Testing playback heads

[pentoad]

I understand that you should not test the heads directly with a dvm or avo as there is risk frying them with the current that is applied during testing.
Is it risk free to test via, say a 10k resistor in series and subtracting that from the reading?

[DMcMahon]

I personally consider this is largely an old wives tale, but there will be people who have experienced it.

There is definitely a risk of magnetising the head especially if using an analogue meter, using a modern digital meter greatly reduces the magnetisation risk. Using a series resistor additionally will reduce the risk. A magnetised head can affect the playback sound and of course can be fixed by de-magnetising the heads and tape path.

I have measured a large number of heads many times using Avo's and digital meters (nearly always without a series resistor) and have not experienced the head magnetisation issue. Once after measuring a Bogen head continuity OK, it subsequently went open circuit some short time later. Because Bogen heads have a bad reputation of going open circuit at the drop of a hat, I do not believe the previous continuity test was to blame.

David

[Sideband]

Yes I would say untrue. During my time at Philips workshop back in the 70's, I tested a large number of heads in cassette and reel to reel with an analogue meter. The only warning was that the head MIGHT become slightly magnetised when the test prods are removed due to the collapsing field. Personally I never experienced any problems and we had demagnetisers anyway which were used as a matter of course but even on the odd occasions when it wasn't possible to use the demagetiser, everything seemed to work ok and apparently the act of switching the machine into record could have a demagnetising effect due to the AC field generated....(a bit like a degaussing coil used in colour TV's).

You certainly won't 'fry' them.......where is all the power coming from to do that?

[Ted Kendall]

Back in the sixties, Mac Hellyer said "it is tempting to check continuity of the head windings with a test meter, but this can magnetise the head, so the general advice is "don't do it!" But if a demagnetiser is available, and the standard clean-up routine followed, there is no harm done."

Since then, electronic test meters have become the norm, putting less current through windings, but Bogens have been failing right and left for a decade at least. I think this is to do with green spot rather than test meters. Any head adversely affected by a modern meter has either failed independently or would have done so spontaneously before long anyway.

[DMcMahon]

Had to Google green spot. So this is oxidation/corrosion of the copper wire brought on by age/damp air ingress (maybe accelerated/induced by lack of use) i.e. a form of Verdigris.

[ricard]

I've heard about 'green spot', but had to google Verdigris ...

Seriously though, I seem to remember reading about damage to phonograph pickup coils from DC measurement. Certainly I remember in my youth being asked to 'have a look at' a gramophone where only one of the channels was working. After measuring the coils with my 20 kohm/volt analogue meter, both seemed to be open, although whether that was due to my measurement or whether they were both on the verge of failing I don't know - I was very young at the time and couldn't do much more about it anyway.

Regarding magnetization, I once had a Beocord 2000, and I think the amplifier input design was such that the playback head coils were floating a couple of volts above ground. During fault finding, I accidentally shorted one of the poles to ground, leaving the head magnetized, as evidenced by the characteristic "frying rumble" of DC magnetization upon subsequent playback of a tape (actually, I think the sound is not heard until the second playback, when the noise has been put on the tape from the first time). However, ordinary demagnetization with a defluxer brought back normality.