Hacking The Teenage Engineering PO-80

[markrob]

Hi,

Looks like it turned out nicely!

I've attached a pdf of the schematic for my amp. I am using an off the shelf amplifier board with a TI TPA3118 chip. Its a 50 Watt unit. You can get instantaneous peaks up to 100 Watts. There are several sources for these boards. Here is one:

https://www.amazon.com/dp/B07Q6RGVHQ

The design is a bit unusual. I wanted to see if I could avoid using a power supply 24Vdc at 2-3A and run with a much smaller supply. I did this because the head can't take more than 3 Watts continuous power. But you need 100 Watt very short duration peaks to get the head moving at high frequencies. So my Idea was to have a bank of energy storage capacitors on the amplifier DC bus charged to 24Vdc. A 10 ohm 1/2W resistor limits the charging current such that only a few watts can be pulled or the voltage will sag. So, you can use a very low power supply of a couple of watts and still get high peak power to the head. The additional components are in place to allow the capacitor bank to charge up to 24 Volts slowly on powerup so that the inrush current is limited. This limit is set by a 75 ohm resistor. Once the bus is at voltage, a MOSFET switches on and removes the 75 ohm resistor out of the circuit. It works well and adds some protection for the head if you drive it too hard. I'm not sure its worth the effort since 24Vdc 2A wall wart supplies are pretty low cost these days. But feel free to use it if you like. You could really just use the amplifier module off the shelf with a suitable supply and be off and running.

EZ_Amp.pdf

Mark

[kugelblitz]

thanks for the info and tip towards a good module.
i wondered what those huge capacitors in your video were for - makes your circuit look like it really means business

[kugelblitz]

i just realized your removable stylus holder is aimed towards cones, i have a tungsten embossing stylus from epos, not easy to get it straight with the flat-side of the stylus a 1/4 turn from the screws

DSC00657.JPG

funny how these things are so obvious when it's all printed out (for me at least), but hard to spot when looking at the stl files
guess i'll have to print it out again with the bottom hole turned a 1/4 turn as well

[markrob]

Hi,

Here is a version with an extra hole on the side of the pivot. Due to the pivot hole, I could only fit one setscrew on this side. I think it should work out fine.

Holder Body Ver3.zip

[kugelblitz]

wow, now that's what i call service!
thank a lot!!

[mrd]

Hi Mark, Looking at your platter, how do you decouple the original lower mounting point part (with the screws which attach to the plastic case) from the original? or what is the mounting method in your case please? I'm having serious issues with my original platter not spinning freely and cannot work out how to get it to behave itself! but interested in the way in which you reuse or don't the existing parts

[markrob]

Hi,

You might have a tolerance issue between the bottom white pulley and the platter. The shaft of the white pulley has a key slot that engages the platter. The metal center pin is also keyed and should be a bit shorter than the surface of the white pulley. When you add the keyed overhead pulley and screw that into the metal center pin, the entire assembly is locked to the to bearing upper and lower surfaces.

If you disassemble everything make sure both bearing are not binding. Then, lay the platter in place, it should rest on the inner bearing race and spin freely.

Now add the white pulley and align it to the platter via the key slot and see if this pair can rotate freely. If it binds in this state, remove the platter and make sure the white pully rotates freely on its own.

Drop the metal center pin from the platter side and check that there is a slight (~1mm) indent where the overhead pulley will attach. If it is proud of the surface, you might need to add some shim washers to the metal center pin on the platter side to move the pin up.

I added a metal stiffener to my base to reduce the flex. I don't think this can work with the stock platter as it sits too low and would hit the stiffener.

PO-80 Base Low Res.jpg

PO-80 Base Stiffener Lo Res.jpg

[mrd]

Many thanks Mark, that's really helpful! I've never had the bottom part fully apart, looks a bit different to your image - made up of two bearings plus a locking plate with screws and an outer drive rim for the belt plus the pulley for the overhead. Maybe a revision or difference between the gakken/PO-80? or yours shows a mod already? It's weird as disassembly and reassembly fixes it briefly and then it returns to binding. I'll try with a shim

[markrob]

Hi,

Next time you have things apart, post a picture. There may well be a difference.

Mark

[Estacaco]

Really cool stuff Mark! Gonna pick up one of the Gakken ones to fool around with while I continue to looks at diy lathes and how I want to go about mine (those linear rails are expensive especially in Canada!) I have a few 3d printers (both resin and fdm) as well as a bunch of spare printer parts im hoping to recycle into a lathe (smooth rods, lead screws, linear bearings etc). I figure since im completely new to the art that the Gakken would be a good learning tool without breaking the bank. I was kinda disappointed in what I saw in youtube reviews about the Gakken/PO-80 until I found this thread. Thanks for all the work you've done!

[markrob]

Hi,

Should be a good primer before you get into the big build. Let me know if you have any questions.

Mark

[Semar]

Super travail