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SX-727 Saga & Recap Advice - Replace all transistors, diodes too?

Christa

Christa

New Member
Hi all!

I'm working on recapping a receiver and its my first complete refurb project. I'm a complete novice - prior to this I've repaired an a faulty phono input SX-525 by replacing a few transistors, replaced the lamps and cleaned the pots, but that's about it. I actually started on this SX-727 a few years back but I gave up due to shoddy solder skills and equipment. Just updated my equipment - got a good soldering iron, got quality solder and flux, a simple desoldering pump, good test leads, nice helping hands - and what a difference! I was using just a desolder wick and a radio shack soldering iron before and it was a mess.

In 2017 I packed up the SX-727 in the original box (without the case) and put it in the back of our storage room. In the intervening years a mouse made a home inside the amp - right behind the tuning dial. It ate through the tuning dial cord, chewed up some wires and generally made a mess. I figured since I had ruined it, I might as well practice my soldering skills and give it a go. First, I restrung the dial cord then fixed the wires and cleaned it all up. I have replaced all the capacitors on the AWR-011 Power Supply board and the caps and all of the transistors on the W21-002 Head Amp. Hooked it up to my homemade DBT and then to some speakers and it sounds good! Still have some scratchy pots - but that's to be expected since it was a home to Mr. Mouse for a few years. I'll be cleaning all the controls with deoxit/faderlube.

So my main question for you experts is moving forward - I'm going to go through the service manual tonight and read all of the voltages and adjust the bias. Assuming I can get good results I'd like to finish the refurb, but I'm concerned with replacing too many parts and affecting the sound. I know that all of the electrolytic capacitors need to be changed, but should I also be replacing all of the transistors or just the noisy ones? Also, I was thinking of leaving the tuner board alone - it works well now and sounds good, but are there known problematic parts that I should replace? We listen to a lot of music so I'd like this to be our main amplifier if I can get it up to snuff and I'd hate for a night to be ruined with a bunch of crackling or worse.

Sorry for such a long post, but thanks for reading!
 
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Do a search for sx 727 rebuild or recap several threads here on that model. Use Google if the site search feature doesn't work well for you (I find it isn't very good, if you search for sx 727 it'll give you every sx model and everything with a 7 anywhere in the text.)
Welcome to AK, by the way.
 
Always better to replace too much, than too little.
New components, if the right substitute, will always have lower noise and linearity.

A design creates a sound signature, components can only detract from it.
 
All diodes and transistors on the power supply for sure should be replaced. The only transistors I don’t replace on audio boards are ones that don’t have subs that I know of, or the weird JFETs that you don’t replace. I replace a fair amount of diodes but I don’t go nuts with it.
 
Hi Mark - thanks for the advice. I am going to leave the tuner board completely alone. I do have a question for you though, and I almost found an answer in the forum, but my Power Amp Unit Board (AWH-011 B) lists Q1-Q4 as 2SA572, but what is actually installed is 2SA726. I found one other post referencing this but I didn't find the answer: https://audiokarma.org/forums/index.php?threads/pioneer-sx-727-transistor-help.528724/

Do you think I should replace these with KSA992FBU or leave them be?
 
Yeah, I don't touch tuner boards except occasionally for the blue Sanyo caps.

Christa said:
Hi Mark - thanks for the advice. I am going to leave the tuner board completely alone. I do have a question for you though, and I almost found an answer in the forum, but my Power Amp Unit Board (AWH-011 B) lists Q1-Q4 as 2SA572, but what is actually installed is 2SA726. I found one other post referencing this but I didn't find the answer: https://audiokarma.org/forums/index.php?threads/pioneer-sx-727-transistor-help.528724/

Do you think I should replace these with KSA992FBU or leave them be?
Click to expand...

Replace with A992.
 
Replace ANY and ALL 2SA725 and 2SA726 period (considered a non negotiable 1st strike) with KSA992's. FBU or FTA no difference except packaging.
 
Thank you everyone - I really appreciate all the help. I'm learning as I go and this forum has been a great resource.

Last night I replaced most of the caps on the the AWH-011 and swapped all of the 2SA726 with A992's I was missing two of the small caps, but they should be here tomorrow. I was going to swap out these other transistors:

Q5 & Q6: KSC2690 for 2SC1124
Q7 & Q8: KSC2383 for 2SC984-B
Q9 & Q10: KSC2690 for 2SC1157
Q11& Q12: KSA1220 for 2SA647-D

I'm leaving the big output transistors alone - should I leave any of these others I am planning on replacing?

I was also going to replace the small diodes D1 and D2 with 1N5243B

I ordered the small pots too. Do ya'll think I should replace those as well or leave them be?

Here's some in progress pics from last evening. Sadly, I couldn't work on the thing all night - had to go inside to fix supper.

AWH-011B:
20200114_183650.jpg

Completed W21-002: All new caps and transistors.
20200112_110946.jpg
 
I'm doing a restore of one these as we speak. You're getting good advice from the fellas.

One thing I would have pointed out if I'd got here sooner is that the replacement transistors in the power amp at Q1/Q2 and Q3/Q4 should be gain matched (hFE). You'll need a DMM with this functionality or a transistor tester which can provide this information. Often original matched transistors will have a painted dot atop them. When you see this it is a good indicator that what you put back in should be matched as well. Also it should be pointed out that there are gain variations of KSA992 so you want to select the potentially highest gain variety which is KSA992FBTA, not KSA992FTA or KSA992FBU.

As it is I'm not sure it's worth redoing, but if you feel inclined at least now you have the knowledge to do so.
 
hopjohn - thanks for the insight. I did notice the painted dots and black legs on the 2SA726's - but like other things I don't understand I just disregarded it. Great policy I know, haha.

My DMM is pretty basic - voltage, current, resistance and testing diodes - I did get better leads for it, but overall its pretty lame. I put the KSA992FBU in - I still have the 2sa726's with the dots though - do you think it would be better to put them back or order some of the higher gain variety 992s?
 
Christa said:
hopjohn - thanks for the insight. I did notice the painted dots and black legs on the 2SA726's - but like other things I don't understand I just disregarded it. Great policy I know, haha.

My DMM is pretty basic - voltage, current, resistance and testing diodes - I did get better leads for it, but overall its pretty lame. I put the KSA992FBU in - I still have the 2sa726's with the dots though - do you think it would be better to put them back or order some of the higher gain variety 992s?
Click to expand...

The Q1/Q2 and Q3/Q4 are the differential pairs and even without the dot present you'd want to match them, the dot is just a giveaway that they were done so at the factory. As far as putting the 2SA726 back in, no you wouldn't want to do that, they are problematic. The KSA992F are just a little lower in gain (hFE) than the KSA992FB (in my experience somewhere in the neighborhood of 370 vs. 460). The significance is pretty minimal and probably not worthy of a 2nd go round in placing an order unless you're being OCD. The main priority is to get the 2SA726s out of there. Whether to change them to the FB is entirely up to you.

Harbor Freight sells a cheapo meter that can do hfe readings or another cheap option is one of the little Chinese transistor testers from the famous auction site for around 15$ that are pretty useful.
 
Leave the 2SC984s alone if they work. I tried the KSC2383 as a sub in a few other receivers and couldn’t get the bias high enough.
 
EastPoint said:
Leave the 2SC984s alone if they work. I tried the KSC2383 as a sub in a few other receivers and couldn’t get the bias high enough.
Click to expand...
Thanks for the tip - They do work now and I was actually planning to leave them alone, mostly because I couldn't figure out how to extricate them from the metal housing/heatsink without forcing them, but now I have a more valid reason.
 
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