Restoration of an Eddystone Model S.750 Gerry O'Hara VE7GUH Apr 2009

	Restored Eddystone S.750.

The British-manufactured Eddystone S.750 communications receiver is a classic dual-conversion superhet with IF's at 1650KHz and 85kHz, the latter having variable bandwidth. A really neat feature of this model is the linear tuning arm ('ratio arm') fitted to the sliderule dial - a simple mechanism that provides almost linear tuning across the dial (no cramping at the HF end of the dial here). This S.750 was kindly donated to the SPARC museum in Coquitlam, BC, in January, 2009 by Fred Kapogines in Guelph, Ontario. The completed receiver is shown with a matching S-meter and speaker, owned by Pat Jones.

This set was one of a production run of only 79 model S.750's built that were fitted with a 110v 25Hz power transformer. This variant on the standard production was needed to suit the Ontario mains power system in the 1950s - the set's serial number indicates that this 25Hz set dates from February, 1954.

After much careful cleaning this S.750 now looks factory-fresh again, although there is some minor discolouration of the scale plate under certain lighting conditions. For set cleaning use 75% rubbing alcohol (general cleaning), acetone (removal of solder flux residues) and lighter fluid (for wax and grease), mildly-soapy warm water (dial glass and scale), extra fine grade steel wool/Brasso (tarnished metal), and Silvo (silver plated parts), applied/removed with Q-Tips, cotton wool balls and/or several cloths as needed. Although the main S.750 chassis units are nickel-plated, it should be noted that steel wool is not a good idea for cleaning-up rusted chassis because of the steel wire fines residue (that can get into places it does not belong). Instead try proprietary rust removers (eg, 'Naval Gel') and scouring pads (the plastic type) work wonders on tarnished/slightly rusty chassis.

Front panel before ...

... and after.

	Ratio arm for linear tuning.

This S.750 was not working on arrival due mainly to an open-circuit HT choke: the fault was found to be a corroded wire internally on one of the connection posts. The choke can be opened-up quite easily by cutting the solder seal with a knife blade and then re-sealed by soldering and/or with epoxy. The electrolytics were re-formed, the dropper resistor to the VR150 voltage regulator changed-out (an incorrect value resistor had been fitted in a previous repair) and the cathode resistor in the RF stage was replaced (a 68 ohms resistor had risen to 230ohms) and a silver mica capacitor that had come undone in the coilbox was re-soldered. Other components were found to be within tolerance and valve voltages tested ok when 110v AC was applied to the set (unfortunately power these days in Canada is between 115 and 120v, so this increases the HT voltage significantly and also adds some 10mA to the HT current draw).

The front panel casting was re-finished in black wrinkle finish powder-coat and the cabinet also underwent a similar treatment. The knobs were cleaned/polished with Novus #2 and #1, a mongrel knob from an HRO was replaced with a correct Eddystone type, the finger-plate was touched-up with black and silver marker pens, the BFO valve base was repaired (the skirt superglued back in place) and a new 5Z4 rectifier tube fitted. The tuning drive gearbox was completely stripped-down and re-furbished (there is an article on this downloadable from the EUG site) and two replacement spool-pulleys/dial cord were fitted from a 'parts set' owned by another SPARC member (one of the original pulleys had badly-worn teeth and the dial cord was fraying). The tuning is now beautifully smooth and the set works very well.

While the set was being re-aligned, it was noticed that the AGC line was going positive by a couple of volts with no signal applied to the set. This fault also had the undesirable effect of increasing the HT current draw by several mA under weak/no signal conditions. Having checked the AGC line passive components, which were all ok, it was suspected that one of the RF or IF valves could be faulty - replacing a 6BA6 (V5) in the 85kHz IF stage cured this.

Front removed.

Chassis surgery.

Inside the choke.

Dismantled gearbox.

Two views of the restored chassis.

The set was returned to the SPARC museum on April 19, 2009 - first though and we spent the morning listening to 20m on the set. There was quite a bit of activity and the S.750 gave a very good account of itself - excellent sensitivity and stability, with the variable-selectivity filter proving very useful. SSB was easily resolved by winding the AF gain almost fully up and adjusting the RF and IF gains to ensure adequate BFO injection, offsetting the BFO about 1.5kHz from the received signal.

Matching speaker ...

... and S meter.

For more reading:

• Restoration of an Eddystone S.750 (PDF)
• Eddystone S.750 at SPARC (PDF)