Ultra Coronation Twin R786 restored

[Phil G4SPZ]

This little set was donated to the Black Country Living Museum, where I work as a volunteer, by my friend Mike G3TDG from the Malvern Hills Radio Amateurs’ Club, and I have just finished restoring it for demonstration purposes at the Museum’s new radio & TV shop, Stanton’s.

The Coronation Twin dates from 1953 and is a mains-battery valve set with a series filament chain. Due to the live-chassis design, the set could not have been made to pass a modern PAT-test, so I opted to modify it for battery-only operation. Someone had previously removed the integral mains-battery switch together with much of the power input wiring anyway, so it was a case of removing the remaining mains wiring, re-wiring the switch contacts and tidying things up. I left the original finned selenium rectifier in situ. The main reservoir/smoothing capacitors tested out okay and reformed with zero leakage.

The valve filament supply requires 7.5 volts at 53mA. On mains, this was derived via a large 1,690 ohm dropper direct from the rectified and smoothed HT, an arrangement I find potentially risky. Valve V3 (1FD9) was absent, and a quick glance at the National Valve Museum showed that a suitable equivalent was the DAF91, which fortunately I had in stock.

When servicing mains/battery portables I prefer to do all fault-finding and testing on batteries, so on this occasion I made up a couple of replica batteries first. These are Ever-Ready AD39 7.5 volts for the LT, and B129 85 volts for the HT. For the LT side I used five AAA alkaline cells in series, and for the HT, nine alkaline PP3s in series which with fresh batteries gave me just under 85 volts. I deliberately kept the capacity quite low as this set is only likely to be switched on for a few minutes each week on demonstration. The LT battery is the limiting factor but should give around 30 hours’ playing time.

For the battery cases I used packing-case type corrugated cardboard, cut to size using a Stanley knife and glued together with PVA woodworking adhesive. The artwork was downloaded from the internet and re-sized to give the correct final dimensions. Somewhere along the line, my first version of the B129 ended up too big to fit into the radio… I made the second attempt slightly too small, but it goes in! A little space around the batteries is a good idea anyway to accommodate the leads.

I cleaned all the valve pins and valve sockets with switch cleaner and checked that there were no shorts on the HT line, then applied HT and LT supplies whilst simultaneously monitoring the current in each using two milliameters. The set came to life straight away, but no signals were heard - a squirt of Servisol on the wavechange switch immediately restored signals on both bands. The LT and HT currents settled at 53mA and 11mA respectively, virtually exactly the figures quoted on the manufacturer’s service sheet. After a decent soak test, the currents hadn’t changed so I felt confident that the circuit was in good order.

Cosmetically, the Bakelite cabinet just needed a good polish with Greygate Paste Polishing No 5, the carrying handle was removed and scrubbed clean as were the control knobs. The glass dial was gently cleaned and the set was reassembled.

Overall this has been a nice straightforward restoration and the set will be able to safely be demonstrated to visitors. My thanks go to Mike G3TDG for the generous donation.

Phil

[Gabe001]

A nice job with the batteries. You were lucky there were no faults, mine had quite a few issues.

Didn't you have to replace any capacitors?

Also did your set come with the original plug in mains lead?

[Phil G4SPZ]

Many thanks! No, all the capacitors seem fine. I understand from the donor that the set had not been used for many decades. There are no ‘waxies’ in it.

It arrived minus any mains lead or connector, and indeed missing the actuating mechanism which switches the set to ‘mains’ when the connector is inserted.

[Gabe001]

From memory I think they've got quite a few hunts mouldseals which are as bad as waxies. Give some thought to replacing at least the grid coupling cap and the mains filter cap (type X) at some point if you're going to use the set regularly.

Are you only going to use it on battery?

[Phil G4SPZ]

Yes, battery only. As I wrote in post #1, “I opted to modify it for battery-only operation.”

Good point about the Hunts. If there were any leakage happening in those capacitors with HT across them, including that capacitor, I’d have expected excess HT current, HT current flow with the LT off, audio distortion or other symptoms, but there are none of these present. However I’ll keep an eye on it weekly and take action should the need arise.