Eddystone S750 Renovation

Posted on Updated on

For my next renovation project I would like to have a go at an Eddystone receiver. These classic receivers were produced in Birmingham just after the war and they produced many different models. The early models used a mains transformer for the PSU, but later ones were AC/DC which meant that they did not have an isolating mains transformer and used a mains dropper like the old TV sets. These models are dangerous as the chassis is not earthed and is liable to become live. After some careful research I decided that the best model for renovation would be the S640, S680 or S750. I particularly like the S750 because of the classis Eddystone 898 style of dial.

After a while I came across a very sad looking S750 on E-Bay that needed a lot of TLC, but for £50 I thought it would make an ideal project. I managed to secure it and was delivered in a huge box!

Cosmetically, it was in a bad way. The receiver had been stored for a long time, the finger plate, selectivity knob and drive coupling were missing and the paintwork was very poor. Inside, the chassis and valves were all in good condition and underneath the original components were intact, but the soldering of some of the wires needed some attention. The drive mechanism was very dirty and not working correctly. This was going to be challenging project!

So where do start? Well, first of all removing the case and front panel so that I can see clearly the work involved. There are lots of information about restoring Eddystone receivers on the Eddystone Users Group and forum. Some great stuff written by Gerry O’Hara, which takes you through the renovation of an S750. At this stage I had no intention of powering up the receiver. Instead I checked the resistances of the mains transformer and choke, everything seems fine.

The front panel includes the 898 style drive mechanism, which of course will need a lot of work later. I decided to put this to one side and concentrate on the main chassis first.

The chassis was caked with dirt and nicotine and quite smelly, a good clean was necessary. Using a wire brush, isopropyl alcohol and metal polish I set about cleaning above the chassis.

Inspection of the Power Supply unit made me realise that it was a rats nest of wires that needed removing or replacing. The smoothing capacitors were bubbled and needed replacing. The smoothing choke was not original and had been poorly fitted above the chassis, likewise the audio output transformer. I decided that a complete rebuild of the Power Supply unit was needed.
I drew lots of diagrams and took many photos of the wiring before removing the PSU from the rest of the receiver. I was then able to work on the unit by itself. A lot of the wiring was to the octal sockets on the rear panel which is no longer necessary, so I could remove most of it, which I did. The octal rectifier valve had been changed for an unknown four pin rectifier valve. I removed this and used a couple of IN4007 for the rectifiers instead. A new smoothing block and new capacitors and resistors used to replace the existing ones. I also replaced the wiring sticking to the original colours. Testing the Power Supply gave me 300v HT and 150v regulated along with two 6.3v heater supplies.
While rebuilding the PSU the underside of the chassis was cleaned.

Now it is time to move on to the RF / IF and audio stages. The RF unit is in good condition, the components are very difficult to get to, so I will leave this alone unless there are any faults that emerge when it is tested. The IF and audio unit can be separated from the rest of the chassis (like the PSU) this needs most of the resistors and capacitors replacing. It was quite easy to work on and most of the decoupling capacitors were bolted onto the chassis. They just needed unbolting and then replacing with up to date components.

Once this was completed I could assemble the whole chassis back together again and re-fit the front panel ready for testing.
I Powered up the receiver while monitoring the HT voltage, after a while I heard a faint hum from the speaker, but nothing else. After some investigating I realised that I had made a wrong connection to one of the switches. When I repaired the connection, the receiver sprang into life!
All the switches and controls appeared to work apart from the BFO. I was able to listen on 80m, but couldn’t resolve the SSB stations. Note that the drive mechanism was not working, I tuned in stations by moving the arm on tuning gang.
One thing that I did notice was that audio was quite low and also when I switched on it took a long time before coming on. I checked the heater voltages, there are two 6.3v windings on the transformer, one gives around 3A and the other, which only supplies the noise limiter valve, is only rated at 300ma. The main heater voltage was down to 4v, but the other was fine. It turned out that the two 6.3v windings were connected the wrong way. This was wired like this when I got it. Swopping the windings over cured the problem, now I have lots of audio and the valves warm up quickly.

I now moved on to looking at the cosmetic problems and drive. I became aware from the Eddystone User Group that some components are still available for these receivers from Ian Nutt, who has a stock of parts. My first step was to message Ian to see if he had any spare parts to replace the ones missing from my receiver. I needed a finger plate, drive coupling spring, selectivity knob and also two drive chord spools. Ian managed to find some of the parts, particularly the spools, which would have been difficult to replace. Ian didn’t have a finger plate, so I solved this by using a template from Gerry O’Hara to have an aluminium sign made by a printing company called ‘Helloprint’. This was not perfect, but acceptable.

The drive mechanism is quite a challenge, stripping down the front dial and drive chord is a bit scary! Never the less I managed to complete it, replacing the chord spools and also the drive chord, which was too thick. I bought some 7 strand, steel Pike wire from a fishing shop. This was used in the original receivers. The drive mechanism itself was thoroughly cleaned and polished. I lubricated the bearings with machine oil so that it would run smoothly. The drive is pleasure to use!

Connecting the drive to the tuning gang needed some thought, the spring that holds the ratio arm to the spindle is still missing. I managed to fabricate a new one from some steel wire. Again, not perfect, but acceptable.

I painted the front panel and case with VHT Wrinkle paint, it worked quite well and I now have a nice wrinkled finish. Before re-assembling the front panel I replaced all the original switches with new ones.

The receiver is now working fine apart from the BFO and a problem with Band 1, which doesn’t seem to be working. First I had a look at the BFO module and stripped it down, it appears to be working, but may not be on the correct frequency which should be 85Khz. Checking the frequency on an oscilloscope confirmed that the BFO is working, but why can I not hear a beat?
Well, this turns out that the S750 has a terrific amount of RF and IF gain, so having the gain controls turned up completely swamps the very low level BFO injection. Gerry O’Hara explained to me that the correct use of the receiver is to keep the RF and IF gains turned well down and the Volume Control turned up. The volume is adjusted using the RF and IF gain controls when listening to SSB or CW. This solved the problem!

Next, why is Band 1 not working?

Investigating with my oscilloscope revealed that the mixer oscillator on this band was not working. Further investigation tells me that the coupling winding on the oscillator coil is open circuit. I decided to strip down the coil assembly, but the faulty winding was buried under wax. I made my own coupling coil of 4 turns and fitted it to the assembly, it works perfectly!
I finished by aligning the RF and IF stages as best I could.

Now I have a wonderful, fully restored Eddystone S750 in true BBC Repair Shop manner!


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s