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Sansui 8080db hum in Dolby only

grindfix

grindfix

Full time hobby
Subscriber
Hello everyone.
I'm working on Sansui 8080db. Originally, it was (you've guessed it) stuck in protection mode. Resistor replacement on the driver board and all 4 outputs took care of that. Next problem found once out of protection was no Left channel. Sound was there just barely. Found it to be broken green PCB type flex ribbon between Dolby board and push button board. Replaced PCB ribbon with individual wires and got Left channel to work equally to the Right channel. Everything is good except any of the Dolby functions. No FM in Dolby mode.
Any position of the Dolby selector EXCEPT ''OFF'' would hum and pop in any input equally synchronous Left to Right channels on level meters. if volume level is raised it will go into protection. When Dolby selector is in ''OFF'' position there is no hum or protection issues. Sound is clear as it should be. The hum WAS there before PCB ribbon was replaced with wires. I'm not sure if Dolby FM issue was there or happened after ribbon replacement. All switches and pots were washed with Deoxit. Does it sound like DB issue or just one stubborn dirty set of contacts on the DB did not get enough TLC?
Any ideas?
Thank you,
 
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The Dolby board was one of the very first PCBs in consumer electronics to be double sided.
They have plated feed throughs that tend to fail.
Chances are you will need to reflow the solder on them or put a piece of wire in there to insure continuity.
BTW - nobody uses the Dolby function.
 
I don't think that any stations broadcast in Dolby FM. Could be wrong but wasn't Dolby FM used for a very brief time in the late 70's and then ditched?

Great job on gettin' her back up to snuff.
 
Dolby FM was a chicken/egg situation.

No station would broadcast in Dolby if the receivers didn't exist and I suspect that receiver manufacturers were eager to offer the next great thing to differentiate their products from the competition.
I know that Marantz and Sansui offered it, I don't recall if Pioneer (the big dawg) ever offered it though.
Ultimately it all imploded.
 
I've re-soldered (or attempted to) those plated feed throughs before i've discovered PCB ribbon issue. They all looked like broken cold solder joints with trace of the ring. For some reason all of those points boil/bubble when you take solder pencil off. What is the deal with that? I'm not planing to use DB but want to be sure the amp won't burn up because of that hum/pop source. Besides, wouldn't you want to own vintage piece fully functional?
Thanks,
 
There's really no use for the Dolby FM. Dolby encoded FM never caught on and was abandoned almost as soon as it was introduced. The only use for the Dolby functions on this receiver is if you are recording to a tape deck that doesn't have Dolby on board. Since virtually every tape deck manufactured after the late 1970s already has it built in, the only time you would need this feature is if you are recording to a tape deck older than the receiver. Other than that, turn the Dolby feature off and and enjoy the receiver. You are lucky you didn't disturb a pass through for the rest of the audio sections.

- Pete
 
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