Roberts R600: Fades

[Mike Watterson]

My R700 is fine. Much better RF on AM and FM than the R600. Though my Hacker RP18 and RP25a are both better. The reduced VHF coverage isn't an issue as: (1) I don't use them much, (2) There aren't any stations here above 101 MHz that I listen to. RTE1, 2FM, Lyric, RnaG, Clare and Limerick are all 89.4 to 99 MHz.

I think I have two other Roberts: A Vidor styled battery valve model that maybe is octal and the not actually Roberts rebadged Sangean Radio-Cassette with a BFO. I'm not planning on more. The R700 and R600 were nostalgia, the other two were bargains. I'm really concentrating on other things, I have loads of sets sitting here for years to sort.

I've encountered plenty of whiskers and this fault wasn't like any. It wasn't affected by tapping. Also I never saw one exhibit the operating profile.
The fact that it only happened if the set was unused, initial perfect audio, sudden silence and then after a minute fading to normal like a valve set warming and thereafter absolutely fine for hours, and even if off for maybe 10 minutes. But leave it off an hour and the cycle repeated. I'll see what inside the AC128 eventually. Certainly two capacitors were marginal, the o/p speaker decoupling being the worst. It's in a feedback path too.

[Mark Hennessy]

Yes, the strange/unexpected behaviour is why I've explained it so extensively - I'm hoping others won't have to go through the same pain I did. Whether it's whiskers or just a heavily leaking junction is almost irrelevant - the symptoms are similar, and the AC128 was the cause. It's tempting to look at the schematic and see that the AC128 is being used well within its limits in that VAS position, so look elsewhere first, but I'm always suspicious now. Later Hacker sets used a BC159 or similar in that role - I've no idea why the AC128 was chosen, apart from familiarity, perhaps. When I've run out of AC128s, I'd have no hesitation in using a BC178 or similar - even a BC558 or BC327 if looks aren't important. No modifications necessary.

The output cap is only bootstrapping T3's collector load. If the cap was included in the global NFB loop, then that loop would go some way to mask any issues caused by that cap. Personally, if the ESR was reasonable, I'd have a go at reforming it - in my experience, slight leakage in those Philips caps usually improves with use.

Trevor, I agree that lifting the screen lead is not elegant. It only works while the whiskers are in contact with just one of the electrodes. As soon as other whiskers contact one of the other two electrodes, then you've got a short that goes via the case. So it's a short term solution at best - I prefer to think of it as a confirmation exercise. Not that I bother with that these days - if there's any sign of misbehaviour, all the AF11x get zapped before I do anything else. In the case of the TO1 types, then yes, if not mounted on a non-isolated heat sink, then they are normally OK until the second electrode meets with whiskers - which of course completes the path between that and the first electrode to meet whiskers via the case. But it depends...

As I think I've probably stressed enough, there are so many faults that go away after zapping, even if the transistor didn't seem to have much, if any, of a short to case, because the whiskers can recede from the heat of soldering, or perhaps handling. It can be really confusing or even unsatisfying to remove, test, find no significant leakage, zap anyway and reinstall, but then find the set works perfectly. However, I've stopped questioning it, and now just accept that you might never find conclusive evidence that it was a whisker, but if zapping works, that's prima facie evidence that a whisker was indeed the cause, and that's good enough for me. Of course, it depends on the circuit - so the mixer/oscillator will generally be more fussy than the final IF, and outputs are less likely to be troubled by weaker whiskers than the preceding VAS. The only times I've been led towards a whisker in an output transistor, it's been a dead short that creates quite a spark when you do the zapping - any fine whiskers are usually blown away instantly at first contact by the available current at that part of the circuit. Usually.

Mike's fault could be either (whiskers or junction fault), but we'll probably never know for sure. My previous experience is screaming "whiskers", but I'm trying to keep an open mind, so I'll give both equal odds at this stage.

Anyway, I'm glad my previous experience of troublesome AC128s in the VAS position helped to speed up the fault-finding process. You're welcome...

[Mike Watterson]

It's a Roberts R600, seems a very much less powerful amp than on the Hacker, also 9V rather than 18V.

It could be something other than whiskers or junction. Whiskers are obvious tin dendrites due to too much pure tin plating. I've repaired a leg snapped off at the body, but I don't trust zapping. What about 6 years or 10 years? The tin dendrites grow slowly, but EVENTUALLY they will come back. It's a production defect and NOS Russian military and other parts are available cheaply. I'd have a different view on replacing parts in a pre-1929 set, and I'd do replacement alternate valves with an adaptor rather than changing sockets or cathode resistors.

Anything I've suspected of whiskers has had them when opened. Maybe tomorrow I'll cut the lid off. If it's not whiskers, and it's not like a whisker fault, then we'll never know why it's bad.

[Mark Hennessy]

Perhaps surprisingly, they're much the same power; the RP25 might well be 18V, but it uses a 15Ω speaker, whereas the Roberts is 5Ω. But where the Hacker should score, of course, is the much bigger 5" by 8" speaker, which should be more efficient than the Roberts one, so more SPL for your electrical input.

The RP18 uses a 30Ω speaker. That amplifier doesn't produce half the power of the RP25 because, of course, the lower current means a higher voltage swing. The spec says 1.2W, whereas the RP25 claims 1.5W. Obviously, that difference is pretty much inaudible in practice. And back in those days, you'd see similar or bigger variations in loudspeaker sensitivity from nominally identical units - consistency wasn't brilliant back then in the hi-fi market, let alone in the cheap transistor radio space. Remember how BBC loudspeakers had tweeter level adjustments in half-dB steps? And they're all set differently. But talk to people who make BBC-licensed speakers today, and they'll tell you that they rarely if ever need to move them away from the 0dB position - the new version of the Audax HD 13D34H, for example, is impressively more consistent that the original.

I'll try to dig out my two R600s to how they compare to the units used by Hacker (there were 4 different types used in the RP25 - the A models usually got a silver Celestion) - I'll stick them in front of my loudspeaker measurement setup. It might take a while, as my Roberts sets are in a part of the attic that isn't very easy to access, but I'd quite like to find my R600s as a result of this thread - not least to see if the Lockfit installation is as bad as you reported earlier...

[Murphyv310]

I've an R606-MB which is used regularly. Still completely original.
It's not a bad radio at all.

[ppppenguin]

Like Trevor I also have a 606 MB . Untouched and used regularly.

[Mike Watterson]

Yes, now that the R600 is fixed, it's working well. I suppose it's really a Philips (Mullard was Philips since 1928) Radio and my introduction to electronics was the Philips EE set with Germanium transistors with long leads. A Christmas present from my Godfather. I hardly met him more than 1/2 dozen times, but for years he did send a Christmas present. Maybe that was the last one.

[Murphyv310]

I also had a Philips EE8. It was a big surprise for me as mother was a Jehovah's Witness and I never got Christmas presents or birthday gifts. Some time later I remember one of the legs came off the AF116 and I got upset. Fortunately my older brother got me one locally.