Thursday, 10 December 2015

Ferranti 146 Radio.

A little while back, a colleague bought this radio to me...

"Can you look at my late father's radio? I've found it in the attic. I think it stopped working in 1964. I'd love to hear it going again"

... and, as regular readers will know, I love a challenge, and hate seeing this stuff go to landfill....

It's a Ferranti 146, manufactured from around 1946. Electrically, it's almost identical to the 145, and the circuit can be found here. 

The case has a nasty crack across the bottom, and it's missing it's back and two knobs. The mains wiring is lethal, and there's a lot of rubber insulated wire that the rubber has broken down, and cracked, so that will require replacement, but it's complete....
Here's a really bad picture of the underside of the chassis...  There's plenty of horrible wax caps, one "traditional" electrolytic, and, (you can *just* make them out on the right hand side of the picture), some electrolytic caps, in wax covered cardboard boxes! , a quick check shows these to be really badly leaky. No matter how long the ole' dreadnaught is left connected to these, they're never going to be any good. Replacement is the only course of action. 

Warming the boxes up gently with a hot air gun (not too hot!) allows the wax to soften, and the old capacitor to be removed from the box. It's best to allow plenty of ventilation when doing this. I'd hate to think what the chemicals given off will do to you! Rubber gloves are another sensible precaution.   
One the old capacitor is removed it's simple enough to solder a new capacitor to the wires, and slide it back inside the box. This preserves the "look" of the set. 

It's a run-of-the-mill job to replace the other capacitors with vintage looking modern equivalents, changing any rotten rubber wiring as we go...

It's obvious that there have been a few repairs made in the radio's long past. There's a "Radiospares" capacitor in there that dates from the late 50's. There's also the anode resistor to the audio preamp, which has been replaced, soldered at one end... and "wire-wrapped" with a bit of fuse wire at the other! A quick check and resoldering sorts that!
The chassis top is cleaned up a little, it's just a bit grimy and dusty. It's not rusted or pitted, so a clean is all that's required...

Checking for obvious short circuits first, mains is applied via the variac...                                                                              ... and almost nothing ....

Tuning up and down the medium waveband produces one very weak local station. (Note: This set has no ferrite or internal aerial, the green wire you can see connected to a socket on the chassis is a long wire, draped around the workshop) Checks in the RF stages, and testing the valves shows nothing is amiss. Alignment is performed as shown in the service sheet, and... bingo! Dozens of stations on Medium wave, a couple on long wave and a few on Shortwave....

Worth noting that the early versions of this set (and it's earlier brother, the 145) use a 6V6G output valve. This set is fitted (from new) with an EL33B. There's some global negative feedback added in to tame it's output.

After a while of testing, it suddenly crackled and went silent... everything appeared to still be working except there was no audio. There was nothing present on the anode cap of the EBC33. Wiggling the wire temporarily restored operation. The wire is screened, so a new one was fabricated from RG59 co-ax and fitted. 

Next thing to sort out is the dial-cord or string. This is in a mess. The tension spring is rusted and has lost most of it's springy-ness. At some stage the string has broken, and been shortened, which has meant it's strung around the shaft which connects to the tuning knob the wrong way round. It slips because of this. New drive-cord is obtained from eBay here, and a suitable spring found in the junk box. Re-stringing is not an easy task, and involves a lot of cursing, but we got there! It now functions smoothly. The string connects the knob to the variable capacitor, and via 4 pulleys to the dial pointer.

Some thin super-glue is poured into the crack in the case from the inside and allowed to harden. As the crack is along the bottom edge, it's out of sight.

The case is cleaned up using some Servisol "Foam Cleanser 30" (other foam cleaners are available, although this stuff is good!) and 69 years worth of grime from coal-fires, fags and lord only knows what runs off in a brown slurry. I thought the case was a dark brown Bakelite, but it's jet black! Quite the handsome set! 

Now to fabricate a back. I obtain a piece of hardboard (the type that's shiny on both sides) and laid the case of the set on it. I draw round the outside, and work out I need to make it about ~6mm smaller so it fits inside the recess of the set. Now some measuring to get the various holes in sensible places, and cut it out with a craft knife. I screw the back to a piece of stout board to stop it warping, and paint it with two coats of 50-50 PVA/water mix to prime it, and once this is dry, I lightly spray the back with some left over black paint. The set is transformer isolated, so the chassis is not connected to mains, so it's safe to operate without knobs until something suitable turns up. 

The result is not so pretty, but functional!
Here's a video of it in action...

1 comment:

  1. My old Grandma's KB radio had similar electrolytics in waxed card. And a field coil on the speaker. Sounded pretty gravelly in its lattter days.