Floppy Drive Cleaning

IMG_2937My NorthStar Horizon came with two Tandon TM100 floppy drives. As I mentioned previously, these were jumpered for drives 3 and 4. I don’t have any other drives to plug in to this system, so I need to re-jumper these for drives 1 and 2.

Even though the service manual is easily downloadable from the net, I don’t have an alignment floppy disk, so I needed to be careful disassembling and cleaning those drives. I took great care not to take apart any assembly that would alter the alignment of the heads or the track 0 sensor. This limited me to basically removing the circuit board to gain access to the mechanism below.

IMG_2821The floppy drives were in remarkably good condition. The belts are snug, with no signs of slipping. The circuit board was in good shape. I removed the strange ID4 add-on wire (mentioned in an earlier post) and returned the jumpers to ID1 and 2. I had no terminator resistor pack, which needs to be installed in the last physical drive in the chain, so I had to manufacture one. The service manual mentioned a value of 150Ω so I grabbed some 1/4W resistors, cut them to length, and inserted them into the terminator socket. The on-board electrolytic capacitors were checked for tolerance and ESR, and all passed. I couldn’t locate any tantalum capacitors (good).

IMG_2945The drive rails were in good condition, but needed a clean and some lubrication. The heads had a bit of oxide build-up, so had to be cleaned. I placed a lint-free wipe soaked in isopropyl alcohol in between the two heads, and closed the mechanism so the heads pushed up against the wipe. The bottom head is a nice solidly mounted head that I’m used to seeing in 5 1/4″ drives, however the top head was a very delicate unframed head held in place with a fragile-looking spring mechanism. Any lateral force (such as pulling the wipe sideways) would possibly bend this mounting mechanism, so I had to be careful. Several open-close motions cleaned the oxide buildup from the heads quite well.

IMG_2948After re-assembling the drives, I powered them up using a current-limited supply, to make sure there were no shorts. Everything looked good, so it was time to mount them back in the case. It took me a little while to determine NorthStar’s factory placement of drive 1 and 2, but after some digging, I was able to find out that drive 1 was the right-most drive (not what I would have guessed, but I’m trying to be authentic).

IMG_2955For such a spacious case, the floppy drives are quite a squeeze, and it was a careful dance to get the drives partially in place, then the power and data cables connected, then sliding the drives all the way in. I learned the hard way that this procedure had to be performed with both drives at the same time, after many attempts at doing the drives one at a time.

The machine is really starting to look like a real computer now. Once I get the floppy controller back in, I should be able to attempt a boot!

My Memory – It’s All Coming Back To Me Now!

IMG_2935As I mentioned in my last update, I was without any NorthStar RAM for this machine. I spent several hours trying to get the Dynabyte 64k board running, but didn’t get much success. Unable to find any documentation for the board online, I had to try to reverse engineer the bank switching scheme used. I managed to figure it out, and was able to use my monitor to switch banks on and off, I was unable to get the RAM to actually work. I could read just fine, but writing always resulted in the value FF being deposited.

I was starting to think that this might be the end of my Retrochallenge (what use is a machine with no RAM?), so I put out a plea looking for S100 RAM boards in a couple of retro mailing lists. I received a couple of offers go help, and incredibly I was able to locate not one but TWO NorthStar memory boards right here in my small home town of Kelowna (population approx. 120,000). He even threw in an S100 extender board!

IMG_2919These memory boards – part number RAM-32-A – were much better to work with because of the ready availability of documentation. The first steps were familiar to me now – test all of the tantalum capacitors for shorts. One of the boards had a tantalum that had already experienced a catastrophic failure – half of it was missing. I dropped in a replacement, then tested the power rails by bringing each one up on a current-limited supply. Both boards passed these tests, so now it was time to test each board in the Horizon.

IMG_2927One board failed completely – the red LED didn’t even light up. However, the second board showed very encouraging signs of life. Using the documentation to enable each 8k bank one at a time, I was able to deduce that all banks were working, however not all banks worked in both bank locations (each bank can appear in one of two locations, or be switched off completely). I suspected the DIP switches as the likely culprit, so I tested the switches with my meter – yup, several switches had a very high resistance or complete open circuit. A quick spray of contact cleaner and some vigorous switching should clean those up. I could only locate my lubricated contact cleaner, which is not ideal for this application (it’s more useful for things like relay contacts), but because my regular contact cleaner was MIA, this would have to do.

IMG_2930Sure enough, this fixed it! 32k of beautiful error-free RAM. My monitor ROM has a memory testing function, so I left this running for several hours, and not a single memory fault! I will save the debugging of the second board until later, but for now I have a respectable 32k to play with. I think the next step is to get the floppy drives going, and eventually get to the point of writing a boot disk.

First Light!

This weekend I’m spending at the lake, so I’m nowhere near the Horizon (849km away according to Google), but I had a small flurry of success just before I left, and I have a few minutes relaxing by the bonfire with a drink in hand, so I might as well grab the photos from my phone and bring my project up to date.

IMG_2889Now that I have a power supply, motherboard, and CPU board with monitor ROM, I have enough pieces to see if I can get some signs of life out of the machine.

I pulled out my trusty VT220 and hooked it up. At this point, all I have in the backplane is the CPU board with the boot PROM. There is no RAM yet, but according to the documentation for the monitor ROM, it operates with no need for RAM – which is very handy for testing. My motherboard is strapped for 9600 baud, so I set the terminal to the same speed, and I power everything on.

IMG_2888A prompt! Well, not right away – I had to exchange my serial cable for a null-modem cable, but then – a prompt! The ‘]’ symbol is the appropriate prompt for this monitor. No fancy sign-on bender (what was I expecting for a 1K monitor?)

I can examine memory (everything is FF except for the 1K of monitor ROM), and depositing doesn’t change anything, but it’s working! What does this success tell me? It tells me that the serial port is working (and if the serial port is working, reading and writing to I/O ports is also working). The address and data busses are also working. This makes me happy!

As I have mentioned previously, the biggest challenge I’m going to face with this project is the lack of any NorthStar memory boards. The machine didn’t come with any, but I did acquire a Dynabyte 64k memory board of unknown operational status. I made a request last week on some mailing lists, and amazingly I was able to find someone willing to part with two 32k Northstar memory boards – and the amazing part was they were located in Kelowna! I grabbed them (and a nice S100 extender card) just before I left for the lake, so I can’t wait to get back and test them out!

IMG_2923Now back to the task at hand…

Missing Power Rail

After re-assembling the motherboard inside the cassis and hooking up the power supply, it was time to check the power rails and make sure everything was working.

I threw the switch and the power supply came to life. I probed the power supply voltages as they arrived at the motherboard and everything looked fine. However, when I checked for power at the backplane sockets themselves, I discovered that the -18v rail was missing from about half of the sockets.

Because the rail was there for half the sockets and not there for the other sockets, it must be a fault on the motherboard.  Several dozen acres later (actually, only 8, but it feels like more), I pulled the motherboard out and did a visual inspection of the underside traces.

IMG_2894I’m not sure how I missed it when I was cleaning the board, but this time it was obvious – a burned out trace. There’s a serious design flaw in the NorthStar Horizon – the power rails from the linear power supply go straight to the backplane without any in-line fuses. Later S-100 machines have fuses on the backplane which would have protected the motherboard. If this was my machine 30 years ago, I probably would have retrofitted some fuses, but this machine has gone from a functional tool to a museum curiosity, so now isn’t the time to start making modifications.

IMG_2897I identified three areas in the trace that had burned through, and carefully scraped away the destroyed trace. The trace winds its way between connector pins, so I used a small piece of insulated wire to re-connect the trace.

A quick test with my multimeter made sure that the trace was reconnected and there weren’t any shorts to adjacent pins. I reinstalled the motherboard with it’s several dozen screws, and re-connected the power supply. This time, all the voltage rails were now visible at all the connectors.

I am now ready to start plugging in boards and getting the old beast to show some signs of life.

Power Supply

IMG_2865After reconditioning the capacitors and cleaning out the chassis, I continued working on the power supply. There’s actually very few components, so it was pretty easy to check.  The bridge rectifier and two rectifier diodes were a simple matter of testing with a multimeter.  The transformer was tested for open or shorted windings.  After cleaning and re-assembling the back panel, I noticed that the fuse holder wasn’t closing correctly.  A quick examination showed that one of the metal fuse contacts was broken. A quick rummage around my junk pile found a suitably aged replacement.

IMG_2882I reinstalled the capacitors and it was time to apply power.  No smoke! All three power rails came up beautifully. The fan – an item I was sure would need replacing – is remarkably quiet and smooth running.

The icing on the cake for this part of the project was getting the front panel LED mounted correctly. The original plastic clip that held the LED in place was broken. Fortunately, I had some very similar LED clips in my parts bin, so with a little bit of gentle persuasion I was able to get the new clip installed.IMG_2886 I’m suspecting that over the period of 36 years, the industry standard sizing for LED packages may have become ‘metrificated’ (I know it’s not a word, but I’m owning it) so it was a little bit of a tight squeeze, but it’s in there and it looks beautiful.


The NorthStar Horizon is unique among S-100 machines because the motherboard is more than a passive backplane.  The Horizon’s motherboard contains circuitry for two serial ports and two parallel ports (one in, one out).  There are voltage regulators on the motherboard that supply power to the serial and parallel circuitry, and they also provide the power for the two floppy drives.

IMG_2869My motherboard had the same corroded IC pin issue that I found with the CPU board.  A quick brush with a glass pen took care of that.  While taking great care to make sure each IC was put back in the correct socket with the correct orientation, I discovered that the original builder had inserted a socket in backwards.  Trying to de-solder the socket and put it in the right way round could potentially cause damage to the board, and with an IC in the socket, the incorrect orientation isn’t noticeable.

While the motherboard is out of the machine, I took the time to note the jumper settings for the serial ports – knowing the serial configuration will certainly be useful when it comes time to talking to the machine :)  The first serial port is configured for 9600 baud, and the second serial port is configured for 1200 baud.

IMG_2884These values seem like a reasonable place to start, so I’ll leave them alone.

Next, the capacitors needed testing.  There are three 6.8μF tantalum capacitors on-board.  All of them measured within tolerance.  The ESR (equivalent series resistance) seemed a little high, but a quick search of Digikey shows ESR values for modern tantalum capacitors for this value ranging from 2 to 8 ohms.

Capacitor Value ESR
C3 6.9 μF 3.44 Ω
C7 7.7 μF 6.55 Ω
C10 7.3 μF 1.85 Ω

I then brought up each of the power rails with a current-limited supply to make sure there were no shorts.

Rail Input Rail Output Current Draw
+8v 5.00v 500mA
+16v 11.85v 80mA
-16v -11.49v 50mA

I now have to wait until I’ve reconstructed the linear supply in the chassis before I can re-install the motherboard. So far, everything is looking good.

CPU Board

IMG_2835The first board to tackle is the CPU board – part number ZPB-A2.  The CPU board, the motherboard, and the chassis all have a number engraved – “CA W417963″.

Tasks performed on this board:

  • Clean socketted IC pins with glass pen
  • Test electrolytic and tantalum capacitors
  • Clean edge connector
  • Install EPROM circuitry
  • Change boot address jumper block

IMG_2848The legs on the ICs were quite corroded on this board (see pictures). Each one was removed from its socket, and given a quick brush with a glass pen (like this).

Next, I tested each of the electrolytic and tantalum capacitors.  I used an ESR meter to check that the components were still within tolerance, as well as making sure that they had not developed any shorts.  All of the capacitors passed, so the next step was to power up the board with a current-limited supply and allow it to warm up.  The total draw on this board is 570mA.

IMG_2864To address the biggest problem with resurrecting a NorthStar Horizon – how to make a boot disk – I needed to install a boot monitor that will allow me to hand-enter (or machine-enter over the serial port) a program to write an image using the disk controller.  Following the instructions for the original CPU board, I populated the optional monitor ROM components.

The boot ROM lives at address EC00, so I had to adjust two sets of jumpers on the CPU board – the ROM base address, and the boot address.  By default, the CPU board boots to address EB00 (a small boot PROM on the disk controller), but we need to boot the monitor at EC00.  As you can see from the pictures, I’ve added headers to (hopefully) address the PROM and boot the right address.

Once the machine is complete and boot disks have been written, I can change the boot address back to EB00 to make the machine boot from floppy.


Capacitor Reconditioning

The power supply in the NorthStar Horizon is a linear supply (common for the day), consisting of a transformer, rectifier, and capacitor.  The electrolytic capacitors in a linear supply are large, and the electrolyte inside the capacitor has a tendency to dry out as the capacitor ages.  It is important to test and condition old electrolytic capacitors before putting them back in to service, otherwise a catastrophic failure may occur, which could take out other components.

Capacitor ReconditionIn order to wake the capacitors from their sleep, and to test their effectiveness, I like to place a voltage across them (with a current-limiting resistor), and bring up the voltage over time to their maximum working voltage.  This process can help re-form the aluminum oxide layer in the capacitor.  At each step, the current should drop as the capacitor charges up.  Once the capacitor is charged, the current should be minimal.  If there is more than a couple of mA flowing, the capacitorIMG_2794 should be replaced.

In my case, the two smaller capacitors pass with flying colours.  The large capacitor is still leaking 0.96mA after 2 hours of conditioning at 15v.

Oh, and remember to discharge the capacitor after reconditioning it.  There’s a lot of joules stored in there!


Floppy Drive Strangeness

In my furious attempt to not do anything constructive until the Retro Challenge officially starts, I’ve completely disassembled the NorthStar Horizon to prepare everything for cleaning and testing.

IMG_2814During disassembly, I discovered that the floppy cable that runs from the floppy controller board goes to the two Tandon TM-100 floppy drives, then continues on and exits the back of the chassis and terminates in a dangling connector.  I can only assume that this was for additional floppy drives, but the cable harness is certainly not the original configuration for this machine.

IMG_2822I only ever intend to have this machine run with two floppy drives as it cam configured from the factory, so I will probably modify this cable to only have the two floppy connectors on it.

Neither of the internal floppy drives have a terminator resistor pack, so I will have to either purchase one, or make one up with 150 ohm resistors.

Next, I got in to checking the jumper settings for the floppy drives.  The first drive was jumpered as drive 3.  The second drive was different.  It had a long wire soldered on to edge connector pin 34 and ran over to pin 9 of the jumper configuration socket.  There were no jumpers in any of the drive select positions.

This puzzled me for a few days, until I happened to stumble upon a paragraph in the Horizon documentation:

  To program a drive as drive number 4 perform the following
  instructions on the printed circuit board mounted on top of
  the disk drive.

  a. Solder a jumper wire between pin 34 of edge connector J1
     and location 1F pin 6.  Take great care to connect the wire
     to the very tip of pin 34 so that the ribbon cable
     connector can slip as far onto J1 as possible.
  b. Solder a jumper wire between 1F pin 9 and 1F pin 11.
  c. The uninarked strap location between MX and MH at location
     1F is now the location for programming drive 4.  It can be
     labeled DS4.  This strap should remain connected and the
     straps labeled DS1, DS2, and DS3 should be disconnected.
     Be sure to disconnect this strap if the drive is ever
     programmed to be drive 1, 2, or 3.


For some reason, the NorthStar Horizon puts DS4 on pin 34 instead of the more normal pin 6.  These drives were jumpered for drives 3 and 4.  I will need to change this back to drive 1 and 2 for my configuration.

Missing Pieces

The Retro Challenge is getting closer.  It’s tough not to just dive right in to the project, but I’ve got to wait until July 1.  In the meantime, I’m slowly putting together the missing pieces that I will need to get my Horizon up and running.

I have sourced some precious hard sectored floppy disks – an entire box of 10!  I have a tracking number, so these should be here soon.

Hayes Microcoupler

The other item I thought might be difficult to locate was a micro coupler for the Hayes Micromodem 100.  I found one on Ebay for a price less than the cost of shipping it to me!

I also ordered the components I need to add the boot PROM socket to the processor board.  Those arrived overnight from Digikey.

I’m not going to know if there are any other missing pieces until I start tearing the machine apart.  Soon…