AFZ12 for Roberts R300?

[Boater Sam]

My old Roberts R300 was suffering from poor sensitivity and volume, time to give it a boost.
The Af117s were still in working order but suspect, the screens on two of them had been removed some time ago.

The mixer seemed to be the worst performer. Swapping in a BF450 stopped the radio altogether.
Time to find some germanium devices rather than struggle getting silicon to work.

My box of oddments has some AFZ12s, these have the screen lead too, good Hfe figures and they are VHF transistors.
Unfortunately the lead arrangement is the complete reverse of what is required to fit the R300 pcb. The leads are not long enough to fit them can down and bend the leads over the top so I fitted them on the reverse of the pcb, plenty of room.

Fitted 3 new transistors but performance was still not good.
Now this radio has many Hunts horrors and Plessey problems. I don't usually worry with transistor radios but I suspected that the history of a leaking battery before I bought it may have done something bad to affect them.

Some of the Plessey electrolytics thought they were diodes according to the Chinese Magic box. The odd ones that were still capacitors were leaking with high ESR figures, they had to go. I have some very cheap Chinese midget electrolytics with low voltage ratings, they proved to be excellent when tested.
The Hunts plastic caps had gone up massively in value and leakage. Odd ones shed their plastic shells when moved.

A complete recap restored excellent performance considering that this is a 6 transistor set missing the audio amp transistor.
The Mullard driver and output transistors were as usual perfectly fine. Just reset the quiescent current at 4mA just to be sure.

The IF transformers had not been touched so they were left alone. RF trimmers were still bang on so the set must like its new front end devices!

A quick check of resistor values in situ showed them all to be near enough in tolerance.

I was impressed with the AFZ12s, the mixer/oscillator works just as well as with the AF117. This is the Achilles heel of replacing AF117s, many transistors that will be fine in the IF amps just will not work in this position, particularly silicon substitutes.

Another one for the top shelf, really must sell off some of the stock, need a few new challenges.

Sam.

[Herald1360]

AFZ12 should be fine- it's somewhat higher Ft and lower min hfe but might be marginal on voltage ratings in a set running at more than 9V supply (it has only a 10V Vceo rating).

[Boater Sam]

All very true but R300 runs on a PP9 so eminently suitable and in stock, big advantage.

Decided to test and Zap the old AF117s.
Test showed that they still worked but had shorts/leaks to screen lead.

Gave them a 12V zap through the Avo 8, big first current kick then nothing.
Now they are all OK, but for just how long would I trust them? Not long enough.

Sam.

[Maarten]

Zapping has the distinct disadvantage that it doesn't do anything to 'almost touching' tin needles. I used to think that melting the tin would be the most durable (least undurable) method, but I read about rinsing them which might be better.