Hacker mini-Herald Transistor Radio model RP17a

Serial number 81503

Portable Transistor radio Made by Hacker Radio Ltd, Maidenhead UK. The radio is housed in a dark blue leatherette covered wooden case. It was given to me by Roddie MacKenzie. Although the Hacker Logo is missing, the strap is in good condition.This is a good quality radio with 7 transistors covering medium and long wavebands. It has sockets for headphones and external aerial (on left and right hand sides). This radio is normally powered by two 9 volt PP9 batteries in series. now working Maidenhead, UK 1962 approx.

Perspex Dial

The radio was connected to a variable voltage power supply as I did not have two 9 Volt batteries and wen the radio was first energised there was complete silence. The loudspeaker and the earphone socket proved to be OK. As I have another RP17A it was an easy matter to test the output stage by using the front end of the second one to provide the signal and power for the output stage. In this way it was established that the fault lay in the main part of the circuit.

It was apparent that the pushbutton switches were not working properly, as it was not possible to switch the set off. I left the set connected on the work bench, switched off the main power supply and spent a week doing other things. When next energised the set came to life (at least on the Long Waveband). This was a relief, I had suspected that one of the dreaded AF117 transistors might be at fault and this seemed to be the case as tapping one of them caused the radio to become silent again.

These germanium transistors manufactured by Mullard are housed in tin plated cans. The cans are connected to a fourth lead out wire which is normally connected to the "earthy" side of the circuitry, the purpose being to act as a screen or shielding. Unfortunately the tin grows whiskers a few microns thick inside the casing and these, if long enough, can make contact with the germanium junction and effectively short circuit the transistor.

There are several remedies:
1. Replace the transistor
2. Cut the fourth lead
3. Blow up the whiskers by connecting an electrolytic capacitor charged to 50 Volts between the casing and all the other leads connected together.
4. Separate the can from the base by unsoldering and physically removing the whiskers.
5. Merely heating the top of the can with a soldering iron has been said to do the trick.

I tried method 3 on the oscillator transistor without success and ended up using method 2 on all three AF117 transistors. The set then came to life and has been working for some hours.

Of course there is the possibility that there might be two or more whiskers touching the transistor junctions and merely cutting the screen lead wire might have no effect as the short circuit would be within the casing.