Elusive noise on left channel

Im having trouble chasing down this noise Im getting on the left channel but only on Tape 1? Phono,Tuner,Tape 2 are all OK its just this damned Tape 1 thats doing it? The sound is an intermittent crackling sound like a bad transistor or a broken solder joint,Ive DeOxited all the pots and changed the switch on Tape 1 just in case it was faulty. Could it be coming from LC7815H? Its a 2 pole 4 position Analog Function Switch thats obsolete now.The amplifier is a Sansui A-909.
 

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Mooly

Administrator
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2007-09-15 8:14 am
There doesn't seem much else tbh. You should as a definitive test perhaps desolder tape 1 L and R inputs at the chip and then carefully crosslink them so that L feeds into the R chip input. That would rule out anything weird like conductive spillage or leakage. The problem should remain on the same left channel if he chip is faulty. Might also be worth isolating that 4066 in case its backfeeding anything into the audio path.
 
There doesn't seem much else tbh. You should as a definitive test perhaps desolder tape 1 L and R inputs at the chip and then carefully crosslink them so that L feeds into the R chip input. That would rule out anything weird like conductive spillage or leakage. The problem should remain on the same left channel if he chip is faulty. Might also be worth isolating that 4066 in case its backfeeding anything into the audio path.

Ah yes! Good one Mooly.Why didnt I think of that? Thanks mate, Ill get right onto it.
 
I disabled the VDD and VSS power by de-soldering two links to LC4066, it got rid of that noise,the volume was really low but only on Tape one and if I changed to say Phono I get a big thump in the speakers. It looks like these LC4066 chips are still available. The tape monitors not needed but muting that thump certainly is. I found some badly wave soldered ribbon connections so I might pop those links back in and see if it was that causing the noise?
 
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The 4066 is a common CMOS chip available anywhere. It doesn't have to say LC on it. The 4016 is similar apart from having a slightly higher 'on' resistance.

The 4066 chip just arrived in the mail I installed it but the noise is still there? Ive replaced the momentary switch on that function as well so the only thing left is the LC7815 chip. It seems strange that it would only fail on one function but the tuner and phono go through it OK? Ive just remembered I desoldered a 6 pin ribbon lead that was for a retro remote on Phono,Tuner and Tape 1. Ive marked it on the schematic. It hasnt affected the Tuner or Phono though?:crazy: lol
 

Mooly

Administrator
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2007-09-15 8:14 am
Its probably going to be the LC chip. Replacing or isolating the 4066 was just to prove (or disprove) that it wasn't influencing anything.

Might also be worth isolating that 4066 in case its backfeeding anything into the audio path.

If you have done other work (the ribbon) then it might be worth putting all back to original condition to see if that proves anything.
 
I found a new LC7515H chip in my unsorted junk pile so I installed that and BINGO! No more noise on the Tape 1 channel! I mustve ordered it for something else years ago,I usually order two if I just need one just in case? Anyway thats Murphys law in action,it just had to be the last bloody thing didnt it.Thanks for the ideas and suggestions mate,it really helps when someone else takes an interest in the problem and can point things out that I have no experience off.:D
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
The old TL0 series opamps fell from fashion many years ago and today are often thought of as inferior but I wonder how many that say that have actually used them properly and listened to them. I still rate then very highly indeed for subjective sound quality.