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.

FailedFilterCaps
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.

OldandNewElectrolytics
‘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.

PSUwithHybridCap
‘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.

RepairedPSU
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.

SmallCapDeepInside
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

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