Model 4P returns "Seek Error" (9) on startup

[clh333]

When I power up the unit the screen displays a seek error message in three languages which is the result of a jump in the BIOS code based on an error number (9) returned to the startup routine. From what I read in the Western Digital 1773 data sheet (p. 1-7) this error is returned to the CPU by the FDC during a RESTORE command, presumably at startup. If after 255 step commands are issued to the drive without Track0 going low the FDC ceases operation and issues an interrupt. In operation the drive 0 light flashes briefly, both spindles turn for about three seconds but neither head moves.

This unit, which would not boot when acquired, has been disassembled, cleaned and the PSU AC capacitors replaced. The two Tandon TM-50-1 drives have been examined, cleaned and tested independent of the unit and found to be operational, i.e. motor light comes on, spindle rotation starts and stops as expected, rotation speed within 1% of 300 RPM, head steps to track 39 or 41 and stops at track 0. Both drives have the same configuration with the Tandy cable-select system used to differentiate them. A 180 Ohm resistor array terminates the last drive.

I would like to ask for advice on how to track down the source of the problem. I could start replacing ICs until something happens but it would be nice to have an idea of where to start. Thank you for your suggestions.

-CH-

 

[ldkraemer]

Charles,
With Power off move the heads to their center position. Then when you power up listen to make
sure they are stepping. They should step away from the Hub to the Outer Track. Another thing
you can do is test the Track 0 Switch to make sure it is LOW when the Head Carriage Assembly
is at Track Zero. Plus VERIFY the Index Pulse is going LOW while you are testing.

Larry

 

[clh333]

Larry, thank you for your reply. Using your suggestions I verified that the heads do not step toward Track0 when the drive is powered up. The motor spins but only for a few seconds; it is an "automatic" feature of the drive. (Tandon says this ensures the hub is seated properly.) During those few seconds as a disk rotates I can see the index pulse on a scope.

When I manually positioned the head back to the T0 stop, one of the two drives (was in position 0 or last on the chain) did not drop its voltage. There was voltage at the test point with the sensor uncovered and with the sensor covered there was the same voltage. I will need to decipher the schematics to figure out why this is so. If the signal is buffered perhaps an IC has failed. The other drive (1) did not display this behavior; sensor voltage dropped when the head was in the home position.

At power up I read the "Step" signal on the drive with the properly-functioning T0 sensor. I got a blip, no more. I have a nagging feeling the signal from the FDC to the drive is flawed. I've been using Tandon's documentation from the 4P service manual but not everything is covered in detail. I don't have a dedicated exerciser or Dysan test disk but maybe I can scrounge something up.

I'll let you know if I discover anything. Thanks.

-CH-

 

[ldkraemer]

Charles I sent a DM with link to Tandon TM-50 Info. When you boot, the Drives should step back to
track 0, not on Powerup. I stated that incorrectly. But, then the Head carriage is at Track 0, the
signal should be LOW (which is lower than .7 Volts.)

Larry

 

[clh333]

For the record: I get the seek error message no matter whether I power up with a disk in the 0 drive or power up, then insert the disk, then hit the reset button. The error message is there both before and after the reset, in other words.

-CH-