Help In Setting Bias/Eq on Revox A77?
Bruce A. Rothwell
I've got an old Revox A77 that's in great shape, and I've got the service
manual for it.
I want to use Ampex 456 tape, but the A77 seems to need adjusting
because I'm hearing some serious distortion in high-end material such
as hi-hat, acoustic guitar pick noise, etc. _especially_ if I get
anywhere NEAR 0 db on the VU meters.
I'm not sure I fully understand what I need to adjust for the bias, but
I've been told that if I feed the deck white noise, I should be able to
do it by ear.
And as far as setting the eq, I don't know what needs to be done here
at all.
So, before I go off and start twiddling pots and such, I'd appreciate
some tips.
Thanx!
Bruce Rothwell
--
Bruce Rothwell / baro...@cca.rockwell.com / Cedar Rapids, IA
-------------------------------------------------------------------------
Rockwell Int'l/CCA Employee by day......... Songwriter by night. <><
Mike Rivers
> I've got an old Revox A77 that's in great shape, and I've got the service
> manual for it.
>
> I want to use Ampex 456 tape, but the A77 seems to need adjusting
> because I'm hearing some serious distortion in high-end material such
> as hi-hat, acoustic guitar pick noise, etc. _especially_ if I get
> anywhere NEAR 0 db on the VU meters.
>
> I'm not sure I fully understand what I need to adjust for the bias, but
> I've been told that if I feed the deck white noise, I should be able to
> do it by ear.
You can, but not in the way you might think. The conventional way to
set bias with modern tape is to put 10 kHz into the recorder, put it
into RECORD, and monitor the playback. Adjust the bias until you see
a peak in the playback level, and then go past that peak until the
level drops by 2-3 dB, depending on the tape and the heads. You
should find this spec in the service manual, but use 3db if it doesn't
say. With an A77, you'll need to run about 10 dB below maximum level,
so unless you have a lab-type voltmeter, you'll probably want to run
the output of the A77 into something with a VU meter on it (another
recorder, a mixer, etc.) and watch that meter.
The right way to adjust equalization (and, in fact, the proper thing
to do FIRST before messing with the record electronics) is to get a
standard alignment tape and set the playback head alignment, playback
equalization, and playback level to the standard tape. That way you
can use the playback electronics as a reference when making record
adjustments.
Once the playback is properly calibrated, set the record head
alignment. You can get it close by adjusting for peak output when
recording a 15 kHz tone, but the accurate method is to use an
oscilloscope or phase meter, and set it so that the two channels are
in phase. Then you're really ready to set bias, record equalization,
and record level.
You may want to just take it to a shop. It's not a good thing to try
to do without the proper test equipment and standard test tape.
------------
I'm really Mike Rivers (mri...@d-and-d.com) On the road.
Salt Lake City, home of the Mumblin' Pumpernickel Choir
P Stamler
pink noise actually). You need to:
1) Adjust play head azimuth (need alignment tape.)
2) Adjust playback (repro) level (ditto)
3) Adjust bias level (Need oscillator & voltmeter; set osc. to 10kHz, turn
bias pot clockwise until output level, monitoring "tape" not "source",
rises to a peak and then drops back by 5 dB)
4) Adjust record head azimuth (Need oscillator & voltmeter)
5) Adjust record level (ditto)
6) Adjust record EQ (ditto)
So it's not all that simple. If you don't have an AC voltmeter, alignment
tape and oscillator, you're better off paying someone else to do it. It
should cost about $100.
Audio Amateur published an excellent article on the subject by Craig
Stark, back in 1976. Back issues are available; write Audio Amateur, P.O.
Box 576, Peterborough, NH 03458.
Write if you have questions.
Paul Stamler
e-mail: Psta...@aol.com
P Stamler
Paul Stamler
e-mail: Psta...@aol.com
David J. Carlstrom
> I want to use Ampex 456 tape, but the A77 seems to need adjusting
> because I'm hearing some serious distortion in high-end material such
> as hi-hat, acoustic guitar pick noise, etc. _especially_ if I get
> anywhere NEAR 0 db on the VU meters.
These symptoms are not signs of mis-calibration. Analog decks, like
digital ones with pre-emphasis, cannot record that type of material at or
near 0 VU.
The solution is to reduce the record level until these sounds are clean.
> I'm not sure I fully understand what I need to adjust for the bias, but
> I've been told that if I feed the deck white noise, I should be able to
> do it by ear.
Not on a deck that has record eq adjustments. You will need instruments
to properly cal this deck.
--
"The more cutting the verbal thrust, .-. / \
the more elaborate was the courtesy .-. / \ / \
employed." _Up At Oxford_ -Ved Mehta / \ / \ / \
' \ / \ / -DJC
Bill Vermillion
>
>In article <D5Fv7...@lazrus.cca.rockwell.com> baro...@cca.rockwell.com writes:
>> I've got an old Revox A77 that's in great shape, and I've got the service
>> manual for it.
>> I want to use Ampex 456 tape, but the A77 seems to need adjusting
>> because I'm hearing some serious distortion in high-end material such
>> as hi-hat, acoustic guitar pick noise, etc. _especially_ if I get
>> anywhere NEAR 0 db on the VU meters.
>You can, but not in the way you might think. The conventional way to
>set bias with modern tape is to put 10 kHz into the recorder, put it
>into RECORD, and monitor the playback. .....
And I am going to present an unconvetional way to align tape
recorders, that is partially by ear, and works very well.
This is an article I wrote for the Orgeon Triode Society's
newsletter POSITIVE FEEDBACK 4 years ago. (Checking the date - the final
of this article - a re-write from a previous - was written on
March 22, 1991). This works for reel-reel tape only, and is a full
article/tutorial on complete tape recorder alignment the way I like to
do it.
Experience to bring me to this point came from such legendary and some
eminently forgettable machines as the Magencord PT6's, some 10.5" reel
Magnecord whose number I've forgotten, Ampex 601s, PR10s, 350s/351s, AG350's,
440s, ATR102, a pair of MR70's, a brief encounter with a 300, Berlant
Concertone, Pentron, Teac, MCI, Scully, Crown 712/814, Electrosound
505, Studer B67's and A80 & A-800s, Stevens 821B, RCA & Collins
cartridge tape machines, and utter fascination with the first
'professional' 10.5" tape recorder I saw a teenager (or even pre-teen)
a genuine Presto. There may have been one or two others somewhere
along the line. (My first recorder was a Webster Chicago wire recorder).
(They were discontinued and about 1/4 the price of current
tape recorders. As a high-school kid I couldn't afford a
real (reel) machine.
------------ cut here ----------------
This is an article on tape alignment that comes from doing this more times
than I would care to admit, on 1, 2, 4, 8, 16, 24, and 32 track machines!
Of course be sure to clean the tape path before starting.
De-magnetization is optional in my opinion. There are some that insist it
be done every time, but in our studio we felt that it was easier to
introduce problems with it. We also checked the machines with a
magnetometer, but typically we thoroughly demag'ed the machines only about
once every 1 to 2 months.
Some machines were re-aligned several times in one day. This was because we
would be working with source tapes recorded on different media, in different
studios at different operating levels. I had one machine that I did a
complete re-alignment on three times in the course of one day. Two
different 24 track tapes, one Dolby, and a 16 track setup. After doing this
that often we got to be able to set up a 24-track machine in 30-40 minutes,
which is about the length of time it took me to do my first alignments on two
track machines.
Be sure to use a good monaural alignment tape. (They should all be
that way. There are some multi-track tapes that are recorded monaurally and
then have guard bands erased after recording. This will make a difference
on low speed playback EQ - but I use a method that elminates any problem
with low-frequency fringing effects.)
The steps used to align are as follows and are to be done is this order.
1. Physical head alignment (azimuth alignment)
2. Playback level
3. High frequency playback response
4. Bias adjustment
5. Record level
6. Record high frequency response
7. Low frequency playback
Standard procedure for record head alignment was to align play head,
and then adjust record head while during recording watching the
playback head. Most multi-track machines and a good many two-track
machines now have the ability to put the record-head into the playback
circuit (often called Sel-Sync*). This is a much more accurate and
prefered way to do physical head alignment.
The first tones on the alignment tape are HF alignment tones. If you have
a scope you can ajdust by watching the phase. If not available you watch
for maximum output at this frequency. I had the luxury of having phase meters
at the studio, this makes it all so much easier.
Playback level is then adjusted. Alignment tapes are manufactured to
different recording levels. Assuming you have a tape for the level you
wish to record, adjust the playback levels to a 0 reading on your meter.
To be precise however you should look at the tables for fringe effect that
should be included in the manual with the alignment tape.
Fringe effect is a higher output caused by playing tapes that have a wider
record track than the width of the playback head. At our studio we
compensated for this as we had standard two-track machines, and we had
"stereo" machines. The latter use the European spacing for 1/4" two
channel tapes of .75mm. The standard two-track spacing is 2mm. If
compensation was not made the tapes will vary in playback when played on a
different two-channel format machine. Most studios ignore this, however
at the higher speeds it is important. At 30 ips the fringe effect will
give a 1db too high a reading at 1000Hz! This will throw your alignment
off by 1db at the high end of the spectrum.
Once playback level is adjusted it is important to MAKE SURE that this is
not touched during the remainder of the alignment procedure.
At this point you can then make your HF playback adjustments to give you
flat(est) response from your alignment tape. These playback calibrations
are critical as all the rest of the alignment depends on them.
Now that playback is done the next step is bias, which will affect level
and record response.
For bias I ALWAYS adjust for minimum modulation noise as that to my
ear is the worst part of mag tape.
To do this pick a nice low freqency tone (I always use about 7Hz)
and listen to the noise components when playing back. Cut the
bottom end of your playback amps if you must because you only want
to hear the noise components.
As you start below the bias point you will heard hf noise pulsed at
the 7Hz modulation frequency. As you increase the bias the noise
will decrease to a point and then start increase with a change in
the "tone" of the noise. Go back to the minimum point. If you do
this, you will find that you can set the bias more accurately by
ear than you can doing the typical 1-3 db overbias at 1kHz or 10kHz
(depending on your choices).
The only tape I have found that is can not be done on is the Agfa 468.
Minimum modulation noise will overbias the tape 6 to 9 db on this tape.
I had a machine shut down as I overloaded the bias amps when trying this.
(NOTE - Since this artilce was written, new tapes such as the Ampex 499
have become available. I do not know if this procecure will work
properly with them. wjv)
Many recommend 1db overbias at 1kHz. I find that you should use
10kHz for anything less than 15 ips, and for 30 ips use 20kHz.
Typically you overbias 3db at these frequencies, but if you have
the chart for your particular brand of tape you can find the exact
point. However, except for 468 I always use minimum modulation
noise. Agfa recommends a 14Khz audio signal for 468, and I don't remember
the exact amount of overbias. They will furnish to you if needed.
After the bias is set, the next step is record level. All you do at this
point is set the output to be 0 vu by matching the output level to the
level you had previously set on playback of the alignment tape.
At this point you can calibrate the record meters if your machine has
record calibration controls. Since the PB level has been matched you goto
the record metering side (up to this point you have been watching playback
metering). You then adjust the record CALIBRATION (not level) so that the
input levels read 0.
Now that you have the record level set you can adjust the HF record
response. The standard that many use is to set 10kHz to be at the same
playback level of the 1000Hz tone. I personally disagree with this.
I try to make the 20kHz level 0 IF the 10Khz region does NOT increase too
drastically at this point. In other words, if I can get 20hKz to zero and
keep 10Khz no higher than 1.5 to 2db I'll go that way. I would much rather
have SMOOTH wide response, than FLAT response that falls off at the high
end.
Depending on the ability of your machine, you MAY have to make the high
frequency record alignment adjustments at a lower level. Because of the
high freqeuncy pre-emphasis some machine's record amps may not take too
kindly at 0db at 20Khz before being overdriven. On the other hand, some
pro machines, Studer in my experience, have absolutely NO problems at
recording at 6db over standard level at 20kHz and above when running at
30ips. The slower the tape speed the more critical this becomes.
Since we have now calibrated the record level against the play level, if
your machine has problems in this area, we can now change the levels. Turn
the record level down about 10db. Turn the playback up so that your output
at 1kHz is zero, and then proceed to do the HF record EQ as this level.
If your machine requires this you should also make sure that you are
carefull not to "go into the red" when recording on this machine.
One point I disagree with in most the procedures I have seen, is
that many say to touch up the alignment (mechanically) after this
you have completed these steps.
I have seen that you can change the physical alignment of the
playback head or record head at this point and further peak the
output (or bring the phase close if monitoring phase).
However, since you have aligned with a stock tape originally I
disagree with this. (And this is my own method that we used at the
studio - and it seems to work. However I have not had the time or
opportunity to test this theory of mine so I may be way off base).
After all this check the phase response at 20kHz, and carefully
adjust the bias on one of the record channels to bring the phase to as
close to 0 deviation as possible. You may have to touch up rec eq just
a bit because of the bias chnage, and you may have to do this once or twice.
I attribute this phenomenon (In my head at least) to the "bias
bubble". The signal is recorded on the trailing edge of the
record gap, and the amount of bias will affect just exactly where
the trailing edge "seems" to be. This varies with the frequency being
recorded. Changing the bias while watching the HF phase will show that
this does affect the phase response.
EQ doesn't enter into it. Changing bias will affect the HF record
response however, but this should only be level sensitive.
The only thing I can think of that would account for this is the
slight displacement in the track caused by bias.
(As I say - this is my own "theory" and I have never had the
time/resources to check it thoroughly - so I may be way off base,
but record/play sounds really great done that way. I guess all of
us a permitted to have at least one ecentricity :-) ).
After you have done the final hf rec eq, you do the lf pb eq. Never
set the LF playback eq from the alignment tape. Except for a
very few machines, you have NO control over the low frequency
record characteristics of your machine and you want to align your
LF playback to your LF record.
Doing the above on an ATR102 - I could get the machine flat within
+- 1db from about 30Hz to 20kHz. Tweaking the bias, as I mentioned
above, I could typically pull in the 20Khz with less than 5 degrees
phase shift. I really loved having a phase meter, as opposed to trying
to interpolate on a scope. On a Studer A-800 at 30ips, I
could get within about +- 1db from 30Hz to well past 20kHz. The
Studer 3db down point was at 33kHz. (One hell of a machine!)
If you align your machine very carefully you will hear great
differences between tape brands/types.
I have not had a chance to hear the new Scotch high output tapes,
but of the rest of the tapes these would be my choices.
For such things as strings, but without any high level peaks,
Scotch 250. Wonderfully quiet. Has more print than I'd like.
That's why I stay away from this tape on msuic with large peaks or big endings.
You will get echo on the first playback. There is very little modulation
noise on this tape.
For voices, horns, accoustically generated music, Scotch 226.
Doesn't print like 250, very low modulation noise, minimal asperity
noise. Good tape.
For pop/rock. (Things that don't have big gaping holes (rest) or
low level vocal tracks) Ampex 456. Seem a bit "brighter" or
"harder" than the Scotch tapes. Great rock'n'roll tape. But tends
to have more asperity noise (but only noticeable in quiet
passages).
All the tapes measure the same (in frequency respone on a give
machine) but all have slightly different sound characteristics.
(For those who are not familiar with the term "asperity noise",
this is a low frequency noise component. We called it "rocks" in
the studio because it is what you would imagine large -really
large- boulders to sound like hitting one another. This is cause
by slight uneven-ness in the oxide coating. You hear it when you
have such things as a soft vocal group - or soft horns. Virtually
inaudible unless the music has pauses. More noticeable the higher
the recording speed).
And to explode one "myth" here. Worn tape head don't ALWAYS show up by
having a degraded HF response. In pro machines the heads have a very deep
gap. As the head wears the depth of the metal is less while the gap is
the same width. The tip-off here is that you INCREASED HF output.
I found this out when fighting a problem on an Ampex MR-70 and Ampex tech
support pointed this out to me. (The MR-70 has to be my favorite all
time tape recorder - followed by the AT102 for 1/4" and the Studer
A-800 and Stevens 821-B for 2". The latter is a true 'hackers'
machine. You have to KNOW the machine to love it, otherwise you'll
hate it.)
So while the above procedures may not always follow the book, they are the
ones we adopted in the studio and the machines always sounded good.
A lot of this information came from trial and error, but to give credit where
it is due, some of the best information I have received working in the business
came from conversations with John Stevens, who built an amazing tape
recorder, John French, of JRF in New Jersey, who remanufactures and builds
magnetic tape heads, and Gordon McKnight, of Magnetic Reference Laboratories,
who make MRL alignment tapes.
* Sel-Sync is a Trademark of Ampex
(Copyright 1991 & 1995 by W.J. Vermillion.)
-------- cut here -------
With very minor corrections and one paragraph addition, this is the
same article I wrote as it appeared in the Oregon Triode Society newsletter
"POSITIVE FEEDBACK" in the spring of 1991.
Bill
--
Bill Vermillion - bi...@bilver.oau.org | bill.ve...@oau.org
douglas....@crc.doc.ca
A flux loop is much more accurate than a standard
test tape for setting playback level and e.q. (not
that one can set playback e.q. on an A-77 anyway..)
It is also far cheaper.
I have no idea where to send you for references on
this one, although Studer in Nashville might be a
good place to start.
The amount of overbias depends upon the head-gap,
so it is machine specific.
Bill Vermillion
<douglas....@crc.doc.ca> wrote:
>One other note:
....
>The amount of overbias depends upon the head-gap,
>so it is machine specific.
It is also tape specific too. To be 100% 'spot on' you need to
get the curves for your particular tape from the manufacturer.
Most are similar. It just depends on how picky you want to be
I'd guess.
Mike Rivers
In article <1995Mar17.0...@bilver.oau.org> bi...@bilver.oau.org writes:
(lots of good info deleted)
> The first tones on the alignment tape are HF alignment tones. If you have
> a scope you can ajdust by watching the phase. If not available you watch
> for maximum output at this frequency. I had the luxury of having phase meters
> at the studio, this makes it all so much easier.
For those with minimal test equipment, combine the channels of a two
track recorder (or a couple of channels on a multitrack recorder) into
mono, and watch for the peak. The phase shift will make the peak of
the combined mono signal sharper than watching for peak output on a
single channel. If the recorder has a balanced output and the
channels can be combined 180 degrees out of phase (with a Y-adapter
and polarity phlopper), at proper alignment, you'll get a very sharp
null. This is the easiest way to detect correct phase alignment if
the tape machine is capable. Running through a mixer and pushing the
"phase" button to flop the polarity of one channel is of questionable
value since going through the mixer may be introducing slightly
different phase shifts in the two channels.
> For bias I ALWAYS adjust for minimum modulation noise as that to my
> ear is the worst part of mag tape.
> The only tape I have found that is can not be done on is the Agfa 468.
> Minimum modulation noise will overbias the tape 6 to 9 db on this tape.
> I had a machine shut down as I overloaded the bias amps when trying this.
I recall attending an Agfa-sponsored workshop when 468 was introduced,
and they emphasized that minimum modulation noise, lowest THD (maybe
that's "third" rather than "total" harmonic distortion), and correct
high frequency response coincided at the same bias point. They made a
big deal about the fact that this was different from other popular
tapes of the time (Ampex 456 and 3M 250) - that with Agfa 468, you
didn't have to compromise one parameter if optomizing for another. I
found this to be pretty much true with Agfa 468 on my Ampex MM-1100.
I normally aligned 15 ips to 3 dB overbias at 10 kHz, but found that
this was also the point of lowest THD, and within half a division off
the 0 dB calibration mark when biasing by ear for minimum modulation
noise. These days I use Ampex 456, so I don't know what the current
Agfa (I guess it's BASF now?) does.
Bill Vermillion
>
>In article <1995Mar17.0...@bilver.oau.org> bi...@bilver.oau.org writes:
>
>(lots of good info deleted)
>> For bias I ALWAYS adjust for minimum modulation noise as that to my
>> ear is the worst part of mag tape.
>> The only tape I have found that is can not be done on is the Agfa 468.
>> Minimum modulation noise will overbias the tape 6 to 9 db on this tape.
>> I had a machine shut down as I overloaded the bias amps when trying this.
>I recall attending an Agfa-sponsored workshop when 468 was introduced,
>and they emphasized that minimum modulation noise, lowest THD (maybe
>that's "third" rather than "total" harmonic distortion), and correct
>high frequency response coincided at the same bias point. ....
This may have been totally machine dependant, but we found that
if we used the minimum modulation noise using the 7Hz signals,
that 468 took well over 6 db more bias than others - extreme
overbias would be an accurate description.
We always experimented with tapes, and when we tried this with
the 468 (using the Stephens 821B), we put all 32-track into
record. We got well over halfway there, we may have even
passed the 24-track mark, when all of a sudden, we cranked one
more channel's bias up and the machine shut down. We were
trying to suck more bias current out of that the machine would
allow.
We had 16/24/32 stacks, however the machine was wired with the
40 track harness and could have gone to 40 tracks like the
machine Queen used. I don't know if there were any other
40-track units made except for theirs.
John was delighted when installing our machine we finally got
the specs on the Telefunken 32-track unit and found the head
specs matched exactly. But what can you really change when
stuffing that many tracks into a 2" format?
Mike Diack
any other 3 head machine) :
feed inputs with subsonic tone (say 20Hz)
Record, and listen to monitor output on phones.
Slowly adjust bias for minimum modulation tone (wooshhhhh)
Thats it !!!!
How it works : serindipidously, the modulation noise curve hits
a minimum at exactly the right bias point for 456 @ 15 ips.
Doesn't necessarily work for other tapes or speeds.
cheers
Mike
Bill Vermillion
Even if the recommended bias and the minimum modulation aren't
at the same point, I always go for minimum modulation.
One of my biggest complaints with analog is modulation noise.
I hate hearing 'furry' pianos. Used with some NR systems
the modulation noise becomes more apparent.
The bias point also depends on your head gaps - so unless you
read the manufacturers specs and see what head-gap was used to
make the charts and you know what your gap is - you don't know
FOR SURE that the point is correct.
Analog recording is a series of compromises. Underbias and
distortion goes up, overbias and distortion goes down but so
does output.
In the past with recorders with pre-distortion circuits - I've
used distortion analyzers to set up recorders - but that was
long ago - and on machines that thankfully aren't made anymore
:-) I've learned a bit more about audio since then too.
Since we are listening to music (for the most part :-) ) - we'd
like to have the music and not have the recorder contributing
it's own artifacts. Modulation noise and asperity noise are
the two components of analog that I dislike the most. That's
why I prefer minimum modulation settings. I think the worst
case of modulation noise I've ever heard on a commercial
release is the late Harry Nillson's recording of "Jump Into the
Fire". Others may make their alignment choices based on other
criteria.
Thats one of the things with analog (whether it is a nice thing
or a bad thing - that's for you to decide). You can change the
sound of the recording by changing your alignment procedures.
The noise contributed by the medium and the difficulty in
getting repeatability in analog is why I prefer digital to analog.
Take a look at a typical analog tape spec sheet sometimes and
look at the numbers. There are specs for variation from one
end of the reel to the other, and specs for variation from
reel-reel. To be 'perfect' you'd need to align your machine
for EACH reel - but we settled for each batch :-)
David T. Medin
In article <znr795534496k@trad>, mri...@d-and-d.com (Mike Rivers) writes:
>
> In article <1995Mar17.0...@bilver.oau.org> bi...@bilver.oau.org writes:
>
> (lots of good info deleted)
>
> > The first tones on the alignment tape are HF alignment tones. If you have
> > a scope you can ajdust by watching the phase. If not available you watch
> > for maximum output at this frequency. I had the luxury of having phase meters
> > at the studio, this makes it all so much easier.
>
> For those with minimal test equipment, combine the channels of a two
> track recorder (or a couple of channels on a multitrack recorder) into
> mono, and watch for the peak. The phase shift will make the peak of
> the combined mono signal sharper than watching for peak output on a
> single channel. If the recorder has a balanced output and the
> channels can be combined 180 degrees out of phase (with a Y-adapter
> and polarity phlopper), at proper alignment, you'll get a very sharp
> null. This is the easiest way to detect correct phase alignment if
> the tape machine is capable. Running through a mixer and pushing the
> "phase" button to flop the polarity of one channel is of questionable
> value since going through the mixer may be introducing slightly
> different phase shifts in the two channels.
One item left out of the discussion so far, is that you should to do
the alignment starting at low frequencies (several hundred Hz) and
move up to the 15 KHz regime in steps. This is because of ambiguity
due to short wavelengths at higher frequencies being greater than the
azimuth range of the head assembly. If you start at low frequencies
where the possibility of being a cycle or more out of phase is least,
then move up, you should minimize the chance of this ambiguity
affecting the alignment. If you align only at one high frequency, then
accidentally end up a cycle or two out of absolute alignment, you
create a comb filter for mono images (when both channels are summed).
The amplitude difference between false peaks (looking at single
channel amplitude) are sometimes not great enough to tell you when you
are truly in absolute alignment.
Since most decks are reasonably aligned anyway, this is usually a step
you can breeze through with spot checks. After head replacement
though, it is a must.
--
-------------------------------------------------------------------
David Medin Phone: (319) 395-4498
Rockwell Collins ATD Internet:
Cedar Rapids, IA dtm...@cca.rockwell.com
P Stamler
you can't adjust PB head azimuth without a tape.
Paul Stamler
e-mail: Psta...@aol.com
Bob Olhsson
to do without the proper test equipment and standard test tape.<<
Not to mention having someone show you how the first couple of times!
Most people don't realize that everything mechanical on a tape deck
interacts with the bias and eq. allignment. The tensions and transport need
to be checked to determine WHY the azimuth or bias or eq. might be out
rather than just putting a simple Band-Aid fix on it with a few tweeks of a
screwdriver. This seriously risks incompatability with other machines and
premature head wear.
General allignment instructions only really hold true for large studio
console tape machines that are heavy-duty enough to rarely require
mechanical adjustment beyond a slight touch-up to compensate another
studio's azimuth allignment. (Even then, 2" machines are very tricky to get
right.)
That said, the number of service shops I've seen (especially at dealers')
that haven't a clue about properly alligning analog decks is frightening.
It's like front-end work on a car, when it's done wrong, you quickly eat a
set of heads even though it all "seemed ok"...
Bob Olhsson
Jeff Evans
: Flux loops are great for setting playback EQ, head resonance, etc., but
: you can't adjust PB head azimuth without a tape.
Nor can you correct for HF rolloff due to gap length and wear, if you
do have adjusters to get to it
--
========================================================
Jeff Evans jev...@netcom.com
Audio Intervisual Design phone 213-845-1155
Los Angeles, CA fax -1170
========================================================
josephma...@gmail.com
Scott Dorsey
>I have a revox a77, when on playing mode, the speed runs too fast, also when switching between 7 1/2 to 3 3/4, the casptain motor speed doesnt change.21 volt in 3 3/4 at pin ae 1 but speed not reduced
transistors in the servo circuit, also check for bad decoupling capacitors
if the machine hasn't been recapped in the past decade.
The manual has a good discussion on locating servo problems. The scope will
tell you what is going on. In a pinch, Jon Hall in Charlottesville, VA might
still be willing to work on them if you catch him on a good day.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."