an amplifier with a speaker.. this single device combines everything I intended to build into one small capsule that would (most probably) outperform any speaker I source to use in this project.

The speaker is pretty easy to take apart as well. Remove the four screws on the bottom part of the accordion and the base will drop out, revealing the PCB.

Remove the one screw on the PCB and everything will be exposed. I had no need to modify the upper half of the speaker where the driver is so I left that on its own. I planned to remove the switch, potentiometer and 3.5mm plug and extend it out of the box so I would be able to power on and off the speaker, adjust the volume, and plug in whatever music player I want to without opening the box to do so.

I set about desoldering the components from the board, but I found it extremely difficult to do so because the components were mostly soldered on both sides via through holes and they were a real pain to remove. I yanked out the potentiometer together with its traces and I had to improvise with thin strands of wire to rebuild the traces. It wasn't pretty but it works.. hopefully if your project requires you to repurpose the X-Mini like this then you would have a good desoldering tool to help you.

With the switch removed. The speaker uses a Double Pole Single Throw (DPST) switch but it only actually uses a single pole, thus a simple single pole switch would work.

The black tape hides my demolition job on the volume potentiometer traces. They also act as a strain relief so I don't accidentally rip out their tracks again by moving the potentiometer around.
I desoldered the male 3.5mm connector and soldered a female connector at the end of the longer wire. This female connector will be accessible together with the on/off switch on the outside of the box. I also extended the LED indicator so I would be able to see if the speaker is on or off. I did not touch the USB connector that is used for charging the device, but I ordered a microUSB extension (that has yet to arrive) that I will plug into the port on the speaker and fix the female end to the rear of the speaker box.

TLDR; I will eventually be able to access all the features of the X-Mini without needing to open the speaker box, making it a fully portable standalone speaker.

While the speaker has a 40mm driver, the entrance of the horn is a lot smaller at about 18mm. This means that simply placing the speaker at the horn's entrance would result in a lot of sound energy (volume) being wasted because the sound waves do not all enter into the hole.

I basically needed a small horn to guide the sound waves emitted from the X-Mini into the horn's entrance. I found these hard cardboard cones from the shop "Spotlight" where they sell all sorts of handicraft materials and cut it down to size:

I put everything together as shown below before closing it all up to give it a try, and it doesn't sound half bad! But I would prefer a metal cone rather than a cardboard one so this would only be a temporary fix just to get a feel of how it would fit and sound like. I will be taking a good look around to explore other options over the next few days/weeks and see what I can find.

I made a simple recording of the capability of this project with my phone's camera, and I will make a higher quality video when I am finally done with this project.