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Old 23rd Sep 2018, 11:07 am   #1
NorfolkDaveUK
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Default Revox A77 Bias

Can someone help me out a bit here, I'm having trouble with the bias coils on this machine. What would make the coils themselves not adjust? Does dirt affect them maybe? I'm trying to adjust the bias on the playback cards but adjusting the coils does absolutely nothing on both coils. I'm hooked up to the output with the 'scope. have the machine in record and nothing at all is happening when I adjust the coils.
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Old 23rd Sep 2018, 11:57 am   #2
lesmw0sec
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Default Re: Revox A77 Bias

Unless my memory is fading, I recall that the bias adjustment for A77 is via a potentiometer. The coil would just set the frequency, rather than the amplitude.
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Old 23rd Sep 2018, 12:16 pm   #3
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Default Re: Revox A77 Bias

Sorry I mean the bias trap, it's set via the coil for sure. There is only one pot on the play back cards and that's for level adjustment.

You are supposed to adjust the coil for minimum voltage (<50mV) but these just don't do anything at all.
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Old 23rd Sep 2018, 12:30 pm   #4
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Default Re: Revox A77 Bias

What are you measuring the voltage with?

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Old 23rd Sep 2018, 12:33 pm   #5
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Default Re: Revox A77 Bias

scope (tek 2213)
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Old 23rd Sep 2018, 12:45 pm   #6
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Default Re: Revox A77 Bias

Strange they don't respond, all I can find is in this article (7):

http://hccc.org.uk/revox/a77servicingman.html

Lawrence.
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Old 23rd Sep 2018, 12:50 pm   #7
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Default Re: Revox A77 Bias

Thanks Lawrence I'll go through that and see what comes out.

Cheers.
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Old 23rd Sep 2018, 1:09 pm   #8
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Default Re: Revox A77 Bias

They are definitely not responding to anything, all a little weird really. The record bias trap I set with no problem but the playback I'm just getting nada it's not the cards either because I swapped them out, same thing.
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Old 23rd Sep 2018, 1:24 pm   #9
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Default Re: Revox A77 Bias

It seems that I'm not measuring the bias voltage, but the output voltage. Moving the output voltage trimmer changes the waveform so I'm doing something wrong here. I just can't flippin figure out what. haha. Probably 'scope settings?
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Old 23rd Sep 2018, 1:52 pm   #10
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Default Re: Revox A77 Bias

OK after reading a bit more of that link you sent me, I'm moving on. He indicates it really doesn't matter all that much so to hell with it.

Thanks for the link Lawrence.
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Old 23rd Sep 2018, 7:24 pm   #11
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Default Re: Revox A77 Bias

The bias trap is for cutting out the HF signal from the audio. So you must be recording and then level the HF signal to a minimum.
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Old 23rd Sep 2018, 9:07 pm   #12
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Default Re: Revox A77 Bias

The bias trap coils are set at the factory, unless someone has misaligned them, why would you need to adjust them?
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Old 23rd Sep 2018, 9:39 pm   #13
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Default Re: Revox A77 Bias

Quote:
Originally Posted by Michael Maurice View Post
The bias trap coils are set at the factory, unless someone has misaligned them, why would you need to adjust them?
Because someone has misaligned them. LOL.
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Old 23rd Sep 2018, 11:47 pm   #14
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Default Re: Revox A77 Bias

Well either I'm losing the plot or this thing just isn't doing what it supposed to.

I'm now trying to do the bias via the osc card, I've set the signal up at 1k @ 200mV, flipped over to 10k, I have the machine set to stereo, full vol, NAB centered, AUX full, AUX full. I have the machine in record and I have a new old stock tape (Scotch) in the machine. The signal I have coming out is reasonably clean but the trimmers are not doing what they should. Here's what they are doing:-

3.5 left - acting like a volume pot, i.e. just increasing in amplitude all the way from left to right.
3.5 right - nothing at all

7.5 left - same as 3.5
7.5 right - nothing at all .

I dunno what the heck is going on with this thing, but it's not at all right and 'm pretty sure it's not me or the two 'scopes I'm trying to do it with.

I know I have to find the peak then drop it off 4db, but there is no peak.
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Old 24th Sep 2018, 8:08 am   #15
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Default Re: Revox A77 Bias

I'm confused. This is nothing to do with the trap adjustment. What are you doing?

Jack Marotick.
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Old 24th Sep 2018, 10:35 am   #16
NorfolkDaveUK
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Default Re: Revox A77 Bias

Quote:
Originally Posted by Marotick View Post
I'm confused. This is nothing to do with the trap adjustment. What are you doing?

Jack Marotick.
well if you follow the thread ,and read the first line of the last message "I'm now trying to do the bias via the osc card," i attempted the bias trap , got nowhere then moved onto the next step . which is biasing for tape . Which is where I`m at now . I would be happy for someone to show me I`m doing it wrong , I just want to get the darn thing off the bench now I have other stuff I need to be doing but I`m using a 50ohm out from the Sig gen , into the aux input (parrallel) , I have the machine set up as stated in the manual , I have it in record with good scotch take and nothing is working as its supposed too . I`m beginning to suspect the OSC card is not doing what it should . btw , this machine has brand new trimmers , so its not that .

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Old 24th Sep 2018, 10:53 am   #17
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Default Re: Revox A77 Bias

My suspicions were correct. I just swapped out the OSC card and now it's doing what it should. I needed to swap the transformers over because the other card I had was half track, but it's doing what it should now. Hopefully that will be the end of the issues and I can just get on and finish this darn thing.
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Old 24th Sep 2018, 2:04 pm   #18
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Default Re: Revox A77 Bias

OK all done . Pesky OSC board . Sounds great now , no real difference between input and tape out so I would say that`s mission accomplished . Thank god for that ..haha . Well at least that`s the first one out of the way . can only get easier now ....right ? hehe . Thanks everyone .-
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