Quote:
Originally Posted by DMcMahon
In an earlier post I did reference that there was no sound or tape hiss in Forward and Reverse Play. The finger test on live sides of the playback heads wiring resulted in hum and only hum for the correct head depending upon whether Forward or reverse play selected, which to some extent partially checks the wiring/switching etc.
Last night did the head demagnetiser test, once again got loud hum when the probe was close to the respective head with Forward or Reverse Play selected.
For completeness I will do a full continuity resistance check of the all the associated wiring including the hum cancelling coils and the track switching.
After that will scope probe the head signals going to the electronics to see if any actual sound signal and if so, then see why it is not getting trough to the output stage, when tape is played.
Of course the playback heads could be open circuited. In that case I might still expect to get a hum signal when head live wiring touched and when demagnetiser used. So also may check the continuity of the playback heads.
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The results of your tests will be very interesting. Pace Ted Kendall, it will be strange if the P/B heads give no output
at all from tape if they aren’t o/c – even if hopelessly low-level and frequency-limited. When I first saw your photos in #113 I didn’t know what to make of them – I’m only familiar with 1970s Bogen heads, which had a 1 micron gap – which is hardly visible to the naked eye. I assumed that there must be something very different about the construction of the heads in your picture with what looked like huge gaps. If that really is the state of their gaps, then I can’t imagine how they got as bad as that – they would surely have been abandoned long before reaching that state.
My (perhaps simplistic) view is that, if your defluxer is able to induce hum in them, then there must be a complete circuit there of some sort. So, a bit of several mysteries so far.
I also have heard that the Bogen heads (your Uher)were notorious for going o/c (as were the Papst motors that often sat close to them – the Papst capstan motor on my Brennel failed at about 3-4 years from new).
I await the denouement with bated breath!
Mike