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Old 6th Nov 2018, 12:32 am   #41
NorfolkDaveUK
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Default Re: Revox A77 calibration.

Where would you recommend I look . The input signal is fine so its only happening on record , if the record signal is weak at the record head , would that not cause the amp to appear weak ? The problem DOES NOT change sides if I swap the cards . or is it the amp causing the weak voltage at the head ? I ruled that out because its not swapping sides

Last edited by NorfolkDaveUK; 6th Nov 2018 at 12:43 am.
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Old 6th Nov 2018, 12:54 am   #42
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Default Re: Revox A77 calibration.

Personally I think using a scope is somewhat misleading you. I use an audio test set which has a very accurate meter and oscillator, I find I have no trouble when using this to set up any tape recorder.
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Old 6th Nov 2018, 1:18 am   #43
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Default Re: Revox A77 calibration.

no its definitely not doing that I`ve calibrated probably 5 of these a77`s , 6 B77s and countless sonys using this scope and this one is just not playing ball . channel one is fine , channel 2 is just way too low , maxing out at 550mV peak to peak on the trimmer , channel one is perfectly fine and can be set mid sweep so there is definitely something amiss here , its not the record cards because the problem doesnt swap sides when swapping the cards over , its not the playback cards because they spec up just fine , its not the playback head because it plays perfectly ok and with the calibration tape it comes into spec nicely , so I`m getting closer and closer to thinking the record head might be the cause here , but we will keep going and see .
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Old 6th Nov 2018, 1:49 am   #44
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Default Re: Revox A77 calibration.

Gosh darn it I just realised the record light isn`t coming on on the duff side .. sigh man i need a day off .
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Old 6th Nov 2018, 3:46 am   #45
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Default Re: Revox A77 calibration.

well . I got there in the end . So the bias trap coil was the issue , one still is it wont quite go down to <50mv , not too sure why its at about 70 -80 i would say , and there is still a lot of noise of channel 2 while i will sort out in the morrow , but the record levels ARE now adjusting into spec , just noisy on one side and a little high on the bias . thanks again every one , the moral support for someone like myself with the health problems i have is paramount , so thank you .. hopefully the B77 i have coming up will just behave ...haha . Time for a well earned coffee and bed ...night all
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Old 6th Nov 2018, 11:40 am   #46
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Default Re: Revox A77 calibration.

If this is just on one channel, I'd guess your input level and equalisation pot are cranked up too high.

Or maybe a ferrite core controlling your bias is busted, I've seen that happen when using the wrong tools, although I don't work on A77's myself so who knows?

Taking a break between calibration might be a good idea.
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Old 6th Nov 2018, 5:46 pm   #47
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Default Re: Revox A77 calibration.

Give that man a cigar ^^^^ , the ferrite rod had a little chunk out of it right in the middle where I couldn`t see it , replaced it , now in spec ! Cheers mate !
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Old 6th Nov 2018, 6:32 pm   #48
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Default Re: Revox A77 calibration.

Out of interest which bias trap coil was causing the low output on one channel?

Lawrence.
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Old 6th Nov 2018, 7:25 pm   #49
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Default Re: Revox A77 calibration.

The one on the record card , I think it was more that A) it has a chunk out of it so it couldnt be put into spec (and it still wont but its a little better) and B) i was relying on "auto" on the scope for everything , so last night I sat and played with the scope and I now understand it much much better , those weird patterns I was getting you were all (of course ) right it wasnt triggering properly , but now I understand how to get the thing to trigger manually its a lot lot nicer to use , I can hit auto to get it within the ball park and then manually get it where it needs to be , much better . So it was half my scope ignorance and half the broken ferrite core ...

NOW I have a question . I cannot get the channel 2 bias lower than about 80mv . It should be under 50 , what could be causing that and secondly , how do I work out how many mV i need to drop the voltage by when adjusting the bias setting on the oscillator card . Its peak is at 0.5vrms and I need to drop 4db .

Thanks again chaps .
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Old 6th Nov 2018, 7:41 pm   #50
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Default Re: Revox A77 calibration.

OK i think i got the second question , 0.5V is -2 db , so I need to drop to -6 db which is 0.3v or there abouts ....man I love learning ... cool .

EDIT :
NO , those figures aren`t right , thats peack to peak figures , but I`ve sussed the process thats the main thing .

Last edited by NorfolkDaveUK; 6th Nov 2018 at 7:52 pm.
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Old 6th Nov 2018, 8:31 pm   #51
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Default Re: Revox A77 calibration.

Nailed it Sounds amazing ! Happy days !
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Old 6th Nov 2018, 11:37 pm   #52
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Default Re: Revox A77 calibration.

Right on, nice work.
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Old 6th Nov 2018, 11:54 pm   #53
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Default Re: Revox A77 calibration.

Thanks mate.

I actually found that using my ears to set the bias up worked so much better than using a meter/scope. I set the cores, set the balance levels, set the output levels and set the record levels, all with the scope, then just matched the bias to the input using a good quality Scotch tape, a few different kinds of music and my ears. Now it sounds amazing. I'm going to do it that way from now on on these A77`s . They are temperamental at best sometimes.
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Old 7th Nov 2018, 12:11 am   #54
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Default Re: Revox A77 calibration.

The 50k pots are not so good for the bias. 100k is better.
I don't used a scope, better is to see the levels on a good dB meter so as an RTW meter. It's easier to see 4 dB down than on a scope.
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Old 7th Nov 2018, 12:22 am   #55
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Default Re: Revox A77 calibration.

It was easy enough to see on the scope, mine is analogue/digital so it has numbers on the screen, but it sounded better when I just did it with my ears, night and day in fact. When I did it as per the recommended figures there was a definite frequency shift between input and tape, but doing it by ear, it was simple to just match it using good Scotch tape and some good quality flat response studio monitors. Sounds much nicer than it did using the numbers. I guess there are many different ways to do things and you just have to go with what works for you.
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Old 7th Nov 2018, 8:22 am   #56
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Default Re: Revox A77 calibration.

Quote:
Originally Posted by NorfolkDaveUK View Post
but , it sounded better when i just did it with my ears ,



when i did it as per the recommended figures , there was a definite frequency shift between input and tape , but doing it by ear , it was simple to just match it using good scotch tape and some good quality flat response studio monitors . sounds much nicer than it did using the numbers . I guess there are many different ways to do things and you just have to go with what works for you

It depends I suppose on who you are doing it for. If it's for yourself then OK, if for a paying customer then you should do it by the book, using a dB meter.

When I do them using a meter, there is no difference between input and tape.
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Old 7th Nov 2018, 8:36 am   #57
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Default Re: Revox A77 calibration.

^ Yes. A scope is useful for setting azimuth on stereo machines and for general waveform checking but an AC Millivoltmeter or some other sensitive device calibrated with a dB scale is essential for proper alignment of any tape machine.
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Old 7th Nov 2018, 11:11 am   #58
NorfolkDaveUK
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Default Re: Revox A77 calibration.

^ I agree , and I have one enroute . I also have my eye on a Ferrograph RTS2 which would take the need for the scope out all together .

Last edited by NorfolkDaveUK; 7th Nov 2018 at 11:25 am.
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Old 7th Nov 2018, 11:28 am   #59
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Default Re: Revox A77 calibration.

You can use a 'scope or a meter with a db scale the end result will be the same if done properly, using your ears to set bias I wouldn't recommend for a proper job, when I was in the trade doing tape decks the set ups were all as per the manual, it's very easy to do there's nothing complicated about it, switching between source and tape and seeing the VU meters pegged at 0db (or whatever) on both accounts is very satisfying. I've used both the 'scope and the AC millivoltmeter meter method, on my bench the two main bits of setup kit were a 'scope and Ferrograph RTS test set.

If the OP isn't too sure about what's what with tape deck setups in general I would recommend downloading a Sony tape deck manual to study, the TC-366 is a good place to start, the instructions and functions are easy to follow and most Sony manuals include level diagrams for both record and playback, with those diagrams you can practice your db to voltage or voltage to db calculations just to get the hang of it and compare the results with those given in the diagrams, the instructions are very clear and include pictures of what's connected to were and the signals journey via the tape where applicable, the level diagrams even give the levels in and out of the record volume controls when set for 0db line out at the prescribed input level and frequency.

Anyways back to the fault, I'm still puzzled as to why the low level output from one channel turned out to be a problem on one of the record boards but swapping the boards over didn't make any difference to which channel the problem was on? (if I've read things correctly)

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Old 7th Nov 2018, 11:36 am   #60
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Default Re: Revox A77 calibration.

For now i have this enroute.
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