CX065B (was Re: Osborne Executive CRT circuitry)

Tony Duell ard at p850ug1.demon.co.uk
Fri Feb 24 18:19:32 CST 2006


> 
> Hi Tony --
> 
> Well, I couldn't find the datasheet for this IC
> but I did locate a few of them if you need 'em
> (I deal in Japanese semiconductors for the consumer
> electronics repair trade).

Well, I finally got the thing to work (based mostly on the info in the 
SAMS book on the 1541 [1]) And it was quite a job.

[1] I bought this in a second-hand bookshop in Bristol over 10 years ago. 
I am not much of a Commodore person, but I always buy old computer books 
that contain schematics...

Anyway, back to the Epson disk drive story. The drive I'm working on is 
an SD320, used in the TF20 drive unit for the HX20/PX8/PX4 laptops. If 
you want to follow along, I think the techncial manual for the QX10 is on 
the web somewhere. This machine uses SD321 drives. The difference is that 
the SD320 ahs a head load solenoid, the SD321 doesn't. Otherwise they are 
almost identical (the positioner, spidle motor, and most of the 
mechnaical parts are the same. There are small differences on the main PCB 
(as to which components are fitted), but it's the same board, same ASICs, 
etc.

So the scheamtic of the spindle motor in the QX10 manual will help.

OK, it's a farily simple design. There's a 4 pole stator with a rotor 
around it. The inside of the rotor has a pair of permanent magnets on it. 
These are pulled round by the stator coils, they also trigger a pair of 
hall sensors on the PCB, which control transistors that drive the windings.

Inside the housing, on top of the rotor, is the FG (frequency generator) 
coil. This produces an AC signal, the frequency of which depends on the 
speed of the motr. This is fed to  the CX065B chip, the output of that 
controls the motor drive amplifier. If the motor is running too slowly, 
the output voltage of the CX065B increases, making the amplifier drive 
the coils harder.

So far so good. Anyway, I had the motor out of the drive (I needed to 
clean the old greae off the eject linkages...). I was pretty sure I knew 
how it was used. so I linked up the 3 pins on the connector to a 12V 
bench supply (motor enable anf ground to -ve, 12v to +ve). The result was 
that the motor started, ran for a few seconds, and stopped.

OK, I thought, a faulty motor. So I tried the one in the other drive. It 
did the same thing. I even pulled a drive from my 'spare' QX10 and tried 
the motor from that. It did the same thing.

So I thohght that perhaps the enable line wasn't just a DC voltage -- 
ground to enable. Maybe it was some kind of clock input.. So I put a 
drive back together, cabled it up to a drive exerciser, and tried again. 
You guessed it, motor runs for a few seconds and stops. And the signal on 
the enable pin is just a DC voltage, it's pulled to ground when the motor 
should be running.

So I investigated the motor further. And discover the transistor Q23 on 
the schematic. Its purpose is to stop the motor if the output from the 
CX065B rises too high. It'll do that if either the FG signal is missing 
(so the CX065B thinks the motor isn't turning at all), or if the motor is 
overloaded, so the CX065B has to drive it harder. 

The only prroblem is that if the bearings get a bit stiff, this circuit 
triggers too easily. Replaceing the bearings is a right pain (I know 
_how_ to do it, but you almost always damage the FG coil and have to 
unwind at least one turn and then reassemble that part, etc). There's an 
ARD-unofficial-kludge, though. There's a 75k (100k on some drives) 
resistor on the motor PCB, near to the connector and at right angles to 
it. Reduce that a bit (68k, 56k). This makes the protection circuit a bit 
harder to trigger (but it will still trigger if the motor is stalled).

Moral : If 3 identical old computer parts appear to malfunction in the 
same way, they might all be faulty!

-tony



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