Z-100 Hard Drives

Since the first 5, 10, or 12Mb hard drives (called Winchester Drives by Zenith) were placed in the Z-100 in the mid-80s, there have been several more advanced types developed - MFM, RLL, SCSI, IDE and others. The drives usually found in the Z-100 were the first type - MFM - and the subject of this article.

Another drive type - SCSI - with considerably larger capacities, became available for the Z-100 in the late 80s with a SCSI Controller marketed by C.D.R. Paul Herman, editor and publisher of the “Z-100 LifeLine” at the time tried to get a special order of boards from C.D.R. adapted specifically for the Z-100. However, it soon became evident to Paul, and several volunteers working as his staff, that they needed to develop their own controller, and the new Z-100 LifeLine SCSI/EEPROM board was created.

As the MFM and newer SCSI systems became more scarce, attention turned to the newer IDE technology and another group of volunteers; John Beyers, Charles Hett, and I. We researched and developed the new Z-100 LifeLine IDE NvsRAM board, shipped in late 2008.

You can find additional information on these newer systems elsewhere on this site. This article will concentrate on the use of the initial Winchester hard drive.

The Heathkit/Zenith MFM hard drive installation was comprised of a Z-217 Winchester Controller Card in the card cage, a separate, unique Data Separator Board that was normally mounted near or over the hard drive and the MFM hard drive itself. These pictures show the drive configuration possible for the Z-120 All-In-One Computer:


   Z120DriveBay3a      Z120DriveBay4a


Note:  For full procedures on installing an MFM Hard Drive in the Z-100 series computer, please see the article, MFM Hard Drive Installation.


Preparing the Hard Drive



IMPORTANT: Early MFM hard drives are fragile and can be damaged easily. In all hard drives, while the drive’s platter is spinning, the read/write heads float on a very thin layer of air, separ-ating the heads from the platter’s surface. However, the read/write heads on these early drives came to rest on the surface of the disk platter when rotation stopped. Any bumping, knocking, or dropping may cause the heads to bang against the surface of the platter. A severe bump, especially while the platter is spinning, could actually damage or gouge out a small area in the platter and cause a “crash”, where an important portion of a program is unreadable and lost because the disk surface was damaged. Further, the read/write head may also be damaged.



For early hard drives, it is CRITICAL to run a disk utility that parks the heads in an unused portion of the disk - a storage or parking area - before the heads come to rest. Such a utility is SHIP, an external command packaged in CP/M and MS-DOS operating systems. Later MFM drives had an auto-park feature that placed the heads down after the last usable sector of the drive, in an unused area. But even then, the heads could be damaged from a sudden drop of the drive.





MFM drives are recognized by their two ribbon cable card edge connectors, one with 34 conductors and the other with 20 conductors. RLL drives also have these but the drive model number includes an R. For example, an ST-138 is an MFM drive, while an ST-138R is an RLL drive, with different formatting, capacities, and controller boards. ESDI drives also have similar cable connections, but cannot be used.


MFM drives are becoming available from Ebay and the used market, sometimes at ridiculous prices and there is no guarantee that any of these will work.

But let’s assume that you find one with possibilities and want to try it. What is involved?



CAUTION: You cannot just slap an MFM drive from another computer into your Z-100 and expect it to work, without completely reformatting the drive. It will require low level formatting using the Z-100 PREP command, partitioning using the PART command, and a high level formatting of each partition using the FORMAT command. These commands are unique to the Heath/Zenith CP/M and MS-DOS (now referred to as Z-DOS) operating systems.

There are numerous manufacturers of MFM drives, each with different sets of programming plugs, jumpers, and terminating resistors. If you have a specific brand that you can’t figure out, try emailing me at the “Z-100 LifeLine”.

As I mentioned earlier, please refer to the MFM Hard Drive Installation, which was published in Issue #59, for the full procedures for installing a hard drive in the Z-100. Here are views of my hard drive setup on my test bed Z-100:


HDriveTest1     HDriveTest3     Note the Cabling.


However, there are a few specific reminders:

*  Try to install the new drive alone and boot to a floppy to run the Winchester Disk Utilities. It can be run from another hard drive, but you would hate to accidently PREP the wrong drive!

*  If the new drive is installed alone, insure the terminating resistor pack is installed. If it is the second drive, only install the terminator resistor on the hard drive installed last on the 34-pin connector ribbon.

*  Double check that the ribbon cables are installed per the directions in Issue #59. Insure all connectors are fully seated and that pin one of each ribbon connector (the ribbon cable may also have a red edge) is at the correct end of the connectors on the drive, controller, and data separator. The 20-pin cable connector can go to either location on the Data Separator Card.

*  Before running PREP on a hard drive, you must install the Format Enable Jumper on the Z-217 Controller Card. It is located under the power connector in the upper left corner of the Z-217 Hard Drive Controller, and may be stored anywhere along the top row of pins of the VI* jumpers, when not in use.

*  Check for a programming plug on the new drive before installation and make a note of the position of any jumpers. Try this setting first and if unsuccessful, try the other positions before giving up.

Hint:  Drive Select (DS) pins may be labeled DS0 through DS3 or DS1 through DS4, if labeled at all. If this is the lone drive, DS0 (or DS1) will become drive unit 0 and after programming, drive E:.

Hint:  Before changing any positions on the programming plug, install the hard drive temporarily, boot up the computer, and run ASGNPART 0:. If the drive is already setup as drive 0: the drive LED should light. If it does not, check ASGNPART 1: and even 2: and 3: before giving up. If the LED will not light in any position, check the cables for an improper connection and finally try a different position on the programming plug.

Note:  If partition info is displayed after running ASGNPART, do not proceed with PREP until you have tried other options. For example:

-  Try running ASGNPART X:(Partition name) E:, where X: is the drive unit number. Then do a directory listing on E:.

-  Try running DETECT or VERIFY to see how many bad sectors are found.

-  Try reformatting the partition with FORMAT to isolate those bad sectors.

The PREP and PART utilities were available on a special disk entitled ‘Winchester Utilities Disk’, distributed separately from the earlier MS-DOS versions. The disk and instructions are available from the Z-100 LifeLine Library.

CAUTION:  Using PREP is the last resort. It will destroy all the files that may exist on the hard drive. If the disk is from another Z-100, you may need to use PREP only if you consistently encounter an unreasonable number of disk access errors. Do NOT use PREP until you have backed up all important files you wish to keep to floppy disks.

PREP has been updated by John Beyers in the Z-100's Z-DOS v4 that allows it to be much more flexible in its operation - another reason to upgrade.

All versions of the PREP utility enable you to:

*  Initialize the surface of the hard disk.

*  Test the data retention capabilities of the hard disk.

*  Isolate questionable disk sectors.

*  Divide the surface of the hard disk into 2 partitions (Z-DOS and CP/M).

PREP takes a long time to run. Expect it to take about 1.5 hours for every 10 megabytes in hard drive size. It runs seven surface passes. However, with the version 4 PREP, you can set the number of passes to make.

If the hard disk does not contain initialization information (from a prior PREP operation), PREP will prompt you to enter characteristics (in hex) in order to identify the type of hard drive that is being installed in the computer. While Issue #59 has a list of common drives that were used in the Z-100, I have updated this information for many more manufacturers. Please see the PDF document: Hard Drive Specs.

Note:  Several of the drives are too large for normal use in the Z-100. Early hard drives and Z-DOS versions in the Z-100 were limited to 32Mb. Later, with the addition of the PREP /k switch (which uses 1024 byte sectors rather than 512 byte sectors), the limit was extended to 64Mb. As I understand it, Z-DOS version 4 can go higher, though I don’t recall the limit. Just remember, larger than 64Mb will ONLY work with Z-DOS v4. The fix is easy - just reduce the number of heads being used by PREP until the number of megabytes is where you want it.

Once PREP has completed, if you run ASGNPART 0: you will see the two partitions created: Z-DOS and CP/M. If you are satisfied with these two partitions, you will not need to repartition the disk with PART. However, if you wish to change this partition information, you must run the PART utility.

The PART utility is self explanatory. Just follow the procedures as given to change the partition names and sizes as necessary, then choose a default boot partition and save the configuration to the hard drive. When complete, you may need to reboot the computer to the floppy drive again.

Next run ASGNPART 0: to confirm the partitions are as you required.

Before we can use the new partitions, you need to assign drive letters to them and then run FORMAT to do a high level format of each new partition. Run ASGNPART 0:(partition name) E: to assign the drive letter E: to the first partition. Likewise, assign succeeding drive letters (F:, G:, H:) to the remaining new partitions (up to four at a time). Run FORMAT X:/s/v to format and load the system files on each new partition, where X: can be E:, F:, G:, or H:.

If successful, you are now in business. Email me if you have any difficulty. I hope this helps clarify the use of Z-100 MFM hard drives.


H/Z-100 Seagate MFM Hard Drive Repair

General Notes:

  • Many, many years ago, I had run across an article in one of the computer magazines serving Heath/Zenith computer users; REMark, Sextant, or Capital HUG (CHUG), that reported that the H/Z-100 had poor regulation in its power supply and suggested adding a 47uF capacitor to the +5Vdc line of each hard drive.
  • This rather unalarming message has increased in importance over the last few years because of the aging hard drives being used in the H/Z-100.
  • The number of Seagate hard drives with no-spin problems has increased dramatically over recent years.
  • Many articles in various magazines have been attributing this problem to STICTION - the drive heads sticking to the very smooth, polished surface of the drive platters, which become even more polished with age/use.

WARNING:  The following fix requires some knowledge of electronics and soldering skills! Try this only on otherwise unusable hard drives and ONLY AT YOUR OWN RISK!


Possible Solution to Stiction Symptoms:

Over several years of experimentation, I had found that the poor power supply regulation and the stiction problem seemed to be RELATED! Further, if you touch a 5 volt jumper lead to a couple of pins (I forget which ones now) on the controller chip of the drive's attached controller board, the drive will indeed spin! This is NOT stiction, but a board issue.

I've now played with more than 10 drives, mostly ST-251s and ST-138's that wouldn't spin. Of these, several were repaired by replacing the 22uf surface mounted capacitor on the +5Vdc line with 47uf electrolytic capacitors. On the two other drives, the heads were so stuck, that turning the platters by hand broke off one of the heads on each! (Now, that's STICTION!).

Before throwing away any old drive that does not want to spin any more, if you are knowledgeable with soldering and want to try to salvage your drive, try replacing this capacitor. For the +5Vdc line, I recommend a 10v or 16v, 47uF electrolytic capacitor with the two leads out the same end. If you want to also replace the +12v capacitor (I believe it improves read/write reliability), try finding a small 25v, 47uF capacitor, no bigger than 1/4" in diameter. They can be found at most electronics stores.

While there have been numerous models of controller boards on the ST-138 and ST-251 drives, generally the correct capacitor is easy enough to locate as they are always located next to the power supply connector. Check which colored lead is the 5Vdc line from the computer power supply (red on the Z-100) and using an ohmmeter, locate the positive lead of the nearest 22uF capacitor connected to that connector. Fold the negative lead of the capacitor back along the side of its case and solder the leads to the vacant pads of the removed capacitor. The capacitor should lay on its side against the board. The positive lead MUST go to the pad connected to the 5Vdc line, generally marked with a plus (+) sign, and the negative lead MUST go to the pad connected to ground.

This fix may be applicable to other computers and Seagate drives. You'll find that some other drive manufacturers already use 47uF capacitors.