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.