Marconi TF2438 Repair

I recently got an old 80’s vintage Marconi TF2438 Universal Counter Timer from a fellow forum member.  The unit looked very good and powered up but was sometimes intermittent.  The unit has an oven-stabilized 10 MHz crystal oscillator fitted which is a bit erratic when warming up but fine after 5 minutes or so.  The following faults still exist even though the previous owner had already done some fault finding.

1.   It either says 0Hz    
2.  Or reads high (approximately double the frequency, but not stable). The former, when it occasionally happens, is fixed by gentle "percussive maintenance", so that's clearly an intermittent connection.

After a bit of googling, I found out that a typical failure mode of these instruments made by Marconi Instruments at the time used double-sided PCBs with the two sides connected with what could only be called rivets. Over time, with thermal expansion and contraction, the solder on these rivets would crack and they became very unreliable. Some of the rivets had already been re-flowed.

Anyway I digress. I got the unit home, powered it up and connected the 10MHz standard output to channel B, it did measure the frequency but unfortunately it was up to its old tricks of reading high approx. double the frequency and despite some “percussive maintenance” I couldn’t get it to behave. I found a legible copy of the schematics, courtesy of UK Vintage Radio Repair and Restoration forum.  I guess it was time to check some voltages, there are four main d.c. supply rails,   +5.1V,  -5.1V, +12V and -12V.

The joy of fault finding

Function Board

Control Board

All of the supply rails were reading low approx.  +/- 4.0v.  I checked the rails pre-regulator and these seemed to be correct.  Inspired by a post on the UK Vintage Radio Repair and Restoration forum,  I started checking for shorts and faulty components on the bottom function PCB but I couldn't find anything wrong. I also checked components which share both +5.1V and +12V rails and removed the 10MHz oscillator but this made no difference.   Following some helpful suggestions, I ran some tests with and external power supply connected across D21 (a temperature-stabilised 6.2V Zener) and the + -/ 5.0v rails were ok.  Several suspect tantalum capacitors were changed, two pass transistors in the power supply were changed and an IN4148 diode.  Finally I replaced the IN4148 diode again and repaired a broken track/pad which had lifted when I had removed the original.  The supply rails then sprang back into life. 

These instruments have a nice feature which allows you to plug the top control PCB with display and switch board into the bottom function PCB via a second set of connections which improves access for servicing, therefore you don't need an extender card or any special cables.  

Having sorted the power supply issue my attention moved to the other annoying intermittent faults, which I suspected was due to a bad through-hole rivet somewhere on the control board.   I re-flowed every through-hole rivet I could find and cleaned the push-button switches with some switch cleaner.   Success, I tested the unit again and it now worked correctly.

Overall I’m very happy I managed to repair the annoying intermittent faults.  It is a beautiful instrument and well-made though I suspect that those through-hole rivets will come back to haunt me one day.

Sucess

BBC micro:bit Pedestrian Crossing

I started this project by trying out the pre-written code that came with the IET Pedestrian Crossing resource. The aim is to use the BBC micro:bit to develop a prototype for a pedestrian crossing for a local secondary school.   

Parts List

You will need the following:

• 1 x BBC micro:bit
• 1 x Kitronik Prototyping System
• 2 x Red LED’s
• 2 x Green LED’s
• 1 x Orange LED
• 1 x Piezo Sounder
• 1 x NPN Transistor
• 1 x 2.2kΩ Resistor
• 5 x Resistors suitable for your LED’s (I used 220Ω resistors)
• 7 x MF Jumper wires
• 3 x MM Jumper wires 

Stage 1 

First, I connected three LED’s to P0, P1 & P2 via series resistors.

Stage 1

BBC micro:bit Edge Connector Breakout Board	Breadboard Connection
P0 pin	Red Traffic LED via 220Ω Resistor
P1 pin	Orange Traffic LED via 220Ω Resistor
P2 pin	Green Traffic LED via 220Ω Resistor
0V Pin	Breadboard negative rail
3V Pin	Breadboard positive rail

Stage 2

This worked well so I thought I would try adding two more LED’s via series resistors to ports (P8 & P12).   I modified the pre-written program so that the two additional LED’s would also be controlled.

Stage 2

BBC micro:bit Edge Connector Breakout Board	Breadboard Connection
P0 pin	Red Traffic LED via 220Ω Resistor
P1 pin	Orange Traffic LED via 220Ω Resistor
P2 pin	Green Traffic LED via 220Ω Resistor
P8 pin	Green Pedestrian LED via 220Ω Resistor
P12 pin	Red Pedestrian LED via 220Ω Resistor
0V Pin	Breadboard negative rail
3V Pin	Breadboard positive rail

Stage 3

The final part of this project was to add a piezo sounder to simulate the crossing beeper.   I connected a piezo sounder to port (P16) via a driver transistor.  I modified the code once more to add this functionality. 

Stage 3

BBC micro:bit Edge Connector Breakout Board	Breadboard Connection
P0 pin	Red Traffic LED via 220Ω Resistor
P1 pin	Orange Traffic LED via 220Ω Resistor
P2 pin	Green Traffic LED via 220Ω Resistor
P8 pin	Green Pedestrian LED via 220Ω Resistor
P12 pin	Red Pedestrian LED via 220Ω Resistor
P16 pin	2.2k Resistor for Transistor
0V Pin	Breadboard negative rail
3V Pin	Breadboard positive rail

Stage 4

I have added an illuminated external push button which is connected to the button A pin (P5),this lets you trigger or detect a button "A" click externally.   The LED inside the switch is connected to port (P13) via  series resistor.

Stage 4

BBC micro:bit Edge Connector Breakout Board	Breadboard Connection
P0 pin	Red Traffic LED via 220Ω Resistor
P1 pin	Orange Traffic LED via 220Ω Resistor
P2 pin	Green Traffic LED via 220Ω Resistor
P8 pin	Green Pedestrian LED via 220Ω Resistor
P12 pin	Red Pedestrian LED via 220Ω Resistor
P16 pin	2.2k Resistor for Transistor
P5 pin	Button A – connected to an external  push switch
P13 pin	Pedestrian wait LED via 100Ω Resistor
0V Pin	Breadboard negative rail
3V Pin	Breadboard positive rail

Code

The code was originally written in Microsoft Touch Develop but this legacy editor is being discontinued so I have migrated the code to the new Microsoft MakeCode editor.  The code can now be found here.

Racal 836 Repair

Racal 836 

I picked up an old Racal 836 32MHz Universal Counter-Timer (circa 1972) at a radio rally last year. The unit looked good, powered up and all the digits worked. However having carried out some of the preliminary tests as outlined in the technical manual the unit would not perform the self-check function.   

Top side showing the various plug-in assemblies

Underside

I found this thread on the UK Vintage Radio Repair and Restoration forum and posted a reply for some help, I also posted on the Yahoo Racal Dana group.   Various helpful things were suggested to check which I did. All of the supply rails were correct, the 1MHz frequency standard was running and I replaced several tantalum capacitors. I finally discovered that motherboard was plagued with dry joints and that there was a dry joint on the gating control board connector so the signal wasn't getting to the high speed decade counter board. Having re-soldered these dry joints all of the functions now work correctly.

Overall I'm very happy with the unit, it is beautifully made though it can still be a bit temperamental. Usually re-seating the plug-in modules fixes the problem.  I have recently acquired a OCXO 10MHz frequency standard to check my counter with, it seems to be pretty much in calibration still after all these years.

RasPiO Pro Hat

I recently backed Alex Eames (@RasPiTV) Kickstarter campaign for a RasPiO Pro Hat which is a small add-on board for the Raspberry Pi and is designed to protect and put your GPIO ports in numerical order.   The Kickstarter campaign has been successfully funded so boards are starting to be shipped out, mine landed on the doormat the other day.  

The RasPiO®1 Pro Hat has the GPIO pins connected via a protection circuit to female headers but is also broken out to un-protected connections. There is a small breadboard for prototyping circuits.  The PCB is very nicely made and is complete with gold and white silkscreen text.

I tried out my board using the included RGB LED and a simple GPIOZero Python script. There is well written manual which contains some structured experiments for the included components.

Overall I very impressed with this great little add-on for the Raspberry Pi,  if you want one for yourself then you can find out more on the RasPiO website. 

My First Electronic Kits

During a recent exploration of the loft I came across two of my electronics kits I was brought by my parents during my teens.

Salter Science Introduction to Electronics 

I was given this kit for Christmas or perhaps a Birthday during the late 80’s.  I remember spending many hours playing with it and trying most of the 25 fascinating experiments,   I liked the Radio and noise making projects the best.  The simple board consists of one single transistor, a couple of capacitors, resistors, diode, tuning capacitor and aerial coil complete with spring terminals to join the components together.

Science Fair 200 in One Electronic Project lab (circa 1981, 1987)

Sometime later in the early 90’s I was given this kit for Christmas or perhaps as a Birthday present.   I spent many happy hours trying out some of the 200 projects, mostly LED or light projects.  I made the DC Meter lot as I didn't have a Multimeter.   The large board has a large selection of resistors, capacitors, LED’s, semiconductors and other miscellaneous components.   Again spring terminals are used to connect the components together.

Vintage Science Fair Electronic Project Kits

I have recently acquired the following. 

• 65 in One Electronic Project Kit (circa 1972)
• 75 in One Electronic Project Kit (circa 1976)
• 150 in One Electronic Project Kit (circa 1976)
• 160 in One Electronic Project Kit (circa 1982)

These kits have a good selection of resistors, capacitors and germanium transistors on the early kits only the later kits had a single silicon transistor. The later kits included a 7 Segment display and some sort of integrated circuit.

Croc clip connector for the Raspberry Pi

Andrew Gale (@pocketmoneytron) from pocketmoneytronics recently sent me a couple of his brilliant croc clip connector boards for the Raspberry Pi he had made.

The board is very simple, well designed and has three large holes so you can easily connect crocodile clips to the board. GPIO pins 23, 24 and 25 have been brought out to the edge via a series resistor along with a ground connection so that LED's can be attached.

I used a simple Python script using the new GPIO Zero library to test out the board, which is really easy to use and the boards work well. Overall I am very impressed with this great project.

Croc clip Parts

Croc clip PCB Assembled

Traffic Lights