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PS/2 Floppy Drive Capacitor Replacement

I was able to successfully change the SMD electrolytic capacitors on a PS/2 floppy drive used in the P70/P75 luggable computers. These capacitors are prone to leaking and may eventually cause damage to the circuit boards. The floppy drive manufacturer is ALPS and the model number is DFP723D15C. It is also marked with IBM numbers "P/N 79F3224" and "FRU P/N 79F3225". Since PS/2 floppy drives have a slightly different connector they cannot be easily replaced with a standard PC floppy drive.

Here is the list of electrolytic capacitors:

|Location        | ID  | Value                 |
|Interface Board | C3  | 10uF 16V              |
|Interface Board | C4  | 10uF 16V              |
|Interface Board | C5  | 10uF 16V              |
|Interface Board | C6  | 10uF 16V              |
|Interface Board | C7  | 10uF 16V              |
|Motor Board     | 6C  | 10uF 16V              |
|Motor Board     | 8C  | 4.7uF 16V Unpolarized |
|Motor Board     | 9C  | 4.7uF 16V Unpolarized |
|Motor Board     | 10C | 4.7uF 16V Unpolarized |
|Motor Board     | 7C  | 4.7uF 25V             |

And here is an overview image:

Capacitor overview

For replacements of the 10uF caps I used 35V rated "RND 150VVT035M100CA1L" and for the 4.7uF polarized cap a 50V rated "RND 150VVT050M4R7CA1L". I did not find any unpolarized SMD type electrolytic caps so I used ceramic caps "Murata GRJ32DC72A475KE11L" instead.

Here are some images of the replacements:

New capactors spot #1

New capactors spot #2

New capactors spot #3

It is also worth mentioning that I had to lubricate the floppy drive as well, since it seemed to have trouble moving when I tested. I used white lithium grease on the stepper motor drive shaft and silicone grease on the plastic surfaces where the head assembly slide back and forth.

Topic: Repair, by Kjetil @ 25/11-2022, Article Link

CentreCOM MR820TR Hub Repair

I found a broken "Allied Telesyn CentreCOM MR820TR" network hub in the trash:

MR820TR Broken

When power was applied the power LED would just blink rapidly. The internal PSU is supposed to supply +5V and +12V on two rails, but when I measured these with the logic board connected they were just +1.6V and +8.1V. Disconnecting the logic board showed +4.8V on the +5V rail and variying voltage between +7.3V and +7.6V on the +12V rail, so likely something wrong with the internal PSU.

I measured the ESR of several electrolytic capacitors in-circuit and one of them (C06) in the center of the board had bad readings. I unsoldered this one and measured it again, and it was still bad:

MR820TR Bad Capacitor

I replaced it with a new one:

MR820TR New Capacitor

This network hub is now working again:

MR820TR Fixed

Topic: Repair, by Kjetil @ 05/08-2022, Article Link

Telephone Handset Connector Replacements

I have some old telephone handsets, where at least one of them seems to date back to the 1930's based on some patents ("British Patents 319837.328926.433234") that I found. All of them had strange connectors incompatible with modern equipment:

Old connection on handset #1

Old connection on handset #2

Old connection on handset #3

But since the speaker and microphone technology has remained mostly the same for over a century, I decided to simply replace the connectors with modern mini jack plugs:

Mini jack plug replacements

This makes it possible to connect them easily to a modern PC and even use them for voice chat.

On the first one the original cable disintegrated when I tried to re-use it, so I replaced the entire cable. This one also has a switch that I wired to pin 8 and 7 on a DE9 connector, so that I can use the CTS/RTS trick to read it through a standard PC serial port.

Handset #1 finished

On the second one I was able to re-use the original cable. This one also has a switch, but I did not wire that up to anything.

Handset #2 finished

On the third one I had issues with a broken wire in the original cable, so that had to be replaced as well. Here I ended up simply re-using the entire cable with connectors from a broken PC headset.

Handset #3 finished

Topic: Repair, by Kjetil @ 13/05-2022, Article Link

Commodore PC 20-III CGA Composite

The Commodore PC 20-III is a XT-class 8088 clone PC running at 4.77 to 9.54MHz with 640KB of RAM. It also features an on-board Paradise PCV4 chip that provides CGA graphics on both D-Sub/RGBI and RCA/Composite connectors. While the RGBI worked fine, I could not get any picture with the composite connection.

I measured with both an oscilloskope and a multimeter and the composite signal was around 5 volts, which is not correct at all. Fortunately, circuit diagrams for this PC can be found on Bo Zimmerman's web pages, where I found something interesting:

Commodore PC 20-III Circuit Diagram Snippet

There was a high chance the 5 volts came from the main VCC voltage rail through this transistor, which could be shorted. I confirmed a short circuit between collector and emitter using a multimeter and proceeded to remove the transistor. As expected the short was now gone. I did not have any 2N3904 in my stash, but used a BC549B as a replacement, which should be equivalent. Here is the replaced transistor in the middle:

Commodore PC 20-III Circuit with Transistor

I powered on the PC and measured with an oscilloscope again, which now shows a proper composite video signal:

Commodore PC 20-III Composite Video Signal

I also ran the 8088 MPH demo to test for more colors on CGA:

Commodore PC 20-III Composite Video Signal

Topic: Repair, by Kjetil @ 11/12-2021, Article Link

Fujitsu FKB8530 Keyboard Repair

I got hold of a strange looking modular keyboard, the Fujitsu FKB8530, which I wanted to refurbish due to its unique construction. Unfortunately I discovered that it had sustained some kind of liquid damage, which in turn had caused corrosion on the membrane. The result was that some of the keys did not work. Measurements showed that the resistance had gotten too high between some of the points on one of the traces on the membrane.

I was able to repair this by gently scraping of the top layer on the membrane trace with a knife, then applying conductive silver paint (Loctite 3863 Circuit +) on top. I used regular Scotch tape to mask off the relevant area:

Silver paint applied

The keyboard after re-assembly:

Re-assembled keyboard

Topic: Repair, by Kjetil @ 22/09-2021, Article Link

Tandon TM9362 Data Recovery

I have a Tandon TM9362 hardcard. Such hardcards are notorious for failing, a common problem is that the heads and other mechanical parts get stuck over time. Despite this I managed to fully recover the data from it. The actual hard drive on the card is a Tandon TM362, which is a 21MB MFM drive.

Tandon TM9362 and Bike Oil

The shaft of the motor controlling the heads on these drives are actually accessible, which makes things easer. I managed to turn the shaft manually with my fingers to loosen it. Afterwards I applied 2 drops of "Bike Oil", which is similar to bearing oil, into the motor through the shaft and wiggled it some more. The card was then connected to a 486-class PC. Still, even with this oil applied it would not always power up on startup, but some more wiggling and power cycling made it spin up in the end.

This card contains it's own BIOS routines for accessing the disk, so I removed the existing hard drive on the PC to prevent any interference. It booted up with MS-DOS 3.20 and timestamps from 1990, so it could have been over 30 years since it was last accessed. I think MS-DOS 3.20 has issues with newer 1.44M floppy disks, so I booted DOS 5.0 from a floppy instead, still with the hardcard connected of course.

This PC has a 3Com 3C509 Ethernet card, so I loaded the Crynwr Packet Drivers for this and configured mTCP for TCP/IP networking. I setup the mTCP "ftpsrv" FTP server to host ALL files from the C: drive and used "ncftpget" from a Linux box to recursively download everything.

An additional cool but not really necessary step is to get those files onto a virtual disk image that can be loaded in QEMU. This particular hardcard has a CHS configuration of 615 cylinders, 4 heads and 17 sectors (per cylinder). Multiplying this with the standard sector size of 512 gives 41820, so a blank disk image can be made by:

dd if=/dev/zero of=tandon.img bs=512 count=41820

Importantly, in order for QEMU to properly forward the same CHS settings into the emulation, it needs to be specified on startup like so:

qemu-system-i386 -drive file=tandon.img,format=raw,if=none,id=dr0 -device drive=dr0,driver=ide-hd,cyls=615,heads=4,secs=17 -boot c

So actually get the files over I booted QEMU with the virtual disk image and another copy of a MS-DOS 3.20 floppy boot disk image that I found online. From the emulated DOS i ran the DOS "fdisk" and "format c: /s" to format the virtual disk in a compatible manner. Then I ran the Linux "fdisk" on the image and noticed the FAT partition was placed on sector 17 and onwards. Multiplying this by 512 gives 8704, so the partition on the virtual disk image can be loopback mounted like so:

sudo mount -o loop,offset=8704 tandon.img /tmp/tandon

Afterwards the recovered files can just be copied into the mounted loopback drive. The result is a bootable (in QEMU) virtual disk image with the correct MBR and VBR.

Topic: Repair, by Kjetil @ 07/08-2021, Article Link

Retrobright Experiment

I finally got around to trying the Retrobright technique to remove yellowing. You need UV light, so doing this in Norway is a bit tricky because the weather is quite unpredictable and we do not have a lot of sun. Also, the Hydrogen peroxide (H2O2) needed is expensive and only comes in maximum 6% solution. I have an old computer mouse which was very yellowed and a perfect candidate for this experiment due to its small size.

There are several different methods and recipes to use for Retrobrighting, but I chose the plastic wrap method and using only H2O2 and corn starch. I put the 6% H2O2 into a cup and gradually added corn starch until I thought the mixture was thick enough. Afterwards I poured the mixture on the plastic wrap, put the yellow part of the mouse on the mixture and wrapped it together.

It was supposed to be a sunny day so I put the wrapped mouse outside at 08:30 in the morning, and then retrieved it at 18:30 in the evening, so it got around 10 hours of sunlight in total. All in all I was quite satisfied with the result.

Here is a picture showing the before and after and the different stages, you may notice the small bubbles appearing which is a sign the process is working.

Retrobright steps of Brother Mouse

Topic: Repair, by Kjetil @ 19/06-2021, Article Link

Gould 1604 Oscilloscope Repair

I have gotten hold of a Gould DRO 1604 oscilloscope from 1989. It would power on, but the display was severely distorted and looked like this:

Gould 1604 Before

I cracked open the case and first of all removed a leaking battery, which is not related to the display problem, but important to fix as soon as possible:

Gould 1604 Battery

Luckily there is a service manual available for this particular oscilloscope, which helps a lot in giving troubleshooting tips and places to measure. I first discovered that pin 4 of U803 (the Y DAC chip) had -3.3V but the service manual said it should be either -2.8V or -4.2V, depending on the "dot joining" state. This led me to Q811 (a MOSFET controlling the "dot join") which had a strange voltage of -3.41V on it's gate, where it should have been either 0V or -5V. According to the circuit diagram the gate of Q811 is pulled by a resistor connected to the -12V power rail.

I measured the -12V rail, and this was only around -11V which is too low. This rail is controlled by a LM377T linear regulator, which is adjusted by a set of resistors and capacitors. I checked the three associated resistors R1, R2 and R16 and they all had the correct values. However, the two capacitors C15 and C16 had correct capacitance but horrible ESR value of 23 and 33 ohms.

Here are the two bad capacitors, with blue casing:

Gould 1604 Old Capacitors

Which i promptly replaced with new ones:

Gould 1604 New Capacitors

This fixed the -12V power rail, which now measures around actual -12 volts, and the display is now working:

Gould 1604 After

Topic: Repair, by Kjetil @ 04/12-2020, Article Link

Amitech Amiga Mouse Repair

I have an Amiga mouse that behaved so poorly that it was unusable. The symptom was that movement was barely detected, if at all at some times. After digging around on the Internet I found that one possible cause is the IR emitter and IR photo-transistors that are used to generate the quadrature encoder pulses.

I ordered replacements from my local dealer, specifically:
* OP 750A - IR phototransistor 850 nm 30 V Side Looking, Optek

IR phototransistors

* IRL 81A - IR emitter 860nm 100mA 1.3V THT, Osram Opto Semiconductors

IR emitters

And soldered on these replacements:

Amitech mouse replacements

This is indeed what was causing the problems and the mouse is now working again:

Amitech Denmark Logo

Topic: Repair, by Kjetil @ 03/10-2020, Article Link

Amiga 500 Capacitor Replacement

I recently replaced all the electrolytic capacitors on my Commodore Amiga 500. This was originally for troubleshooting and diagnostics purposes, but sometimes it is a good idea anyway because electrolytic capacitors are prone to drying up and become worse over time. I understand that this varies a lot between manufacturers and production batches, so it's not always the case. As for the Amiga 500, there seems to be some dispute.

Anyway, in this process I had to come up with a easy way to track the work while soldering. So I made this "map" with all the capacitors and their values. It might be helpful to others later:

Amiga 500 Capacitor Locations

Note that this is a Amiga 500 revision 5 board, so others might be different.

Topic: Repair, by Kjetil @ 01/05-2020, Article Link

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