Unstable Workld Radio Playmate Portable

[Old Sparky]

This is a mid 1960s AM portable radio from the then makers of Motorola Car radios, and uses 3 of the dreaded AF117 transistors in the RF/IF stages.  It is very unstable and bursts into oscillation at various points on both MW & LW.  I don't have a diagram for this radio, but it appears to be a conventional design, except for a 'car/portable' switch, and I haven't checked exactly what is switched by this.  Changing several electrolytic caps. hasn't cured the problem, so my suspicions fall on the AF117s, though the fault could, I guess, be caused by defective components in the F.C. stage.  Maybe another forum member is familiar with this model, and knows what is the most likely cause of the instability?

[Refugee]

I had that problem on a French built 1960s set and it was a Callins capacitor. It decoupled the power rail on the IF amp stages and had spat its insides all over the PCB.
If there is plenty of gain there the AF117s will be just fine.

[Murphyv310]

Hi.
Make sure that no one has discreetly disconnected the screens form the AF117's. I have also had a couple of them with OC screens which were not snipped off.
Measure from the can of the AF*** to chassis for continuity.

[Mark Hennessy]

After checking/changing caps (which you've done), my next move would be to "zap" those transistor. Even if they're not shorted, there still could be leakage. And as Trevor says, what if the screen leads have been cut? In which case, a mild leak from can to base could cause all sorts of weirdness.

If that doesn't help, then at least you know you've returned the transistors to their original spec, along with the electrolytic caps. No point trying to apply logic until those steps have been done IMHO

[Old Sparky]

There are a couple of electrolytics I haven't yet change, neither sre Callins types, but I'll still replace them. As to the AF117s, none have had their screen leads cut, and, having removed one, the 1st I.F. Amp., and checked it with no sign of leakage, I'll check the other two, but may well replace all 3 anyway, never having been a fan of 'zapping' suspect transistors, if only because thee's always the possibility they'll fail again later. I have some Russian transistors on order , and will no doubt replace all 3 AF117s when they arrive.

[Geordie McBoyne]

There is a Telefunken transistor, the type AF137 which can serve as a replacement for the AF117. Ft = 15mhz. The leadout wires are arranged as the AF127.
http://www.radiomuseum.org/tubes/tube_af137.html

I'm sure I've got some in the shop.

Geordie McBoyne.

[Murphyv310]

Hi.
I bought a job lot of Russian Germanium RF transistors a couple of years ago, these work beautifully as AF117 replacements. Same case as the AF12* types but different leadout.

[Old Sparky]

I have some GF322As on order, which should arrive in the next few days, so will replace all 3 aF117s with those and hopefully cure the fault. Thanks for the info. about AF 137s, Geordie. They seem to be an equivalent to AF127, too.

[Old Sparky]

Since my last post I've replaced VT2 with an AF127, but the set is still unstable. I now have the GT322A Transistors, so will replace VT's 1 & 3 with two of those. There are also a couple more electrolytic caps. to change, and I've also found the circuit diagram on Radiomuseum.org, and since then discovered that it's also available from Paul Stenning, but is listed there as 'Playmate 800T', rather than 'Wolrd Radio Playmate 800T', which is how I expected it to be catalogued, WR being the maker's name. It's a fairly standard 7-transistor radio with single tuned IFTs and a push-pull output stage, but has seperate switched aerial coils brought into circuit to replace the ferrite rod aerial when the 'Car' button is pressed.

[Murphyv310]

Hi.
I've just repaired a Roberts R300 that was unstable. All three AF117's had their screens removed but reconnecting them made no difference. I replaced all three with Russian GT332 transistors and the instability is gone!