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Pioneer SX-680 recap

RFmaster

RFmaster

Member
After doing a recap on a unit do you ever wonder what is removed??? Wonder no more - here is a family portrait of a typical SX-680 recap. In addition to changing electrolytic caps it is highly advisable to R&R noise prone 2SA725, 726 and 2SC1919 transistors in the FM and phono sections. In the power supply 2SD712 and 2SC2073 should be replaced for good measure.. And now for group photos.
Caps
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Transistors
crop_20151120_135925.jpg

2SA725, 726 => KSA992FBU (pin order ECB)
2SC1919 => KSC1845FTA (pin order ECB)
Replacing above transistors quieted FM and Phono sections!!!

In the power supply section
2SD712 => KSC2073TU
2SC1284 => KSC2690AYS - no PCB board modification required leads are small enough.

Recap list can be found here.
http://www.audiokarma.org/forums/index.php?threads/my-attempt-at-a-sx-680-recap-list.489129/
 
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Nice! I'm building up the nerve (well, skills mostly) to re-cap a 780 that's being used in the bedroom. If it's not too different from your 680, I think it should be a good project to start with. Overall, can you tell a difference in the sound quality?
 
bando281 said:
Nice! I'm building up the nerve (well, skills mostly) to re-cap a 780 that's being used in the bedroom. If it's not too different from your 680, I think it should be a good project to start with. Overall, can you tell a difference in the sound quality?
Click to expand...

Believe it or not SX-780 is easier to re-cap than SX-680! Main Board GWM-118 on SX-680 has tightly packed components and sandwiched between front panel and heatsink wall. In some places caps and transistors are positioned well under the front panel over hang. On SX-780 there are a handful of these 'hide-away's', but it is easier to get hands in and out. Besides, on SX-780 phono front end is based on a pair of HA1457 OP-amps while SX-680 is based on discrete transistors: 1x 2SA726 and 2x 2SC1919/ per channel. That portion of the board on SX-680 is packed like a commuter train in Tokyo. My guess is that HA's were a bit more money back in 1978 vs. all discreet line up.

On the sound quality - yes the phono section was the biggest difference after rework. When I first got this unit (Ebay - not working for repair or parts) phono section sounded like a soda can - fuzzing and raspy sounding. After re-cap and changing out 2SA726 and 2SC1919 it is nice and quiet. On top of it all one of the two main filter caps was leaky. So all together it made a huge difference. Unit was very dusty inside - I used compressed air to blow away dust and cobwebs. DeoxIT D5 all pots and switches made a difference, however it would start quiet and after about about 3 to 4 hours of playing one channel would get fuzzy. Cycling function selector would cleaned it up, but it shows that one application of DeoxIT D5 is not enough - DeoxIT Fader is needed for contacts lube. So all together I put about 10 hours into it over period of two weeks.

Just take your time - order more caps and transistors than you need for spares in case if one decides to go for a walk, make sure that you have a quality soldering station, solder wick, and hand tools. De-soldering tool, for example Hako DR-300, makes this job a breeze, but it is pricey!
 
Yeah, the 780 isn't really that bad. After talking about it for many years, I finally recapped one. I also kept all the old caps for posterity's sake. RFmaster, what did you use for the filter caps? I didn't replace mine (yet). They are 8000uf 50V. I can find several 8200's but I need to check the lead dimensions to find one that will work.
 
Schanz said:
Yeah, the 780 isn't really that bad. After talking about it for many years, I finally recapped one. I also kept all the old caps for posterity's sake. RFmaster, what did you use for the filter caps? I didn't replace mine (yet). They are 8000uf 50V. I can find several 8200's but I need to check the lead dimensions to find one that will work.
Click to expand...

On SX-780 I used 50V/10,000 uF from Nichicon 647-UFW1H103MRD - Mouser has them for a very reasonable $5.23 each. Direct fit - with correct lead spacing, but slightly smaller. OE ELNA cap uses third lead for mechanical retention.

20150911_134833.jpg
 
OK, I've seen others use those but was reluctant to go more than 20% above the originals. That would be the easiest of the ones I looked at. Thanks.
 
Schanz said:
OK, I've seen others use those but was reluctant to go more than 20% above the originals. That would be the easiest of the ones I looked at. Thanks.
Click to expand...
Why are you reluctant to go with higher value filter caps? Original value was 8,000 uF +/-20% on a good day 36 years ago. From electrical aspect you want as much charge storage with lowest ESR as possible - this will improve low frequency response especially during heavy bass passages. So far I have re-capped 3 units with these cap without any issues.. .
 
Cool. I am placing the order tonight.
I pulled out the old ones and they measured .58 and .62 mf on my tester so they were pretty low anyway.
 
I just replaced the transistors 2SA725, 726 and 2SC1919 in my pioneer sx-680 with the ones mentioned above. The hissing distortion I was experiencing in the phono section stopped, but now I’m getting a repetitive thumping noise like someone is slapping a microphone.

Has anyone experienced this issue?

Thanks!
 
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