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Pioneer SX-3700 repair/maintenance assistance request

Ruslan_603

New Member
(Reposted from the solid-state forum.)

Hey all, new kid, so fresh I just tripped out of the account signup and faceplanted straight into the boards for assistance with my reciever.

Now, normally i could have dealt with its minor problems such as burnt indicator lights, but things have just gone south.

See, when i originally got this, I had found it neglected and in the trash at a condo complex, it still worked pretty decently too! There was a missing knob and a single switch cover I salvaged off some 1960's sears budget clunker that had been completely totalled on some other curb i found a month or two later.

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Now, to get into the problems, when I found it; It's one major issue was that the audio on the right channel would not function easily (in stereo mode. In mono they both work fine), unless I jiggled the stereo/mono, and Tape monitor switches until it clicked into connectivity.

If that were still the problem, i wouldn't be here. Today, i had a rude awakening as i discovered my shop radio would no longer amplify the AUX circuit, as in I'd have to max out the volume knob to even get a reasonable volume, and even then it didn't sound right.

So, If any of you have suggestions I'd love to hear them.

- None of the caps look to have bulged, surprisingly, but they may have dried!

- There is this mysterious blue wire which was cut when i opened the reciever on the tuner/AUX/PHONO board, which makes no sense as every single other bridge wire on the boards is your typical non-insulated bars from A to B.

- I don't own an oscilloscope yet so, i can't really check much else as easily.
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General overview

So, for those who want a few more details about me:
I've surprisingly had four pioneer recievers over the years, starting with cheapos.

a Pioneer SX 3400 from the rubble of an abandoned house.

An SX-550 I bought for $20 at a pawn shop which has similar problems to the 3700 (given to a friend as a gift, and to fix)

A meh SX-303 reciever which was lackluster in every spec compared to my previous owns, found in the grass near a recently demolished abandoned house (along with a technics tape deck, I didn't bother keeping it, gave that away.)

And finally, this 3700 I've found being yeeted onto the curb on a rainy day. (luckily I got my hands on it JUST as rain started...)

I think as this is my most expensive reciever to date, and I hear it's worth at least maybe 1-200, perhaps more, I should make a concerted effort to at least try and repair this one myself.

I hadn't really been all that into audio just for the sake of being vintage, but with time bought some fitting AKG-K240 MKII headphones, and kinda grew to love the warmer sound of an old stereo compared to the sterile digital sounds. When my 550 would no longer work on one channel, I bought a cheap $25 home theater system from the thrift store as a temporary fix, Sony.

I hated everything about it, from the synthetic echo and filters over the music i could not turn off, to how complicated the settings were without appropriate buttons to easily work the equalizer. Lately the thing's just been giving me surge protection notices out of nowhere and automatically shutting off though i haven't moved it in about a year. Really not worth the effort.

I figure it's about time i restore this anyways, since for the past few years there has been an increased interest in creating synth/wave music once again, which in my opinion is one of the best things ever.

This 3700's been plugged into my computer, until it started to get a little wild with the speakers not doing their job in stereo, I moved it with the intent to repair it, but now I have no option, I can't wait any longer, think of all those tracks that are going unplayed!
 
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I recently inherited a SX-3700. It had been neglected for years. Full of dust and dirt. I took the cover off, used a soft paint brush and vacuum and cleaned it as best as I could. I also used Caig Deoxit on all the buttons and controls. I really worked these quite a bit. Once done, I went back and cleaned some more. Only then did I turn it on. Once I did, everything worked. I was amazed. I’m betting you may find your problems mostly dirt and corrosion. I wish you well.
 
I recently inherited a SX-3700. It had been neglected for years. Full of dust and dirt. I took the cover off, used a soft paint brush and vacuum and cleaned it as best as I could. I also used Caig Deoxit on all the buttons and controls. I really worked these quite a bit. Once done, I went back and cleaned some more. Only then did I turn it on. Once I did, everything worked. I was amazed. I’m betting you may find your problems mostly dirt and corrosion. I wish you well.
I've been told i may want to re-cap everything, which would be a pain, but i could also try it out. Mind you it was on the curb so, literally putting any cash into it would just increase its value, thankfully as a former mechanic i also have an air compressor, so this is going to be cleaned REAL GOOD with a little air pistol.

I also found a can of magic juice in someone's bin! still full, too.

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That should solve my corrosion problems at least. I'm wondering if using one of those WD-40 straws would be good enough to just spray into the switch and work it back and forth. Perhaps that might clean it without needing to take it apart fully.

My main issue is the suddenly blown amp.
 
My main issue is the suddenly blown amp.

How do you know the receiver is blown? I'll bet you have a dirty pot or switch issue somewhere so I would definitely clean them to rule them out.

You can use a WD-40 straw if it will fit. I'm always losing those damned straws! :D
 
The cut wires are jumpers across 82 ohm resistors (R319 & R320) right next to them, and is used to set idle current in the output transistors for each power amplifier channel.
Higher model numbers use trim pot resistors. If at the factory the idle current is too low the wire jumper is cut - allowing an additional 82 ohms of resistance added to the 1k ohm R317 & R318.

Your contact cleaner will NOT solve your corroded contact problem, cleaner will remove junk from the contact and leave behind CLEAN corroded contact material.

DEOXIT attacks the bond between the corrosion and the good metal, loosening the corrosion so that the repeated operation of the switch allows the contact friction to scrub off the corrosion.

ANYTHING that goes in n out, up n down or roundy round need to be deoxited - darn near ALL of them carry the 150 mV audio signals.
As an additional step - ALL of the potentiometers (volume, balance, tone etc) need to be LUBRICATED after DEOXITING - or the pots will wear out VERY quickly.

Most of your issues are corrosion and DEOXIT related.

Just as a note, the MONO/STEREO switch combines the audio (L+R function) right after the switches (which are giving you problems) and sends it to the volume, balance controls, then both tone control / power amps.
 
you uh, you guys aren't gonna believe this. I haven't plugged it in since i made the post, until now, was testing the headphone outs, (with my AKG's no less.), they work fine

tested the speaker outs

SUDDENLY THEY WORK AGAIN.

IN FACT SO DOES THE STEREO SWITCH. I have literally done nothing apart from take it apart and flip it all about, then put it back together when i got a little miffed that I couldn't find anything. Probably gonna recap it someday, my guess is that the issues may return sooner or later, they seem to be coming and going, but the amp thing, this was the first time. Def gonna unplug it and spray down the switches just for posterity sake, as well as take it out to the garage and air dust with the compressor. i doubt dust was causing some sort of short, but, you never know considering this was on the desk with a whole bunch of tiny screws from nerf blasters, and with the open top who knows, something could have fallen in.



Edit: after cleaning it seems the stereo switch has abso-freaking-lutely zero problems being flicked up and down like a cat with a spring toy.

there was fairly minimal dust inside too, I'm amazed how easy this was to fix... though a good recapping is due sooner or later, maybe if it starts acting fisher.
 
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Greetings All,

Performing my first post and vintage receiver restore/recap

Concerning the SX-3700 Output Transistors on the aluminum heatsink:

Does any one know what's up with the parts list/ Interior Components List for the sx-3700 Output Transistors?

Parts List shows that part 49 is a 2SC2525-G and part 50 is a 2SA1075-G.

On the Interior Components list it shows, from left to right 49 50 50 49, On the actual unit, from left to right the actual physical placement is 2SA1075 2SC2525 2SC2525 2SA1075?

I am using the hi engine manual and the Interior Components list is on page 38.

I have 2 of these receivers, one working and one with the two transistors pairs on the left (2SA1075 2SC2525) shorted. It bought a 2 sets of 2SA1216 2SC2922 and replacing all 4 and I want to make sure I place them in order.

Thanks in advance.
 
The differentiating factor is the C or the A in the transistor number.
2sc2525, 2sc2922 is all "C" - which means a NPN transistor
2sa1075, 2sa1216 is all "A" - which means a PNP transistor.

PNP and NPN are opposites of each other, and thus cannot be mixed.

Also the transistor mountings are "hot", as in electrically conductive and they MUST MUST MUST be electrically ISOLATED from the metal of the heat sink they are mounted on.
Write down on the heatsink the A or C status (or PNP / NPN) of each location.

when replacing, (NO endorsement!!) trade 2sa1075 with 2sa1216
and on the other polarity trade 2sc2525 with 2sc2922.
 
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