Roland PA-250 Mixer Amp Power Supply.

[Boik]

Hi guys. Have one of the above, but it's probably a more general question. I want to remove the 125watt power amps and the huuuuuuge and heavy transformer and rehouse the mixer section it in a smaller, lighter cab. I have traced the power supply where I can, and the links to the mixer unit seem to be a simple +15vDC -15vDc 0v and one other small orange lead from the power supply board to the main mixer board. It seems to carry a stable 0.67vDC, but does anyone know what it might be?

Also, how precise would my replacemnt -/+15vDC suuply have to be. Could I get away with -/+12vDC from a computer supply or is that likely to be too low?

Any guidance very gratefully recived.

Boik

[Enzo]

+/-12 would work, but you may lose some headroom on peaks.


I have no idea what your orange wire is doing, my first guess would be a power indicator maybe.

Welcome to the forum. Good form would be to link us a schematic for the unit. Do you have one?

[Boik]

Hi Enzo. Thanks for the welcome and advice. I wish I did have a schematic, it would make things a lot easier. I did pay for a copy of the schematic for the PA-400 hoping that it used the same power supply, but they are completely different. I would be happy to mail anyone a copy who wanted it.

I'll try the +-12v with the orange lead disconnected and see how it responds.

[g1]

It's also possible the orange wire is some kind of mute line. See if the .7V changes at all while the unit is powering up.
Or see if it goes to a power indicator on the mixer board as Enzo suggested.

[J M Fahey]

Those 0.7V look like some current (say, a resistor connected to +15V) into a remote transistor base, so the transistor "knows" that power is on and does something: light a LED, turn a mute off, whatever.
What I mean is that the mixer or something else (PSU / Power amp) might NOT work with orange wire disconnected.
Trace it to see where it goes or what it does.

As of the PSU, build a dedicated one.
In a mixer there´s a BIG difference between clean regulated rails (say, with 7815/7915) and dirty unfiltered SMPS voltages .

[Boik]

Really appreciate the replies guys. Not had time to do much with this due to a run in between fingers and circular saw. Only one winner :-( I have snipped the orange wire and things seem to still work. Cant remove the PCB one handed, but orange wire appears to come from same area as reverb unit is connected (which I don't use). Will try more in a couple of days and report back.

[Kees88]

Kicking this topic, because i'm in pursuit of the same goal to replace the power amp of my Roland PA-250 with a smaller power supply for mixer only functionality. I have made a Ken Stone +15/ -15dvc power supply to see if the unit would power up.
I also included a 0,67v line using a voltage divider for the 'mute line'. However, this does not seem to work. Powering with just the +/- 15vdc powers the mixer and headphone output, yet, the master and monitor outs are muted and VU-meters do not work.

With the 0.67v mute-line included, the voltage of this line drops to 0V when powered.

The original power supply seems to deliver 40V to the orange mute line, when it is not connected to the mixer. Once it is, the voltage is 0,67V and the mixer outputs work correctly. The original power transformer has both a center tapped 20vdc and 45vdc line.

I am not a electronics expert, and haven't stumbled upon any schematic for this nice piece of vintage kit.

Is there any possibility to find out how to imitate the mute-line, or perhaps bypass it in any way? Or would this be impossible without a 40v power source?


btw, i hope Boik is all well after his saw accident.

[J M Fahey]

I have made a Ken Stone +15/ -15dvc power supply to see if the unit would power up.

Good. That's a clean supply, not like a dirty buzzy PC supply.

I also included a 0,67v line using a voltage divider for the 'mute line'. However, this does not seem to work.

And it won't.
It is not a "0.67V" source but a **current** source, feeding "something" on the mixer board which drops 0.67V when receiving such current, a very different thing.

That voltage perfectly matches a "diode drop" , the voltage you get when current passes through a diode ... or a transistor BE junction.

Never saw the schematioc but because of what you measure and functionality, i strongly suspect some current comes through the orange wire, goes to a transistor base, and said transistor unmutes the mixer Master stage, probably to avoid turn on pops and thumps.

So you need to provide same *current* .

Powering with just the +/- 15vdc powers the mixer and headphone output, yet, the master and monitor outs are muted and VU-meters do not work.

Matches what I suspect.

With the 0.67v mute-line included, the voltage of this line drops to 0V when powered.

Not sure about this, please recheck.
I expect 0V there when unpowered (duh !! ... 0V everywhere ) and 0.67V when powered.

The original power supply seems to deliver 40V to the orange mute line, when it is not connected to the mixer. Once it is, the voltage is 0,67V and the mixer outputs work correctly.

Which I read as: you have an "x" ohms resistor at the power supply, connected to +40V and the other end to the orange wire "there" , said orange wire "here" goes to a transistor base, passing "y" current.

Ok, measure "y" .
Still with the original supply lift the orange wire end at the mixer side, set your multimeter to the 200mA scale and measure current to ground.
I expect something like, say, 1 to 5 mA ... shouldn't need more than that.

Once you measure it, it's easy to calculate a resistor which will provide same current when fed +15V (which you now have) instead of +40V (which you won't)

The original power transformer has both a center tapped 20vdc and 45vdc line.

Power transformers do not have DC voltages anywhere, please check and rewrite this.

Is there any possibility to find out how to imitate the mute-line, or perhaps bypass it in any way? Or would this be impossible without a 40v power source?

Oh, there's light at the other end of the tunnel, just read above

btw, i hope Boik is all well after his saw accident.

Same here

[Mick Bailey]

Don't forget that as well as the opamp rails, most mixers will also have a supply for phantom power.

[Kees88]

Good news:-)

She is alive!

Juan Manuel, you are a hero, deriving to a good conclusion without concise info from my side;-) I followed your instructions closely, and measured a current of 0.23mA on the orange wire. Therefore, i used a 68k resistor in series with the 15v dvc supply to derive to this current. The orange line indeed feeds a transistor.


Both master and monitor outputs are alive now.

"Luckily", this unit lacks phantom power, dedicated mic XLR inputs and whatnot:-)

I could now safely use the grinder to get rid of the residual space in the back of where once the power amplifier was. It is way lighter to carry now, without the huge transformer in the back.




Thanks again, guess i'll do some further reading about current controlled circuits.

[paleeman]

For future reference (if ever needed)

pa-80_150_250.pdf