Bush Transistor Radio TR132 Repair and Revival

You never quite know what you may find at Ardingly antique and collector fairs. Early this year, as a post lockdown day out, we rummaged through hundreds of stalls and uncovered a few classics for Radio Retro. One of the items was a ‘well loved’ (tatty, neglected) Bush TR132 in muddy blue leatherette. The others included a Sony portable black and white TV and an HMV record player — more on those two in later post.

When looking for 1960’s memorabilia that truly evokes the mood of the swinging sixties you can’t go wrong with a transistor radio. Well that’s what I tell my wife when she wisely says haven’t we got enough? £5 seemed a reasonable punt to take on this unloved classic although not as popular at the smaller TR130 that’s already been featured here at Radio Retro.

Inside the radio

Inside the TR132’s wooden case houses much the same circuitry as used in the TR130 and so I guessed the usual unreliable suspects would be found. The chassis is fixed in place by two screws accessed from inside the battery compartment. Removal of the two speaker wires separates case from chassis to begin repair.

An initial check revealed no sound whatsoever with power applied from a bench 9v power supply and no tell tale click or hiss from the speaker. The on/off switch – part of the tone control – was open circuit and no amount of switch cleaner would restore life. Fortunately Bush has chosen to fit a double pole switch, see below, and the second pair of contacts worked, so a simple swapping of the switch wires powered up the radio.

Internal view showing tone control with on/off switch. Also shows two AF117 transistors mentioned later in post.

TR132 Faulty On/Off switch

The TR132 features just three wavebands medium wave , long wave and ‘band spread’ – for the 208m band Radio Luxembourg used. Tuning the radio was almost impossible even with the large diameter control knob — the mechanism was very tight. Inside the tuning capacitor was sitting at an odd angle to the chassis – the rubber mounting grommets has long since perished causing the shaft to pull towards the dial by the tuning cord tension.

Internal view showing tuning capacitor pulled off square by tuning cord tension.

Tuning capacitor out of alignment

Small cardboard wedges, 3mm thick, held by double sided tape restored the tuning capacitor to sit at right angles. Removal of hardened grease along with switch cleaner and light oil freed the spindle and allowed the tuning dial to work again. I wondered how many years since it had been last used.

Stations could be heard but faintly on medium wave and hardly on long wave. More switch cleaner on the push button wave change switches restored some sound to the long wave and band spread but all very feeble. Touching the car radio aerial input produced more volume as did a finger near the ferrite aerial. You can’t go poking fingers inside valve radios but I’d grown up fault finding transistor tech by touch and instinctively prod with digits before reaching for oscilloscopes and ‘grown up’ test gear.

Low gain – poor sensitivity from the RF or IF stages I wondered? Bush’s design uses three AF177 germanium transistors – one for the RF oscillator / mixer stage and the other two for the 465kHz IF amplifier. Powering up the Advance RF signal generator – another ‘well loved’ piece of kit – caused more confusion. There was no 400Hz modulation tone to be heard on any waveband. The TR132 dial is marked in wavelength – metres whereas the Advance uses KC/s or frequency. So it takes the little grey cells a few seconds using the speed of light to work out 1500m is 200KHz and should be booming through. It took a little longer to figure out the modulator had stopped working on the Advance after 70 years and needed repair too.

It made me think philosophically about radio repair or any diagnosis really – it’s all about having a known reference or starting point. How did I know the switch was open circuit for sure? I used a multimeter that measures resistance as a reference. How did I know the tuning capacitor was in the wrong position? I used angle as a reference. How did I know the signal generator had stopped modulating? I used an oscilloscope to look at the waveform and the pure 200KHz carrier frequency as a reference.

So how can I find out why the volume seems to quiet on this radio? If I had another one I could use it as a reference and compare the two. And many service engineers do just that – they don’t fault find to component level – it takes too long or needs more training and skill – it’s easier to swap out complete circuit boards or the whole item and replace it. Often at much higher cost to the customer and the environment as more electrical waste is created from the faulty units. But I digress and leave WEEE for another post. Back to the faulty radio…

Suspect the AF117 transistors as they are notorious for intermittent failure due to microscopic conductive filaments that grow between the connections inside the device. It’s possible to melt these with a short sharp shock of current between the electrodes (thanks to the Dark Side for reminding me). And I’ve successfully restored a previous TR130 with just such a fault. But this often does not last — over time the microscopic tentacles grow again and the radio looses volume and fails.

So I tapped along the row of AF117s with a small screwdriver searching for a clue as to which may be faulty. Third time lucky, suddenly voices boomed. Then after a short time returned to whispers. I tried wiggling the suspect transistor via its long leads to check for dry solder joints that could also be the cause. No change in volume, but several sharp taps on the can and it crackled into life.

I decided to replace all the AF117s with AF125, a physically smaller germanium RF transistor that has a 4th screen electrode like the larger canned AF117. If you can’t source AF125 transistors it’s possible to use a more modern silicon PNP transistor like the BF450 without changing the bias resistors in this circuit. Bipolar silicon transistors need a higher operating voltage between base and emitter of ≥0.6v rather than the ≥0.2v for germanium. BF450s don’t have a metal can – there’s no 4th screen wire – so internal capacitances are a little different and may need stage realignment. Also for restoration purists silicon is not authentic although it won’t alter the 60s sound unless you follow silicon through the audio stages and replace the OC81s as well. I’ve rarely needed to swap the audio stage transistors in these radios and prefer to keep the mellow sound germanium devices exhibit.

Outside the radio

I decided to remove the rear plastic panel and front metal speaker grille to allow a deep clean. The back panel is held in place with clips – prise off gently with a flat bladed screwdriver. The front grille has folded wire spikes recessed in the cabinet – prise and straighten using screwdriver and pliers. A few spikes may detach from the grille surround so will require soldering back in place.

Years of grime and mould patches covered the leatherette case. I tried detergent on cotton cloth to gently clean the surface and this helped a little. I found methylated spirits best to restore the blue colour beneath. Using a clean cotton rag with fresh meths it’s possible to restore the sheen like a french polish and remove the muddy brown stain and mouldy appearance. After leaving to dry for a few hours I finished the whole case in wax polish.

Soaking the rear panel and speaker grille in detergent for an hour helped remove the years of dirt and grime. Car paint restorer like T-Cut helps restore shine to the grille surround. A couple of tiny dents in the grille needed a careful press from behind and a dab of off white paint – I forgot about these in haste to reassemble to will sort at some later point.

Revived radio

Reassembled after the deep clean, with a new set of RF transistors this classic Bush radio is restored and ready to bring back the sound of the sixties. Shame the slow fade of Radio Luxembourg is no longer with us to complete the nostalgia trip. It’s powered by a 9v PP9 battery, still available from Wilko, that lasts for ages if you remember to switch it off using the tone control rather than using the volume control that became for usual.

If you would like to own one of these to bring back your times past, post a comment and I’ll see what Santa can do. Alternatively look for one as you browse collector’s fairs or emporiums and try a restoration yourself. Good luck.