These comments have been due to the AK link on the OP's first post which shows how to bypass the Dolby board on a Sansui 9090db receiver. That thread shows the tape functions have been eliminated with the mod implementation. So...to aid us all, would you be able to share with us how you accomplished this mod while keeping the tape functionality?
Then the mods done on yours are different than all of the other Dolby board bypass mod in the OP, and the others that have been proffered up here over the years. It would require manually rewiring all of the tape ins/outs in order to bypass that board and retain their function. I'd love to know more.
He talks about pinning the vias (pass throughs) in that post, which is the standard fix and has been for as long as I've been doing this.
The task of manual re-wiring of the tape ins/outs is the reason most settle on re-pinning the board. Which is why I'm interested in seeing what @z-adamson has to share with us. It's possible it could forever alter the way we repair the Dolby board.
Whatever floats your boat man. I really do not care.
Unfortunately the fellow who did the work is no longer with us. I wasn't being facetious when I said I would love to know more. Based on the one post we have, I would guess that what was done to z's unit was the standard pin and solder, along with maybe a permanent disabling of the Dolby function itself. But that's only a guess.
From the QRX restore site....
I explained earlier in this thread...
Basic stuff for some is brain surgery for others. My first foray into repairing the 9090db Dolby board years ago ended unsuccessfully, as the problem reared it's ugly head a few months after the re-pin. The only thing stopping me from bypassing the board was the fact that it would disable the tape functions, which I use.
Does this make tape function not work though?Hi all, I looked at this issue recently as I have an 8080DB (although that works fine), and it was a bit of a 'puzzle challenge'
I looked at the schematics, the F-2655 board already has a bypass function, position 3 on the dolby selector knob.
If that doesn't perfectly bypass it, then there is a fault with either the selector switch (the signal goes through it 3 times) or the ribbon wires have cracked, or the tape monitor 1 or 2 are faulty. Or a combination.
As for the actual bypass, if for some reason cleaning the switch or replacing the ribbon is not the favoured approach - it can be fairly easily bypassed from the parent F-2654 board, without touching the F-2655 board, that can stay in there safely preserved.
The pin numbering of the ribbon is amusingly 'non-sequential', and the service manual has some errors, which may make the task of computing a bypass seem tricky, but once the correct S01 switch elements are identified, along with the correct ribbon pins, the task is straightforward.
- Simply remove all the ribbons, then on the F-2654 use 4 jumper wires in place of the ribbon (to emulate the dolby 'off' position), and then unsolder the f/g blue and grey signal input wires from the F-2655 daughter board, and solder them instead straight to the F-2654 board.
That's it.
Doing this, (in theory, I've not done this to mine as it works), gives the behaviour of 'dolby Off', without any signal touching the F-2655 board.
I post the image I made as I worked it out for anyone who is interested - I don't think bypassing is the best fix, but at least my method preserves the F-2655 board for the future, and allows the tape loops to work as in the original) so it's Ok I guess
If anyone does this bypass, please post up how it works for them
The tape monitors etc. work as the receiver did before in 'Dolby off' mode.
No, the F-2654 PCB has no ground plane, and all the ribbon wires were just hanging in the air, so I would just use some convenient wire to hand, perhaps a different colour for each channel (as per red and blue above).
This is awesome!!! Thank you.No, the F-2654 PCB has no ground plane, and all the ribbon wires were just hanging in the air, so I would just use some convenient wire to hand, perhaps a different colour for each channel (as per red and blue above).
The grey and blue fly leads (incoming signal from the main selector switch) that need to be removed from the F-2655 dolby board and soldered to the pin 1 and 2 on the F-2654 switch board will need their ground connecting to 705 as shown in the diagram.
That ground meanders all over the place in the ribbon and F-2655 so just make it neat, don't worry about the shielding, Sansiu didn't and all was fine
My bypass is actually very straighforward - relocate the input signal (grey and blue shielded wires from the main selector) to pins 1 and 2 (which is where 'dolby off' sends them), one pair of jumpers bypasses one bit of the dolby board (emulating one of the dolby-off switch - the 2nd pair of jumpers does the same for a different part of the dolby off switch, and then the signal wriggles through the mono and loudness buttons and out to the level and balance as before.
The reason people didn't solve it before is simple: The diagram and numbering was very confusing, as is the switch designations, all switches are S01, but each used toggle is lettered a,b,c etc. Once identified, the bypass is obvious - but without that first step, it's very complex
