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SX 750 restore

calman46

calman46

AK Subscriber
Subscriber
I'm working my way through this cosmetically challenged SX-750. I want to add a Triac can someone check my work please. I had to use snip it to make the diagram. PN# Q4025L6TP (isolated tab). I disconnected the AC and power transformer leads from power switch. AC lead (orang.) goes to M1, transformer lead (green) goes to M2 and added lead back (black)to power switch. Add lead and 100-ohm resistor to Gate and back to power switch's original AC terminal. Thanks
Screenshot 2022-10-04 110505.jpg
 
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Super Noob said:
Triac up next... this is detailed by others previously. Bottom line it reduces arcing of the power switch contacts by changing the switch contacts just to sending over a lower power signal to the Triac gate to put the 120VAC mains in the on-state.

Looks like I missed the after picture... but as LeeStereo post directs, there is a easy place to mount on the chassis right above the switch. I applied some heat compound - no insulator needed on the Triac used here. Note = you will need a 100 ohm 1/2 watt resistor, and you should remove the snubber cap on the switch contacts.

ACtC-3dAgqhrz-7PONq6TcUo9K10VS7MGSIHZGdXCb7epeOK4REYMK7Q4vu5mgg5RNb-tiHaYjuDGlWzJbAuqijCk76huEPosPV4k3JpfMvK5waehwjX4XNL973KRQFSa4ozT77F2e4VgmFxtkjNg_3PWOXr=w900-h732-no

ACtC-3d5CcwFKgJva6fRomBVlhZrGhm5wnHNMIbH9Cd4pYLatW91vnjap19ExG8f_W9NZfM6niHefeT67JaBUoXQqabOb6Pp05qFtzFCbCifvTiD2cNiKEU8AWdRFviQachxQomBSUl9lY8uSHDMEohrHFtz=w661-h881-no
Click to expand...
 
A relay in lieu of a triac.

Those "E" coil transformationators don't always appreciate the little switch lag at the zero line that some triacs can create. Remember your basic training - ELI the ICE man says that the potential for the lagging current spikes in a linear power supply could be problematic in the secondary, and might be audible in an audio system. There are several example of that right here on AK. There are also a lot of examples here where people got away with a triac without noticing any noise.

If you drop in a triac and notice any new noise, you will know why. If it is quiet enough for you and doesn't stress the filter caps, you're all good.
 
The drawing in the opening post isn't correct. The original power switch contact should be in series with the gate resistor.

I have done the triac mod on three SX-750 receivers so far. I mount the triac along side the power transformer using one sheet metal screw.
I attach a small solder terminal strip (with a sheet metal screw) behind the power switch to collect the end of the 100 ohm gate resistor.
This is the layout I settled on:
(EDIT: Thinking more about the physical layout, it might be better to mount the triac to the sheet metal directly behind the power switch. You'd only need to add one short wire, from the fuse to the power switch, and the resistor of course)

20221005_143315.jpg

20220501_161600.jpg
 
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Granted my drawing is not the best but Its right. There are two wires on the switch. AC and transformer/fuse/switch I removed the AC from switch and connect it to M1, connect the gate and resistor to the switch where the AC was connected. Then remove the other wire from switch connect to M2 and add wire from M2 back to the switch. What is wrong with that? I did this before I think mounted it on side of the chassis, but I don't remember Do you have the fuse connected to the terminal strip? yellow wire in your drawing
 
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Anyway, I haven't done the triac yet, but the electronics are done, bias set 30mV, DC offset 19mV left 23mV right. Testing now sounds like a restored vintage Pioneer, excellent! The top of the faceplate was severely scratched and a good candidate for me to try removing them. I started with a wire wheel and took off the anodizing. The aluminum is not perfect underneath there some "grain" marks? and plus I had the swirls from the wire wheel. Next 220gr in a drill rotary pad, then 400gr pad then 800gr on a block. Some parts of the scratches were just too deep to remove, and block sanding aluminum with 800gr is tricky as the paper clogs easy and makes its own marks in the aluminum. So, I left well enough alone and spray the top with some satin clear. I like it. Now on to the wood veneer. I get a lot of light reflections off the faceplate top. I need to fix up previous lamp replacement.
IMG20221014101544.jpg.
 
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