Fidelity RAD11 Repair Advice

[TheMightyMadman]

Hi all,

I've got a Fidelity RAD11 Germanium transistor radio from the 1960's that I'm trying to repair, and I could use some advice please.

It gives a pop from the speaker on power-on, and some scratchiness from the volume control, so it seems that amplification is working - there is also buzzing when I put an inductive instrument (i.e. my finger) near the detector diode. Tuning doesn't seem to have any effect on any AM/FM band.

The detector diode seems to test OK; all of the transistors seemed to test OK out-of-circuit (Peak meter recognises them as Germanium transistors); all of the electrolytic capacitors were either open, shorted, or electrically leaky, so I've replaced them all - seemingly to no effect. Some of the resistors seem to have drifted in value, but none seem to be open; I've replaced a couple. All of the solder joints seem OK. I've cleaned all of the controls with contact cleaner.

Are there any common failure modes with the RAD11 that I'm missing? It uses three AF117 Germanium transistors which are apparently unreliable (typically internal shorts occur), but they seem to test OK with the Peak meter, and there's no change in symptoms with some percussive maintenance (i.e. whacking them with a screwdriver).

Many thanks,

Adam

[Restoration73]

Failure of the AF117 style devices by internal leakage can be confirmed by cutting the
screen lead of the transistor - this often restores operation.

[agardiner]

Although your peak unit is showing the transistors as OK, they have probably shorted to earth as mentioned in the post above. You can check with a simple meter, measure resistance between the can and each leg.

There is lots of information in this forum about these devices and the various ways of solving the problem, but as mentioned above, you could start by disconnecting the earth leads.

[chompy1]

one trick I have used before is to remove the transistors, connect base, emitter and collector together, charge up a 47uf 50v capacitor and discharge the capacitor between base, emitter, collector (connected together) and the screen lead which should burn up the tin whiskers inside. You may have to repeat the process a few times on each transistor. Paul

[marceljack]

Originally Posted by TheMightyMadman

Tuning doesn't seem to have any effect on any AM/FM band.

This radio does not seem to have FM.
Regarding the reason of the problem, it may be the oscillator which does not oscillate.
Try to tune it near 600 kHz (about 500 m) and put another receiver on MW nearby, if the oscillator works you should receive it around 1055 kHz (slightly less than 300 m).
f you dont receive anything the oscillator does not work.

[Techman]

It's likely to be one or more of the AF117 transistors.

Testing this type of transistor with a transistor tester is a complete waste of time, as it won't take into account the screen, so will always test them as good.

I always test transistor junctions with an AVO (or similar) - a far more reliable method.

Just refit them with the screen connection disconnected (ie, bent back out of the way) and the set will probably work.

[Mr 1936]

Hi

Just to add to the above. Metal can transistors like the AF117 suffered from the slow growth of incredibly thin crystalline whiskers of tin, a bit like miniature stalactites. After decades they can bridge all the way from the inside of the can to the working part of the transistor, and operation is then upset. They usually sit within a filling of silicone grease, so are supported and immune to bashing !

If the can (shield) connection is disconnected from the PCB or just snipped, it allows the can to sit at the same voltage as the internal part it's shorted to and there is a good chance operation can be restored.

I have not tried melting whiskers with a pulse of energy stored in a capacitor, but it sounds like it should work. They are effectively little fuses with very thin wire and ought to blow at a low current.

[TheMightyMadman]

Hi all,

Thank you all for your input and advice, it's much appreciated

I disconnected the screen pins on the three AF117s and cleaned out the band selector switch block, and the RAD11 seems to be working again.

Many thanks,

Adam

[Techman]

Good news and well done. I thought it would work just fine with the screens disconnected.