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Old 18th Jan 2019, 2:21 pm   #1
NorfolkDaveUK
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Default Revox A77 DOLBY calibrating .

OK so I`m trying to calibrate this machine and getting completely lost . haha .. nothing new there of course .

I just wanted to know if anyone here had calibrated one of these in the past and can tell me if I`m reading these pots as correct . Thanks folks .
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Last edited by NorfolkDaveUK; 18th Jan 2019 at 2:35 pm.
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Old 18th Jan 2019, 2:38 pm   #2
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Default Re: Revox A77 DOLBY calibrating .

Also , can someone tell me what "turn FET bias off , set gain to minimum amplification" is referring too please . Thanks .
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Old 18th Jan 2019, 4:52 pm   #3
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Default Re: Revox A77 DOLBY calibrating .

OK . I get it , FET is P101p , gain is P102p . Now it makes sense .
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Old 18th Jan 2019, 5:07 pm   #4
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Default Re: Revox A77 DOLBY calibrating .

Ok I think I`m getting there , you have to clip onto the dolby record processor board on the Dolby input with test clips , and read the level from the dolby output using test clips , directly on the board . Thats where I was getting messed up . This is quite a complicated process when you don`t read "tech" . I think I understand it now though . The "P1 , P2 , P3 " etc relate to the pins on the board connector. OK got it . phew . Oh well you only have to learn it once right ..hehe .
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Old 18th Jan 2019, 7:59 pm   #5
monaro0162
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Default Re: Revox A77 DOLBY calibrating .

Looks like you are on the right path. I did one of these a while back, and it took me way longer to do compared to a regular A77. Yes you have to monitor whats going on by probing on the Dolby boards, and on the usual TPs on the amp cards. I snapped a pic of the signal routing that I had worked out on a whiteboard at the time for future reference.
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Old 18th Jan 2019, 10:36 pm   #6
NorfolkDaveUK
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Default Re: Revox A77 DOLBY calibrating .

Thanks mate i wrote on the interconnect board for the dolby boards , and added some connection hooks to clip on to , was easy once i has figured it all out . Bit of a head scratcher before I did though haha
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Last edited by NorfolkDaveUK; 18th Jan 2019 at 10:43 pm.
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Old 19th Jan 2019, 2:47 pm   #7
NorfolkDaveUK
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Default Re: Revox A77 DOLBY calibrating .

OK I am REALLY struggling here .

I`ve done the dolby and i`m now trying to set up this ****** rec-cal nonsense .

It`s hard to explain but I will try . Here`s what I am doing . It says to "press the -6db button on the t19" , i`m assuming the t19 is a function generator and the -6dB button is an attenuator . I`m not using a T19 but I am attenuating the signal . OK .

1, adjust the sig-gen to read -1dB on the VU`s (what is the point of this ?)
2, rec-cal pots fully ccw
3. increase sig gen to +10dB (so why go to -1 VU in the first place ?)
4. Adjust the P503 on the record cards to read 0dB VU.
5. Reduce the sig gen to -10dB
6. Adjust the record level back to 0 dB using the rec-cal knobs

So far everything above (although makes no sense to me) does what it should ..

7. turn record level pots (main ones) fully ccw
8. turn monitor selector to "cal" , and adjust P103r to obtain 0dB on the VU..

number 8 is where this all goes to hell .
When i select "cal" on the monitor switch, the needles fall to 0 and I get nothing from the P103r pot because the signal is way too low.

This process is a NIGHTMARE !

Anyone know what the hell I`m doing wrong here ?


EDIT........I think I know where I am going wrong ......You go to -1 on the VU`s . the you increase the signal BY +10 dB , not TO 10dB.....I`ll try that and see where it gets me . ....

Last edited by NorfolkDaveUK; 19th Jan 2019 at 3:07 pm.
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Old 19th Jan 2019, 3:05 pm   #8
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Default Re: Revox A77 DOLBY calibrating .

It may be a bit late now but I would normally not touch those PCB presets unless there was clear evidence they were out. I would first align the machine (with Dolby bypassed) so that record/play response was as flat as possible. For the Dolby to track correctly, that condition must first be met. That applies to all tape machines when using Dolby.
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Old 19th Jan 2019, 3:35 pm   #9
NorfolkDaveUK
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Default Re: Revox A77 DOLBY calibrating .

The sweepers were all missing on the pots , hence the chaos . And i would have calibrated it any way , if a job is work doing ....etc ....the learning may be stressful , but the end result is always worth it .
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Old 19th Jan 2019, 5:04 pm   #10
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Default Re: Revox A77 DOLBY calibrating .

Eureka , i`ve got it as near as dammit , what a complete pain in the watsit that was .
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Old 19th Jan 2019, 5:56 pm   #11
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Default Re: Revox A77 DOLBY calibrating .

They're not much more fun to use, so that's why dobbly revoxes are uncommon.

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Old 20th Jan 2019, 4:22 am   #12
NorfolkDaveUK
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Default Re: Revox A77 DOLBY calibrating .

Well I got it in to order so I`m not worried too much haha . It`s not going to be here much longer . It sounds really nice now its working properly and now the Dolby is in spec its not acting like a "tone control" like it was when i got it , it is just removing the tape noise . So in spec they actually sound really good , the saving grace for this one is the heads on it are immaculate. One of my customers in Sweden wanted one for his collection so I found him one . I guess now I know how to do it the next one wont be so bad (not that I`m in a hurry to do another one ..haha)
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Old 21st Jan 2019, 11:41 am   #13
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Default Re: Revox A77 DOLBY calibrating .

I trust your client realizes that for good performance, Dolby machines are quite fussy about using the same type of tape as they were set-up with. So, just out of interest, have you set it up for the type of tape he/she is going to use?

Last edited by SteveCG; 21st Jan 2019 at 11:41 am. Reason: added word
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