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Revox B790 Variants

Q

QuadMusic

New Member
Does anyone have any information on internal variants on the Revox B790 turntable? I'm thinking of two so far:

(i) Power Supply Board

I've replaced the power supply caps. on 5 or 6 B790 in the last few months. Around half of them have a small daughter board 179.161-12 on the power supply board. It seems to mainly consist of a BD135-16 transistor on the input to the 24V voltage regulator.

Does anyone know why this was installed? Is this from a certain serial number onwards?

(ii) Arm Control Board

Some have the EMK PCB daughter board modification that is mentioned in the B795 manual.

Does anyone know the purpose of this? Again, is it from a certain serial number onwards.


Regards,

Peter
 
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Hi Peter -

Sorry for such a long delay in replying - don't visit the board all that often and this just slipped by. There were 2 "Service Bulletins" that addressed your questions - most mods to the B790 that were Service Bulletin "Improvements" can be seen in the "Revisions" noted in the Service Manual PCB schematics. After production stopped in '81-82, the subsequent mods could only be determined via the Service Bulletins that ReVox internally to repair stations issued - I'm lucky to have scored a complete set of these B791 and B790 SBs! I don't think any of this SB info is available online in PDF form - unfortunately I don't own a scanner so I can't copy these for you.

i) The Power Supply mod was addressed in ReVox's "B790 Tech Bulletin #3". It said:

"New 24 Volt Regulator

The 24 volt stabilizer UA78HV24C (50.05.0261) will soon be no longer available from our supplier. This regulator will be replaced by 50.05.0240 which also requires the piggyback circuit 1.179.161. This new circuit will be mounted on the heat sink of the power supply."

Not really sure myself why the circuit was necessary since I've replaced the 3 regulators in my B790 with new ones and haven't need any extra "circuit", but maybe ReVox had a bunch of regulators that needed this mod and didn't want to source new parts!

ii) The EMF circuit was addressed in ReVox B790 Tech Bulletin #4. It reads as:

"EMF Circuit 1.179.251

There have been some reports of intermittent lifting of the tone arm for no apparent reason and at no particular point in its travel. If it has already been determined that the adjustment of R54 on 1.179.150 is correct, the fault could be the follow-up motor in the tone arm. Sometimes the 366:1 gear assembly binds, causing the motor not to correct the tone arm's position with the result that more and more error accumulates until finally the comparator, IC7, triggers "end-of-record". An easy installed circuit which analyzes the counter EMF of the follow-up motor is now available to eliminate this problem. Complete mounting instructions are included with each kit; order number 1.179.251."

I've built this small circuit myself from the info in the B795 SM and installed it in my B790 - the precision of tracking and end-of-record sensing was definitely improved.

Hope all of this helps you,

Dale
 
boyerd001 said:
I've built this small circuit myself from the info in the B795 SM and installed it in my B790 - the precision of tracking and end-of-record sensing was definitely improved.
Click to expand...

Hello Dale,

Do you know if the board was only for the 1.179.150 (B790) and 1.179.250 (B795)? I have a 1.179.152-12 board spare although I'm not sure what is inside my B790 at present.

Regards,

Peter

(Yes, it's a 2 year-old thread. And it's a 33 year-old turntable.)
 
I quote everything Dale wrote six years ago, just adding that the Bulletin infos have been included in the latest service manual: you can tell it because it's printed in color, and there are orange pages between chapters.
Go to page 48, they noted all the modifications applied until 1982 to the Arm Electronic board, including the EMK daughter-board.
I just finished to build mine: I'm posting this "report" because, after I replaced all caps and trimmers, and changed a few resistors according their specs, the arm stopped responding to commands... I don't know if I zapped the IC7 op-amp or the new resistor values altered some sensor condition, but I was quite worried. Nevertheless, I decided to follow the upgrade path until the end, and managed to build my EMK-clone board: good idea, because when I put it in the IC socket the arm came back to business :jump:
The end of record trigger is fairly inconsistent, but I guess it's because I didn't have the RC4559 op-amps that Revox recommends as replacement for 4558. As soon as I find them, I will replace both IC6 and IC1 on the EMK-clone board, then I'll proceed with alignments. If anyone is interested in building its own board, just let me know: I can post a PDF, an EAGLE layout and I even got a couple of spare PCBs. BTW, I managed to keep it on a single-face board: harder to draw but much simpler to build.
 
Chartz, I'm so sorry, I read your post just today... If you're still interested let me know with a PM and I'll send you a small ZIP with all the related files.
 
j-man said:
...If anyone is interested in building its own board, just let me know: I can post a PDF, an EAGLE layout and I even got a couple of spare PCBs. BTW, I managed to keep it on a single-face board: harder to draw but much simpler to build.
Click to expand...

Hi j-man
I'm just reading your post. Would be very happy if you could send me the mentioned ZIP. Thanks you very much!
Dieter
 
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