AMPEX MM1200 / MM1100 Bias and Erasure Adjustments and Tips
Bias Tips for Ampex MM 1200 / MM1100 machines
Here are some things I've picked up over 25 years of professional
audio recording on Ampex machines.....
BIAS FREQUENCY ADJUSTMENT
First you MUST check to see that the Bias Frequency is 150 khz .
Use a frequency counter, and make sure that it's within 1000 hz
The Adjustment is in the 39 Volt Power Supply Card, and you
would adjust the "slug" core of the single Transformer slightly up and
down until you get the correct frequency. NOTE THAT THIS
ADJUSTMENT MUST BE MADE WITH A PLASTIC TRANSFORMER SLUG ADJUSTING TOOL...
if you use anything containing metal you will get incorrect readings !!!!
BIAS CARD TRANSFORMER ADJUSTMENT:
You cannot correctly bias the machines unless both Bias
Transformers are correctly set in EACH Bias card. Under "Normal"
conditions, these transformers, once set correctly, will almost never
have to be re-adjusted, so pay particular attention when you set them
up.
You'll need an Extender Card for these adjustments, so borrow one if
you don't have one !!!
1) Put the Bias card on the extender, and insert it into the Bias slot.
Turn the machine back ON. You can "fool" the mahine into thinking
there's tape on it by using a rubber band and looping it around the
Counter and the Tape-Break Arm, positioning the arm near a normal
loaded position. BEWARE that you CANNOT do this for long periods of time -
audio tape also acts to wick away some of the heat that builds up in
the heads when the system is in record mode. You should be using
tape when you perform Erasure adjustments - almost any tape will do
since this first step is not 'tape-type sensitive'.... of course when
we do the final adjustments and alignment you MUST use the correct
type of audio tape you are going to be using.
2) Engage Record mode. Select BIAS on the VU Meter selector.
3) Adjust BIAS CAL on the Bias Card so the VU Meter reads "0 VU"
4) USE ONLY A PLASTIC TRANSFORMER ADJUSTING TOOL.
if you use anything metal you will be completely changing the action
of the coil and cannot correctly make these adjustments. Radio Shack
should sell these core adjustment tools, so go get 'em.
NOTE: Sometimes the slugs in the transformer cans will stick and won't
easily turn... do NOT force them - they will break!!
IF you cannot turn one, leave it as it is and move on to the
next can and just adjust that one...
5) Adjust the REAR coil first, turning the adjustment "slug" inside the
transformer (access is thru the hole in the metal cover over the
transformer). Turn the slug until the VU meter reads maximum bias.
6) Adjust the FRONT transformer the same way, again until the VU
meter reads maximum bias.
7) Re-adjust the REAR transformer. Then re-check the FRONT
transformer. If there's a broad area in which the VU meter doesn't
move try and set the slug into the center of this area.
8) If you have an oscilloscope, monitor the Bias waveform at the Bias
Cal pot, and adjust the transformers as above, then check that you
have a good undistorted bias waveform. Make minor changes to
obtain the best sine wave you can.
9) Turn the machine OFF, and move on to the next channel. You
should make these adjustments will ALL channels in RECORD, even
though you're making adjustments on only one card.
ERASURE Adjustments
The machine's Erasure is dependent on obtaining maximum current
flowing through the Erase Head. To get this happening, you have to
adjust the ERASE Adjust capacitor on the front of each bias card. But
this needs to be set for the particular head stack you're using..... the 24
track adjustment is different from the 16 track adjustment, so if you
switch heads, you've got to re-adjust the ERASE Adjust cap.
For MOST 24 track heads...
(for 16 track heads see the note at the "5a)" below...)
1) Thread tape and begin recording or"Fool" the machine into
thinking tape is on the machine (as above
in the Bias Transformer adjustment procedure...) - again be aware
that you cannot have the machine in Record mode for long without tape running!!!
I HIGHLY suggest you run tape!!!
2) go into RECORD on all tracks
3) Select BIAS on the VU meter selector
4) Turn the ERASE ADJ cap (Erase Peak) COUNTER-CLOCKWISE until the VU
meter drops a few db from the maximum level
5) Now start turning the ERASE Peak ADJ cap CLOCKWISE until you get
to the maximum VU meter reading. Now turn CLOCKWISE until
the VU meter falls by about 1.5 db from the maximum reading. In
most machines the fall will be a rapid drop, usually more than you
want, so you'll have to slowly turn the adjustment cap back until you
get a 1.5 db reading lower than the max reading.
5a) FOR 16 TRACK HEADS YOU ADJUST THE ERASE
ADJUSTMENT FOR MAXIMUM VU METER READING.... not the
1.5 db drop!!!
6) adjust the other channels..... there is some inter-play between
channels.... this is nothing to worry about UNLESS you see that a
large change happens on another track as you're adjusting any one
card. If this happens you'll have to go back to that particular card and
re-adjust the ERASE ADJ again.
7) Once you have adjusted the entire machine this way, you MUST
check the machine for MAXIMUM ERASURE>>>>>>>
MAXIMUM ERASURE TESTS
Demag the machine, clean the heads and guides, and use good quality
NEW tape. I'm assuming that the machine is somewhat correctly biased for
the type of tape you're using... if not, you should bias the machine
BEFORE making these final Erasure Tests !!!
1) Record a 1 khz tone for a few minutes on all tracks
2) Rewind to the head of this section
3) Monitor REPRO
4) Remove any signal going to the recorder
5) Begin recording
6) monitor each track one at a time, and listen for any residual tone...
you should hear only a very very small amount of tone, if any at all,
and this should be buried in the hiss of the tape itself... if not, make
small adjustments to the ERASE ADJUST cap until you get the
LOWEST amount of tone as you're erasing it. Also note that the
ERASE ADJUSTMENT also controls the "quality" of the erased noise,
and you want to adjust the ERASE ADJUST for both Maximum
Erasure AND Minimum tape "gravel" noise.
BIASING for Specific instruments
Here's a real trick that few know about.
One of the best ways to bias a machine is to know that the "normal"
bias is an "overall" good point, but not the best for certain
instruments... in some cases you are not looking for great frequency
response, but a specific great "sound". Kick drum and bass guitar are
examples of instruments that really do want to be "incorrectly biased"
rather than set up at the bias for everything else...
So if you can, get the drummer to hit the kick drum, record this onto
the track you're going to use for the kick drum, and Monitor REPRO,
and now slowly turn the Bias adjustment way down (counter-
clockwise) then begin turning the bias back up..... listen for changes in
the sound, and adjust the bias for the BEST SOUND.... You'll often
find that you'll get to a bias point way beyond "normal" for low end
things like kick and bass.... say 5 or 6 db over at 10khz rather than
the "normal 3 or 4 db overbias. Try this, it works really great!!!
But do this ONLY with low frequency instruments, for anything else
use the "normal" bias of between 3 and 4 db at 10 khz (15 ips).
USING THE BIAS CAL
Bias Cal is just a small amount of the actual bias signal that's sent to
the VU meter when you select BIAS on the VU meter selector. It has
NOTHING to do with actual bias, it's just a convienent way to check
bias levels. And it's something that you should set when you're biasing
machines. That way, if something goes wrong, you can quickly look
at the Bias Cal reading and know if that track is putting out bias to the
heads....
NOISE ADJUST pot...
on the later Bias Cards there is a small trimpot in the middle of the
bias cards. This is to adjust the amount of bias noise that's printed
onto tape. To adjust this, monitor REpro while recording a black
signal onto tape, and adjust for minimum noise. If you've got the
proper test equipment, you can adjust this for minimum second
harmonic distortion, but if you've got that type of test gear around you
already know what the heck to do don't you?
PURC BIAS CARDS...
completely different, and beyond the scope of this discussion folks...
MM 1000 and AG 440's
Basically the same as above. Erasure adjust is set for Maximum VU
readings, and the maximum erasure tests should be performed.