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G-9000 No Right Channel - Microphone ribbon cable?

Catfish Phil

Catfish Phil

New Member
New here and amateur at repairing electronics - started by dissecting radios and TV's as a kid in 1972 or so, even tapped the phono inputs of our family's GE phonograph to work as a guitar amp - Mom was mad at me!

This lovely, clean G-9000 was obtained from the original owner about ten years ago and have finally gotten serious about getting it functional and in regular use.

This forum is fantastic! I've searched and read every G-9000 (and similar model) thread I could find on the subject of restoring dropped channels and just cleaning and general maintenance as well. My girlfriend has an audio generator and oscilloscope on her deceased husband's workbench and I would be willing to learn how to use test equipment to get this repaired.

The unit is clean as a whistle - the problem started out before I got it with an existing scratchy volume control and other pots and switches. Disassembled and sprayed out everything when I first got the unit, and that helped, but it was not enough.

Second time I sprayed out everything, about two or three years ago, the ribbon cable from F-2805 Mic. Amp Circuit Board had become disconnected where it is soldered to F-2795 Mic. Amp. Circuit Board.

This is a photo of the damage to the pcb traces and a photo of the schematics.

08-Sansui G-9000.JPG

Mic Circuit photo of damage on flipped schematic G-9000.jpg



Would not having the Mic Jack Circuit Board properly connected to the Mic Amp Circuit Board cause the right channel to drop out?

Can I simply remove the ribbon cable from F-2795 and perhaps solder in a jumper so the amp card sees "no mic in mic jack?" (If this could possibly be causing the right channel to drop - I cannot figure this out for myself by reading the schematic).

Or, should I repair the traces and keep the mic feature functional? I am never going to plug a damned microphone into this unit. Perhaps Sansui should have utilized a ribbon jack/plug here to prevent this - I was careful as could be and the damage still happened. Saw a mention in another thread about this same unfortunate thing happening. Bending the ribbon stressed the edge of the board, widened the solder holes, etc. The copper traces leading up to the ribbon cable are broken away from the board .

I am convinced something in the preamp is causing loss of right channel signal going to the amp.

Every toggle switch, button, rotary switch and pot has been triple cleaned with DeOxit, worked and worked, and there are no wonky switches or cutting out of audio on the left channel, which sounds great. Before the latest cleaning last night, there was just a little cross talk in my headphones from the left channel to the right, and that has disappeared. The "connect" switch, power switch and headphone jack are all clean.

Audio through a mixing board was run directly through the amp to good speakers and all amp channels are great - A left and right, B left and right - the amp output is nice and clean, loud and lovely. The protection circuit and A/B relays are good - tapping on them changes nothing.

G-9000 radio audio was pulled from the G-9000 pre-amp outputs to a different, working amp/speakers - FM and AM function are great, but there's no audio from the right channel - good, scratch-free audio does come from the lleft channel. Connected a turntable - no right channel but again left is fine. So, I'm convinced it's clearly a preamp issue.

I've spent five days going through the unit, inspecting solder joints, cleaning plugs, examining pc boards for leaking capacitors or heat damage from resistors - not seeing anything out of the ordinary other than the damaged pcb.

Check out how the ribbon cable damaged the traces - on the right side of the F-2975 board. Simply resoldering is not going to fix it.

10-Sansui G-9000.JPG

Could it be the mic ribbon cable damage, or is this inconsequential and I need to continue looking elsewhere?

Thank you for your help!!

(ETA: clarity about the traces being broken away, relocating the photos)
 
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Did you clean the "Connect/Disconnect" switch on the side of the receiver? These cause a lot of problems. A thorough deoxit cleaning is where I would start.
 
You need to be careful with the G-9000. Many here consider it a gateway drug into the world of collecting Sansui Receivers. A lot of information in your post. Lets summarize it. One channel is not working and the ribbon cable for the mic amp is disconnected in some way. Is that right?

It's unlikely the mic amp issue is causing the right channel to drop. Most channel drop issues are in the pre amp signal path. As you may already know, every knob, toggle switch and push button on the front panel has a pot or a switch behind it. The signal can drop out at almost any one of them. What DeOxit product did you use to clean them? Caig Labs has done us all a disservice by tagging the DeOxit name on almost every product they make. Each has a specific purpose and may not be correct for cleaning the range of pots and switches on that receiver.

If you have access to a scope and audio generator, you have the tools to trace the signal through the pre amp. Sansui was very consistent about using blue and gray wires for the audio signal connections from board to board. Feed a 1KHz signal into the aux in and trace it from board to board.

As far as repairing the ribbon cable, just skin back the insulation and solder it back in.

- Pete
 
lbcgav said:
Did you clean the "Connect/Disconnect" switch on the side of the receiver? These cause a lot of problems. A thorough deoxit cleaning is where I would start.
Click to expand...
Yes, sir - everything has been cleaned at least three times and cycled like fifty times or more.



LBPete, here is a detail photo of the broken mic preamp board:

08-Sansui G-9000 broken pcb detail.jpg


DeOxit D5 was used during the last cleaning iteration - before that, Radio Shack contact cleaner/lubricant was used; the D5 is a much more effective product!
 
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LBPete said:
If you have access to a scope and audio generator, you have the tools to trace the signal through the pre amp. Sansui was very consistent about using blue and gray wires for the audio signal connections from board to board. Feed a 1KHz signal into the aux in and trace it from board to board.
Click to expand...

Most excellent LB :)
Let us know if you have a scope. Did not see where you mentioned cleaning the 2/4 channel switch?
 
As far as the broken mic pads, you will need to clean all the residue off the wires. I don't remember how much slack you have to work with on that cable however if there is enough you will have to carefully scrape off some of the green trace mask and directly solder the wires to the cleaned traces.
 
Overundr1 said:
As far as the broken mic pads, you will need to clean all the residue off the wires. I don't remember how much slack you have to work with on that cable however if there is enough you will have to carefully scrape off some of the green trace mask and directly solder the wires to the cleaned traces.
Click to expand...

Lee, thank you for your tips, as well. You jogged my memory - I've repaired something that way before, long ago, soldering the wire along the trace. Might try that, but if not having the mic board wired in is inconsequential to the operation of this rig, I just might leave it alone.

Noticed your ham call sign! AF5JS de AA5FJ tnx es 73 om
 
I had the same problem, I noticed hiss on both channels when the mixer was turned on. I found the ribbon cable had pulled off the tabs, similar to you. The purists here will cringe when I say this, but I stripped the insulation back enough to feed the conductor through to the traces and soldered them to the traces after scraping off some of the the green enamel coating with an Exacto blade. I have a punch list to repair the pads the next time I take it apart, but for now it works fine. PCB repair kits can be found online if you want to repair it the correct way.
 
Just skinning back the cable and the remaining pcb traces will work. If you want to get a little more professional, you can get adhesive backed copper foil that's made for repairing PCBs. I bought some locally that has small die cut "donuts" to replace missing pads. I've been looking for it on line and can't find it but any well stocked electronics shop will have it.

- Pete
 
LBPete said:
Just skinning back the cable and the remaining pcb traces will work. If you want to get a little more professional, you can get adhesive backed copper foil that's made for repairing PCBs. I bought some locally that has small die cut "donuts" to replace missing pads. I've been looking for it on line and can't find it but any well stocked electronics shop will have it.

- Pete
Click to expand...
Thanks, Pete.... I repaired the connections from the ribbon cable to the board. That ribbon cable has six leads, and only five are used, so I stripped off the sixth, so it's easier to move around the mic jack board and the Mic Amp (selector switch) board and hopefully this won't happen again. There is a similar problem with the two wider ribbon cables on the other side of the selector switch board - these two ribbon cables carry audio from the 4 ch NR adaptor board to the Mic Amp board, and one or two of these leads have damaged a trace on the Mic Amp Board. I repaired that but it was inconsequential to the No Right Channel Audio.

I've tried visually looking for broken traces on all the boards in the preamp section - that's how I found that one.

So, still no right channel audio from the preamp. Until my girlfriend returns from vacation, I won't be able to get at that oscilloscope and signal generator. I will have to learn how to set up and use the oscilloscope in order to trouble shoot on the component level.

Gotta love a challenge! Been repairing all sorts of stuff lately that I've been procrastinating over for years - gonna re-lube my old Dual 1237(early model - original owner) and working on a Craig H260 8 track recorder that won't kerchunk through one particular track.

Thanks to all who have commented so far! Just gotta chase down this problem. Again, the amp section is just fine - the preamp is okay on the left channel but will not pass a signal through the right channel.
 
Overundr1 said:
Most excellent LB :)
Let us know if you have a scope.
Click to expand...

Lee, I now have an oscilloscope, a B+K 2021B and probe with alligator clip, and am researching how to use it. Downloaded a Signal Generator, need an 8 Ω dummy load. Need to know what settings to use on the oscilloscope so I can trace the signal from inputs forward. We know the problem is in the preamp.
 
Okay, I can trace a L/R from the Aux inputs through the 2795 Mic Amp circuit board (selector/tape monitor rotary switch board), then onto the 2797 FM Accessory SW circuit board and through that to the 2799 Tone Control board.

What's weird at this point is, a Stereo signal will pass through 2797 to the input of 2799 - but when switched to Mono, no signal reaches the input of 2799.

I checked functions of the Stereo/Mono switch with a continuity meter and it seems to function as intended.

Either way, only Left channel audio is reaching the amp. Right channel still out.

Moving on to 2799 Tone Control Board next. Maybe that Mono signal enters 2799 somewhere else on the board than the side that faces the 2795 Mic Amp board.

Happy to know the selector and tape monitor switches are okay. Perhaps the amateur trouble shooter is flaky, but having more success using the continuity meter than the oscilloscope at this stage. The 10x probe is calibrated and grounded to chassis. The laptop Signal Generator is operating on battery power to prevent a ground loop, and the 1/8" to RCA stereo cable is good. Sometimes the scope's sine wave is nice and other times, not so nice, either at the RCA jacks or elsewhere. Maybe the oscilloscope is flaky, but it's my first time using one.

Either way, it's an engrossing task, and fun applying brain power to finding the problem. Hey, I just want it to work, and I intend to keep it forever.
 
Hmmm, will dig out my schematics. Usually the scope doesn't lie, not getting a good probe contact with the point under test when you lose the pattern or testing the wrong point is the cause.
 
With stereo turned OFF we will start walking through the circuit. Do not apply power until we get through these first readings---
After verifying scope is picking up the sine wave pattern 1kz 500mv input to aux from sig gen probe pin 1 left input and pin 2 right input on the F2797 fm switch board. Got signal? if so---
On F2797 board probe pin 3 lleft out and pin 4 right out, got signal, continue
On F2799 Tone board sub board F2800 probe left side and right side of volume control pot, got signal, great

Here is why I recommend unit being off :)
With probe on volume control output pins left then right rotate volume control from min to max, you should go from 0 signal on scope to full signal on scope as you turn the control. got signal, onwards we go
With volume set to max (power off) probe the input and then output side of balance control watching to see the pattern on both left and right channels. For fun, since you are learning what goes on with scopes turn the balance control back and forth showing signal drop with attenuation. Still got signal? report back and we continue.

Note to lurkers, be gentle with the OP, he is dipping his feet into scope use and we don't want to scare him off lol.
 
Lee, just read through your post - had to go to work, will hit it when I return. It all makes sense.

Am I clipping my probe ground to chassis ground? (unit off)
 
Yes. Without getting into the weeds here lets say that having a ground point provides a common reference point for continuity of readings. And its good bench technique too.
 
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