SX-636 static only

[InstaBram]

Hi,

I just cleaned out and replaced the fuses on a SX-636 that had not been working.

Now the right channel works great (both A+B) but I get heavy static on the left channel (both A+B) as soon as I attach a speaker wire, even with nothing playing.

Can anyone give me some guidance on what could be the problem here?

Thank you,
Bram

 

[Watthour]

Welcome Aboard!

The SX-636 has some notoriously noisy devices in the audio amp section (AWK-035), namely the 2SA726s and 2SC1344s in posisitons Q1 through Q6. Both of those have a history of generating random noise and static with no signal applied. That might be a good place to start invenstigating.

 

[parman]

Welcome Aboard!

The SX-636 has some notoriously noisy devices in the audio amp section (AWK-035), namely the 2SA726s and 2SC1344s in posisitons Q1 through Q6. Both of those have a history of generating random noise and static with no signal applied. That might be a good place to start invenstigating.

WOW! I have a 828 doing exactly the same thing.

 

[Watthour]

And the SX-828 has some of the same historically trouble-making devices.

 

[markthefixer]

The 2sa725 and 2sa726 are annoyances, while the 2sc1451 is a definite HAZARD - as IT will overload SMOKE your output circuits.

 

[parman]

I had a friend take a look inside my 828 and he said he could see signs of someone soldering on the control circuit board (I think). He didn't know if they created the left channel buzz while working on something else or if they were trying to fix the buzz. It didn't do it when I first bought it 5 years ago but I hardly used it and then when I hooked it up recently it had the buzz

 

[markthefixer]

OP: you need to replace some transistors... A sx-636 with HEAVY (mind melting) static is a 2sc1451 working itself into a full furious meltdown.

parman with sx-828 problem, you need to start your own thread as your problem is completely different from where this thread is going.

 

[dly66]

Picked up an SX-636 from a fellow AKer put it on CL cheap after he tested it and got crackling. He put it on CL for cheap as a fix and repair. Never heard that part myself, just took their word. Opened it up, saw the 2SA726 differential pairs looked like one transistor had been replaced with what I gather was an NTE replacement, and the 2SC1451s in each amp channel. Replaced those 6 transistors (with KSA992 and KSC3503), put a new power cord on it since unit was bought cordless untested. Brought it up for the first time for me, checked the DC offset was below 20 mV in each channel, checked the bias, unit sounded good through headphones, then with speakers sounded great. Apparently extracted off a scrap pile behind a Goodwill, had issues got its cord cut and tossed. With the new transistors it works well and the case was in good shape.

I like the old blue faced Pioneers.

The 2SA726 and 2SC1451 are known troublesome transistors good chance they could be your amp problem. Pioneers of this vintage and the generations before usually have some other transistors not known for aging well.

Picked up a fix and repair Sansui 771 badly crackling - pretty much drowned out the tuner and sometimes tripped the protection circuit when I demoed it. Checked with DVM when I got home and the DC measurements on both channels were going all over the place. This one had the 2SA726s in the differential pairs replaced them and the transistors on the tone board. I had the tone board loose to clean the controls. Could then adjust the now stable DC and bias, been enjoying the Sansui for the last week or so. After the transistor replacement it works well too.

 

[InstaBram]

Welcome Aboard!

The SX-636 has some notoriously noisy devices in the audio amp section (AWK-035), namely the 2SA726s and 2SC1344s in posisitons Q1 through Q6. Both of those have a history of generating random noise and static with no signal applied. That might be a good place to start invenstigating.

Thank you for the quick diagnosis. Though I have to admit I am so novice that I don't even know how to test these transistors. Is there a basic tutorial or other reading you can recommend?
Also, I hooked this receiver up to a different system today and I thought I would give the left channel another try while playing music from an aux source and when I went to connect the speaker wires I actually got sparks at the connection. Is that common? Or perhaps an indication of a different problem?

 

[dly66]

You are not connected the speakers with the unit on, are you? At the very least speaker selectors off so the speakers outputs are disconnected from the amp circuit, but it is much more conservative to power down the receiver. The amp is ready to deliver power out so any short across the output of a powered amp circuit can quickly blow something in the circuit from the current rush. You should not see sparks.