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Author Topic: REVOX A77 ALIGNMENT QUESTION  (Read 8984 times)

Offline demto

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REVOX A77 ALIGNMENT QUESTION
« on: March 18, 2010, 08:41:06 AM »
I have just completed the alignment of my Revox A77 after changing many of the components on the boards and relapping the heads. No problem, everything works fine but I have a query about the alignment proceedure and wondered if anyone can throw some light on the subject.

The revox manual says that many of the adjustment when aligning should be carried out at a voltage of
155 milivolts which is -20db down from the reference 0db level. I have done this but it is a bit fiddly and you have to be very carefull not to make a small error when adjusting at -20db. For example, reading the db scale when setting the bias is not easy as the scale is compressed.

I have read somewhere on this site that one of the members adjusts the bias on his Revox B77 at 0db and not at -20 db without any problems. Apparently the original reason for adjusting at -20 db was to avoid tape saturation on the tapes that were available 30-40 years ago.

It would be much easier all round if all the -20 db adjustments mentioned in the manual could be carried out at a higher voltage, say 775 millivolts or -6db. In this way small errors of adjustment could be avoided and it would be much easier to read the db scale.

Has anyone tried this or have any thoughts on the subject as I would find yor comments extremely helpfull. Thanks  Mark




Offline Ki Choi

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Re: REVOX A77 ALIGNMENT QUESTION
« Reply #1 on: March 18, 2010, 10:59:44 AM »
Hi Mark:

Although I have not had a chance to align a Revox A77, I have set up Studers many times.  A77 being a consumer deck, its normal operating level would be 0.775V for 0VU.  I would suggest you try the -10dB. 

In addition, the VU meters on Studers and Revox machines are good but not necessarily accurate.  I use the milivolt meter instead of built-in VU meter for calibration.  As Paul had mentioned in the other thread regarding setting bias, the VU meters get less accurate as you move away from 0VU region.  BTW, if you use the 1Khz bias tone method Paul had suggested the that thread, using 0VU for bias adjustment would make better sense.

Ki
Ki Choi

Offline demto

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Re: REVOX A77 ALIGNMENT QUESTION
« Reply #2 on: March 18, 2010, 01:29:04 PM »
to Ki

Thanks for your reply.

On the Revox A77 0db is definately 1.55 volts [it used to be 2 volts till Revox revised it]

You are probably right about the built in Vu meters not being accurate enough but I have never used them as a tool when doing any aligning. In any case they do not work in replay mode so it would be impossible anyway. Revox designed the A77 like this, I have always used an audio millivolt meter. I have never tried biasing at 1 khz, i stuck to what the manual said.

Getting back to my question as to whether or not the alignment adjustments could be done at a higher voltage than 155 millivolts [-20 db] Do you think it would be OK to do them at -10db if it were not a good idea to do them at 775 millivolts [-6db]

thankyou    Mark


Offline sound signal

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Re: REVOX A77 ALIGNMENT QUESTION
« Reply #3 on: March 18, 2010, 03:50:34 PM »
Hi,

That could have been one of my posts you read about the B77 alignment.  I use 468 tape (previously Agfa, then BASF, then EMTEC and now RMGI) and set a reference level of 355nW/m on the B77 Mk.II, which is 3dB higher than the 250 recommended by Revox.  I also set my meters to read 0dB at reference level, whereas Revox recommend a "ballistic lead" of 7 dB if I recall correctly.  So my setup is a total of 10dB "hotter" than the recommended Revox setup.

I reckon that about half of that 10dB difference is accounted for by the difference between 468 and the typical 1970's consumer tape, and the other half by the use I put the machine to.  I record mostly chamber music and small acoustic ensembles without percussion, so there's not a lot of high frequency or transient energy there to cause overload problems and I can run the tape hotter to get lower noise.

Whatever blank tape and whatever reference level you choose at whatever tape speed, you can quickly see if you can make a bias adjustment at that level on that tape at that speed.  Record 1kHz at your reference level, and set the bias for maximum output off tape.  Then switch to 10kHz at the same recording level and set the bias for maximum output off tape, before increasing bias to reduce the output level off tape to the prescribed 10kHz drop-off.  If you can't find a 10kHz maximum that is fairly close to the 1kHz maximum, then the tape is saturating and you need to use a lower level for the bias adjustments.  You can double check this once the HF record equalisation has been set, after setting the bias.  Tweaking the bias for a maximum again, after setting the HF EQ, should give a maximum at 10kHz which is higher than the maximum at 1kHz, dialling in the prescribed overbias will restore flat response.

Using 468 in the B77 at 15ips and at 7 1/2ips, I could get a 10kHz maximum close to the 1kHz maximum at my reference level which is 10dB higher than Revox' recommendation.  I wouldn't think you'd choose a higher reference level than that, so I would think you should be OK to carry out your bias alignment at any reference level you choose.  And if you're in any doubt as to the dynamic capabilities of an A77 with 468 tape, find a copy of the the famous audiophile favourite recording "La Spagna, Siglos XV - XVI - XVII" on the BIS label, made in 1979 or thereabouts - I think the CD is still available.  BIS used an A77 at 15ips with 468 tape to capture that, and anyone who's heard it can vouch for the dynamics and the frequency range.

Good luck and best regards,

« Last Edit: March 18, 2010, 03:54:57 PM by sound signal »
George Karaolides
Nicosia, Cyprus

Offline demto

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Re: REVOX A77 ALIGNMENT QUESTION
« Reply #4 on: March 18, 2010, 11:04:16 PM »
Dear Sound Signal

Yes, I think it must have been one of your posts I read. Thank you for yor reply.

You way of setting the bias is very interesting.

The problem with the A77 is that some of the other adjustments that are necessary when aligning the machine have to be done at very low voltages, mostly 155 millivolts or -20 db compared to the reference level.   Bias is just one of them.  Others include setting the record level in and out and HF equalization.

As i said in my post it is easy to make a small error when adjusting at these low voltages that is magnified when the level is cranked up. Here's an example.  When balancing the record level in and out at 155 millivolts I get the meter needle to read as close as possible to 155 millivolts on both in and out although the meter scale is cramped. If i then leave these settings and crank the volume  up to show the same ins and outs at reference level [1.55 volts] there can be a difference of 1db or so between the two voltages.

OK its no big deal and it is an old machine and i dont expect perfection. My reasononing for posting my query was twofold. Firstly the meter has a bigger scale at about 775 millivolts [-6db below reference level] and is easier to read, particually the db scale for setting the bias, and secondly i would prefer the A77 to be more accurate in its midrange recording level than miles down at -20 db.

This is all probably splitting hairs but maybe that is what we are here for. I very much doubt that Joe Public would notice any difference in a recording made on a machine that was spot on and one that was a bit out of alignment.

I have aligned my A77 according to the Revox manual and I am very happy with the results. My query is really to find out if there is a more convenient way of doing things if i have to redo the alignment in the future.   Mark


Offline demto

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Re: REVOX A77 ALIGNMENT QUESTION
« Reply #5 on: March 19, 2010, 10:32:48 AM »
Thank you to all who have replied to my query. You have been very helpfull

I have now got the answer to my question.

I rang a Revox specialist who told me that it was perfectly OK to carry out the adjustments i asked about at 775 millivolts instead of 155 millivolts. Question answered.  Mark