SX-750 - Left Channel Buzz

Similar story to many of you, I came across a vintage Pioneer turntable and decided to partner it with a similar vintage receiver. I managed to find a very clean SX-750 on CL and while demo-ing with headphones I noticed some minor noise in the switches and pots but otherwise sounded fine. I deoxited and hooked up some speakers to test and I no longer have switch noise but it does have a buzz in the left channel. Note, however, that the buzz does not exist when listening through headphones. Do any of you masters have a suggestion for further investigation or can you recommend a good service tech in the Houston area?

Thank you..

Does the buzz occur in all modes of operation, or only in the PHONO mode? If only in PHONO, there is a chance that the input RCA connectors may not be properly grounded to the unit chassis. It's also possible that one of your interconnect cables has a broken shield connection. If the PHONO input leads are exchanged (right for left) does the buzz move with the cables or remain in the left channel? If both PHONO input leads are removed, does the noise still occur?

The 750 used two different types of phono amplifier stages, and if the noise is in the phono section only, a bit more information would be needed before the analysis could continue.

Watthour said:

Does the buzz occur in all modes of operation, or only in the PHONO mode? If only in PHONO, there is a chance that the input RCA connectors may not be properly grounded to the unit chassis. It's also possible that one of your interconnect cables has a broken shield connection. If the PHONO input leads are exchanged (right for left) does the buzz move with the cables or remain in the left channel? If both PHONO input leads are removed, does the noise still occur?

The 750 used two different types of phono amplifier stages, and if the noise is in the phono section only, a bit more information would be needed before the analysis could continue.

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The buzz occurs in all modes of operation. I removed all cables and tried each input separately and the buzz occurs consistently for all inputs. As mentioned the buzz does not come through the headphone output which told me that it was not a cable or rca connector issue. Tried plugging speakers in on A and B speaker leads and buzz/scratchiness is output through left channel consistently. It is not a constant buzz but rather a scratchy/static in combination with the music. Thanks for the feedback.

make sure you are not plugged into an outlet that has a dimmer control on it.

elvi said:

make sure you are not plugged into an outlet that has a dimmer control on it.

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Definitely not... Nice avatar!

If the noise occurs in all modes, and you haven't already done so, download the service manual and schematics for the unit. Even if you do not perform the work yourself, reading through the manual and studying the diagrams should provide a good clue as to what might be involved with the analysis and repairs.

Does the noise occur with the volume at minimum? Does the noise occur if the TAPE MON is turned on?

If the noise does not occur when volume is at minimum, there may be a problem with the protection relay speaker contacts. That can be verified by temporarily and CAREFULLY jumpering the left channel contacts at the relay while the unit is making the static noise to see if the noise is eliminated with the jumper. If this is beyond your comfort level, since sticking a jumper onto a live circuit board does pose some risks, it may be best to seek service.

A second treatment of DeOxit is also sometimes needed to fully clean and flush deposits from very contaminated controls and switches.

Since the SX-750 has some suspect devices in the audio path, but only in the MIC amplifier and in one version of the PHONO amplifiers (AWK-065 boards). These should not affect any other modes of operation unless they are dragging down a power supply.

There are many other possibilities, but eliminating those might make the analysis simpler.

Those are confusing symptoms. The schematic shows that the headphones and speakers get the exact same signal. The audio goes from the output transistors to the relay then out to both the headphones and speaker select switches. A relay problem, or any upstream problems in any boards or any switches should be audible in headphones as well as speakers. Looks like the speakers and headphones have a different grounding path, but the A and B speakers each have a different ground as well. If it were a ground, I'd expect it to occur on only A or B but not both. A dirty contact or loose connection at pin 7 on the rotary switch at the bottom of the attached schematic would affect the left channel on both A and B, and would not impact headphone operation.

two.dogs said:

Those are confusing symptoms. The schematic shows that the headphones and speakers get the exact same signal. The audio goes from the output transistors to the relay then out to both the headphones and speaker select switches. A relay problem, or any upstream problems in any boards or any switches should be audible in headphones as well as speakers. Looks like the speakers and headphones have a different grounding path, but the A and B speakers each have a different ground as well. If it were a ground, I'd expect it to occur on only A or B but not both. A dirty contact or loose connection at pin 7 on the rotary switch at the bottom of the attached schematic would affect the left channel on both A and B, and would not impact headphone operation.

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First, thanks to all for your insights. To add, there is no noise when volume is at zero and the noise only occurs with the music. It occurs for all inputs including tape. As mentioned there is no noise through headphones and noise occurs for both A and B speakers. I have downloaded the service manual and I did get a look at the rotary switch while dexoit-ing. I did the initial jiggle on this as I first thought the issue may lie here.. that made no diff. I will dig into that and take a closer look as it seems that this is a weak point in this particular model. Thanks again..

You probably, already swapped the speakers from left to right?

zebulon1 said:

You probably, already swapped the speakers from left to right?

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Sure did.. Thanks..

King of the obvious here, but intermittent scratchy static is classic sign of leaky transistor. But if that were the case it should be heard on A, B, and headphones. Are you listening to headphones and speakers at about the same volume control setting?

two.dogs said:

King of the obvious here, but intermittent scratchy static is classic sign of leaky transistor. But if that were the case it should be heard on A, B, and headphones. Are you listening to headphones and speakers at about the same volume control setting?

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Yes, the volume level was about the same.

DOG+DOG - Think of load. The current flowing to headphones through the dropping resistors is minimal compared to that going to speakers straight from the amp. If the speaker relay contacts had oxidation, poor spring tension, etc., the minimal current to headphones would have a lot less significant effect upon the load through the contacts and thus their tendency to increase in resistance and break signal. A size 5 starter with really bad contacts will run a 1HP motor all day long with no problems, but may go completely open trying to run a 200HP motor.

Again, jumpering pins 1 & 2 on the power supply/protection board will test the theory very quickly.

Watthour. Makes sense, thanks for the explanation. If OP is nervous about jumpering, would listening through headphones while also powering the speakers be something to try? It looks like plugging in headphones doesn't disable the speaker select switch.

Good thought. If the speaker is loading down a poor connection somewhere, it should be evident in the phones while it is happening. If not, it turns into a real head-scratcher.

I can understand if there is hesitation of the OP reading the suggestion of some buffoon on the interwebs (like me) telling him to stick wire into his live receiver, but the two terminals are right there, staring us in the face at the top rear edge of the card, just asking to be shorted by an alligator clip:

two.dogs said:

Watthour. Makes sense, thanks for the explanation. If OP is nervous about jumpering, would listening through headphones while also powering the speakers be something to try? It looks like plugging in headphones doesn't disable the speaker select switch.

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Before i go bridging the pins i thought i would give a try with speakers on and headphones on. Results - No static in headphones but static in left channel. Switched speakers - results still static in left channel. This occurs if speakers are plugged in as A or B.

Hanger18 said:

Before i go bridging the pins i thought i would give a try with speakers on and headphones on. Results - No static in headphones but static in left channel. Switched speakers - results still static in left channel. This occurs if speakers are plugged in as A or B.

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As mentioned in a previous post i deoxed the switches and knobs but I could not figure out how to get the top off of the relay and I was a bit nervous about prying on it. Can you enlighten me to this process? Also, should i be concerned about the brown "substance" that is under the capacitors shown in the red box below?

Watthour - to confirm - you are suggesting I bridge pin 1 and 2 together and test the output (see yellow box below)?

The substance is pioneer glue. No worries.
Pins 1 and 2, correct.
"I wanted to answer the post so we can be enlightened."

Good idea to short the pins to see if the relay is creating problems, but your headphone/speaker test makes me think the relay is working. You should have heard the same problem on both headphones and speakers. I looked at the bottom of my SX750 and took a few pics.

On the first pic, the blue wire is the left audio coming from the relay where it connects to the speaker select switch. You can see it is soldered directly to the thin yellow wire that connects to the headphone jack. The second pic is the underside of the speaker terminals. The washed out yellow wire is left speaker A, the white wire is left speaker B. The teminal I'm pointing to shows that there is a ground jumper that connects both left speakers A and B. This ground wire is only for the left channels of A and B (though the pic makes it look like the same ground might serve the right channel that isnt the case). The last pic shows where this ground wire connects to the chassis. Maybe take a nonconductive instrument and put pressure on these ground wires and terminals while the receiver is operating?

Hello,
i have some similar problem ( http://www.audiokarma.org/forums/showthread.php?t=636016)
I show you what i can found on my unit, perhaps it can help you in some way...
Look at the picture below and read the note i took, and see if there is some different wire in some place.
Notice that the caps on awr099b are original but without glue.
I deoxide whole relays but no effect, noise still remains, BUT noises disappear suddenly i don't know why a few days ago. I'm still investigating on it. Now the unit sounds good but the wrong voltages remains.
Take a look even at the heatsink of q8 and q10 if they seems too hot.

Hope this will help.