A Transistor Tester Kit

MyGoodLadyWife told me recently that she would like to assemble another electronic device kit of some sort.  How could I say no? A quick trip to Ebay resulted in the purchase of an absurdly cheap little transistor tester kit including a clear plastic case. Even if it didn’t ultimately work I could probably re-use some of the parts in other projects.

A few days later, the package arrived and ended up on the G7IVF workbench readyfor MGLW to have fun with…

A random-looking jumble of parts in a clear plastic bag.

MGLW then set about inspecting all the parts of the kit for any damaged items that might need replacing. Although an IC and its corresponding socket looked like they had been trodden on, thrown across the room, thrown back and finally trodden on again, MGLW was confident all would be well after some TLC applied with some suitably small pliers.

Most components have survived their postal adventure, but the IC and its socket needed some TLC.

Next up, MGLW sorted the components and checked against the parts list downloaded earlier for correct values.

Checking resistor values and placing them on the sort list.

Assembly proceeded quickly – MGLW was pleased with the quality of the circuit board. Here’s how things looked after all the resistors had been placed.

All resistors soldered to the board.

Very soon after, the main board was fully assembled and ready for the display to be fitted.

Main board now assembled and waiting for the display screen to be fitted.

Once the screen was in place, a suitable 12 Volt DC supply was hooked up and… it lives!

MGLW powering up the tester for the very first time.

Now we knew the tester was powering up, we fitted a 150nF capacitor and ran the self-test and calibration program. After a short while, all the tests had passed and the device reported the capacitor was measured as a 140nF device. That’s less than 10% off… not bad!

Self-test completed and first measurement made – a 150nF nominal capacitor coming up as 140nF so well within spec.

Build and test complete, it was time to case up the unit. Here it is!

Cased up and ready to make useful measurements on the G7IVF workbench

We finished up by picking a random transistor from the junk drawers and ran a test. It passed!

Testing an old BSY25 transistor found gathering dust in a junk drawer. It still works!

I was very impressed with this little tester. It was very cheap to buy and simple to build and MGLW had great fun assembling it for me. I suspect there will be another order for a different kit very soon…

Marconi Signal Generator Part 3 – Repairs and Testing

David 2E0DYN and myself spent two days in the lab working on the signal generator, removing old capacitors, cleaning here and there and finally fitting new components.

One problem we encountered was the poor state of the original mains filter capacitors. The plastic bodies had either cracked and broken, or the ends of the bodies were coming away from the wire. These would all be replaced.

The mains filter caps were in a poor state
The original devices were replaced with suitable safety rated devices
The original devices were replaced with suitable safety rated devices

In many cases we had to replace old ‘can’ style electrolytic capacitors that would have dried out over time. These would be replaced with modern ‘motor’ capacitors that have similar high voltage and capacitance characteristics.

‘Old’ and ‘new’ style electrolytics side by side

The old ‘can’ in the above picture would pose a challenge, as it contained multiple capacitors in one device. We would have to find some way of replacing it with a bank of individual capacitors.

‘Hybrid’ capacitor replacing original multi-device ‘can’

There were also some alterations made underneath the power supply. The original capacitors had large lugs on them that allowed other wires and components to be directly soldered to them. The new ‘motor’ capacitors had a pair of flying leads instead. It was necessary to fit some tag strips to allow everything to be reconnected.

Tag strip allowing connections to be made
Tag strip allowing connections to be made

Finally the power supply section was ready, having had the necessary component changes and tag strips fitted.

Underside of completed PSU section

There were of course many other components to replace, including several that required some major disassembly of the front panel to allow access.

There were some hard to get to places needing lots of work with screwdrivers and small spanners

Things were progressing well by now. The replacement components were all fitted and work began on the physical reassembly of the unit.

The unit is reassembled prior to testing
The unit is reassembled prior to testing

Finally after a few hours of the second session in the lab, we could do a test power-up of the device. This would be the first time in several decades that this unit had come alive.

The device is working for the first time in many years
The device is working for the first time in many years

Here is the output on the oscilloscope. (Yes, the Gould scope is back in the equipment stack. The HP is fun to use but it is very noisy). A nice clean waveform was observed. The set was then hooked up to my DX394 receiver where we were able to test the output of both continuous wave and amplitude modulated signals from the unit.

A clean CW waveform
A clean CW waveform

Marconi Signal Generator Part 2 – Assessing what needs to be done

I knew that when the signal generator arrived on the bench, it would not be safe to just plug it in and ‘suck it and see’. There was a very good chance that some components had failed with old age and this could result in some fireworks and a severely damaged unit if power was applied.

Unfortunately I have little experience in restoring old valve-based equipment. However, I was able to call on the expert advice of David 2E0DYN who is quite the boffin on valve equipment in general and even better, he has recently renovated his own TF995A. My unit was in safe hands!

We set to work, removing the six bolts in the front face which allowed us to slide out the main chassis from the case. The power supply is built in to the bottom of the case and is connected to the chassis by a six-pin plug.

The chassis connects to the PSU with this six pin connector plug

One interesting aspect of this renovation is that unlike a lot of modern equipment, the TF995A is clearly designed to be disassembled for servicing and repair. We had access to the original Marconi instruction/service manual which allowed us to compile a list of components that would need replacement.

Capacitor Cans
Typical old electrolytic capacitor cans scheduled for replacement with modern parts

A decision was made at the outset – old capacitor cans such as these would be replaced with modern parts. This would not be a ‘historical’ restoration trying to return it to some sort of factory finish, but a repair with the intention of putting this unit back in to service to do useful work and add more to its life story.

Capacitor Bank
Two banks of capacitors that all need to be replaced

David’s eagle eyes spotted some failed capacitors deep within the mains filter requiring the disassembly of the filter.

Mains Input Filter
The mains input filter before disassembly

There was also work to be done underneath the power supply section. This was removed from the case and inspected for components needing replacement, as well as planning the routing of replacement cabling.

PSU underside
The underside of the power supply showing some of the wiring

Finally, it would be necessary to strip down part of the front panel. This would permit the removal of a sub-assembly attached to the back face of the panel. Sandwiched in between this sub-assembly and panel were more components needing attention.

Stripped Panel
The front panel was stripped down to allow access to screws holding the sub-assembly in place

After a few hours of work, we had a list of components to buy and jobs to be done. The next post will show the work being done.

Marconi TF995A/5 Signal Generator

I have an uncle who for for many years up until his retirement worked as an engineer for the UK Civil Aviation Authority. At some point during his career he was able to purchase an old Marconi signal generator that has become surplus to requirements.

Years passed and the signal generator quietly gathered dust up in the attic until a recent clearout saw it delivered to the workbench of G7IVF to see if possibly, just possibly,  it might find a home here. Silly question really…

However, because the unit hasn’t been powered up for a considerable amount of time, it is safest to assume that some investigation and repair work will be necessary. More to come on this…

Meanwhile here are some pictures of the generator as well as an interesting piece of history in the form of a label that was attached to the unit showing it was withdrawn from service and placed in to storage in 1976 before being sold on.

Marconi signal generator image 1
The Marconi TF995A/5 sits hopefully on the bench of G7IVF
Marconi signal generator image 2
Bright sunlight after so many years in the attic. The Marconi is suddenly full of hope…
Marconi signal generator image 3
Yes! It’s going to become part of the radio station G7IVF. A happy signal generator!
Marconi label front
Part of the ongoing history of this unit – the original label from the CAA.
Marconi label rear
The last time this unit was used properly – 1976. Forty years on, the story is about to continue…

Preparing and Fitting the Edelux II Headlamp

Having unboxed and inspected the new lamp it is time to prepare it for fitting to Rose, my trusty Thorn touring bike. First, I used the old wiring harness from the original headlamp as a template to cut the new cable to size:

The new lamp cable cut to the same length as the old one.

Next, I stripped away a few centimetres of the cable sheathing to expose the two inner conductors:

The conductors after the cable sheathing has been stripped away.

Because these two conductors will be close to the road surface at the centre of the wheel hub they will need protecting with some heatshrink tubing that is supplied with the new headlamp. I use an electric heat gun to do the shrinking. It is like a hairdryer… but much much hotter!

The hot air gun ready for action.

After cutting the heatshrink tubing to the correct length, make sure to use a suitable tool for holding the work while heat is applied. Remember, the air from the gun is very hot and it only takes a few seconds to complete the task.

The first heatshrink piece cut and ready for heating.

Here is how it looks after shrinking down:

The protected conductor cooling down.

Once cooled, another piece of heatshrink tubing is applied at the point where the two conductors enter the cable sheathing.

Extra protection for the conductors.

Now the conductors are ready to have the crimp connectors fitted. It is easy to fit them with a small pair of long-nose pliers.

The crimp terminals fitted ready to attach to the hub generator.

As you can see, the crimps each have two sets of tabs. One set of tabs is bent over the copper conductor to create the electrical connection. The second set of tabs wrap around the insulation to add mechanical strength to the connection.

Preparing the crimp connectors to survive the open road.

Time to add more heatshrink tubing, this time to protect the crimp connectors from damage.

Ready for the road!

The electrical connections are now prepared. We can now fit the headlamp to the bike…

Rose has her new headlamp.

The new headlamp has been fitted. As you can see, I had to replace the mounting bracket as well as the headlamp.

Wired up and ready to power the headlamp.

The crimp connectors take their place on the hub generator ready to supply power when needed.

The cable is secure against the front fork.

The cable runs up the front fork as before and is held securely in place to prevent it fouling the brakes.

Ready once more for the daily commute.

Rose is now ready for the next commute although it looks like she could do with a bath (the riverside path that forms part of my commute has been quite muddy recently). You can also see the rechargeable headlamp on the top of the handlebar that provides extra lighting and some redundancy if one lighting system should fail (I also run two tail lamps for the same reason).

Rose has a broken headlamp

Poor Rose. Years of cycle commuting has taken its toll and her hub-powered headlamp has fallen to pieces.

Disaster strikes! Rose has lost her headlamp.

I have chosen to replace the headlamp with a Schmidt Edelux II LED light from St John’s Street Cycles.

The new lamp has arrived.

I like the simplicity of packaging. It suggests that all the effort has gone in to designing the light, not the box it comes in.

The box contents at first glance.

The new headlamp is unveiled. What else has come with it?

Everything is supplied to complete the replacement.

That’s everything needed to complete the replacement. The extra coiled cable is for running to the rear of the bike to power a tail light. However, Rose has two large Cateye LED lamps powered by batteries that help keep us visible at night.

As you can see, the lamp supplied is the version with the 140cm cable which will need to be cut to length and prepared for fitting to the front fork. More on this in another post…


Changing my Oscilloscope

I have a few days off from work and it seemed like an ideal opportunity to tidy up my electronics bench. This would allow me to change out my old Gould 20MHz oscilloscope for something different.

In this case, the something different is a Hewlett Packard 54200A Digitizing Oscilloscope. I know it powers up, but as yet I don’t know if will serve as a reliable instrument for my experiments. Time will tell…

HP 54200A
The new oscilloscope installed on the workbench
The tidied and reconfigured workbench