SX-750 Left Channel Missing thru Speaker but Present in Headphones

[buckEye6]

Hello from a brand new member!

I have an SX-750 which I purchased brand new in 1978. It hasn't been used in several years but has always been stored indoors in my home office. I recently fired it up after deoxing the pots and tuning capacitor. It works quite well with the exception that there is no output in the left speaker even though the left headphone channel works fine.

This condition exists for both the A and B speaker channels as well as the AM, FM and AUX inputs. I currently have no suitable PHONO device. All of the capacitors look to be in great shape with no swelling or leakage noted.

I have a copy of the service manual and a rudimentary knowledge of circuits, devices and schematics. My only diagnostic equipment is a hand-held digital multimeter.

I'm looking for advice on how to proceed in correcting this condition.

TIA

 

[larryderouin]

Did you DeOxit the SPEAKER SELECTOR Switch?????? If not do so 1st. This may solve the problem. If not some troubleshooting is in order.

The headphones are connected between the RELAY output and the SPEAKER SELECTOR Switch (See picture of the pertinent area) BEFORE THE SPEAKER SWITCH. If you have NO LEFT Channel on ANY SETTING OF SPEAKERS, then look for a broken connection of the LEFT OUTPUT wire at the Relay (presuming the HEADPHONES are wired separately), at the speaker selector switch(Pin 7 of Section "D") or at the take off for the Headphones If on a terminal strip or wired junction). S-10-D is the SPEAKER SWITCH(S-10), "D" is the 2nd wafer (from front) Back side. Specifically you'll be looking for a broken wire from pin #7 (INPUT from Relay). Pin 8 is output to "A" spkr, while Pin 10 is OUTPUT to "B" SPKR. The other thing it cold be is a lifted wiper on the "D" side. Set to "A" spkr and check continuity from pin 7 to pin 8. Then switch to "B" spkr and check btwn 7 and 10. If good continuity between 1.) pin7 to 8 & 10, and 2.) pin 8 and the "A" spkr "HOT(RED) or Pin 10 to "B" spkr Hot (RED), the problem is between the switch and the RELAY. Check continuity between pin 7 and the RELAY. Do the same for the Headphone jack to the Relay.

 

[merlynski]

One thing to check before digging into the amp is making sure both speakers work: swap the speaker connections on the back of the amp. If the problem switches to the other speaker the amp has the problem, if the 'dead' speaker stays silent it is the speaker or the speaker connecting wires.

AND Welcome to Audiokarma!

 

[buckEye6]

Did you DeOxit the SPEAKER SELECTOR Switch?????? If not do so 1st. This may solve the problem. If not some troubleshooting is in order.

The headphones are connected between the RELAY output and the SPEAKER SELECTOR Switch (See picture of the pertinent area) BEFORE THE SPEAKER SWITCH. If you have NO LEFT Channel on ANY SETTING OF SPEAKERS, then look for a broken connection of the LEFT OUTPUT wire at the Relay (presuming the HEADPHONES are wired separately), at the speaker selector switch(Pin 7 of Section "D") or at the take off for the Headphones If on a terminal strip or wired junction). S-10-D is the SPEAKER SWITCH(S-10), "D" is the 2nd wafer (from front) Back side. Specifically you'll be looking for a broken wire from pin #7 (INPUT from Relay). Pin 8 is output to "A" spkr, while Pin 10 is OUTPUT to "B" SPKR. The other thing it cold be is a lifted wiper on the "D" side. Set to "A" spkr and check continuity from pin 7 to pin 8. Then switch to "B" spkr and check btwn 7 and 10. If good continuity between 1.) pin7 to 8 & 10, and 2.) pin 8 and the "A" spkr "HOT(RED) or Pin 10 to "B" spkr Hot (RED), the problem is between the switch and the RELAY. Check continuity between pin 7 and the RELAY. Do the same for the Headphone jack to the Relay.

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wow! thanks you for such a speedy and detailed answer. this is exactly the advice i was looking for. looks like we have the same schematics, so that helps. i did deox the speaker switch but maybe another shot is in order. i'll post back after i've completed what you suggest.

thanks again...

 

[buckEye6]

One thing to check before digging into the amp is making sure both speakers work: swap the speaker connections on the back of the amp. If the problem switches to the other speaker the amp has the problem, if the 'dead' speaker stays silent it is the speaker or the speaker connecting wires.

AND Welcome to Audiokarma!

that's most certainly true. both speakers do indeed work. i borrowed them from my home theater system this morning for this very reason. this is the 'is it plugged in' scenario.

 

[Watthour]

Another item to check is the speaker relay. Since there is audio on the right channel through speakers, we can presume the relay is actuated. However, if the relay contacts for the left channel are deteriorated, they may have sufficient contact to operate headphones at 150Ω impedance but might not carry enough current to operate a speaker at 8Ω. Since the realy is relatively accessible with the unit cover removed, gently tap on the clear relay cover with the handle of a small screwdriver while the unit is powred on. If this produces any signal through the left channel speaker, that could indicate a failing relay contact.

You could also carefully jumper Pins 3 & 4 on the power supply/protection board (AWR-099) and power up the unit with speakers connected to check for audio in the left channel.. There is slightly less risk tapping on the relay cover, but it may not be as definitive.

 

[Watthour]

Also, verify that the speakers you "borrowed" from your HT system are at least 6Ω rated, otherwise you risk damage to the 750.

 

[EastPoint]

Try turning the volume up a bunch and see if that does anything. If the relay is defective usually it will work at louder volumes.

 

[buckEye6]

Hello from a brand new member!

I have an SX-750 which I purchased brand new in 1978. It hasn't been used in several years but has always been stored indoors in my home office. I recently fired it up after deoxing the pots and tuning capacitor. It works quite well with the exception that there is no output in the left speaker even though the left headphone channel works fine.

This condition exists for both the A and B speaker channels as well as the AM, FM and AUX inputs. I currently have no suitable PHONO device. All of the capacitors look to be in great shape with no swelling or leakage noted.

I have a copy of the service manual and a rudimentary knowledge of circuits, devices and schematics. My only diagnostic equipment is a hand-held digital multimeter.

I'm looking for advice on how to proceed in correcting this condition.

TIA

SUCCESS! many thanks to everyone who replied. i followed larryderouin's advice and deox'd the speaker selection switch once again paying close attention to the areas he identified. it's now working like a champ! i'm currently playing my 12-year-old ipod through the aux input and it sounds... wonderful! i'd forgotten just how good this unit is. sometimes the solution is just that simple!

@Watthour: the speakers i'm using right now are two JBL 150SISAT with an impedance of 8Ω.

i look forward to hanging out in this area and hope to contribute going forward...https://audiokarma.org/forums/index.php?members/larryderouin.42316/

 

[larryderouin]

HOT DAMN! The smoke leaking from my ears actually produced results. Fiancee almost called the Firehouse for a response..