SPX 90 Power Supply - Page 1
Forum section: electronics / repair

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2016, 12:02:25 pm
content: 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.pngThanks So much

author: johnkenyon
author_rank: Regular Contributor
timestamp: January 28, 2016, 12:40:38 pm
content: 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.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2016, 12:53:42 pm
content: Would it still work with out it for testing purposes

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2016, 01:00:24 pm
content: And i don't have access to a scope at the moment. any Ideas ?

author: johnkenyon
author_rank: Regular Contributor
timestamp: January 28, 2016, 01:41:49 pm
content: Quote from: Jbliss on January 28, 2016, 12:53:42 pmD'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.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 28, 2016, 01:52:58 pm
content: Quote from: johnkenyon on January 28, 2016, 01:41:49 pmThanks 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.

author: johnkenyon
author_rank: Regular Contributor
timestamp: January 28, 2016, 02:34:28 pm
content: Quote from: Jbliss on January 28, 2016, 01:52:58 pmYes 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

author: Jbliss
author_rank: Regular Contributor
timestamp: January 29, 2016, 03:23:24 am
content: In the end i decided not to use a toroidal transformer due to spacing issues

author: Jbliss
author_rank: Regular Contributor
timestamp: January 29, 2016, 06:32:07 am
content: Quote from: johnkenyon on January 28, 2016, 01:41:49 pmwhat 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

author: Jbliss
author_rank: Regular Contributor
timestamp: January 29, 2016, 07:32:07 am
content: would this work

author: johnkenyon
author_rank: Regular Contributor
timestamp: January 29, 2016, 11:46:37 am
content: Quote from: Jbliss on January 29, 2016, 06:32:07 amThe 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 failsPulses stopMute circuit is activatedThe transient generated by the analog DC power failing is mutedSeveral milliseconds later, when the DC eventually fails, the relay will drop outIf you feed the optocoupler LED from the +5v line you getAC power failsThe transient generated by the analog DC power failing is NOT mutedThe +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."

author: Jbliss
author_rank: Regular Contributor
timestamp: January 31, 2016, 11:28:40 am
content: 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 brokenlike Q101 and Q102 Q103.

author: Jbliss
author_rank: Regular Contributor
timestamp: January 31, 2016, 11:53:16 am
content: Ok I belive ZD 101 is blown would this stop the relay from closing

author: Jbliss
author_rank: Regular Contributor
timestamp: February 01, 2016, 06:40:23 am
content: 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.

author: ilcaccillo
author_rank: Contributor
timestamp: May 17, 2017, 07:01:21 pm
content: 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

author: ilcaccillo
author_rank: Contributor
timestamp: December 22, 2017, 06:25:22 pm
content: Hello Jbliss, were you able to make your SPX90 work with a Linear PSU?thanks