Decca TP50 Debonette Radio

I looked at this a few times before I made up my mind that I 'needed it', the voice in my head kept saying things like "knob broken, never find another' and 'It's a bit expensive for faulty' or un-tested is the usual description. There were two for sale at the time one was working and with a broken knob for £20 and the other was £5 starting bid with and faulty. So I went for the faulty one, cost me £12.50 + £2.79 postage. Ooh well something to do, it's a little like being addicted to crossword puzzles, but both keep you mind active I suppose.

Component side of the print board, showing HT test across C26

 The repair
First thing as usual was to take lots of photo's before and while I stripped it down. It was then cleaned and broken pieces of cabinet repaired. In this case the channel selector knob had been broken in the past and the 2 halves glued back together. Sadly removing it broke it apart again. The safest way to remove push-on knobs is to use two screwdrivers one each side of the knob and make an even pull of it. Many radios like this have been left unpaired because they wont't come apart without force, but it has to be done.

Stripped down showing original faulty loudspeaker with glue attached

Somebody in the past had been a little ever enthusiastic with the Evostic glue and had fitted the speaker surround back because they had lost the sir-clips that  hold it back in place. I cleansed off the glue from the Rexite vinyl covering with a small modeling knife and a scouring pad, the metal type. There was lots on the rear cover too.

rear cover after clean-up

I use a toothbrush and kitchen cleaner  to remove ground-in dirt, test on bottom first in case the colour comes off. If it does used a weaker cleaning fluid. The two halves of the channel selector knob were now glued back together and I made two matching caps to cover the volume knob and the selector knob as one was missing. I kept the old good one, perhaps a knob may turn up at a latter date. Now the chassis is open I can connect two PP3 batteries to the radio to test it, the larger PP6 type are no longer available, but the 18 volts provided by the PP3's supplies ample current for the 6 transistors. 

Circuit diagram for Decca TP 50 radio

When I put my ear to the speaker there was complete silence, suggesting ear plug or on-off switch were possible suspects. I put my test meter across  C26 (100uF - HT decoupling) and found 17V, so on-off switch was OK. Next I tested the loudspeaker as the set has no earphone socket with a possible doggy switch (quit a common fault), on the circuit diagram the one battery connects to the center point of the two sound output transistors and this speaker was found  to be 'open circuit' (no resistance at all)

Component side of print showing new speaker fitted and  foam support for smaller batteries

It doesn't give a value for the loudspeaker's resistance on the circuit diagram, but I found a 3' round Sony 8 ohm speaker worked fine, it was a little smaller than the original but fitted nicely through the hole on the printed circuit board. In actual fact this LS came from an expensive Sony portable radio that I had scrapped due to a broken cabinet (Good to hang on to some things) I made a fixing for the speaker by wrapping some 18 SWG wire around the magnet and attaching it the the plastic speaker cover clips by melting a tiny hole in the plastic using the heat from my iron and pushing smaller thickness wire through the clip. The sound quality now far outshone the original sound I'm sure.