Video Head Evaluation

VIDEO HEAD EVALUATION

In all the service manuals there is an evaluation section that outlines a procedure for checking the condition of the video heads. On the left in the illustration above is the SL-5151 video head checker by Sony. On the right are two charts that accompanies the test method outlined in the manuals. Setting up the checker is paramount prior to testing if you ever going to be able to trust the results. Examine the test instrument on the left. Notice the slide switch on the meter marked MODE and also the BATT area on the scale. This device is battery operated and prior to any testing the condition of its batteries must be tested. With the power on and the mode switch in BATT the batteries are good when the pointer goes into the red. Next it must be calibrated by setting the slide switch to "CAL" and rotating the calibration knob to the place where the pointer is directly over the "CAL" line on the scale. With these steps completed we can now slide the switch to the "MEAS" (for measure) position. This where the chart comes in. There are two. If the until is two heads the manual will include the top chart. DA+1 and DA4 (dual azimuth) models have the bottom one. The only difference is the addition of a fourth area in the bottom chart for testing the A' and B' half of the DA heads. To see the testing process click on the photo. The leads coming from the heads are unsoldered from the round printed board and the test leads of the meter are placed on the end each lead. Polarity is not a concern even though the leads are colored. Consulting the manual we find it directs us to set the "RANGE" switch (in most cases) to the "B" range. The position of the pointer will now tell us the condition of the heads. Photos illustrate the following: top left good A or B head, top right good A' or B' head and lower left new A or B head for comparison. Because this method is based on the resistance generated by the small windings of wire around the iron cores the outcome is not perfectly reliable. Heads can be physically damaged or crusted with junk and still check good. So visual inspection is also required, as well as a performance check to evaluate the playback capabilities of the heads. The flying erase head(s) aren't tested in the manuals but I have found from experimentation that they measure similar to the A and B heads. Heads that measure in the B range would be replaced during refurbishing even though they could be considered useable. Click on the picture to see how to tell if video heads are bad by the visual or playback method. This page is taken from the section on refurbishing a Beta VCR. If you wish to go to the beginning of that section click on "Beta Refurbishing" in the navigator bar.