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…