Saturday, 20 August 2016

Radio Rentals Model 218 radio, repairs and restoration.

My good friend Derek, has once again dropped off a challenge.

"Always wanted a set with a magic eye. This one's got no mains lead. Can you sort it?"

Why not...

It's a well designed set, with long, medium, shortwave and FM bands. The FM band only covers 88-101 MHz, as the police used the upper bit to 108 MHz as late as the 1980's in the UK.

It's tone control isn't the conventional variable control, but a three position switch. I can't really see why this was done. It involves a rotary switch, and 3 caps and resistors, rather than a pot, and one cap. Strange... maybe a switch and caps were cheaper than a pot !?

This set is a "purchase model", rental sets were marked as "Property of Radio Rentals." It dates from about 1956/57

Nice un-cluttered chassis...

First off to disconnect the mains smoothing cap and hook it up to the MK87 "Dreadnaught" capacitor reformer, and leave each section to reform.

It doesn't take long, the capacitor isn't original, and is in fine shape.

 Odd "dog bone" resistor (yellow body, purple end, red spot!) ... more the sort of thing you'd find in a 30's or 40's receiver forms part of the power supply filter... can't be original, can it?

A few caps are evicted as a matter of course, and the switches and pots are cleaned up.

Initial results on LW and MW are good, then... POP ... silence.... A new EABC80 valve restores audio. Faults are not going to be easy to find, as I have no service information on this set.
Shortwave works OK too, not much selectivity, but that's only to be expected really. FM is awful. Almost nothing.... it's unstable. A quick sweep with the signal generator shows the thing to massively high in frequency. It's tuning about 140-150 MHz! A long period of head scratching, and tracing signals with the 'scope shows there's a 68pF capacitor open circuit. That brings the tuning back into range. It's not very sensitive, but things improve with a re-alignment of the IF and front-end. One problem remains. It's not good at handling modern broadcasts. The deviation is too wide, and bass is horribly distorted. Not much I can do about that really.

I spoke to Derek, and he's not too fussed about the FM, so it's time to crack on... Derek would like a line-level audio in though, for MP3 use, which is easy on this set, as it's transformer isolated, and has a "gram" input we can use...

A 3.5mm jack is added to the gram input, and shunted with a 10K resistor to earth, and 20K in series. Works really well, but gives us an electrical safety issue, as if a fault were to develop, the chassic could become "live", as would the MP3 player! Thankfully as the chassis is isolated by a transformer, we can add a 3-core mains lead. Testing this out gives no issues :)

Sound quality from it's large Goodmans speaker and EL84 amplifier really are very good indeed.

Magic eye is a little dim, but works well.. The "wings" close up as the signal strength increases to show when the set is correctly tuned.

So to box it all up, and final test. MP3 input works well.... but nothing on the radio ! Back out with the chassis again. Poking about around the wavechange switch, and there's a wire broken! Duly soldered back on, and still nothing! Traced the signal back to the EACB80 again.... (?) Starting to doubt my sanity, when I find another broken wire on the valve base from the IF transformer! I wonder if that was the original problem with the valve, and changing it just caused the broken wire to meet up again? Yep, replacing the valve with the original and it's still good.

Soak test for a few hours, and it seems fine ..!

"Just a wire off" .... sheesh!

Cleaned up nicely too.

1 comment:

  1. I used to really hate it when people come in with non functioning equipment saying "Its probably just a broken wire2! And there you go! A broken wire at last!!!