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General Maintenance & Calibration Procedure for 3 Head Reel-to-Reel Tape Decks
rev 0
12/06
rev 0
12/06
The following is a generalized calibration procedure for 3 head, (3 motor) reel-to-reel tape recorders / tape decks. This procedure is in no way a substitute for the procedures outlined in the Service Manual for a specific model. However, in the absence of such a Service Manual, this generalized procedure may prove useful.
Warning: Before commencing the preliminary portion of this procedure, disconnect the machine from the electrical mains. Dangerous electrical potentials may be exposed and could cause a dangerous electrical shock. The following procedures should not be performed by those unfamiliar with the proper and safe handling of electrical / electronic equipment.
Some of these steps may be skipped if you do not have the proper equipment to perform them. That will simply result in an incomplete tape deck set-up and calibration keeping in mind that it is better to perform some of the steps rather than none of them. Some of these procedures are order dependent.
The procedures assume that you will be working on a ¼ inch pro-grade machine capable of either ½ or ¼ track stereo.
1. Clean all of the tape heads and tape guides with de-natured alcohol and cotton swabs (Q-Tips). Repeat this cleaning process until all traces of oxide are removed as evidenced by the lack of the appearance of a reddish-brown, brown, or dark brown substance on the cotton swabs.
2. De-gauss all three tape heads, tape guides and the capstan using a tape head de-gausser.
3. Using clock oil, or oil recommended by the tape deck manufacturer, lightly oil the following bearings:
A. Capstan Motor (1 – 2 drops per bearing)
B. Supply and Take-Up motor bearings (3 – 4 drops per bearing)
C. Capstan Shaft (1 drop)
Note: Over-oiling can be just as damaging to your machine as under-oiling.
4. Let the machine sit for an hour or two so that the oil soaks into the felt oiling pads on the motor bearings.
5. While the machine is resting and soaking up its oil (step 4), remove the pinch roller. Clean the old oil from its shaft with a dry cotton swab. Clean the rubber of the pinch roller using liquid soap and water. Do not use alcohol on the rubber surface of this part as it will degrade its properties (durometer which is its softness – hardness characteristic).
6. After the machine has finished soaking in its oil, plug the unit into a source of electrical power.
7. Place a take-up reel and a supply reel of blank tape on the machine and fast-forward it to the halfway position. Make sure that the reel size setting are in agreement with the size of the tape that you place on the machine, i.e. 5, 7, 10.5 inch. Also, make sure that the supply reel size matches the take-up reel size.
8. Place the machine in “Play” mode.
9. Using a tentelometer, measure the supply and take up side tape tension. It should be around 45 +/- 10 grams. If it is outside of that tolerance, then adjust the tape tension servo pots until the proper value is achieved.
10. Using the tentelometer, measure the supply and take up side tape tension values when the machine is placed in fast forward and also in rewind modes. The tension values should also fall in the range of 45 +/- 10 grams.
11. Rewind and remove the blank tape and replace it with a 3,000 Hz, 15 ips flutter free test tape. Set the output level control to the 50 % position.
12. This test will confirm the speed of the machine to be within spec. @ the 15 ips speed. Connect the output of the machine to your soundcard or a frequency counter. Play the tape. Measure the frequency of the tape using your Diamond Cut Software’s spectrum analyzer (in high resolution mode) or a frequency counter. The measured result should be 3,000 Hz +/- 30 Hz max. If it is outside this specification, adjust the capstan motor servo control speed potentiometer until the unit is within tolerance.
13. Connect the output of the machine to a Wow and Flutter meter (such as a Varo Fl-30 or equivalent). Play the 3,000 Hz test tape (@ 15 ips) and measure the wow and flutter value using the wow and flutter meter. It should be no greater than around 0.1% RMS.
Note: The following procedures involve setting up the tape heads into proper alignment. Non Ferrous (such as brass) tools should be used to avoid magnetizing the heads. Also, great care should be afforded not to contact the tape head surface with any tools. Rarely do items 14, 15 and 16 need attention except when a head replacement had been performed. But, tweaking item number 17 (azimuth) usually produces an improvement in performance. Generally, 3 screw adjustments will be found on each head (except, often only 2 for the erase head). They form a triangle. Two screws fall in the same plane as the direction of tape travel while another forms the third point of the triangle. The screw at the third point of the triangle adjusts the tape head zenith (16).
14. Adjust the Tape Head Height: This adjustment involves the vertical orientation of the tape head with respect to the tape itself. Proper adjustment locates the tape head stacks in vertically equal alignment with the tape. Tape head gaps should fall in proper alignment with the prescribed location of the tape tracks when this parameter is adjusted properly. No tape head gaps should fall partially above or below the tape edges. If you are replacing a head, adjust the head accordingly visual inspection. Otherwise, this parameter will not require adjustment.
15. Wrap: This parameter defines the arc over which the tape travels past the tape head gap(s). When properly adjusted, the gap should be in the precise center of that arc. If you are replacing a head, adjust the head accordingly visual inspection. Otherwise, this parameter will not adjustment.
16. Zenith: This parameter defines the front to back orientation of the tape head with respect to the tape. In other words, the tape needs to be moving in the same plane as the tape head surface. If it is not, uneven tape head wear will occur and the head will fail prematurely (loss of high frequency response on some channels on a multi channel machine). Adjust this parameter also via visual inspection.
17. Coarse Azimuth Adjustment (Playback Head): This parameter defines the perpendicularity of the tape head gap to the direction of tape movement. If it is not perfectly perpendicular, then high frequency loss will occur. Also, stereo image distortion can occur. To adjust this parameter for the playback head, you will need an azimuth test tape having a 15 KHz tone (or something close to that value). Connect the line outputs of the tape deck to your sound card. Bring up Diamond Cut and the Multifilter. Remove all filters from the filter signal pathway and put the software into “Live Preview” mode. Play the azimuth tape at the machines highest speed and adjust the azimuth adjustment screw(s) on the playback head for the maximum reading shown on the Diamond Cut VU meter while Diamond Cut is in “Live Preview” mode.
18. Fine Azimuth Adjustment (Playback Head): After the Coarse Azimuth adjustment has been completed, bring up the X-Y plotter in Diamond Cut. Play the azimuth tape and fine tune the azimuth adjustment screw(s) until you see a 45-degree line (up and to the right) formed on the plotter; you will not be able to obtain a perfectly straight line, but adjust for the closest that you can get to the ideal.
19. Measure and Adjust the Bias / Erase Oscillator: Connect an oscilloscope or frequency counter across one of the two erase head windings. Be sure that the shell of the BNC connector ground shares the tape deck common side of the tape head connection (the shielded side of the connection to the erase head). Put a blank tape on the machine and place it in Record mode. Adjust the bias oscillator “can slug” (a metallic rectangular device having a round hole in its top) so that it reads the manufacturers recommended bias frequency. Usually, on pro decks, this will be 100 KHz +/- 5 KHz. Be sure to use a plastic “twiddle stick” to adjust the ferrite slug. Metallic adjustment screwdrivers can “pull” the frequency while adjusting and / or damage the ferrite slug.
20. Bias Trap Adjustment: The Recording Head is fed by the Bias oscillator through a trimmer capacitor and fed by the recording amplifier through a bias trap LC tank circuit. The bias trap circuit prevents AC Bias from back-feeding into the recording amplifier, because if it did, high levels of distortion would be produced by the amplifier. So, there is usually a test point (TP-X) located near the output of the recording amplifier and on one side of the bias trap. Connect an oscilloscope to this test point. Adjust the bias trap inductor (with a plastic twiddle stick) for a minimum deflection on the oscilloscope of the AC bias signal. The bias trap inductor looks similar to the Bias oscillator can.
21. Record Head Bias Adjustment: It is very hard to generalize an effective procedure for this adjustment because each machine design requires a different set of parameters to be applied. Also of note is the fact that the record head bias current needs to be tuned to tape manufacturer and type for optimal results. If this procedure is not performed properly, the upper frequencies in the audio spectrum may become overly attenuated or accentuated. So, the best recommendation that I can make, absent a manufacturers procedure, is to leave the record head bias trimcaps alone.
22. Coarse Record Head Azimuth Adjustment: Place a blank tape on the machine. Create a 10 KHz wavefile with the make waves generator in Diamond Cut. Make it 600 seconds in length. Connect the output of your soundcard to the line input of your tape deck. Play the wavefile in Diamond Cut at the same time that you record it onto the tape deck, with the output of the tape deck set for “tape monitor” (not source monitor). Record this signal at around –10 dB as shown on the recorders VU meter when switched to “Source”. Adjust the azimuth screw(s) on the record head for the maximum deflection on the output VU meter.
23. Fine Azimuth Adjustment (Record Head): After the Coarse Azimuth adjustment has been completed, bring up the X-Y plotter in a second instance of Diamond Cut. Connect the output of the tape deck to the soundcard input. Place the second instance of Diamond Cut into “Live Preview” mode via the Multifilter with no filters present in the filter lineup. With the machine still recording and playing the 10 KHz waveform, fine tune the azimuth adjustment screw(s) until you see a 45 degree line (up and to the right) formed on the plotter; you will not be able to obtain a perfectly straight line, but adjust for the closest that you can get to the ideal.