WARNING: The following fix requires some knowledge of electronics and soldering skills! Try this ONLY AT YOUR OWN RISK!

WARNING: All-In-One users MUST KEEP CLEAR of the CRT power cables, the CRT second anode (the thick single cable going to the top of the CRT) and the high voltage transformer on the vertical video board during computer operation. Voltages in these locations are DANGEROUS.

In addition to the typical part failure experienced by any electronic product, the Floppy Disk Controller is subject to drift caused by aging parts on the card. If you are experiencing increasing read/write errors, but the card is still working, it might be prudent to have the card recalibrated by someone with the tools to perform that function or you may attempt to apply the following troubleshooting procedures.

The following information is compiled from the Z-100 Technical Manual, the Floppy Disk Controller Card Model H-207 Assembly Manual, and personal experience. The intent of this article is to provide troubleshooting information for those wishing to repair their own Floppy Disk Controller Card. However, please understand that there is a risk to further damaging your controller and your computer. Please proceed only at your own risk.

Troubleshooting Chart

PROBLEM: Drive access light does not turn on when a diskette is booted.

POSSIBLE CAUSE:

1. Check for proper connections of the floppy cable inside the computer,
    paying particular attention to the location of pin 1 and the red edge of
    the ribbon cable.
2. Remove and reseat the Disk Controller in the S-100 bus connector.
3. Check the S-100 pins for corrosion. Use a pencil eraser to gently
    remove any dirt or staining from the gold fingers. CAUTION: Do not
    remove any more material than necessary.
3. Be sure the DIP switch on the Disk Controller is set to the correct
    address. See the section H/Z-207 Port Address below.
4. Check the connector positions on P1 or P2 on the Disk Controller.
5. Be sure the drive is properly jumpered for unit 0 or 1 as appropriate.

PROBLEM: All diskette access lights turn on and remain on.

POSSIBLE CAUSE:

1. Drive cable is connected with the marked edge on the wrong side.
2. Drive(s) configured incorrectly.

PROBLEM: Two drives turn on when a boot operation is selected.

POSSIBLE CAUSE:

1. The two drives have their selection jumpers programmed the same.

PROBLEM: Computer will not accept a boot command, returns to hand prompt, or starts to boot, but freezes and requires reset.

POSSIBLE CAUSE:

1. Be sure diskette is bootable.
2. Be sure diskette is installed in selected drive before boot
    command is given.
3. Be sure DIP switch on Disk Controller is set at the correct address.
    See the section H/Z-207 Port Address below.
4. Be sure drives are properly jumpered.
5. Be sure DIP switch bits 0 and 1 are selected for the type of drive
    being used. NOTE: This is not applicable to MTR-ROM v4.3 and later.

PROBLEM: Upon Power Up, the message "Primary Z207 Controller Error" or "Device Controller Error" is displayed before the hand prompt.

PROBABLE CAUSE:

1. The controller card of the Default Boot Device is not installed,
    or has a failure.
2. Check the positions of the S101 DIP switch on the motherboard.
3. Check DS1 on the Disk Cntroller for incorrect settings or markings.
    A number of switches have been found to be mismarked, or assembled
    with the cover upside down. A quick check is to check continuity of
    each section. When DS1 is set as required, sections 0-3 and 6 should
    be shorted when positioned toward the bottom of the board.

PROBLEM: Capacitor C35 fails on 85-2807-1 model Disk Controller cards.

POSSIBLE CAUSE:

1. According to Field Service Bulletin FSB-Z100-31, if the Disk Controller
    has the model number "85-2807-1" printed in the lower left corner of the
    board, the silk screen of the board is printed WRONG and capacitor C35
    is installed reversed. Replace the capacitor with the "+" toward the left.

PROBLEM: The disk drive does not stop running or runs longer than about 20 seconds after the last access.

POSSIBLE CAUSE:

1. Check and/or replace C30, a 47 uF, 6v capacitor in the one-shot time
    out circuit. To reduce the run time, a 20 uF capacitor will time out
    in about 9 seconds.
2. Check for model "#85-2807-3" in the lower left corner. If the board is
    a -3 model, the silk screen of C30 is WRONG! Remove C30 and reinstall
    with the positive "+" sign toward resistor R20, which connects to +Vcc.
3. If operating at 8+ MHz CPU speeds, replace U15 N9202N integrated circuits
    with faster 92LS02 chips.

PROBLEM: The computer is reporting soft read/write errors.

POSSIBLE CAUSE:

1. Aging parts on the Disk Controller may have drifted off calibration.
2. Precompensation failure. See the next two problems for further checks.
3. Early Disk Controllers may have a 10uF capacitor at C6.
    This was increased by FSB-Z100-016 to 33uF in later models to
    reduce soft errors. NOTE: All 85-2807-2 and -3 models already
    have this modification.
4. According to FSB-Z100-024, soft errors may be reduced by connecting
    a 10 uF capacitor in parallel with the 0.1 uF capacitor at C32,
    located just above the controller chip U22. Be sure the positive "+"
    lead of the 10 uF capacitor is positioned to the left when facing the
    component side of the board. NOTE: All 85-2807-2 and -3 boards already
    incorporate this modification as a separate capacitor at location C32-1.
5. According to FSB-Z100-019, floppy disk errors in a Z-120 All-in-One may
    occur when a hard drive is mounted above the floppy drive, causing EMI
    radiating from the hard drive to affect the floppy disk drive. Check to
    ensure a shield is installed between the two drives in this configuration.

PROBLEM: FORMAT verification fails after track 23 on 5" drives; after track 43 on 8" drives.

POSSIBLE CAUSE:

1. Failure in the Precompensation Circuit.
2. The Disk Controller needs calibration.

PROBLEM: Precompensation will not adjust.

POSSIBLE CAUSE:

1. According to Field Service Bulletin Z100-041, if the pulse width at
    CP3 cannot be adjusted, or is missing, check for an open circuit to
    U3-17 or an open wiper at R3.

PROBLEM: Erratic Write Operation

POSSIBLE CAUSE:

1. According to FSB-DK-025, early model #85-2807 boards did not include
    base resistors for the Q2 and Q3 transistors. To eliminate write
    irregularities, the newer models included 100 ohm resistors in series
    with the base lead of Q2 or Q3. The bulletin says to install the
    resistors only when needed.

PROBLEM: Running DIAG, the Heath/Zenith Diagnostics Program, shows a Head Load Time Error and suggests checking U33 and U15.

POSSIBLE CAUSE:

1. Replace U33 and U15 as recommended.
2. Replace C48, located between U15 and U21, which controls the timing
    for this circuit. This is a 1.0 uF capacitor of a type with a known
    history of problems; for me at least.

PROBLEM: CP/M will boot from a floppy, but Z-DOS gives an I/O error.

POSSIBLE CAUSE:

1. According to FSB-DK-027, check U31 (74LS365A, #443-1039) for a
    defective gate.
2. Check DS1 switch section "0" in the wrong position or open.

Calibration Procedures

Equipment Needed:

You will need the following equipment to most precisely calibrate your Disk Controller Card:

   * A digital voltmeter (DVM) with at least a four-digit readout.
   * A 10 MHz bandwidth, calibrated, laboratory-quality oscilloscope
      with a sweep speed of 50 ns/division and a vertical deflection
      of 2 V/division, and a low capacitance (X10) probe.
   * A frequency counter capable of six-digit accuracy at 4 MHz.
   * A blank 8" diskette (or a 5.25" diskette if you are using only
      5.25" drives in your system).

NOTE: The H-207 Assembly Manual provided a simple circuit for calibrating the Disk Controller kits. It is provided toward the end of this article.

Precompensation Calibration

Write precompensation corrects for "bit shift" distortion. This bit shift can be seen electronically when you read a data stream from a diskette. Data pulses which are recorded close together when read back, appear to spread apart. Precompensation advances or retards the data pulses as they are written so the pulses are properly spaced when read back off the diskette.

All Heath/Zenith 5-1/4", 48 tpi, floppy disk drives require 120 ns of precompensation on tracks greater than 23. Also, all Heath/ Zenith 8" and 5-1/4" 96 tpi drives require 120 ns of precompensation on tracks greater than 43.

Usually, two values of precompensation are needed: one for 5.25" drives and one for 8" drives. Accordingly, there are two precompensation adjustments on the Disk Controller Card. Potentiometer R3 (PRECMP1) is used to set the lower value of precompensation and potentiometer R4 (PRECMP2) is used to set the higher value of precompensation. Pictorial 1 shows the locations of these potentiometers.

Jumper J2 selects whether the 5.25" or the 8" drive will receive the lower value of precompensation. Pictorial 1 shows the location of J2 on the Card.

While referring to Pictorial 1, perform the calibration as follows:

1. Locate R3 and R4 on the Disk Controller Card.
    Turn R3 fully counterclockwise and R4 fully clockwise.

2. Insert the Disk Controller into the S-100 bus.
    Turn the power on and wait five minutes to reach operating temperature.

3. Attach the oscilloscope's probe to CP3 and the probe's ground clip to GND.

4. Determine the values of write precompensation that the 5.25" and 8"
    drives need from the drive's documentation. H/Z floppy drives all
    require 120 ns of write precompensation. If the value of precompensation
    is higher for the 5.25" drives, or if you only have 5.25" drives,
    go to Step 7. If the value of precompensation is higher for the 8"
    drives, or if you have only 8" drives, go to Step 5.

5. Format a blank 8" diskette in any of the 8" drives by running FORMAT.
    While FORMAT is running, turn R3 (PRECMP1) to adjust the pulse width
    displayed on the oscilloscope to the value of write precompensation
    needed by your 8" drives. Restart FORMAT as needed until you are
    satisfied with the adjustment. If you do not have 5.25" drives, you
    have completed the precompensation calibration; proceed to "Data
    Separator Calibration". If you do have 5.25" drives, continue with
    the next step.

6. Format the 5.25" diskette. While FORMAT is running, turn R4 (PRECMP2)
    to adjust the pulse width displayed on the oscilloscope to the value
    of write precompensation needed by your 5.25" drives. Restart FORMAT
    as needed. Proceed now to Step 11.

7. If you have both 5.25" and 8" drives, perform the next step.
    If you have 5.25" drives only, go to Step 9.

8. Cut the foil on the circuit board that connects the middle hole of
    J2 to the "8<5" hole.

9. Format a blank 5.25" diskette in any of the 5.25" drives using FORMAT.
    While FORMAT is running, turn R3 (PRECMP1) to adjust the pulse width
    displayed on the oscilloscope to the value of write precompensation needed
    by your 5.25" drives. Restart FORMAT as needed until you are satisfied
    with the adjustment. If you do not have 8" drives, you have completed
    the precompensation calibration; proceed to "Data Separator Calibration".
    If you do have 8" drives, go to the next step.

10. Format the blank 8" diskette. While FORMAT is running, turn R4 (PRECMP2)
     to adjust the pulse width displayed on the oscilloscope to the value of
     write precompensation needed by your 8" drives. Restart FORMAT as needed.

11. Remove the oscilloscope probe.
     This completes the precompensation calibration.

Data Separator Calibration

While referring to Pictorial 1, perform the calibration as follows:

1. Turn the computer on and wait five minutes to reach operating temperature.

2. Make sure the disk drives are not selected.

3. Set the DVM's voltage range to 2 Vdc and attach the common lead to GND
    and the positive lead to CP2.

4. Adjust R2 (BIAS) for a reading of 1.400 Vdc.

5. Remove the voltmeter test leads.

6. Set the six-digit frequency counter to count 4 MHz.

7. Attach the shield lead to GND and the signal lead to CP1.

8. Adjust R1 (FREQ) for a reading of 4.000 MHz.

9. Repeat Steps 2 through 8 until there is no further improvement and
    the 1.4 Vdc and 4 MHz readings occur simultaneously. There will be
    some (but not much) interaction between these adjustments.

10. Remove the test leads and turn the computer off.
     This completes the calibration procedure.

NOTE: Format the blank diskettes used in this procedure again before
you use them for recording files.

H/Z-207 Port Address

The H/Z-207 Floppy Disk Controller Card occupies a block of eight contiguous I/O port addresses beginning on an 8-bit boundary. The low-order six address bits in this block of eight ports are actually used.

The port address is selected by sections 3 through 7 of switch DS1 on the Card. Switch section 7 selects the most significant bit. Heath/Zenith Computers use port address B0 hex. Switch sections 0 and 1 are used to control bits 3 and 4 (with 0= least significant bit) of the status port, which can be read at I/O address BASE + 5.

NOTES:

1. H/Z software uses switch section 0 for 48/96 tpi drive selection.
2. Early distributions of Z-DOS v4 used sections 0 and 1 for specifying
    the 48/96 tpi drive mix. See Appendix I: User Configurations from
    "Z-100 LifeLine" issue #79.
3. Z-DOS v4.06 and later ignores switch sections 0 and 1.
    The mix of drives no longer matters to the operating system.
4. Switch section 2 is not used.

Other Configurable Options

Please refer to your Z-100 User's Manual to configure the following two items, if needed.

Disk Controller Clock Speed

The H/Z-207 Floppy Disk Controller Card is configured to operate with a CPU that operates faster than 3 MHz. If the Disk Controller is to be used with a CPU that operates at 3 MHz or SLOWER, a minor modification must be made to the board at J1.

Disk Controller VI Lines

The Vectored Interrupt lines (VI) are properly configured to operate in a H/Z-100 computer. If you will be using the Disk Controller with a different manufacturer's computer, you may need to configure the VI lines by installing jumper wires.

Constructing a Calibration Board

Pictorial 4 shows the circuit board that was constructed during assembly of the H-207 Floppy Disk Controller Board. A similar board can be constructed on a standard plug board.

The parts are easily found from spare H/Z-100 computer boards. DL501 and DL502 are the standard 200 ns Delay Line found on the Motherboard or Z-205 Memory Card. DL502 is optional and is only needed if you intend to use this circuit with drives that require more than 120 ns of write precompensation.

The only critical construction note is to make the wire connecting pin 1 of DL501 to pin 9 of U501 no less than 2" long.

Figure 1 shows the circuit schematic.

Heath Drive Calibration

Caution: The procedures given earlier use an oscilloscope and frequency counter, reportedly making them much more accurate and preferred over the following procedures, which can damage your controller card or even the computer if improperly used. The following procedures are only provided as an alternate method to be used if the special equipment used above is not available. Well, since I wrote this many years ago, I've found that those boards that I used the oscilloscope on were having difficulty formatting drives. I've found that the resistor pots were turned just slightly too far clockwise - maybe my Heath computer scope isn't accurate enough! The circuit board and alternate calibration procedure picked up on this and I've since found that using the circuit board was actually the better procedure!

If a frequency counter is available, perform the voltage and frequency adjustment as outlined in the Data Separator Calibration section above. If the frequency counter is not available, perform the alternate Data Separator calibration procedures, next.

Alternate Data Separator Calibration

Calibration Preparation:

1. Locate R1 (FREQ) on the Disk Controller board and turn it 30 (thirty)
    turns clockwise, or until you hear a clicking sound as you rotate the
    control shaft.

CAUTION: While the following procedures could be used without a spare
socket, inserting wires into the board's IC sockets could severely damage
the socket. Always use a spare socket.

2. Locate a spare 40 pin IC socket and bend all of the pins inward under
    the socket except pins 20, 30, 33, and 37. These are the only pins that
    will be used.

3. Remove U22, the FD1797 controller chip, and carefully set it safely aside.

4. Install the new 40 pin socket into the empty U22 socket, ensuring that
    only the four pins, 20, 30, 33, and 37 are properly inserted into their
    respective pin sockets.

Refer to Pictorial 3 for the following steps.

5. Cut a 1/2" bare wire. Then bend a loop in one end of the wire and
    solder the wire in the CAL hole located between U20 and U14. Be sure
    the wire does not touch any other nearby foils or component leads.

6. Cut a 2-1/4" bare wire, bend 1/4" at each end, and insert the wire
    into socket holes 20 and 37 at U22.

7. Push a length of bare wire into socket hole 33 of U22, solder it to
    the 2-1/4" bare wire already installed, and cut off any excess.

8. As before, push a bare wire into socket hole 30 of U22, solder it to
    the 2-1/4" bare wire, and cut off the excess wire.

9. Remove U9, 74LS241, from its socket and set it carefully aside.
    Locate a spare 8, 14, 16, or 20 pin socket and install it into the
    vacant U9 socket, lining up the notched ends.

10. Install a 3/4" bare wire into socket holes 1 and 20 at U9.

11. Cut and install another 1/2" hooked bare wire at the CAL hole at
    the center and side of U9.

12. Remove U30, 74LS273, and set it aside.

The card is now prepared for the Alternate Data Separator Calibration procedures next.

Alternate Data Separator Calibration:

Refer to Pictorial 5 for the following steps.

1. Install the Z-207 Controller Card, less the three IC chips and
    prepared as given above, into the computer and connect its cables.

2. Using a high impedance DC voltmeter (10M ohm or greater), select
    the voltage range that is most suitable for measuring a voltage that
    is below 5 volts.

3. Connect the common (or negative) test lead to test point "GND".

4. Connect the positive voltmeter lead to test point "CP2".

CAUTION: When the all-in-one computer is turned on, high voltage is
present at the video deflection circuit board and the CRT's anode.
Be careful and do not contact these areas while the computer is turned on.

5. Plug in your computer and turn it on.

6. Adjust control R2 (BIAS) for a meter reading of 1.40 Vdc.
    Switch to as low a range as possible to perform this adjustment.

7. Turn off the computer, unplug the cable, and remove the Controller Card.

8. Cut a 5" white wire, remove 1/4" of insulation from each end, and
temporarily solder the wire between the two wires in the CAL holes
on the component side of the circuit board.

9. Reinstall the circuit board into the computer and reconnect the
voltmeter to test points GND and CP2.

10. Turn on the computer.

NOTE: In the next step you will adjust the R1 (FREQ) control for a meter reading of 1.40 Vdc. It is normal for this voltage to start at about 1 volt. Then as you rotate the control counterclockwise, this voltage may gradually drop to near zero, and then jump to approximately 2 volts. Rotate this control backwards (clockwise) to locate the FIRST place where you can obtain 1.40 volts. If your initial voltage reading is greater than 1.4 volts, adjust the R1 (FREQ) control counterclockwise until you find the FIRST position at which you can obtain a 1.40 Vdc reading.

If you continue turning counterclockwise, you may find another point that will give a 1.40 Vdc reading. This second point is the wrong one; do not use it.

11. Adjust R1 (FREQ) counterclockwise for a meter reading of 1.40 Vdc
     +-0.05 volts.

12. Place the top cover on the computer and allow it to operate for
     15 minutes (to stabilize), then remove the cover and fine tune the
     previous step.

13. Turn off the computer, unplug the cables and remove the Contoller Card.

14. Remove the 5" white wire from the component side of the Card.

15. Remove the top 40-pin IC socket from U22 and reinstall the FD1797
     controller IC. Check that no pins turned under.

16. Reinstall U30, the 74LS273 IC.

17. Remove the top socket from U9 and reinstall U9, the 74LS241 IC.

Proceed with the Alternate Write Precomp Calibration procedures next.

Alternate Write Precom Calibration:

1. Turn control R3 (PRECMP1) fully counterclockwise.
    Turn R4 (PRECMP2) fully clockwise.

2. Connect the red wire coming from the calibration circuit board to
    the indicated lead of capacitor C27 on the Controller Card, push the
    black wire connector onto the GND pin, and press the yellow wire
    connector onto the CP3 pin.

3. Lay the calibration circuit board on the drive shelf as shown if you
    have a low-profile computer, or position it away from your computer
    if you have the all-in-one version. Do not let the circuit board touch
    any metal.

4. Install the Controller Card into any of the card cage slots and connect
    the drive cables.

CAUTION: When the all-in-one computer is turned on, high voltage is
present at the video deflection circuit board and the CRT's anode.
Be careful and do not contact these areas while the computer is turned on.

5. Turn on the computer and boot up either Z-DOS or CP/M.

6. Locate a blank soft-sectored diskette and insert it into any disk drive.

7. Run FORMAT, and as the diskette is being formatted, adjust R3 (PRECMP1)
    on the Controller Card until the calibration board LED, D501, just lights
    and glows steadily. When the FORMAT program stops, the LED will remain
    latched "ON" as long as the adjustment was set between 120 ns and 160 ns
    of write precompensation. Be sure the FORMAT routine is running while you
    make any adjustment to the control.

8. Open the drive door, turn off the computer, remove the calibration
    circuit board, and install the Controller Card in any convenient
    card cage slot.

This completes the calibration of your Controller Card.

NOTE: Reformat all blank diskettes used during these procedures
before you use them.