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Pioneer SX-750 power switch mod on HG model

I

iPhoenixDave

New Member
Hello Everyone!

I'm relatively new to vintage audio but the love is strong with my Pioneer SX-750 from 1977. I started doing a recap on it after numerous hours of research not to do wrong. The power supply and protection board AWR-099 is done, it went smooth, I'm currently waiting for all the other capacitors.

After reading a lot of SX-750 threads here on AK, I'm playing with the idea to do a power switch modification. My switch is working fine currently, but I'd like to future-proof it, because I really like this unit.
I really like the @K7sparky triac mod because its subtle, but some people did relay power switch mods on their 750s.
Unfortunately I'm not that good in circuitry yet, so I'd like to ask for some help before I butcher something, I'm not in a hurry, I don't want to do a "shotgun" approach on the unit.
I have the HG variant, 220/240V and 50Hz. I attached the schematic for it.

Képernyőfotó 2021-02-25 - 12.28.07.png

Could someone please explain me how should I do the Triac mod?
I haven't bought the triac yet, so what specs are required?
Which wires are connected to the 3 legs of the triac?
I'd like to mount it on the chassis next to the transformer like @Leestereo did it. I know I have to use an insulation pad, heatpaste and an insulated screw.

I'm sorry if I sound like a beginner, I am, but I'm trying.
Thank you!
David
 
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I installed one along the rear wall of the unit - Plenty of space. Any DPDT relay with a 240V operating coil will work in an HG type. I used a P&B (TE Connectivity) T92P11A22-series, but it was reidiculously oversized for the application with its 40A contact rating. It still fit nicely. Something like THIS should work as well.

SX-750PowerRelayMounting.jpg
 
If I am correct, the type HG is a dedicated 220/240V source only. A single triac in this case will NOT isolate both power lines from the transformer/load. A pair of triacs with control circuit would need to be installed, and the potential for non-zero crossing clipping would be more than doubled compared to using a single triac. You could attempt it, but I would not at all be surprised if the noise generated would be noticeable in the power and in the audio.
 
Thank you for the detailed explanation! I really appreciate it!
I will place an order for the relay, as you suggested, and keep this thread updated with my installation, in case it helps anybody

Watthour said:
I installed one along the rear wall of the unit - Plenty of space. Any DPDT relay with a 240V operating coil will work in an HG type. I used a P&B (TE Connectivity) T92P11A22-series, but it was reidiculously oversized for the application with its 40A contact rating. It still fit nicely. Something like THIS should work as well.

SX-750PowerRelayMounting.jpg
Click to expand...

Watthour said:
If I am correct, the type HG is a dedicated 220/240V source only. A single triac in this case will NOT isolate both power lines from the transformer/load. A pair of triacs with control circuit would need to be installed, and the potential for non-zero crossing clipping would be more than doubled compared to using a single triac. You could attempt it, but I would not at all be surprised if the noise generated would be noticeable in the power and in the audio.
Click to expand...
 
So the relay has arrived, while I was recapping the tone control board.
Now... someone before me has tinkered with the power switch circuit and oh boy...
One of two brown wires coming from the AC inlet goes to the switch, the other straight into the transformer.
IMG_6307.JPG
I really want to do this right, clean up this mess and install the relay.
@Watthour or someone please tell me how to open the switch and how to fix the wiring. I could do it if i really tried but I’d rather ask the professionals.
IMG_6304.JPG IMG_6306.JPG
 
So I wired up the relay, I’m 99% sure that I’ve done it correctly.
I fired up the amp on a dim bulb tester, and the relay made a “crow-like” crackling sound. The lights turned on and off and on ágain érc and the dbt did the same...
I immediately turned it off. What did I do wrong?
 
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You're right on the money with that. The relay is likely chattering from insufficient voltage. Get off the DBT and I'm guessing it will go right. I've had the same thing on a few that I've converted to power relays.
 
I’m happy to report that it works without the DBT!
Stressful few minutes these were...
Is there any other way than the DBT to safely turn it on after I recap a few more boards? I’m confident enough but just to make sure.
 
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