RL02 write faults, found it, now how to fix it?

Charles charlesmorris at direcway.com
Thu Jan 5 18:24:52 CST 2006

I spent an interesting day in the shack today. After blowing off
the dust, I found that my old 7D01 logic analyzer won't trigger on
the word recognizer but Ch. 0 would trigger, so I started hooking
it up to the RL02 logic board and R/W boards using the drive error
as a post-data trigger and looking backwards from there.

Eventually I learned that Write Gate and Write Data are being
correctly asserted, and the data bursts continue for 500+ uS (one
sector) beginning a couple of uS after the Sector pulse. But
approximately 4 to 6 uS after the Write Gate opens, and before the
data could even begin (10+ uS), the DL5 DRIVE ERR signal asserts!
I worked around the loop from there and found that the *first*
thing to go wrong was the Write Data Error Latch ('LS279, E6 on
logic board) which was setting its output pin 13 (DL5 WR DATA ERR
H). This fatal error signal then latches DL5 DRIVE ERR H via the
other "glue" logic and prevents any further action.

But the R/W board that generates Write Data Error Pulse was *not*
telling the latch to set! There is a 6K8 resistor to +5 and a
0.0056 uf cap to ground as a delay (set only) in the error circuit
(a time constant of 38 uS) so to reach a logic 1 (2.0v) would take
considerably longer than 6 uS anyway.

Further exploration with the logic analyzer on "high" speed (20
ns/tick) shows that on the 'LS279 pin 15 (the latch input, WRITE
DATA ERROR PLS L) there is a small (~180-200 ns) low pulse with a
20 ns glitch in the middle. It is NOT evident to the logic
analyzer when examining the signal's source on the R/W board,
(Write Data Error Detector E3, 7404 pin 8). But it's enough to set
the WDE latch all right!

Right about then, sparks started flying from the brush area of the
spindle motor and the drive faulted on its own and spun down.
Holy crap! =:^O

I pulled the circuit breaker fast and gently spun the motor and
could hear scraping noises. I figured the brushes were shot or
something, so I took the two nuts off the through bolts. Of course
the end housing wouldn't come off, way too tight a fit, and I
didn't want to pull the blower motor. With a bright light I could
see a piece of something blue sticking up, so I pulled it out with
a pair of fine needlenose pliers. It was the tip of one of my
logic analyzer leads that had sneaked into the vent holes unknown
to me, and been cut off by the armature turning! I put the nuts
back on (of course I dropped the second one inside the motor and
had to fish that out too), crossed all my fingers and flipped the
breaker. It spun right up and has been working fine since. Whew.
Never a dull moment in Murphy's Laws.

Anyhow, there is a 330 pf deglitching cap near the input to the
'LS279 but obviously it's not adequate. I need to put a fast scope
on the line and see what the edges actually look like. Some
resistive termination may be needed. This is on a very short
ribbon cable to J6, about 6", inside the drive, and the chassis
ground wires to each board are connected. Or maybe the Berg
connector on one end is defective in the ground lead. More as I
discover it.


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