"Nama" wrote in message
...
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

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'.

Arfa