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Last week a customer called concerning a problem he was having with his reel to reel tape deck. One channel was not producing an output signal and he wondered if he could measure some parameter(s) to determine if the playback head was ok.
I suggested that he first try to inject a 1000 Hz signal into each channel of the tape deck with a signal generator (directly into the playback head amplifiers with the tape head disconnected from the electronics). He did not have a signal generator, so I suggested that he use the "Make Waves" generator of Diamond Cut and then use the output of the soundcard as the injection source. I suggested that both "hot" leads be lifted from the playback head (unsoldered) first before running the test. I recommended using a -40 dB setting of the "Make Waves" signal generator for the test. By injecting this signal into the tape deck input, he should have been able to isolate the problem between the tape deck amplifier(s) and the tape head itself.
Furthermore, I suggested that it is quite rare that a tape deck head fails "open", but it does happen. I have repaired high-end and pro audio equipment over a 6 year period between my sophomore year of high school and all 4 years of engineering school, so I have seen a lot of different types of tape deck related failures, usually involving worn tape heads, not open coil ones. Perhaps, I have seen two or three tape heads actually measure "open" in my lifetime.
Sometimes, tape heads can be re-lapped (ground), but often they need to be replaced because the wear extends deep enough that the gap is increased which kills the high frequency response of the head.
I do not know the outcome of the customers diagnosis of his reel to reel machine. Nonetheless, here is a listing of some tape recorder head coil resistances based on the contents of my tape head junk box:
1. Wollensak reel to reel, 1/4 inch, 1/2 track Record / Playback Head: 317 Ohms
2. Webcor reel to reel, 1/4 inch, 1/2 track:
A. Record / Playback Head: 1380 Ohms
B. Erase Head: 0.2 Ohms
3. Crown reel to reel, 1/4 inch, 1/2 track Playback Head: 268 Ohms
4. Generic replacement reel to reel, 1/4 inch, 1/2 track Record / Playback Head: 520 Ohms.
5. Magnecord reel to reel, 1/4 inch, 1/4 track Playback Head: 370 Ohms
6. Ampex reel to reel, 1/4 inch, 1/4 track:
A. Erase Head: 18 Ohms
B. Record Head: 55 Ohms
C. Playback Head: 44 Ohms
7. Sony TC-200, 1/4 track
A. Record/Play Head: 155 Ohms (each coil)
B. Erase Head: 0.05 Ohms
8. Generic replacement reel to reel, 1/4 inch, 1/4 track Record / Play Head: 200 Ohms
9. Generic replacement reel to reel, 1/4 inch, 1/4 track Erase Head: 3.0 Ohms
10. Compact Cassette, 1/8 inch, 1/2 track Monophonic Record / Playback Head: 250 to 335 Ohms (measured 5 heads).
11. Compact Cassette, 1/8 inch, 1/4 track, Stereo Record / Playback Head: 150 - 211 Ohms (measured 3 heads).
12. 8 Track Cartridge, 1/4 inch, 1/8 track, Stereo playback head: 523 Ohms
13. VHS, 1/2 inch, Linear Track Monophonic Audio playback head:
A. Audio Channel: 350 Ohms
B. Sync / Control Channel: 550 Ohms
C. Erase Head: 1.0 Ohms
14. Micro-Cassette Record / Playback Head: 250 Ohms
Note: After measuring the coil resistance of a tape recorders head(s), it is necessary to de-gauss (demagnetize) the head. A certain amount of residual magnetism will remain on the head because of the d.c. bias applied by your Ohmeter or Multimeter (this is referred to as magnetic remanence). If you do not perform this step, you may degrade any tape played on the machine thereafter. Also, it is very important to make these measurements with the tape deck turned off and also having no tape in the machine. If a tape is in the machine when making these measurements, you will erase (creating a "click" on the tape) where the head gap had been located.
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The best way to evaluate a tape head from an electrical perspective is the measurement of its Impedance. Impedance is sort of the AC equivelant to DC resistance, but it includes the reactive terms as well as the coil resistance. Tape head impedance (or Z) is:
Z = {(R^2 + Xl^2 + Xc^2)}-1/2
wherein,
Xl = 2pi x F x L (where L is in Henries & F is Frequency in Hz)
and
Xc = 1 / 2pi x F x C (where C is in Farads and F is Frequency in Hz)
If one measures well below the self resonance of the tape head coil structure, then it simplifies to:
Z = {(R^2 + Xl^2)}-1/2 (1000 Hz is a good value to use)
I used an impedance bridge or an impedance comparator to make this measurement. I would go to the stock room and grab a known good head of the correct model number and either measure it and then measure the suspect head against that or I would simply connect an impedance comparator between the suspect head and the known good one. If the measurement was more than 15 percent variant, I knew that the tape head gap was beginning to widen, thus the head needed replacement. Another method which can be useful when working with a stereo machine is to measure one channels head Z and compare it with the other. If they are variant by more than about 20 percent, then something is definitely amiss. This method assumes that both channels generally do not fail at the same time which is generally true. Note that as the tape head gap wears and widens, the Xl term starts to decrease with the resistance term remaining constant.
Tape Head Impedance measurements must be made with the head disconnected from the tape deck amplifiers, else a false reading will occur.
Note: I did not include Z measurements of the tape heads listed above because very few folks own impedance bridges or impedance comparators. However, if someone is interested in a Z value for a particular type of head, I can do that for you. Just let me know which of the abovementioned types of heads you want me to measure and I will do so and then I will post the results here.
--------------------------------------------
Tape Head Impedance Measurement Example:
Right Channel Results:
I measured the Xl and calculated the Z of the Magnecord reel to reel, 1/4 inch, 1/4 track Playback Head shown in the above coil resistance listing. With a 1 KHz excitation set on the Z bridge, I measured an Inductance of 0.340 Henries. Therefore, it had an inductive reactance (Xl) of 2135 Ohms. Given that its coil resistance is 370 Ohms, its calculated Impedance (Z) @ 1 KHz is therefore, 2167 Ohms.
Left Channel Results:
The left channel coil Resistance measured 356 Ohms. With a 1 KHz excitation set on the Z bridge, I measured an Inductance of 0.320 Henries. Therefore, it had an inductive reactance (Xl) of 2010 Ohms. Given that its coil resistance is 356 Ohms, its calculated Impedance (Z) @ 1 KHz is therefore 2041 Ohms.
The variance between the left and right channel Impedance is 126 Ohms or around 6% which is within the expected tolerance for a good tape head. Also noted was the self resonance (parallel L C resonance) of the tape head which was 29.5 KHz which is good since it is well above the audio spectrum and nowhere near our impedance measurement frequency of 1 KHz. This self resonance was measured with a signal generator, a coupling resistor (51 KOhms), and an oscilloscope.
--------------------------------------------
Note 1: Most audio tape playback heads Z will fall somewhere within the same order of magnitude as the Magnecord head, as a general guideline.
Note 2: Higher tape head inductance results in better low end (bass) response.
Note 3: Smaller playback tape head gaps results in better high end (treble) response. For more details on this subject, follow this link:
http://www.diamondcut.com/vforum/sho...=tape+head+gap
keywords: tape head resistance, tape head coil resistance, tape head impedance, tape head inductance, tape head resonance, tape head resonant frequency
I suggested that he first try to inject a 1000 Hz signal into each channel of the tape deck with a signal generator (directly into the playback head amplifiers with the tape head disconnected from the electronics). He did not have a signal generator, so I suggested that he use the "Make Waves" generator of Diamond Cut and then use the output of the soundcard as the injection source. I suggested that both "hot" leads be lifted from the playback head (unsoldered) first before running the test. I recommended using a -40 dB setting of the "Make Waves" signal generator for the test. By injecting this signal into the tape deck input, he should have been able to isolate the problem between the tape deck amplifier(s) and the tape head itself.
Furthermore, I suggested that it is quite rare that a tape deck head fails "open", but it does happen. I have repaired high-end and pro audio equipment over a 6 year period between my sophomore year of high school and all 4 years of engineering school, so I have seen a lot of different types of tape deck related failures, usually involving worn tape heads, not open coil ones. Perhaps, I have seen two or three tape heads actually measure "open" in my lifetime.
Sometimes, tape heads can be re-lapped (ground), but often they need to be replaced because the wear extends deep enough that the gap is increased which kills the high frequency response of the head.
I do not know the outcome of the customers diagnosis of his reel to reel machine. Nonetheless, here is a listing of some tape recorder head coil resistances based on the contents of my tape head junk box:
1. Wollensak reel to reel, 1/4 inch, 1/2 track Record / Playback Head: 317 Ohms
2. Webcor reel to reel, 1/4 inch, 1/2 track:
A. Record / Playback Head: 1380 Ohms
B. Erase Head: 0.2 Ohms
3. Crown reel to reel, 1/4 inch, 1/2 track Playback Head: 268 Ohms
4. Generic replacement reel to reel, 1/4 inch, 1/2 track Record / Playback Head: 520 Ohms.
5. Magnecord reel to reel, 1/4 inch, 1/4 track Playback Head: 370 Ohms
6. Ampex reel to reel, 1/4 inch, 1/4 track:
A. Erase Head: 18 Ohms
B. Record Head: 55 Ohms
C. Playback Head: 44 Ohms
7. Sony TC-200, 1/4 track
A. Record/Play Head: 155 Ohms (each coil)
B. Erase Head: 0.05 Ohms
8. Generic replacement reel to reel, 1/4 inch, 1/4 track Record / Play Head: 200 Ohms
9. Generic replacement reel to reel, 1/4 inch, 1/4 track Erase Head: 3.0 Ohms
10. Compact Cassette, 1/8 inch, 1/2 track Monophonic Record / Playback Head: 250 to 335 Ohms (measured 5 heads).
11. Compact Cassette, 1/8 inch, 1/4 track, Stereo Record / Playback Head: 150 - 211 Ohms (measured 3 heads).
12. 8 Track Cartridge, 1/4 inch, 1/8 track, Stereo playback head: 523 Ohms
13. VHS, 1/2 inch, Linear Track Monophonic Audio playback head:
A. Audio Channel: 350 Ohms
B. Sync / Control Channel: 550 Ohms
C. Erase Head: 1.0 Ohms
14. Micro-Cassette Record / Playback Head: 250 Ohms
Note: After measuring the coil resistance of a tape recorders head(s), it is necessary to de-gauss (demagnetize) the head. A certain amount of residual magnetism will remain on the head because of the d.c. bias applied by your Ohmeter or Multimeter (this is referred to as magnetic remanence). If you do not perform this step, you may degrade any tape played on the machine thereafter. Also, it is very important to make these measurements with the tape deck turned off and also having no tape in the machine. If a tape is in the machine when making these measurements, you will erase (creating a "click" on the tape) where the head gap had been located.
---------------------------------------------------------------------------
The best way to evaluate a tape head from an electrical perspective is the measurement of its Impedance. Impedance is sort of the AC equivelant to DC resistance, but it includes the reactive terms as well as the coil resistance. Tape head impedance (or Z) is:
Z = {(R^2 + Xl^2 + Xc^2)}-1/2
wherein,
Xl = 2pi x F x L (where L is in Henries & F is Frequency in Hz)
and
Xc = 1 / 2pi x F x C (where C is in Farads and F is Frequency in Hz)
If one measures well below the self resonance of the tape head coil structure, then it simplifies to:
Z = {(R^2 + Xl^2)}-1/2 (1000 Hz is a good value to use)
I used an impedance bridge or an impedance comparator to make this measurement. I would go to the stock room and grab a known good head of the correct model number and either measure it and then measure the suspect head against that or I would simply connect an impedance comparator between the suspect head and the known good one. If the measurement was more than 15 percent variant, I knew that the tape head gap was beginning to widen, thus the head needed replacement. Another method which can be useful when working with a stereo machine is to measure one channels head Z and compare it with the other. If they are variant by more than about 20 percent, then something is definitely amiss. This method assumes that both channels generally do not fail at the same time which is generally true. Note that as the tape head gap wears and widens, the Xl term starts to decrease with the resistance term remaining constant.
Tape Head Impedance measurements must be made with the head disconnected from the tape deck amplifiers, else a false reading will occur.
Note: I did not include Z measurements of the tape heads listed above because very few folks own impedance bridges or impedance comparators. However, if someone is interested in a Z value for a particular type of head, I can do that for you. Just let me know which of the abovementioned types of heads you want me to measure and I will do so and then I will post the results here.
--------------------------------------------
Tape Head Impedance Measurement Example:
Right Channel Results:
I measured the Xl and calculated the Z of the Magnecord reel to reel, 1/4 inch, 1/4 track Playback Head shown in the above coil resistance listing. With a 1 KHz excitation set on the Z bridge, I measured an Inductance of 0.340 Henries. Therefore, it had an inductive reactance (Xl) of 2135 Ohms. Given that its coil resistance is 370 Ohms, its calculated Impedance (Z) @ 1 KHz is therefore, 2167 Ohms.
Left Channel Results:
The left channel coil Resistance measured 356 Ohms. With a 1 KHz excitation set on the Z bridge, I measured an Inductance of 0.320 Henries. Therefore, it had an inductive reactance (Xl) of 2010 Ohms. Given that its coil resistance is 356 Ohms, its calculated Impedance (Z) @ 1 KHz is therefore 2041 Ohms.
The variance between the left and right channel Impedance is 126 Ohms or around 6% which is within the expected tolerance for a good tape head. Also noted was the self resonance (parallel L C resonance) of the tape head which was 29.5 KHz which is good since it is well above the audio spectrum and nowhere near our impedance measurement frequency of 1 KHz. This self resonance was measured with a signal generator, a coupling resistor (51 KOhms), and an oscilloscope.
--------------------------------------------
Note 1: Most audio tape playback heads Z will fall somewhere within the same order of magnitude as the Magnecord head, as a general guideline.
Note 2: Higher tape head inductance results in better low end (bass) response.
Note 3: Smaller playback tape head gaps results in better high end (treble) response. For more details on this subject, follow this link:
http://www.diamondcut.com/vforum/sho...=tape+head+gap
keywords: tape head resistance, tape head coil resistance, tape head impedance, tape head inductance, tape head resonance, tape head resonant frequency