Blue Ring Tester

Discussion in 'Test & Measurement Forum' started by carlosraj, Mar 8, 2018.

  1. carlosraj

    Thread Starter New Member

    Jan 24, 2018
    5
    0
    Hi

    Can a blue ring tester be used to check the recording head of a Studer reel to reel tape machine. One side of my tape recorder is not recording. I measured the resistance , the good side (can record) shows a resistance of 7.5 ohms and the bad side (cannot record) shows a resistance of 7.7 ohms. Then I used my blue ring tester (since what is inside the head is a coil for each channel). The good side shows all green LEDs, while the non recording side shows only the red LEDs. Can I conclude that the non recording side has a damaged coil. Please assist.

    Thanks

    Carlos
     
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    7,045
    1,994
    I've never used a blue ring tester, but from all accounts this is exactly what it is good at finding. If you are sure about your connections, then the head probably is bad. How old is it? The problem might be erosion of the gap rather than an actual failure in the winding.

    I assume you have completely removed the head's connections to any circuits so they are two stand-alone inductors. If so, can you reverse the head's connections to the channels (swap left and right) to see if the problem moves to the other head? This would eliminate the record drive and bias circuits as suspects. I realize that this is not an easy thing to do, but I've done it and it saved me a lot of money.

    Also, what about playback? Are these record-only heads, or R/P? Even if they are record-only, they will produce a signal during playback. You can verify this "good enough" with a scope set to its highest gain.

    Note - ring testing will leave the heads *seriously* magnetized. Degauss thoroughly before playing anything important.

    ak
     
  3. wayneh

    Expert

    Sep 9, 2010
    14,846
    5,331
    I built a ring tester that uses a similar strategy as the Blue ring tester. I spent some time looking at the ringing waveforms produced when testing different inductors, versus how the rings were counted by the device. I don't know the numbers anymore, but the ring tester can't properly test inductors with inductances too low or too high for the device. It's designed for testing TV flyback transformers. If you test a wall wart transformer, it's inductance is so large that it will still be ringing slowly when the counting period of the device ends. If the tested inductance is small, the ringing pulses may be too fast to count, or have ended before the counting period starts.

    One thing you might do is test a known-good one to find out how it "should" react.
     
  4. carlosraj

    Thread Starter New Member

    Jan 24, 2018
    5
    0
     
  5. carlosraj

    Thread Starter New Member

    Jan 24, 2018
    5
    0
    If I put to "sync" then both sides of the record head can playback. If I shift the record head connections, it still the right side that is recording. Now the right side also has a problem, it records but it is not completely erasing the previous recording. I measured the voltage at TP3 using a scope, the erase head high voltage. I use to get a reading of 6.5V DC on both sides. Now I connect the scope to the right side TP3(test point) and put it on record. At first it shows a high voltage waveform but within secs it drops to a very low waveform. This happens to both sides and both sides are not erasing completely. There is a constant 150Khz sine wave( from the command board) entering the transconductance IC(LM13700), that drives both the erase head high voltage and record head bias coils. I guess there must be a constant high voltage present at both the erase and record head while recording. Pls assist, why is the high voltage disappearing.

    Thanks
     
Loading...