SPX 90 Power Supply

[Jbliss]

Hey all,
The power supply in my SPX 90 (digital effects processor) is definitely on its way out. very noisy and hot. I would to build a nice new linear supply for it Which should be easy as it only has a +5 rail for TTl logic and +- 18v rail (which gets regulated on the main board) for all the op amps and stuff. how ever there are two line coming form the power supply labeled PC and PE which I don't known what to do with should I leave them out or what? I think they may have been used as a feed back for the switch mode supply.

I have included the schematics The power supply in use is the general one.
http://elektrotanya.com/PREVIEWS/63463243/23432455/yamaha/yamaha_spx90_r_sch.pdf_1.png

Thanks So much       

[johnkenyon]

PC could be a low voltage AC feed into an anti-thump circuit which operates a mute relay (box with a X through it marked mute which has D101 across it).

The idea is that when power is lost, this input "fails" before anything else and the mute relay is activated which clamps the the left and right outputs to ground.

Don't know about PE - maybe a return for PC?

My recommendation is that you put a scope between PC and PE, and expect to see AC/unsmoothed half wave rectified AC/ or unsmoothed fullwave rectified AC.

[Jbliss]

Would it still work with out it for testing purposes

[Jbliss]

And i don't have access to a scope at the moment. any Ideas ?   

[johnkenyon]

Would it still work with out it for testing purposes

D'oh - didn't notice the PSU board schematics - PC and PE are just the collector and emitter connections from an opto-isolator.





Would it work if not connected?
Not sure - you would either (a) have the relay permanently energised and get no audio output, or (b) it would never energise and you would get a thump noise when the thing was turned off.

If you got (a) then you could simply short PC and PE together (thus simulating the photo transistor being permanently on, and then you would end up with (b).

If you want the original/correct behaviour I would pick a transformer secondary winding, and drive the LED of an optocoupler from that winding with a suitable current limiting resistor, with a diode wired in in reverse parallel across the optoisolator input. (Look at the "Japanese, US and Canadian Models" PSU design - which uses D1, R24 and PC2 to do the same job - you use a lower value for R24 for a low voltage transformer winding).

There's no smoke and mirrors here - all you are doing is driving the LED of an optocoupler from an AC source without a smoothing/reservoir capacitor.

Also you need to note that the  +/-15v supplies and the +5v supply have separate "ground" connections (AG=Analog ground and DG=Digital ground)- so to keep noise to a minimum, design your replacement to have least separate transformer windings if not separate transformers.

[Jbliss]

Would it still work with out it for testing purposes

D'oh - didn't notice the PSU board schematics - PC and PE are just the collector and emitter connections from an opto-isolator.





Would it work if not connected?
Not sure - you would either (a) have the relay permanently energised and get no audio output, or (b) it would never energise and you would get a thump noise when the thing was turned off.

If you got (a) then you could simply short PC and PE together (thus simulating the photo transistor being permanently on, and then you would end up with (b).

If you want the original/correct behaviour I would pick a transformer secondary winding, and drive the LED of an optocoupler from that winding with a suitable current limiting resistor, with a diode wired in in reverse parallel across the optoisolator input. (Look at the "Japanese, US and Canadian Models" PSU design - which uses D1, R24 and PC2 to do the same job - you use a lower value for R24 for a low voltage transformer winding).

There's no smoke and mirrors here - all you are doing is driving the LED of an optocoupler from an AC source without a smoothing/reservoir capacitor.

Also you need to note that the  +/-15v supplies and the +5v supply have separate "ground" connections (AG=Analog ground and DG=Digital ground)- so to keep noise to a minimum, design your replacement to have least separate transformer windings if not separate transformers.

Thanks So much you have really helped me out !!!!!!
so in order to get a seperate ground could i just use two transformers one for the + - 15v and one for the 5v.

[johnkenyon]

Would it still work with out it for testing purposes

D'oh - didn't notice the PSU board schematics - PC and PE are just the collector and emitter connections from an opto-isolator.

Would it work if not connected?
Not sure - you would either (a) have the relay permanently energised and get no audio output, or (b) it would never energise and you would get a thump noise when the thing was turned off.

If you got (a) then you could simply short PC and PE together (thus simulating the photo transistor being permanently on, and then you would end up with (b).

If you want the original/correct behaviour I would pick a transformer secondary winding, and drive the LED of an optocoupler from that winding with a suitable current limiting resistor, with a diode wired in in reverse parallel across the optoisolator input. (Look at the "Japanese, US and Canadian Models" PSU design - which uses D1, R24 and PC2 to do the same job - you use a lower value for R24 for a low voltage transformer winding).

There's no smoke and mirrors here - all you are doing is driving the LED of an optocoupler from an AC source without a smoothing/reservoir capacitor.

Also you need to note that the  +/-15v supplies and the +5v supply have separate "ground" connections (AG=Analog ground and DG=Digital ground)- so to keep noise to a minimum, design your replacement to have least separate transformer windings if not separate transformers.

Thanks So much you have really helped me out !!!!!!
so in order to get a seperate ground could i just use two transformers one for the + - 15v and one for the 5v.

Yes you could - Two transformers would be easier to source, at the expense of more space used inside the case.

Take care if you are using toroidal transformers - the SPX-90 doesn't look as if it has too much room to spare, and you don't want to create an accidental shorted turn.
http://www.diyaudio.com/forums/parts/30676-toroids-mounting-bolts-shorted-turns.html

[Jbliss]

In the end i decided not to use a toroidal transformer due to spacing issues 

[Jbliss]

Would it still work with out it for testing purposes

D'oh - didn't notice the PSU board schematics - PC and PE are just the collector and emitter connections from an opto-isolator.





Would it work if not connected?
Not sure - you would either (a) have the relay permanently energised and get no audio output, or (b) it would never energise and you would get a thump noise when the thing was turned off.

If you got (a) then you could simply short PC and PE together (thus simulating the photo transistor being permanently on, and then you would end up with (b).

If you want the original/correct behaviour I would pick a transformer secondary winding, and drive the LED of an optocoupler from that winding with a suitable current limiting resistor, with a diode wired in in reverse parallel across the optoisolator input. (Look at the "Japanese, US and Canadian Models" PSU design - which uses D1, R24 and PC2 to do the same job - you use a lower value for R24 for a low voltage transformer winding).

There's no smoke and mirrors here - all you are doing is driving the LED of an optocoupler from an AC source without a smoothing/reservoir capacitor.

Also you need to note that the  +/-15v supplies and the +5v supply have separate "ground" connections (AG=Analog ground and DG=Digital ground)- so to keep noise to a minimum, design your replacement to have least separate transformer windings if not separate transformers.

what is the prupose of the of putting a diode in reverse parallel across the input of the optocoupler. is it for protection. also i was going to use the regulated 5v to drive the LED in the opto with a current limiting resistor. What this Work 

[Jbliss]

would this work

[johnkenyon]

what is the prupose of the of putting a diode in reverse parallel across the input of the optocoupler. is it for protection. also i was going to use the regulated 5v to drive the LED in the opto with a current limiting resistor. What this Work 

The optocoupler is deliberately fed with AC. The reverse parallel diode protects the LED when the AC goes negative.

The optocoupler is used to tell the mute circuit that the power has failed. It provides pulses to the mute circuit.
As soon as the AC supply fails the pulses from the optocoupler stop. Note that at the instant the AC fails your +5v line will probably stay above +4.5v for a significant period due to the reservoir capacitors after the bridge rectifier.

So the sequence of events is:
AC power fails
Pulses stop
Mute circuit is activated
The transient generated by the analog DC power failing is muted
Several milliseconds later, when the DC eventually fails, the relay will drop out

If you feed the optocoupler LED from the +5v line you get
AC power fails
The transient generated by the analog DC power failing is NOT muted
The +5v rail drops to a low level, and the mute relay activates for a few milliseconds (if that)
Several milliseconds later, when the DC eventually fails, the relay will drop out.

Remember what I said earlier "all you are doing is driving the LED of an optocoupler from an AC source without a smoothing/reservoir capacitor."

[Jbliss]

Ok All is working expect the relay is not triggering even when PC and PE are shorted Is it possible that the circuit to drive the relay is broken
like Q101 and Q102 Q103.

[Jbliss]

Ok I belive ZD 101 is blown would this stop the relay from closing

[Jbliss]

OK I replaced the Zener diode and it made no difference i have just put in a jumper where the zener was this leaves the relay always closed and passing audio. is there any way this could further damage the circuit.

[ilcaccillo]

Hi,
I would like to replace my Broken SPX90 PSU for a Linear Power Supply.

Anyone knows how much Power do I need in the different rails?

How many mA for each?

+18V
-18V
+5V

[ilcaccillo]

Hello Jbliss, were you able to make your SPX90 work with a Linear PSU?

thanks