14 Jun
2007
14 Jun
'07
1:19 p.m.
1820-0325.=
MC815P
That one suprised me last night, becuase I couldn't understand why
there'd be a DTL chip on this board. Am I correct it's a type for
1810-0325? That appears on my board _and on the board of one of the disk
drives _ and is a 150 Ohm resistor array (terminator for the floppy drive
cable). I am pretty sure the read clock VSO (and part of the
PLL circuit?) is the
sea of discrete transsitors at the front edge of the board. There's a
twiddlepot which presumably sets the read clock frequency, but I'd not
turn it yet.
HP used a lot of strange circuitry in this sort of thing. Current mirrors
and the like. But let's at least prove your drive has problems in that
area befroe worrying.
All other self-tests pass ok, and if I put a floppy formatted with format
self-test in another drive (I have a 82902 also), I can read it with the
read self-test as described in the service manual.
So I'm almost sure that there is a problem in the read circuit. It can be in
the analog or digital part of it anyway, but there are more probability that
it's in the analog part.
Also, I made some reads with the oscilloscope, and it seems there is a
shorted transistor. I ordered a few in a local store and I'm waiting for
them to replace.
Roberto