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.

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