B750 Almost there after ceveral years

[nijunge]

Hi all

So alot has happend since last, But for those who dont want to use a hour to find prvious posts here is a recap.

I bought a B750 MKII that turned out to be in worse state that expected.

It has been overhauled by a proffesional including new upgrade power supply exchange of faulty transistors etc.

Problem remaning that I would love some inputs on is:

1. the Tone defeat and the level -20dB both have a bad connection issue, it simply seems that they need a good clean. is that possible ? (they loose connection in one channal and if i kind manipulate them a bid it will return.

2. Phono 1 is playing mono meening if input only in left both channels will play (right one the most) if only input in right both will play (right one the most) so same result.

I have added som photos as there is a thing I dont understand with the work arround my technician has made for the Phono 1, he simply connected what I call pin 1 to 4 and 9 to 12

I was of the impression that the AUX board schould work for both Phono 1 and 2 but that seems wrong ?

I thought it was a problem with the input board and found a new one online but that did not change anything!

I hope some can inlighten me Borchee or Ernst56 maybee ;-)

20230503_140508.jpg20230503_140611.jpg

[Epper8]

das gelbe Kabel ist vermutlich falsch angeschlossen.

[nijunge]

So you think that the two cables included in the Yellow cable schould go the othere way round ? but how schould that only have an effect on the Phono 1 input not everything else (Tuner, Phono 2 etc) ?

I could just try to reverse them ?

If you have a MKII your self could you please take a picture of yours ;-)

Thanks

[borchee]

Hi Niels,

1. The Schadow switches can be mended, although it's not guaranteed they'll work properly after.

As you can't squirt contact cleaner inside, you must dismantle the switch partially. I suggest you do it in a transparent plastic bag - there are some tiny springs and if they become airborne, they're not easy to find.

Proceed the easy way as follows:
a) Pull the spring so that you uncover the metal "wire" (it locks the switch) and pull it out with tweezers or simmilar.
b) The switch lid has a narrow notch which prevents the white middle part to popp out due to the spring. Gently pry it open with a needle, or a small flat screwdriver, until the middle part popps out. Be carefull not to break it.
c) Clean the sliders and squirt contact cleaner into the switch's body.
d) Insert the middle part with clean sliders and work the switch multiple times.
e) Remove middle part again, flush inside with contact cleaner and wipe out with cotton swab.
f) Check for contact. If not OK, you can try the "harder" way:

...basically the same, just push the contact legs into the body, so that you can remove the lid, take them out and clean thorughly.

Pushbutton switch Schadow switch in parts_image by @borchee_01.jpg

2. Epper8 is right, although I have no idea if this is the root of the problem. The AUX PCB works only with PHONO 2. Why PHONO 1 slots are being bridged, instead of using the phono card, is a mystery to me - I am not a technician.

Good luck.
Borut

Edit: Here you go...

IMG_20230503_213445.jpg

[nijunge]

Thanks Ebber8 and borchee

I have reversed the cables
The Phono 1 problems changed a bit now its even Mono so to speak.

Button
borchee I think I need to do the Harder way.

Bridging Phono 1
The bridging is done as I need the Phono 1 also to work as a AUX as I use a MC phonostage for that as well as AUX 2 (two arm record player).
To verify that the bridging is done right I need to find my self a data sheet for the phono 1.


Thanks both.

[Broesel]

Manual:

[nijunge]

Unfortunatly I do not find any pin explenaytions in these files

[Taube_Nuss]

Hello Niels,

you probably find what you need here: https://www.revox.name/download/manuals/
I do have a service-manual for the B750 Mk I+II, but the File of ~25MB is too big to be attached.
Let us know if you had success, There has to be a way to bring that manual to you.

Don't give up!!

Bernd
p1230124.jpg

[Taube_Nuss]

Hi again,

this may cool you down!
Screenshot 2023-05-09 at 14-35-02 Revox_B750_Serv.png

[borchee]

Actually you have been given a pins explanation, Niels...some seven years ago. Here.