PCB files contributed for redesigned Sansui 9090DB F-2655 Dolby board
- Thread starter Biroc
- Start date
Tuner module recapped, tuner PS rebuilt, installed and working a treat.
Ran through @LBPete's terrific performance check for the Dolby board
Everything green on my screen 
I honestly don't think I've ever heard FM so absolutely clear, musical, not fatiguing with the Dolby switch to DOLBY FM!! I'll definitely be listening to FM via Dolby going forward. The difference is apples and oranges. It is really worth the effort to get Dolby sounding how it should on this receiver.
This was the final steps in the electronics restoration, done to resistor level on all circuits. My case got smashed in transit so it's time for some woodworking including new real walnut veneer.
Ran through @LBPete's terrific performance check for the Dolby board
Dolby Circuit Performance Check
There has been a lot of discussions about the Achilles heal of the 9090DB/8080DB, the Dolby circuit. It can cause a variety of channel dropping problems and can be hard to fix. In other cases, you may loose some of the Dolby circuit functionality and not know it. Here’s a simple check to see...
audiokarma.org
I honestly don't think I've ever heard FM so absolutely clear, musical, not fatiguing with the Dolby switch to DOLBY FM!! I'll definitely be listening to FM via Dolby going forward. The difference is apples and oranges. It is really worth the effort to get Dolby sounding how it should on this receiver.
This was the final steps in the electronics restoration, done to resistor level on all circuits. My case got smashed in transit so it's time for some woodworking including new real walnut veneer.
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Nice work on a classic receiver. I appreciate the tip of the hat to my old post.
The Dolby circuit will make FM sound "bright" but there is no Dolby FM broadcast. That technology was stillborn, it never got off the ground. For Dolby to reduce noise, the media, in this case the FM broadcast, has to have the Dolby noise reduction encoded into the broadcast signal. The Dolby board is designed to decode that encoded signal.
From Wikipedia:
In the early 1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby B "pass-through" mode. In 1971 WFMT started to transmit programs with Dolby NR,[11] and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.[12] Dolby FM was based on Dolby B,[13] but used a modified 25 μs pre-emphasis time constant[14] and a frequency-selective companding arrangement to reduce noise.
What you are hearing is the 25 μs pre-emphasis. That said, if you like it, no reason not to enjoy it.
- Pete
The Dolby circuit will make FM sound "bright" but there is no Dolby FM broadcast. That technology was stillborn, it never got off the ground. For Dolby to reduce noise, the media, in this case the FM broadcast, has to have the Dolby noise reduction encoded into the broadcast signal. The Dolby board is designed to decode that encoded signal.
From Wikipedia:
In the early 1970s, some expected Dolby NR to become normal in FM radio broadcasts and some tuners and amplifiers were manufactured with decoding circuitry; there were also some tape recorders with a Dolby B "pass-through" mode. In 1971 WFMT started to transmit programs with Dolby NR,[11] and soon some 17 stations broadcast with noise reduction, but by 1974 it was already on the decline.[12] Dolby FM was based on Dolby B,[13] but used a modified 25 μs pre-emphasis time constant[14] and a frequency-selective companding arrangement to reduce noise.
What you are hearing is the 25 μs pre-emphasis. That said, if you like it, no reason not to enjoy it.
- Pete
Thanks for that Pete. I was not aware of the 25 μs pre-emphasis but that certainly makes sense. Also the inclusion of metal film resistors and a few stacked film caps adds to the brightness. I will eventually change out the films for Muse but for now, yep, really happy with the sound
Scott Muni
Well-Known Member
Isn't there a switch on the tuner board for pre-emphasis? Has something to do with european FM broadcast. In addition to the FM Dolby circuit,
there is 75μs or 50μs depending on your region. Australia is 50μs. I understand the Dolby board adds an additional 25μs but Pete can confirm.
indeed it is. Well worth the effort. Brand new F-2624 and F-2655 PCB's are a superb mod. I've yet to try out @Hipocrates F-2619 RF PS but will do at some point.
lowkey next should be the QRX 9001 quad board LOL
PCB files contributed for redesigned Sansui 9090DB F-2655 Dolby board
Transistors: Pulled TR601 (2SC945) from stock board and measured hFE of 258. Went with KSC1815 as replacement, matching hFE of TR01/02 as their collectors are connected. TR01 BCE=64,37.5, 62.5,40, 64,42 2SC945 (Q,P) KSC1815 179hFE TR02 ECB=56,31.5, 59,32, 61.8,31 2SC945 (Q,P)...
audiokarma.org
Nice. I used something similar on the EQ/Phono board to replace the green cables in an 8080DB and they did the trick!
JosieDog2
Active Member
Biroc-
I recently purchased one of these new F-2655 replacement boards from eBay vendor Ben. He had listed a BoM parts list for Mouser. Is there a designator list corresponding to the BoM list? Looking forward hoping this new board will solve my 8080db issues. The service manual lists very few parts on this board. Any help is appreciated!!
JosieDog2
Active Member
Ok, thank you.
Here you go, a little difficult to locate. We should probably rename the file to F-2655.zip as it's flagged as 9090DB Dolby ...