It's beginning to sound like a Read Head problem, after looking at the Schematics. I've located the OHM values for all
the Heads. That should make it easy for you to verify the heads are good.
There is some information I can send you with some software that will help you test the Read operation.
The following Basic program will make Disk Drive B run continuously in the read mode with HEAD 0 ON.
To make Disk Drive A run continuously, in line number 80 change the number 37 (2nd number) to number 20.
Code:
10 DEF SEG=0
20 DEF USR=40000
30 FOR X=40000 TO 40007
40 READ Y
50 POKE X,Y
60 NEXT X
70 A = USR(O)
80 DATA 176,37,186,242,3,238,235,253
Type in and run the program given in the "Direct Operation of Disk Drive" section of this Troubleshooting guide to
make the Disk Drive run continuously in the read mode. To verify that the Disk Drive is operating in the read mode,
check for a low logic reading at pin 10 of IC 2B.
If the reading at pin 10 of IC 2B is not low, check for a high logic reading at pin 11 of IC 2B. If the reading is
high, check IC 2B by substitution. If the reading is not high, check for a low logic reading at pin 13 of IC 3B.
If the reading is low and the logic reading is high at pins 9, 10 and 12 of IC 3B, check IC 3B by substitution.
If pin 13 of IC 3B does not read low, check for a high logic reading at pin 13 of IC 3D. If the reading is high,
check IC 3D by substitution. If the reading is not high, check the cable and connectors going to the Disk Drive
Adapter. If the cables and connectors are normal, check for a low logic reading at pin 15 of Driver IC U7.
If the reading is low, check IC U7 by substitution If the reading is not low, check Control IC U6 by substitution.
If the reading at pin 10 of IC 2B is low put a diskette into the Disk Drive and close the Disk Drive door. The diskette
should be filled with programs to insure that the head will continually read information. Check the waveforms at pins
1 and 14 of Diff Amp IC 4A. If the waveforms are missing, check the voltages and components associated with pins 1,
4, 5, 7, 8, 10, 11 and 14 of Diff Amp IC 3A. Check Diodes CR2, CR3, CR11, CR12 and the resistance of the heads.
A problem in the head switching circuit, Switching Transistors Q6 & Q8, may affect the read circuits. If the problem
appears to be in the head switching circuit, see the "Disk Drive Will Not Switch Sides" section of this Troubleshoot·
ing guide. If the waveforms are good at pins 1 and 14 of IC 4A, check the waveforms at pins 2 and 3 of IC 5B. If the
waveforms are missing, check the voltages and components associated with pins 4,5,7,8,10 and 11 of IC 4A. If the
waveforms are normal, check the waveform at pin 7 of IC 5B while opening and closing the Disk Drive door. A noticeable
shift in the waveform pulses should occur. If no change occurs, check IC 5B by substitution.
NOTE: The waveforms shown at pin 12 of IC 5D and pin 4 of Multivibrator IC 5E should be present even when the disk
drive is not running. If the waveforms are missing, check Capacitor C11 and Resistor R22 and check Multivibrator IC
5C and IC's 5D and 5E by substitution. If the waveforms are normal, check the waveform at pin 5 of IC 5E. If the
waveform is missing and there are pu1ses at pin 1O of IC 5E, check Capacitors C12 and Resistor R24 and check IC 5E
by substitution. If the waveform is present, check for pulses at pin 8 of IC 1F. If the pulses are missing and the
logic reading is high at pin 10 of IC 1F, check IC 1F by substitution.
Larry