Custom/Hack/DIY power supply?

[spacelordmother]

So... I just scored a sweet deal on an old Tascam Midistudio combo mixer and 4 track, but of course there was no power supply and research shows it to be almost completely unobtainium. WELL THERE WAS THAT ONE ON EBAY THAT SOLD TODAY THAT I MISSED BECAUSE MY MOM CAME OVER FOR LUNCH AND THEN STAYED LATE AND THEN MOTHERFUCKER SHIT WAS GONE but I digress.

Anyway -- anyone have ideas about how I could go about hacking one together from an existing power suplpy? The toughest part seems like the special adapter plug, but that could be done with connecters and shrink tube if need be. In the end I want it to 1. not electrocute me; 2. not burn the house down; and 3. be cheap(ish) since at this point I don't even know that the unit works.

The specs I have found online say that the four pins are: +10V (1A), -10V(1A), +7V(10mA) and GND. Here's a pic of the jack:



Thanks!!

[crochambeau]

I'd cobble together a female pin header/heat shrink solution if you know the pin-out. Modern switching supplies purport to tolerate shorted outputs (which I equate to less of a fire hazard for some reason). That +7 at 10 mA is an odd duck, I'd probably just string together a series of diodes hanging off the +10 to shave the 3 volts or so (say five 1N4001 or whatever you have lying around, 10 mA is puny and any self respecting silicon diode should be able to handle it).

So you're down to some parts and two 1 amp 10 volt supplies, just connect the + of one to the - of the other to make ground and you're set. Another option is to find an old +/- 12 volt Power One or Condor supply, those generally have trimmable output so you can dial down to a true 10 volts.

I'd be blown away if the Tascam measured 10, 95% of my supplies from that era are easily 20-30% high unloaded.

[spacelordmother]

Now I will have to go research the things you said since I am a total newb at stuff like this! Would have have a second to draw a quick diagram?

Most of the info I have collected I found here:

http://homerecording.com/bbs/user-forums-by-brand/tascam-user-forum/tascam-644-seeks-power-supply-154000/
http://homerecording.com/bbs/user-forum ... iy-370307/

along with this image:


Thanks for your input!

[crochambeau]

Whoa, that's saying AC signal where I would think your "+7" would have gone.

Question: does this thing have a phantom power switch for the XLR microphones, and do you need/use phantom power? That's the only reason I can see for a small current AC supply...

Unless you need that I would just leave it out and see what happens with something like this:

[spacelordmother]

You're awesome!!

Ah ha! Well, I don't see a phantom power switch but there are two balanced XLR jacks on the back, so that could be it. This is the unit: Tascam 644 Midistudio



You think it would still work/won't blow up if I just wire the +/- 10 and ground?

[crochambeau]

Typically losing a power supply leg does not result in destruction, just lack of function. There are, of course, exceptions - but low volt stuff is pretty forgiving.

Make sure you can measure voltages and they measure correctly from ground before plugging it in.

[spacelordmother]

Typically losing a power supply leg does not result in destruction, just lack of function. There are, of course, exceptions - but low volt stuff is pretty forgiving.

Make sure you can measure voltages and they measure correctly from ground before plugging it in.

So you're saying to hook the 2 DC 10v supplies up as you've drawn, and then read voltage from + to GND, and the - to GND and compare? And I do this with the plugs in the wall?

I have the shrink and I salvaged some pin sleeves from an old computer power supply I had here, so now I just need to find a local source for old power supplies. I'll report back!

[crochambeau]

So you're saying to hook the 2 DC 10v supplies up as you've drawn, and then read voltage from + to GND, and the - to GND and compare? And I do this with the plugs in the wall?

Hahahaha, yeah....

Here's the comfort method: Grab a couple alligator jumper wires, connect your meter to the DC legs you need to measure. Grab a power strip (with a switch), switch it off, plug in the wall wart type power supplies, plug the strip into the wall, turn your meter on, then all you need to touch is the power switch on the strip.

Repeat for the other end of the supply (leave center tap "ground connected to the meter).

If you're using a decent UL power supply the outputs are floating and isolated from the wall. 10 volts won't hurt you, probably won't even feel it unless you put it in your mouth (that hurts, trust me, don't do it).

[spacelordmother]

Nice! This is crazy enough that it just might work.

[spacelordmother]

I'm headed to get the power supplies tomorrow -- when I take the readings each on should read the value as noted, yes? One @ +10, and the other at -10? THANKS!

[crochambeau]

+/- are just relative to one another, you can get two +10 and connect the positive of one to the negative of the other, call that common (ground) and then observe that you have a negative and a positive leg with respect to the common point.

[spacelordmother]

+/- are just relative to one another, you can get two +10 and connect the positive of one to the negative of the other, call that common (ground) and then observe that you have a negative and a positive leg with respect to the common point.

Got it! Unfortunately the place I went to look for used power supplies didn't have 2x 10vdc with enough amperage to supply. I got one with 1400ma and one with 700ma to wire up and test and got the results you noted (yes!) but as expected, no joy when I turned it on so I will have to find another wart and try again.

Though -- I did find a 10VAC 2amp supply and read online that it could be converted into a good hobby supply. Any insights?

Hewlett-Packard (HP) Model 17122B 20VAC CT @2A. Polarity - Center tap wall transformer, manufactured by AT&T. Physical size is 3” X 3” X 2.5”. The input cord is an approximately 80” 115/120VAC, 0.40A, 50VA, 3-wire. Output cord is an approximately 81” 4-conductor female Molex type connector. 20VAC CT (+10v, 0, -10v) @2A. Polarity - Center tap. UL & CSA approved. (96E012).

https://www.physicsforums.com/threads/what-power-supply-to-purchase.709314/#post-4498030

[crochambeau]

You can indeed build a rectifier on something like that and regulate down to a proper +/-10 volts.

But let's pause a moment and consider that power supplies are rarely run at their maximum, the 1 amp rating is going to cover worst case current draw. Current draw at idle should be less, ie: 700 mA available at your lesser supply should have at least powered it up. Sounds like you got nothing at all?

Maybe that 7 volts AC is rectified and supplies some logic IC that is always ON and listens for on/off switch input? I've never been inside of one of these, so I have no idea what's going on in there.

One way to confirm your 700 mA supply is suitable for test is to measure current draw through it, or next best would be to measure its output voltage while under "use", if the voltage drops heavily (below 10 volts) then you do indeed need a more robust supply.

[spacelordmother]

Cool! Backburner/just in case project then. Think that would be able to be the +10, -10, and 7AC all at once?

So you know my process (and that I didn't screw something up) here's what I did:

- Measured both supplies from hot to ground -- both ranged about 10.3-10.4 VDC. Flipped the leads and got the same result, but negative. (obviously, but I am a noob so I wanted to check)
- Wired up the wires as shown in your diagram and tested. Got the same 10.3ish on one side, and -10.3ish on the other.
- Added salvaged connector pins and heatshrinked connectors. Re-measured to make sure contacts were solid and got the same readings as before.
- Using the shematic diagram as a pin map, attached the connectors to the 644, plugged it in, and turned it on. Not even a flicker.

Do I measure draw by switching modes on my multimeter from V to A? How would I test it under use with this unit?




The extra X factor in all of this being that I bought it used and it's possibly just broken.

[crochambeau]

Cool! Backburner/just in case project then. Think that would be able to be the +10, -10, and 7AC all at once?

7AC will need to be stepped down, it's kind of a pain in the ass voltage. I'd look for an old 110 to 6.3 filament transformer, that would supply close to 7 on modern 120 volt mains.

Sounds like you wired it up the same way I would.

Do I measure draw by switching modes on my multimeter from V to A? How would I test it under use with this unit?

Current draw usually wants to be measured *through* the meter, so you'd attach your positive lead to the supply and your negative lead to the unit, then set your meter into the correct range and power the unit up.