I have an old Sharp MZ80k computer (from the late 70's) which unfortunately needs the mono head replaced in it's built in cassette deck. I have found a place online that sells mono heads, but they come in different OHM ratings, from 60 OHMS to 400 OHMS. Unfortunately I have not been able to find out any specific info on the original head (labeled KA30 - OE030). And using my multimeter on the dead head gives me no reading what so ever. I didn't realize heads came in different ratings, so now I'm stuck. I was wondering if anyone here would have any advice for me. Phil
There's usually a head tuning cap connected to the head to help the HF end. If there is and you give the value it might be possible to make an educated guess.
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Dave Plowman dave@davenoise.co.uk London SW
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Likely any of those would work OK, the applciation is not that demanding. If I were to guess (and I am) I'd go with something in the middle of the range, about 200 ohms.
Looking at the schematic, I think it is highly unlikely that the replay circuitry will care less about the head resistance. The record situation is a slightly different kettle of fish in that it uses DC bias supplied by R3007, so the resistance of the head will influence the level of this bias. That said, in all probability, the practical effect of this is likely to be negligible. If it were me looking for a suitable replacement, I think that where DC bias was being used, I would tend towards a higher rather than lower resistance one.
The most challenging aspect of this repair is likely to be setting the azimuth correctly after fitting the new head. I think that I would probably hook a monitor amplifier to the righthand side of C3006, then play back a good quality audio recording - doesn't matter if it's stereo - and adjust for best audio quality. Alternatively, you could use a known good data tape which previously loaded ok, and adjust for best signal amplitude with a 'scope attached to Q3003 collector, or if no 'scope available, listen on the monitor amp as described before, and adjust for 'sharpest' sounding data noise. Remember to seal both the fixed and azimuth adjustment screws with acrylic paint (nail polish or similar), when you've finished adjusting.
If you have a head deguassing probe, I would recommend using it on the replacement head, because depending on how the head has been stored and handled, it is not all unknown for them to be somewhat magnetized 'out of the box'.
Just a comment... Does it have to be specifically a mono head? I suspect that a more readily available stereo head would work with the windings simply connected in parallel (or maybe series), provided the phasing was correct.
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