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