Yamaha Mixing desk ac power supply theory and practice!

[Rhod]

Do let me know if there’s a better place to post this in.

I’ve recently acquired an old mixing desk cheap because it was without its power supply - a common scenario which can be a good way to get a bargain! I thought I had a power supply hence my willingness to buy the desk. Turns out I have a Yamaha PA-10 PS which is an 18.5v AC X2 supply rated at 0.62 amps delivered through a GX16 aviation 3 pin plug. Unloaded it reads approx 21V between pin 1 and 2 (Gnd), the same between 3 and 2, and 43V between pins 1 and 3.

The mixing desk I have purchased was supplied with a similar supply but 17.5v AC X2 rated at 0.94 amps, model PA-20.

I’m happy to buy a replacement - they’re readily available if a little pricey - but it left me with a couple of questions, one practical and one theoretical, that I wondered if the forum members might have views on.

1. Although I’m happy to buy a replacement PSU - the desk cost me not very much after all - I’d like to see if it’s working beforehand. How sensible do you think it would be to temporarily and briefly hook up the one I have - 1V over and underrated by 0.3ish amps - to see if it powers up?

2. How are three pin supplies like this constructed? Is it a 2 phase supply with two ACs out of phase? Is it
just like a 2 pin AC supply but with an inverter included? Are there 2 coils/transformers or any modern switching equivalent needed in a unit like this a - little like a makeshift Eurorack PS I cobbled together to supply +12v and -12v (dc in that case)?

I’d be interested in your thoughts if you’d like to share them. Thank you!

[sugeda]

Simply a transformer with three wires in the secondary

[Rhod]

Ah, thank you. So is it actually a (nominal) 37V transformer with a centre tap? Or dual 18.5V windings in the secondary? Or could it be achieved either way?

[oc disorder]

Yes it's simply a center tapped transformer.

[oc disorder]

To answer the second part of your question could you use your old power supply to test the newly acquired mixer ? ..... well yes with reservations. You can see from the schematics here the power input to the mixers are very similar.
Logically the bigger one has a higher current draw has an extra regulator and extra channels so it draws (consumes) more current. I would say the regulators can cope with dissipating another volt although probably get slightly hotter , but your older power supply the PA 10 won't like to supply nearly double the current for very long and will probably blow the internal thermal fuse in the windings if left on for too long making it useless. If you have an amp to monitor the mixer and a CD player wired mono to a jack plug and you set up all the channels ready to go you should have time to plug into all the channels one by one and see if you get output. The power supply will get warm but I really don't know how long it will hang in ! Of course if there is a nasty fault in the mixer like a short it may have already blown the internal fuses as shown in the diagram. It may pay to check them first .
Good luck.

[g1]

your older power supply the PA 10 won't like to supply nearly double the current for very long and will probably blow the internal thermal fuse in the windings if left on for too long making it useless.

I have no idea why they are using 2A fuses there, but the PA20 supply is 0.94A rating. The consumption watts number given (42 vs 35W) is only 20% higher.

[Helmholtz]

Input power typically is not power consumption.
Rather it should be apparent power calculated from mains voltage times input current.
Real power consumption is always less than apparent power.
The conversion factor (power factor PF) varies with PT design and load.

[Rhod]

Yes it's simply a center tapped transformer.

That makes sense. I’ve also seen dual output transformers with two windings - would those work too if you doubled the voltage by connecting the output in series and taking a Gnd connection from the commoned outputs? Maybe that’s the same in effect as a centre tapped transformer….

[Rhod]

Thanks for all responses… I will open it up, do a visual check and test the fuses and then test with my existing power supply but only to establish if it’s powering up and maybe passing audio to the main outs. As long as I’m happy I’ve got something to work with i.e. it hasn’t been sat in water and all the boards are kaput I’ll buy the appropriate PS to do the detailed checks of all the inputs…
Thanks for your help, particularly oc disorder.

[Rhod]

An update to this story, and a couple of observations about the desk.

I tested the desk with my PA-10 long enough to confirm that the desk was powering up and that it was passing audio.

As for getting the proper PSU, well, good things come to he who waits (!). I was prepared, but a little reluctant, to shell out for a new power supply that would cost more than the desk itself. A bit of searching, and I found another desk on sale on Fb Marketplace less than 30 mins away, so for about the same amount I got a power supply and a whole desk full of spares (and indeed mine was missing the rack mount adaptors and a fader). Both desks actually work. All it took was some intensive cleaning of the pots and switches. If you’re looking at a desk like this note that both the desks had an issue with outputting L and R from all channels. The culprits were the mute buttons (ST On) but after a lot of working with contact cleaner (I used Propower - poor man’s Servisol) these are no longer a problem. The other particularly tricky switch was the Gp to Main output switch, which again took some working. The other switches and pots were a bit scratchy but responded well to cleaning, and it now behaves perfectly.

The desk is decent quality, not too much noise and perfectly adequate for the live sound I’m going to be using it for. And I’ve got plenty of spares for anything that goes wrong!