D07: 3W guitar tube amplifier from old record player based on the ECC83 and EL95 tubes

I found a Record Player by Philips, model EL3514 in the dumpster.

Many parts are missing, but I decide to take this home.

I disassemble it, to my surprise both the tubes are untouched.

This player has a speaker, which is ok too. And three tubes (ECC83, EL95, DM71).

WARNING! - The project described in these pages utilizes POTENTIALLY FATAL HIGH VOLTAGES. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

The DM71 is an interesting tuning indicator tube that can be also use as power output indicator. I'll keep this valve for a future project.

The player also has one PNP germanium transistor, the AC107. I've tested it, seems ok. I'll keep it for some other project.

I don´t have a full tube guitar amplifier, so let's focus my attention on the main components of a small tube guitar amplifier, which are the main transformer, the output transformer, the preamplifier and the output tube.

First thing I've checked was the main transformer. It seems to work. It has high voltage output and low voltage output for tube heater. I just have to rewire it. I decide to keep the original voltage output selector. I will always use it with the 220V setting, but one can change the selector to make it works even at 100V.

Output transformer also seems to be working. At least it was now shorted.

Then I attached the tube heater and check. To my surprise the heating elements works.

I haven't got a tube tester. So I have to build a breadboard circuit to test tubes. Turns out that both tubes works.

 The ECC83 it's branded Philips. This tube is also known as 12AX7 and it can be used as pre-amplifier. Indeed many guitar amplifier use this tube in the preamplifier stage.

As preamplifier reference I decide to use the Fender Champ 5F1 schematic.

The tone stack is indeed referenced from a Fender Blackface.

I can not found any guitar amplifier that use the EL95, which is a power output tube, also brand Philips, but there are a lot of old gear, as radios, that use this tube.

The EL95 can output up to 3W with 250V at plate.

For the output I take as reference the Philips EL3514 schematic.

Thanks to the https://www.radiomuseum.org website one can find a lot of schematic and information about old gears.

Let's put it all together in a case. I've use the original Philips case, cut some piece of new aluminum sheet from a PC case, makes some holes, painted black, and the tube amplifier main case is ready to use.

As guitar amplifier case I use a wine bottle wooden box. It all fits. I can even reuse the EL3514 handle and feet.

Wood is painted light walnut and finished with beeswax. There's a read hole to look at the tubes, the front panel is from the original R-Player.

Notes

• read risk disclaimer
• excuse my bad english

D06: solid state bass amplifier from JBL subwoofer

The D06 is a Bass Guitar Amplifier built on top of the solid state TDA2030 IC.

The project starts from a salvaged JBL subwoofer. The enclose was in wood, front and back cover in plastic. the woofer cone was inside. I decide to check if it works... and it of course works. The speaker was a good 8ohm 8inch subwoofer.

I can't leave the speaker inside the case, so I decide to cut the case in the middle, where the wood that holds the speaker is glued. Now that the case is cut, the speaker can see the sun.
The case was wrapped with plastic, that I have to remove cause it was ruined even before I cut the case.

Once removed I had to cut an hole on the front plate, just over the speaker. There I will install the amplifier controls.

Hole done, now it's time for the plastic holder for the controls, but that was simple.
The back of the amplifier too it's as simple as a piece of wood with some hole for the supply plug and the handle.

One problem I have to figure out was the speaker hole, it was done in such a way that the speaker has to be mounted from the outside of the case, but I don't want to see screw, so I've 3D printed an adaptor, that will also be the holder for the mosquito net I'm using to protect the speaker.

Last but not least is the case finish, I've used a texture glued with vinyl glue.

Inside the speaker I've build a new electronic.

The amplifier main supply it's a 230V to +-14V AC. Rectified to +-18V and 9V. I'm using the dual rail supply for the amplifier IC and the single rail 9v supply for the preamplifier.

The preamplifier circuit first stage is based on J.Tillman Discrete FET Guitar Preamp (http://www.till.com/articles/GuitarPreamp/). JFET used is J201.

The three tone stack can equalize Low, Middle, High frequencies and it's based on a design found in Peavy amplifiers from 80's. After the tone stack a new preamplifier stage is needed in order to mitigate the loss of the previous stage.

I've added here a simple distortion stage, enabled with a switch. It's simply a capacitor that increase the gain of this second stage and a couple of diodes to clip the signal.

The power amplifier it's base on the TDA2030 IC in a simple dual rail configuration.

One interesting thing I've added it's an optical compressor. It's based on the VTL5C7 optoisolator driven by the output of the preamplifier stage. The optoisolator drive a voltage divider that is used to reduce the output signal of the amplifier.

Now here we came to the sound.
It's pretty good, tone stack also sounds good. Volume is not so high of course, we are speaking of a 14W amplifier but it sounds good to my ear.

Hope you find this project interesting. You can find all the schematics and information below.

Notes

• read risk disclaimer
• excuse my bad english

Kevler Pro SubWoofer TPA3116D2 mod

A friend give to me it's 5.1 speaker system subwoofer.

The brand it's Kevler. It has an integrated amplifier for a 5.1 speaker. The problem is that the amplifier does not work. I've checked a few things around but I did not find anything sensational. Also I don't have a subwoofer around to use.

So I decide to repurpose this as a single subwoofer system.

The speaker is rated 120watt, 6ohm.

I've buy a TPA3116D2 cheap board, it maybe does not even have a genuine chip, but for my purpose I will not investigate, I'm happy with that. I'm going to use a 24V supply. I've around one of 3A.

From figure 22 and 26 of TPA3116 datasheet I can only find Maximum Power Output in PBTL for Rl of 2ohm and 3ohm. My speaker is 6ohm.  

So to estimate the consumption I've supposed a 50W max output, and a 85% efficiency. My estimation formula is Power/Efficiency/Vsupply, so 50/0.85/24 = 2.5A almost. My 3A supply should fit it.

I've add a passive third order passive low pass filter, it just uses a few resistors and capacitors. You can find the schematic at electroimc.com website (https://www.electroimc.com/en/doc/projects/audio/3_subwoofer_third_order_passive_low_pass_filter)

Then I've add a 230V switch and a volume control.

The result is a pretty good subwoofer I'm going to use in my gym environment.

Notes

• read risk disclaimer
• excuse my bad english

D05: vintage hybrid guitar amplifier (6AT6 + TDA2003)

D05 is a vintage hybrid guitar amplifier. The preamplifier stage is driven by a vacuum tube, the power stage by the a class AB solid state IC.

It's built on top of an old portable turntable found in the dumpster. This time I've found just a few parts of the turntable. In particular the low part of the wood case, and the electronics.

Unfortunately I've not taken pictures of the system when I've found this, you have to believe if I say it was in horrible conditions. The full turntable mechanism is missing. The amplifier tube is broken, but I've notice the preamp tube was not.

WARNING! - The project described in these pages utilizes POTENTIALLY FATAL HIGH VOLTAGES. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

I've search for the datasheet of this tube, the 6AT6 (https://www.radiomuseum.org/tubes/tube_6at6.html), it turn out it has enough gain to be used as a guitar pre amplifier. Indeed it is used in the Fender "Harvard" 5F10, that's a vacuum tube amplifier made by fender from 1955 to 1963, you can find more info about his amplifier here: https://en.wikipedia.org/wiki/Fender_Harvard.

The preamplifier tube I'm using is manufactured from the Italian company Europhon.

For this design I've decide to keep things clean, I do not want this amplifier to be equipped with an overdrive circuit, all the distorted sounds should came out from the tube saturation.

I've put the Tillman FET preamp (http://www.till.com/articles/GuitarPreamp/) in front of the tube, to prepare the signal for the tube gain. This preamp has a 3dB gain.

After the first preamplifier stage there's the single tone controller, which is derived indeed from the Big Muff tone stage. I like the way this tone stage boost high and bass sounds, also it drops the signal just a little.

Here it comes the heart of this amplifier, the 6AT6 valve preamp stage. Even it is used by the Fender 5G10, I've not just copy and paste this circuit here, I have to bias the valve for the voltage provided by the power stage I've built. This stage has to be clean enough even with full volume coming from the input.

The last stage is based on the TDA2003 solid state class AB amp. This is driven in single rail supply, +12V, in a pretty common configuration, bang on the datasheet application circuit.

Up to here the amplifier sound clean, but I would like to add a bit of saturation if needed. For this reason I've added a booster stage between the tone and the valve pre amplifier. The booster is based on the LBP-1 booster, with just a few values changes.

After the tube preamp there must be voltage divider that arrange the signal for the amplifier, if the signal is too high the poor TDA2003 will clip, and I just want the distorted sound comes from the preamplifier, not the amplifier.

I've finally add a 4ohm 6" speaker I've around, not that big cause the cause is pretty small, for this reason there's also a mono jack speaker output that disable the internal speaker output, this way one can use an external cabinet.

The power stage for this amplifier use a trick to drive the valve with the correct voltage. As you know tubes are driven with high voltage power, indeed other IC use low voltage. I've use a standard 230V to 12V with primary and secondary connected in reverse to pull the voltage up to +170V, used by the valve preamp. Keep in mind that using a transformed in this configuration induct some loos on signal, usually 20%, after all we are using a transformer in a way it was not built for.

Power stage also provide +6.3V for the valve filament, this is done by using a 7805 with the ground biased using two 1N4148 transistors. +12V is provided for the TDA2003 stage and finally +9V for the solid state preamp stage.

As you may notice, there's almost more of power stage than preamplifier and amplifier stage.

All the electronic have been mounted on the original iron holder of the turntable amplifier. I've just added a few new holes for the new board. It has been spray painted black.

Final stage was to build a decent case using the original one. I've cut from the original case the bottom part, the one that holds the turntable mechanism. On top of the case I've pierce the wood to make the space for the controls, Jack input, Volume, Tone control, Gain control, Saturation engage switch, On/Off button. I've pasted and prepared the wood. Then I've used vinyl glue to cover with fabric the top of the wood, like it's done on vintage amplifiers.

The front mask is the one from the original turntable. The hole which contains the voltage selector has been replaced with acrylic glass, behind the tube can be seen. The tube is secured using also a 3D printed holder.

A 230V light bulb it's used for the on/off detection.

At my ears: it sounds good, especially if used with an external case. I've used with a couple of pedals I've built, I must admit it makes the pedals sound really good.

At my eyes: it's a joy! Each time I see this amplifier I'm happy for having built it.

 

Notes

• read risk disclaimer
• excuse my bad english

70's electronic surgical cutter instrument repair

My system is a veterinary, she has a 70's electronic surgical cut, suitable for electrosurgery.

It's a LAEL with the front label electronic transitorized bistury STILUS.

 

She does not use this anymore, cause she had a new one. But last week she want to test it as a spare device. And she get a little shock.

The problem is that touching the ground (or the patient) you get a shock, and of course the patient to is been shocked.

I decide to open it and take a look at it.

First I notice the main transformer, so luckily the shock emitted by this device is not a power mains shock, but a secondary side one. Then I try investigate further.

I've notice a popped transistor. So I removed it.

I can not read the label on that transistor, cause it's popped and cause it seems marked twice or three times over. I just can read BC, and then something. But it does not help.

So, I measure voltage between the three join of the transistor. I suppose the opposite solder join are the emitter and collector, and the voltage between them it's 80V. So based on the device era a possible replacement is a BJT PNP one. I've a few 2N5401 here in my lab, they are PNP high-voltage transistor. Vceo it's -150V, that's enough for that. So cross finger and replace the original BJT with this one, placing emitter to Ground.

And it works!

I've tested from 0 to 10 gain, I can cut a piece of meat without problem, and without any shock.

Of course my sister will not use this device anymore, cause it's too dangerous to use. But at least it's a "on emergency" device to use.

Notes

• read risk disclaimer
• excuse my bad english

Pioneer XL-L77 subwoofer + Creative Inspire T3030 + Bluetooth

This time I've find a vintage Pioneer XL-L77 hi-fi in the dumpster.

Amplifier display is not working, and also the amplifier has a big bump on enclosure. So I decide to keep just the subwoofer and speaker.

The front speaker are not that good, they comes in a small plastic enclosure, and the speaker are 8W small speaker.

The Subwoofer looks pretty good, but it has speaker with damaged ring.

I decide to restore the Subwoofer and make it a 2.1 system our of it.

So, I've replaced both the rings with a couple a new foam based. To do this job I've clean the speaker and then glue the replacement with all purpose clear solvent based glue.

Another think I've done to the enclosure is to remove the adhesive grey paper. I prefer to have it painted like wood.

Then I've to do some woodwork to make the amplifier board fit on it, and to drill out holes and space for the amplifier interface.

The amplifier has line/in, bluetooth, mains voltage in, a volume, a bass level selector, and the output for the left and right speaker.

As amplifier board I've used a Creative Inspire T3030 board I've around. This board has a unusual connector. I don't have the board command interface for this, so I've to find the pinout and the way to add level and volume.

Luckily someone on the https://forum.allaboutcircuits.com forum has trace it out.

I've removed the connector and solder it to my panel case.
It was a bit of a pain to make it fit the Subwoofer box, but in the end I've managed to do it.

Final product looks good to me, and it will be a good replacement for my PC speaker.

Notes

• read risk disclaimer
• excuse my bad english

Pioneer M-J310 amplifier bluetooth and standalone mod

I've found a Pioneer M-J310 amplifier.

This amplifier it's a 90's stackable hifi vintage. It's a 50W+50W on 8ohm output. In class AB.

The problem with this amplifier is that it comes alone without the preamplifier and the other stack elements, and it comes with a custom plug.

So, in order to make this device work I've to find the schematic online and check which is the correct wiring to makes this amplifier goes out from stand-by.

After having found this trick I've tested it with an aux cable. It works good, not the best thing I've heard but clean.

So I decide to add a toggle switch on the input to select between an RCA stereo input plug that I've added and a bluetooth input.

To power the bluetooth board I've take the 15V voltage output from the board into a stepdown converter, to build 5V out of it. Then to decouple the signal I've added a small 1W DC to DC converter. This last one component is really important if you want to keep your bluetooth audio out clean.

I've also added a volume control by using a simple stereo potentiometer.

Notes

• read risk disclaimer
• excuse my bad english

900VA variac rehabbing

Last month I've found a big industrial enclosure with a giant knob on it throwed away. I've take it home and open it. To my surprise what's inside was a variac.

A variac is a type of variable autotransformer in which the output voltage is varied by twisting a dial.

My variac is the Belotti V-10NA 220V 900VA 4A.

WARNING! - The project described in these pages utilizes POTENTIALLY FATAL HIGH VOLTAGES. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

In my small home lab I don't have a variac, so I decide to clean it and prepare a new enclosure. The original enclosure was big, and it has disconnected wires. It was all a wiring jam.

I throw almost all and keep the variac. I clean it and then I select an enclosure that I've around.

I decide to built it using a simple circuit.

It has two fuses, one on the mains input rated 6.3A on the rear side. The other on the front side. This last one is useful if someone want's to test some equipment.

I would like to thank Pier Aisa (https://www.youtube.com/c/PierAisa) that help me sizing the main fuse.

Then I have a main switch. I've around a 25A one. And a de-activate switch, a 16A switch that I use to de-activate the output, when activated a bulb lights up.

The main switch powers the variac, and the two meters I've put. There is one voltmeter that measure the mains line. The other measure the output from the variac. It also features an ammeter. This last one is a cheap two wires meter, for this reason it takes supply from the line and it can not measure voltage lower than 20V. If I want accurate measurements or lower voltage measure I can wire my multimeter on the line output.

It's important not to overtake the limit of 900VA output, otherwise the variac will became hot and eventually burn his coils.

On the output I've put an EU plug, and a couple of banana plug in parallel,

The enclosure I've around it's used so I've to clean it and cut holes. The front panel already has holes, so I've to put a piece of wood in front of it to keep the front clean.

The finish product is pretty good at my eyes, and finally my lab also have a variac.

Notes

• read risk disclaimer
• excuse my bad english

Measure tube amplifier output transformer impedance

A tube amplifier output transformer is a crucial component in vacuum tube-based audio amplifiers. It's responsible for matching the high impedance of the vacuum tubes to the low impedance of the speaker, as well as transferring the audio signal from the tubes to the speaker while maintaining fidelity.

WARNING! - The project described in these pages utilizes POTENTIALLY FATAL HIGH VOLTAGES. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

Imagine you have salvaged one from an old amplifier but you don't know which is the primary side, which is the secondary, and what's the impedance of that transformer.

Finding the primary and secondary side is the first step. You just have to measure resistance between the two wires of each side. In this type of transformer primary side winding has a greater resistance then secondary side.

Once you know which is the primary side you have to measure the impedance of the transformer, this is quite important cause you have to match the impedance of the power tube.

I you take as example a 6V6 tube and take a look at the datasheet, for a push-pull class AB amplifier when the plate voltage is 285 the effective load resistance (plate to plate) has to be 8000ohm. So our aim will be matching that impedance.

We know that the ratio between primary and secondary impedance is equal to the square of the ratio between the turns on each side. So we have to find the turn ratio.

Zp/Zs = (Np/Ns)^2

To find the turn ratio the way I use it to load the primary side with an AC voltage, I use pretty high voltage cause the ratio will lower the voltage down on the secondary side, using an higher voltage makes reading with a standard multimeter more accurate.

So to find the turn ration you just have to measure voltage on the primary side and then voltage on the secondary side, this ratio is equal to the turns ratio.

I prefer to measure the secondary side voltage under load. For my approximation it's enough a resistor even if it's preferable something that is not a "pure" pure resistance load, so a speaker as example. Then I made the mean between the voltage on the secondary loaded and not loaded. So summarizing the procedure

1. Apply an AC voltage on the primary side. Use an high voltage source, like your main amplifier transformer output. Also to be sure not to break anything, put a fuse on the output of your main transformer. I use a 300V AC with 200mA fuse.
2. Measure the voltage on the primary side. Note that it will not be the same voltage of your main transformer output, cause this voltage will drop due to the resistance between the turns. Let's call this value Vp.
3. Measure the voltage on the secondary side without any load. Let's call this value Vs.
4. Measure the voltage on the secondary side with a load with impedance Zs. Let's call this value Vsl.
5. Compute the turns ratio. Let's call this value K. K=Vp/((Vs+Vsl)/2)
6. Now you can find your Zp impedance with the secondary side is on the load of Zs. Zp=Zs*K^2

This way you can match your output transformer with your tube specs.

Notes

• read risk disclaimer
• excuse my bad english

Vintage IBM 5160 Schrack Elektronik 130W PSU repair

I've restored an IBM 5160 XT vintage PC (https://davidegironi.blogspot.com/2023/12/vintage-ibm-5160-xt-pc-restoration.html).

It's based on MS-DOS 3.3, sometimes I boot it up, just to check if it works. After a few month the boot process ends up with an error. It seems the 25Mb Hard Drive has not been recognized, so the O.S. it's not been loaded. The only way to boot it is to turn the PC off and on a few times within minutes.

The first thing I've done was to investigate the Hard Drive. I though it was a problem related to capacitors on the Hard Drive electronic. Soon I've realized it has nothing to do with this. Indeed the power lines of the supply are heavily out from recommendation.

With PSU disconnected I get

• +9.7V (on the +12V line)
• +4.7V (on the +5V line)

With all connected (and PC powered on)

• +10.5V (on the +12V line)
• +4.4V (on the +5V line)

Negative lines are ok.

I've posted an help request on vcfed forum, https://forum.vcfed.org/index.php?threads/ibm5160-psu-output-voltage-issue.1244144/. With the help on people of the forum I've learn that usually that era PSU has to be loaded to pull out the correct voltage. But even loading it does not help, this means the IBM loads are ok (motherboard and peripherals).

So I disassembly the supply on my bench and start the repairing adventure.

My supply is a 130W IBM 5160 Schrack Elektronik EG07168-E PSU.

First step was to check capacitors, they are all ok. I've even momentarily replaced those with new one.  Nothing helps.

I've then posted an help request on the more specialized forum, the eevblog forum: https://www.eevblog.com/forum/repair/vintage-130w-ibm5160-schrack-elektronik-eg07168-e-psu-repair/.

People on the forum guide me on tests. During tests I've made a giant wiring mistake putting the mains on the DC rectified high voltage input of the PSU. Luckily I've "just" fried the BUT11A main switching transistor and the input 2.7ohm resistors. I've then replaced the main transistors and input resistors but this not fix my under voltage issue, it works again but the issue is still there.

Diodes also seems ok.

It's pretty difficult to check this board cause I'm not an expert in swathing PSU and also there is no schematic available. I've ask Schrack for a schematic without luck.

There's a "mysterious" board on this PCU, which have some SMD component, and it seems double sided with carbon printed resistors. I try to reverse engineer it. It seems this board is the one that drive the switching of the transistors, and so the output voltage.

Also I've try to reverse engineer the output stage of this PSU. It's difficult cause there are unbranded inductors with multiple wires and without de soldering it or experience on switching supply, drawing the schematic of the board can be difficult.

Then the turning point. I try to heat the board up. Heating the "mysterious" SMD board makes the output voltage decrease, and it makes sense looking at my reverse engineered schematic.

The heating test effect, in conjunction with the help on the eevblog forum post helps me finding the issue. It was all related to the 1N823 thermally compensated zener diode mounted on the SMD board.

I've changed it with a standard 1N4735A 6.2V zener diode and the voltage goes back to nominal values. Heating it up does not makes the voltage changes too much, so I'm happy with that diode. Below the testing diode, later I've soldered it in a better way.

Fixed issue. After reassembly all the things the Hard Drive keep working again, and the PC can now boot.

Find the complete story on the eevblog forum. Again thanks to the help of people on the eevblog forum and vcfed forum this IBM vintage PC is working again.

Notes

• read risk disclaimer
• excuse my bad english

 

USB to serial adapter to transfer files from a modern to a vintage XT IBM PC

In this previous post I've talked about the restoration of an IBM vintage PC: https://davidegironi.blogspot.com/2023/12/vintage-ibm-5160-xt-pc-restoration.html

Once I've booted this PC the first thing I had to do was to transfer files from and to the Hard Drive of that PC.

My IBM 5160 has a 5 1/4 inch floppy driver. But this is not the way I want to use to transfer files.  At the minuszerodegrees website there's a full page dedicated to the transferring methods, https://www.minuszerodegrees.net/transfer/transfer.htm.

Luckily my IBM pc has a serial board, so I decided to use the FastLynx 3.3 software one. FastLynx 3.3 is a program designed to help you transfer files, directories, or entire hard disks quickly and easily from PC to PC, it can transfer files through USB, parallel or serial port.

I've found a trial version of FastLynx software on https://archive.org/. It is limited to 10Mb files only. But if you consider my Hard Drive on the IBM c is 25Mb, that is more than enough.

As a first test I've built a test serial to serial cable on a breadboard. On the modern PC side I've used a USB to serial adapter while on the IBM side the straight DB connector. This kind of serial cable is called null modem cable.

 One I've realized that the cable works I decided to build an all in one USB to serial adapter.

I take a PL2303 USB serial adapter apart, and wire the DB25 connector as specified on the FastLynx manual. You can find the wiring diagram above.

One connected you have to run FastLynx on the modern PC, then you have to run the Upload Dos Slave function to load the DOS slave program. It will be uploaded through serial port. The process is a wizard, you just have to follow the instructions. On the DOS PC you simply have to run the mode command (mode com1:2400,n,8,1,p) and then the ctty command (ctty com1). Then on the modern pc you can upload the SL.exe slave program.

Once done you can move SL.exe to the directory you want, and run it on DOS.

The master modern PC have now access to the hard drive of your slave PC.

That simple I was able to transfer files on the vintage PC to a dump folder of my modern PC, and then upload a few usefull program on the vintage PC, like the Norton Commander. 

You can find full instructions on the FastLynx page at minuszerodegrees.net here: https://www.minuszerodegrees.net/transfer/fastlynx33/fastlynx33_serial.htm

Notes

• read risk disclaimer
• excuse my bad english

IBM AT (PS/2) to XT (DIN) keyboard conversion

In my blog post here: https://davidegironi.blogspot.com/2023/12/vintage-ibm-5160-xt-pc-restoration.html I've talked about the restoration of an IBM 5160 vintage PC.

The first time I've tried to boot this PC it ends up with an error. The error was due to the missing keyboard. Investigating further It turns out that I need an XT keyboard in order to make this PC works.

IBM PC of that era uses the XT protocol to communicate with the keyboard. From the 1984 IBM starts building AT compatible keyboard. Starting from 1987 IBM also introduce another kind of connector for the AT keyabords, the PS/2 mini-DIN 6 pin connector.

There are a few differences between the XT and AT protocol, the main difference is that XT can only send data, while AT can receive and send data. Under the hood they are completely different protocol, so it's not just a matter of changing the connector wiring, one have to write some logic in order to make an XT keyboard work.

So, in order to boot my PC I need an XT keyboard, which I don't have. I have searched for an used one but I can not find it. Well I've found it later on after a few months, but at that time I don't have that keyboard, so I decided to build it.

There are a few DIY commercialized adapter, but I've an Arduino micro clone and I want to use this cheap board. I've found a PS/2 to XT code for Arduino on github (https://github.com/kesrut/pcxtkbd).

Just I have to wire it according to the DIN connector pinout, XT keyboards uses a 5 pin DIN connector with the following pinout:

1. clock
2. data
3. /
4. ground
5. +5V

Then I've compiled and upload the Arduino firmware and I've tried it using a PS/2 keyboard as the master.
It works!

I don't like to have the adapter flying on the bench, so I've decided to pack it in an IBM 90's keyboard, the IBM KB-7953 model.

I've opened it, and wire the internal PCB to the Arduino and the Arduino to the DIN connector, using the original keyboard PS/2 wire. I've exposed a 5 pin adapter to eventually update a new firmware to the Arduino in the future.

The keyboards is now ready, compact and perfect for my vintage IBM pc.

Notes

• read risk disclaimer
• excuse my bad english