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Fault Finding My FDX


This page describes my efforts to repair two of my faulty FDX systems - the content is "warts and all"

    - some of the problems were "self inflicted wounds" - but some of the other stuff might be of interest to others


- alternatively, you might just want to read about incompetence in action :-)

Status : Completed - skip to the latest update


I had two faulty FDX systems :-

  • one that I bought new back in 1984, let's call it FDX-1, it failed a few years ago with a display problem that looked very much like a loss of sync on the 80 column card, the system booted successfully but the screen was unreadable. I was only able to view the composite video output, as I did not have an RGB monitor back then.
  • a second faulty FDX, which we'll call FDX-2, that I purchased off ebay in 2009 with a view to using it to diagnose the problem with FDX-1. Unfortunately, I managed to break it while trying to diagnose the problem with FDX-1! The FDX is not very "maintenance friendly", you pretty much have to completely dismantle it to access the internal components. With the system open and under power, I seemed to recall that I managed to cause a short on the "good" 80 Column board which put an end to my efforts at that time.

At that point, as well as any other potential problems, the issues with these systems were believed to be :-

  • FDX-1 and FDX-2 - both had faulty 80-Column cards, one with a sync fault and one "dead" from a self inflicted wound
  • FDX-2 had faulty floppy disk drives - the heads appeared to be damaged, costing me 2 valuable FDX system disks!

When I damaged the second FDX, things stalled until I came across the MTX-500 Facebook group in 2011; this is a small, but very active, group of Memotech enthusiasts which include a number of MTX hardware and software experts. Inspired by the appearance of this group, and with their assistance, I made renewed efforts to repair my failed FDX and have documented my experiences, good and bad, on this page.

The information on this page may, or may not, be useful to others, in any event, "your mileage may vary" :-)


Status Update : January 2013

  • At the start of this renewed effort to recover one, if not both, of my FDXs, the status was that FDX-1 was in pieces, you pretty much have to completely disassemble an FDX to remove the drives, and FDX-2 was "dead" - when connected to an MTX, the computer went straight to the MTX "Ready" prompt and failed to try to initialise the FDX. (I now know why this was - if you want to, you can skip the intermediate stuff and find out why here!)

Using FDX-2, I had tried swapping parts in an attempt to identify the failed sub-system; I had tried 2 MTX512s, 2 MTX RS232/FDX interface cards, 2 MTX-FDX ribbon cables, 2 FDX SM1 bus interface cards, 2 FDX floppy disk controllers and 2 FDX 80 Column boards (including the one with the apparent sync problem) in FDX-2. I had also tried to eliminate the possibility of power supply problems by providing power to the MTX from 2 external MTX power supplies as well as through 2 FDX-MTX power cables and had measured the voltages on the internal FDX power supply, all of which appeared to be within tolerances.

Somewhat surprisingly, at least to me anyway, these efforts were unsuccessful and obviously disappointing, I would have expected to be able to identify the failed sub-system in this way, but unfortunately not, the FDX failed to start with any combination of components installed. My next thought was that it may be possible to run some tests with a bare minimum of hardware, i.e., the MTX, RS232/IF card, ribbon cable and FDX SM1 interface card to see if the MTX could at least communicate successfully with the first board in the FDX, although I was not sure how this could be done.

Test Setup

A comprehensive range of FDX photos can be found on my FDX Photos page, but a couple of photos here will help illustrate the difficulty in accessing the FDX internals for testing. (You can also look up my FDX disassembly page.)

Removing one FDX end plate allows the top cover to be slid out to expose the FDX internals.

The FDXC1 disk controller, installed in the top slot of the 6" card cage, is now clearly visible, however, the rear panel must also be removed to gain access to the PCBs and allow their removal from the card frame.

The screws securing the rear panel to the remaining end plate have been removed and the rear panel allowed to swing away, exposing the rear of the card frame.

(This photo is of an FDX with a single floppy drive, the large silver plate is where the second floppy drive in a dual drive system would be mounted.)

Note: the large grey ribbon cable is a non-standard feature of this FDX, it is wired to a connector in the 8" disk connector slot in the rear panel.

Caution !

Exposing the rear of the card frame also exposes the FDX power supply and the additional transformer used to supply power to the MTX. These items have exposed terminals which carry mains voltages - take care when working inside the FDX with the power on, also be aware that the PSU capacitors can hold charge for a few seconds after the power is removed from the system.

With the floppy disk controller removed from the upper slot and the 80 Column board removed from the middle slot, the SM1 bus interface card is visible in the bottom slot.

The ribbon cable connects between the bus interface card and the IDC socket on the bottom of the FDX case. A short ribbon cable connects from the bottom of the case to the RS232/FDX Interface card in the MTX.

The terminal block at the top of the card frame is used to supply power from the FDX PSU.

Card Frame Power Connections
Blue -12 VDC
Red + 5 VDC
Pink -12 VDC
Black Common
For ease of testing, I found it more convenient to use the card frame from FDX-1, powered from the FDX-1 PSU. As well as making it easier to work on, this also eliminated the internal FDX ribbon cable as a potential source of the problems.

I was able to connect the SM1 bus interface directly to the MTX RS232/Interface card using a replacement cable that I had bought off ebay (a mistake - again, you can skip the intermediate stuff and read here to find out why).

For ease of access to the SM1 interface, since the card frame bus is contiguous between slots, I moved the SM1 interface into the top slot.

Using this test setup, the symptoms were the same as those seem on FDX-2, the FDX failed to initialise and the MTX displayed the "Ready" prompt generated by the BASIC ROM.

ROM Access Test Program

Having enquired  in the Facebook group, about the possibility of being able to do a basic level of testing with only the FDX SM1 card connected, Andy Key kindly wrote a small program to test communication between the MTX and the SM1 bus interface card. This tiny program was small enough to type in each time using the MTX built-in Z80 assembler without needing to save and load from tape :-

 Z80 Instruction Description (any errors in the comments are mine)
DI Disable Interrupts
LD A,#40 Load reg. A with 40H, i.e., select ROM4, the CP/M Boot ROM
OUT (0),A Port 0 is the Page Port, switch in the selected ROM
LD HL,#2000 Load reg. HL with start of memory to copy  (8192d)
LD DE,#D000 Load reg. DE with target memory location to store the data
LD BC,#2000 Load reg. BC with the number of bytes to move (8192d)
LDIR Load the address stored in (DE) with the contents of the address stored in (HL), increment (DE) and (HL), decrement (BC) and repeat until (BC) = 0
LD A,(#FAD2) Address FAD2h Stores the current memory page configuration
OUT (0),A Restore the previous configuration, i.e. switch ROM1 back in
EI Enable Interrupts
RET Return

How it works

When the MTX paged memory map structure is in ROM based mode (RELCPMH = 0), ROMs are mapped into memory between 0000 and 3FFFh. The MTX Operating System (OS) ROM is an 8K (8192d = 2000h bytes) ROM which occupies the memory space between 0000 and 1FFFh and the paged ROMs of 8K each are mapped from 2000h to 3FFFh as eight pages, 0 to 7, set by bits R2, R1 and R0 in the page port register.

Page Port Register - Output Port 0 (write only)

D7 D6 D5 D4 D3 D2 D1 D0
RELCPMH R2 R1 R0 P3 P2 P1 P0


Page Port Register

Memory Range Memory Range
Bit 6 Bit 5 Bit 4 0000h to 1FFFh 2000h to 3FFFh
R2 R1 R0
0 0 0 OS


Always present in

RELCPMH = 0 Mode

0 0 1 ASSEM
0 1 0 ROM 2
0 1 1 ROM 3
1 0 0 CP/M Boot
1 0 1 SDX
1 1 0 ROM 6
1 1 1 ROM 7

Normally, ROM0, the 8K BASIC ROM, occupies the memory space between 2000h and 3FFFh, but alternative ROMs can be switched in and out by sending an appropriate command to the memory Page Port (Port 0). ROM4 is the CP/M Boot ROM on the FDX SM1 bus interface card and is also an 8K ROM, it is selected by setting bit D6 in the page port register, bit 6 = 26,  or 64d, or 40h i.e., sending 40h to output port (0).

The program switches in the CP/M ROM and copies its contents to high memory starting at location D000h.

After the program had run, the MTX "PANEL" display could be used to inspect the contents of memory from D000h. The contents of the first 8 locations of the CP/M Boot ROM, and in fact any bootable ROM, are "08, 07, 06, 05, 04, 03, 02, 01", if the ROM had been read and copied correctly, these bytes should have been saved to location D000h and above.

With my first FDX SM1 card, the values reported were "08, 08, 05, 06, 04, 04, 01, 02"

With my other FDX SM1 card, the values reported were "07, 07, 05, 05, 03, 03, 01, 01"

Initially suspecting that the ROMs may have been corrupted, I read their contents on an EPROM programmer and the values stored in the ROM were correct, therefore, there appears to be a problem with the data or address lines between the MTX and the CP/M ROM. But, how to find the problem?


Status Update : February 2013

While discussing my failed FDX on the MTX-500 Facebook group, Inaki Castillo generously offered me a Tektronix TLA714 Logic Analyzer to help me locate the problem(s). Having got this tool, and received offers of help from other group members including Peter and Andy, I am now making renewed efforts to repair my failed FDX.


Additional (partial) Failure - FDX-1 PSU Capacitor

At this point, one of the times that I powered up FDX-1 to get power to the card frame resulted in a slight "pop" and lots of smoke coming from the PSU! I hastily switched off the FDX and waited for the smoke to clear before inspecting the PSU. A close inspection of the PSU revealed that one of the filter capacitors just at the side of the main transformer had  "burst", but there was no obvious other damage to the board. Somewhat gingerly, I powered up the FDX again and the PSU still worked - I measured the voltages and as they appeared to be fine, so I decided to continue to use this PSU for testing

(For the PSU repair, see the FDX Power supply repair page.)


Diagnostics using my Logic Analyzer


Having facilitated access to the card frame components, the next question was which signal(s) to use to trigger data capture by the logic analyzer? A logic analyzer has the ability to capture high speed data from a large quantity of signals, but with a limited amount of data storage for each signal, choosing an appropriate trigger signal to start data capture is critical to achieving good results with a logic analyzer.

Peter suggested that a good trigger would be the CS signal (PIN22) of the BTROM on the SM1 Interface board. However, probably more due to my lack of skill with the logic analyzer, than any inherent problems with using CS, I was unable to successfully trigger on this signal.

However, I was able to use the ROM select bits in the Page Port register, although not readily available on the SM1 card, these bits are available on the cartridge port connector on the side of the MTX. Bit 6 (R2) is set when the CP/M ROM is switched in and I used this as the trigger.

To begin with, I only had a small number of the tiny clips required to connect the leads at the end logic probe to appropriate test points on the circuit boards. I had ordered more (from the US) and until they arrived, I was unable to connect to all of the test points required, i.e., 16 address lines, 8 data lines and up to 8 control bus signals.

Even with the limited number of probe test hooks that I had available, I was able to see that the values on the data bus were the same as those displayed on the MTX PANEL after running Andy's program. However, when probing the address lines on the CP/M ROM while running the memory copy program, errors were visible, the addresses should be consecutive but there were missing addresses and repeated addresses being read. This agreed with the data seen on the PANEL display - suggesting that some memory data was being read from the wrong location. However, when probing the address lines on the MTX cartridge port connector, the addresses were correctly sequenced.

This suggested that the problem lay somewhere between the RS232/Interface card in the MTX and the SM1 interface card. The SM1 interface card includes buffers between the MTX and FDX, for the address lines, this is done using two 74LS245 octal bus transceivers. My initial thought was that the problem may have been due to a failed bus transceiver on the SM1 board, however, further investigation revealed that the buffers are on the output side of the SM1 board and buffer the signals immediately before they are placed on the FDX backplane, therefore, they should not affect reading of data from the CP/M PROM.

Inspection of the RS232/Bus Interface circuit diagram showed that there are also 74LS245 buffers on the address lines on the output side of that board leading me to suspect that this may have been the cause of the fault. Once my additional probe clips had arrived, I was able to probe the low order byte address lines (A0-A7) on the MTX cartridge port at the same time as probing the same lines on the input side of the 74LS245 responsible for buffering these address lines on the RS232 board (IC4).


This trace shows first 8 addresses being read by the test program measured on the address lines (A0 to A7) at the MTX edge connector and at the input side of IC4 on the RS232/IF card - the 74LS245 which buffers address lines A0 to A7.

(Click on the image to see an expanded range of address lines.)

It can be seen that the address coded on the address bus is the same when measured at both locations

This trace shows first 8 addresses being read by the test program measured on the address lines (A0 to A7) at the MTX edge connector and at the output side of IC4 on the RS232/IF card.

(Click on the image to see an expanded range of address lines.)

It can be seen that the addresses coded on the bus are different, i.e., the buffer appeared to be corrupting the address lines. The data values being read by the test program had changed to "08, 08, 08, 05, 08, 08, 08, 05", i.e., only bytes 00 and 03 are being correctly read from the CP/M ROM. This is consistent with data being read from locations 0 (value 08) and 3 (value 05).


Self Inflicted Wound Number 1 - Using an untested "new" cable


As the next step, I was advised to check the MTX to FDX ribbon cable, although I was rather sceptical that this would find any problems, after all, it was a brand new cable, but . . . . . 

It's back to the drawing board! - with the ribbon cable disconnected from the RS232/IF card output, the address bus data values were identical when measured on the MTX edge connector and on the input & output side of the LS245 buffer - confirming that there was not a fault with the buffer after all.

In an attempt to eliminate the original ribbon cables as the source of the problem, I had previously bought two new 60 way ribbon cables, one of which I was using for testing. Would you believe it? Although the other new cable, as well as the old original cables tested fine - the new one that I had chosen for testing the FDX with was faulty!

I removed the bad cable (for later testing) and reconnected using one of my original MTX-FDX ribbon cables. (Subsequent testing of the cable revealed a dead short between lines 4 & 5 of the ribbon cable, this no doubt occurred when the IDC plug had been crimped onto the cable.)

With a known good cable connected, the next step was to remove the SM1 ROM and check the address lines on both sides of the MTX-FDX buffer on the SM1 card to ensure that the address data all the way through from the MTX to the FDX side of the bus were consistent. This exercise led to the discovery that the schematic drawing of the MTX bus interface card in the FDX manual is incorrect.


The 74LS245 in position B1 on the SM1 Bus Interface card buffers the A0 to A7 address lines between the MTX and FDX busses. The schematic in the FDX manual (shown here), indicates that the address lines on J2, the IDC header for the ribbon cable to the MTX, are connected to the "B" side of the buffer on lines "B1" (A0) to "B8" (A7), with the same address lines on the FDX bus connected to the "A" side of the buffer on lines "A1" to "A8".

This is incorrect, in fact, the address lines from the J2 connector are connected to the "A" lines, the A0 to chip signal to "A8" on Pin 9. Similarly, the "output" ("B") signals go to the FDX Bus - "A0" to "B8", on Pin 11.

(These connections are consistent with how it is done on the RS232 card.) The same error is shown for the high order address lines on the buffer chip in position C1 on the SM1 board.


Once this error had been spotted, I was able to confirm that all of the address lines (A0 to A15) showed consistent data when measured on the MTX bus and on the input and output sides of the 74LS245 buffers on the SM1 card.

With the SM1 CP/M ROM re-installed, the MTX no longer booted to the "Ready" prompt and the MTX Video Screen was blank. This is the expected behaviour, with the CP/M ROM working correctly, the FDX should output to the 80 column card and attempt to boot CP/M from the configured disk drive, in this case, a type 03 disk connected as the first physical disk drive.

With the CP/M ROM now apparently trying to boot normally, I thought I'd try installing an 80 column board and see what happened! Although there was some slight corruption of the text on the 80 column screen, the ROM start-up message and RAM test message were OK.


Self Inflicted Wound Number 2 - Not checking the obvious! (Hindsight is wonderful)

As described earlier, this testing was being done with the card frame from FDX-1 out of its chassis and on the bench. I then took a really close look to see what might be different between the card frame being in the chassis (FDX-2) and it not (FDX-1) and discovered that there was a bent pin in the IDC socket underneath FDX-2, resulting in the IDC connection from the MTX not having the A0 address line connected - aaarrgghhhhh!

Although I was worried that the pin might break, I managed to straighten it enough the allow the IDC connector to be properly mated. With the card frame installed in the chassis of FDX-2 and the SM1 Bus Interface and 80 column cards installed, the system now correctly started up and displayed the "Insert System Disk" prompt message.

Conclusion - the identical problem symptoms seen when testing on the card frame of FDX-1 and the complete FDX-2 were both caused by connection problems between the MTX and FDX. In the case of FDX-1, the problem being the faulty replacement cable that I'd used and in the case of FDX-2, the problem being the bent pin on the FDX to MTX ribbon cable  chassis connector. The fact that both test setups exhibited the same failure with completely different hardware and cabling cost me a lot of time - but did help me get a better understanding of the MTX-FDX interface!


The text output from the 80 column board monochrome connector (I do not have an RGB colour monitor) was slightly corrupted - however, it was legible and markedly different from the original video output problem on FDX1. I tried both 80 column boards and they both displayed the same output - it appears that my old 80 column board with the "sync problem" is now working!

I also confirmed that both SM1 bus interface cards were working normally.

At this point, I discovered that my recollection of shorting something on the "good" 80 Column board was incorrect - in fact, the problem had been on the disk controller from FDX-2 - the capacitor in position A9 of the FDXC1 board had been somewhat toasted! - At this point, I did not know if anything else on the board had been damaged though. In fact, it is obvious (now) that the disk controller was the most likely PCB to be damaged while working with the FDX open - it is normally installed in the top position of the card frame.

At last! - Things are looking up

I installed the undamaged disk controller from FDX-1 and tried to boot from both of the floppy drives. I knew that this was unlikely to succeed, having established in the past that both drives on FDX-2 appeared to be faulty. Although the drives attempted to access the disks and the read/write head carriage could be seen to move, the system failed to read the disk and reverted to the "Insert System Disk" prompt.

I connected the "good" "B" disk drive from FDX-1 in place of FDX-2's "B" drive and tried again - success!

The FDX successfully booted a CP/M system disk that was created some 30 years ago!


The photo opposite was taken with my iPhone, so the quality is not that good, but you can see that the quality of the display is very poor, with some corruption or horizontal "bleeding" of the characters (click on the photo to see it in more detail).

This is the same as the "slight corruption" when testing using the FDX-1 card frame that I mentioned earlier . I had only a very basic mono monitor available, and although I recall that this monitor seemed to work fine previously, the monitor was suspected to be the reason for the poor display now, so I wanted to try the 80 Column board colour RGB output.

Not having an RGB colour monitor, I needed to convert and upscale the RGB output to VGA using a Video Converter board as described on my MTX Video Page.

With apologies again for the quality of the photo, you can hopefully see that the clarity of the text is much improved, the photo does not do it justice - the display is much clearer than the photo suggests.

If you click on the photo to see the larger image, you may be able to make out some white dots on the display. This seems to be a "feature of" the video converter being used, I believe that I should be able clean up the display by fine tuning the video board.

There are also other adjustments that can be made to the video converter that I expect will improve the display quality.

The video converter is small enough to fit inside the FDX case, but this may not be practical if frequent adjustments are required.

I then tried the original disk controller from FDX-2 which, despite the "toasted" capacitor, also successfully read system disks and booted the FDX, i.e., appeared to be working normally.


Status Update : March 2013

My working drives have now "failed", the original "C" drive from FDX-1 literally went up in a cloud of smoke, and unsurprisingly, doesn't work any more - the spindle motor does not fire up. (Subsequent testing showed that the drive mechanics, including the spindle motor and OK and the fault is on the PCB but no further fault finding has been done on it at this point.)

The original "B" drive from FDX-1 worked very briefly in my test FDX, it was able to read/write and format disks for a short while before it exhibited the same symptoms as the drives already installed in that machine, i.e., the drives appear to start as and when required, the disk spins, the access LED comes on, but the can't read anything from the disks - even when the disks are known to be good.

However, I have been able to confirm that the rest of the hardware appears to be still OK by using a 3.5" floppy drive. Using an FDX CP/M Boot Disk image from Peter, I was able to use Teledisk to create a 3.5" FDX boot disk on a PC, setting the FDCX1 configuration DIP switch (SW6) for the boot drive to 96 TPI allowed me to boot from and run with the 3.5" disk drive. (As an aside, I am now thinking about installing a 3.5" drive in one of the FDXs (I eventually did do that, see here), even when I  get replacement 5.25" drives.)

Getting replacement 5.25" drives is not proving to be too easy,  they have been obsolete for a number of years are in short supply - even on ebay. There are more available on, but they are pretty expensive to get to the UK. Typical prices seem to be around 25 for the drive, plus 42 for postage, onto that you need to add VAT of 13 and 8 Royal Mail fees for processing the import. That works out at around 88 - for a drive that might not even be guaranteed to be working.

I have been trawling through some of the other retro computer forums and found that Daniel Jameson on the, Stairway to Hell Forum, had a freebie 5.25" drive with a broken door latch available for the cost of postage. The door latch looked to be fixable so I paid the postage and got the drive (thanks Daniel).

I fitted the drive and initially, it worked fine, it was able to read my 1980's floppy disks, boot CP/M, format disks etc. before very quickly exhibiting the same failure as my original "C" drive, appearing to run, but not able to read disks.

So, to recap, this FDX had two drives that were faulty when I received it and my original "B" drive as well as the replacement drive from Daniel had both failed in it too. This was obviously more than a coincidence, and I initially suspected that the disk controller in this FDX had a serious fault which caused drives to fail. This was particularly odd, as the 3.5" drive (with a fixed ID of 1) has continued to work reliably. I now need yet another 5.25" drive to proceed further. I have ordered what looks like an ideal replacement off, a Teac FD-55F, it has taken a while to get delivery sorted out with the seller, but thanks to Peter, it is on its way now.

In the meantime, through a combination of the stardot and Amibay forums, I found another drive for sale by Iain Hancock, Iain also threw in another 5.25" drive free for the cost of postage (thanks Iain). I have also obtained another drive from Tony Brewer (thanks Tony). I hope I now have enough working drives on the way to get my FDXs working.

I am worried that installing these drives on one or other of my disk controllers will cause these drives to fail too. Fortunately, thanks to another Memotech collector, Jan Seyfarth in Germany, I have a full compliment of FDX cards on loan to help me get my FDX working (thanks Jan). When the drives arrive, I will use this card set to make sure that I can get reliable disk operation before proceeding.

Using my original FDX chassis and PSU along with the replacement card set from Jan, I built up a second system to test the new drives. The drive from Tony looks like new and it seems to work quite happily in 80 Track mode but unfortunately has problems with my 40 Track disks, I am currently trying to find out whether it is possible to set the drive to 40T mode. The second drive from Iain turned out to be single sided, so couldn't read my DS/DD disks, but the other drive was DS/DD, switchable between 40 and 80 Tracks. Using the drive, I was able to reliably read and write my original 40 Track FDX disks.

That still left me with the question as to what would happen when I had to return Jan's card set. The only way to know was to take the risk of damaging the replacement drive and try the suspect disk controllers in the second FDX. When I did this, I found that the drive continued to work reliably, i.e., both of my original disk controllers are fully functional after all.

So, why did the first FDX appear to be killing disk drives? The obvious candidate was dirty drive heads, but cleaning the heads was the first thing that I had done when I had problems reading disks with the drives on FDX-2. I had previously used a "wet" diskette cleaning disk and also tried manually cleaning with a cotton bud and isopropanol. Whilst a small amount of deposits were removed, there did not appear to be significant build up of dirt/oxide on the heads. These drives had also damaged (scored) a couple of system disks, after cleaning, so I believed them to have been faulty - probably due to a pre-existing fault, but possibly due to my overenthusiastic cleaning actions!

I decided to try cleaning them again - when manual cleaning again did not help, and having nothing to lose with drives I thought "dead", I got even more "enthusiastic" with the isopropanol and some lint free wipes on the "B" drive in FDX-2! I thought that I was being far too rough and fully expected to remove any lingering doubts about the drive's condition by completely kn*****ing it. However, doing this showed up a significant amount of black deposits being removed from the heads. (Some deposition of the magnetic oxide coating from floppy disks onto the R/W heads is to be expected, but the effect can be minimised by keeping the diskettes as clean as possible, using good quality media and regularly cleaning the drive heads.)

Given the amount of "dirt" that I managed to remove from the heads, it was now no surprise that the drive could not read disks! Having been somewhat "rough" with the cleaning, it was time to see if there was any signs of life from the drive.

Using one of my remaining system disk copies, I was able to successfully boot from the drive! It is clear that the previous disk cleaning attempts had not had much effect in removing the build up from the R/W heads, I think that the "wet" cleaning kits that use cloth type material impregnated with a cleaning solvent are just not tough enough to clear years of deposit build-up. I think that a more abrasive cleaning disk would have been better but I don't know if they are available any more.

Anyway, buoyed by this success, I tried the same rough treatment on the other drives and was able to recover the "B" drive from FDX-1 in the same way. However, the "C" drive from FDX-2 is still not able to read disks reliably and my "exploding" "C" drive from FDX-1 is definitely done for.


Self Inflicted Wound Number 3 - Using ancient, poor quality disks


So, what had caused the heads, particularly those in drives previously known to be working, to get so contaminated so quickly? Confession time again - mea culpa! Along with FDX-2 from ebay, came a number of disks with labels suggesting that they contained various interesting software programs that I did not have. A cursory inspection of these disks suggested that they were OK, and being in a position to look at them for the first time, I had tried to read them in the known good drives. The drives had then worked for a short time, before being unable to read anything - even my own disks. I did not make this connection until after I managed to resurrect the "failed" drives. At that point I took a closer look at the floppies from FDX-2 and realised that they appeared to be of poorer quality than my own disks and a number had signs of deterioration of the surface coating.

As well as preventing FDX-2's floppies from being useable, I guess that when used by the previous owner, they had left significant quantities of oxide coating on the drive heads resulting in the damage to my own system disks when originally tried in these drives.

Using the same diskettes in my replacement drives was obviously a big mistake and led to them being contaminated to the point of being unusable without vigorous cleaning activities!

All floppy disks of this age are very likely to have deteriorated to some extent, but the degree of degradation is very much influenced by the quality of the media and its storage conditions. Close inspection of my own media has revealed no noticeable degradation, they have certainly stood the test of time much better than the media from FDX-2. I am sure that I will find some of my disks to have unreadable sectors too, but so far, the only problems have been with disks damaged by the drives in FDX-2.

FDX-1 Display Problem Returns


The original problem with FDX-1 has reappeared - the output from the 80 Column board is again exhibiting the same problems as I saw initially, an apparent lack of sync. However, having the second FDX, I was able to try the card in that machine where it worked perfectly.

Further investigation has revealed the cause to be the power supply in FDX-1. The voltages do not appear to be too bad when measured with a DVM, but having just bought an oscilloscope, I was able to take a detailed look at the output voltages which revealed large voltage swings at a frequency of 100Hz. Using an old ATX PSU to provide stable voltages in FDX-1 restored the operation of the 80 Column card to normal.

Conclusion - the original problem on FDX-1 was more than likely due to a failing PSU rather than any problem with the 80 Column card.

Rather than attempt to repair the faulty PSU, I have decided to replace it with a more powerful modern PSU from a PC. As well as providing a stable, reliable supply, this PSU will also provide extra power should any of the FDX enhancements currently being discussed on Facebook ever materialise. You can see the details of the power supply replacement here.



Current Status

I eventually upgraded FDX-1 to power it from an ATX PSU and fitted replacement 3.5" floppy drives.

I replaced the blown capacitor on the DCB in FDX 2, thoroughly cleaned the 5.25" drives and returned this one to working condition in its original Memotech configuration, i.e., with 2 x 5.25" Qume Qumetak 142 drives.

Outstanding issues :-

  • Fine tuning of the external RGB-VGA video converter is needed, but using a legacy 15.75kHz monitor will always give a better result.


And Finally . . . .

The above saga could be seen as examples of how not to attempt to fault find and repair a failed FDX!     

However, given all the help that I have received along the way, and in the vain hope that it might actually help someone else, I think that it is only fair that I share my experiences of the long, arduous and error-strewn process here.

Try not to be too critical !   



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