06-08-2020, 02:42 PM
As I said, when I've had this, it's usually been tin whiskers - the intermittent nature of the leakage being the clue. You might get audible (often very loud) cracks on the audio if the path through the whiskers is a fairly low resistance one, but often that is not the case, and the DC conditions just drift about slowly - as you've found, it's quite confusing when you first meet it. Listening to the output while tapping the case can often confirm the whiskers, but not always.
Sometimes, the whiskers can be measured from case to electrodes (has to be more than one, as the can isn't in electrical contact with anything in this position) - a simple voltage measurement of the can relative to earth can also be useful. But the heat and handling from de-soldering can be enough to temporarily clear the whiskers, meaning the transistor works perfectly upon test, but of course a replacement cures the fault. That's also confusing, but you can sometimes find light leakage (>100k) from case to one electrode, which confirms the presence of whiskers in some form.
Zapping the whiskers is a cure that'll last for decades if done properly. Of course, some won't want to do that; that's fine, no need to get into that here. But even if you don't re-use the transistor, it does at least prove that whiskers were the root cause...
I don't know how many times I've seen this exact fault with the Hacker circuit, but at least a dozen. But I've had a couple of instances where it wasn't whiskers - it was a genuine fault with the junction leaking very heavily. Not surprisingly, the leakage amount was highly temperature dependant, and it didn't change at all after zapping. In circuit, it worked fine when very cold, or when blasted with freezer. The Hacker circuit runs this transistor at about 4 times the power level than the Roberts - they even fit a small heat sink - so the time constant with that fault was pretty short. It's possible that the variations you're seeing in the static test are temperature related - that's easy to investigate should you be curious.
I've found that the Peak Atlas transistor tester is a lot better with germanium than the Chinese testers. Although occasionally - perhaps one time in 20 - the Chinese tester gives a sensible result when the Peak Atlas did not. I'm glad I have both, but use the Peak most. I got mine from Maplin for only £20.
Taking all the TO1 and TO7 transistors together, I'd say that actual junction failure is very rare indeed - certainly in my experience of transistor radios. Ignoring cases of incorrect battery polarity killing output transistors and similar abuse, I can probably count on one hand the number of dead junctions. I've even had germanium transistors with broken encapsulations that were working just fine. But I have had countless apparent junction failures that went away completely when the whiskers were treated - even though the measured resistance of the whiskers to might have been hundreds of kΩ, which could easily have been dismissed as insignificant. I've "zapped" hundreds of them now, and never damaged one in the process, and as I haven't had a reoccurrence ever, I'm confident in giving a 5 year warranty on whisker treatments (I give the customer the choice, and most of the time they prefer originality, though I might steer them towards replacement if there's lots of labour involved in getting to them - e.g. Roberts R707). Some time ago it got to the point where if I have any RF/IF fault at all in a radio using TO7 transistors, I remove and zap before doing anything else. Apart from a quick check to leakage to case, I don't bother testing before or after - I just zap and put them back in. Offhand, I can only think of one occasion where that caught me out because of a "real" junction fault - there must have been some others, but I really can't think of any. Luckily, that was in a Hacker where the transistor was very easy to access.
Yes, the R700 came first, as per RM.org and various brochures, etc. You can tell it's an older design - all germanium, for a start. It's perhaps the only Roberts portable to have Hacker-style separate AM and FM IFs stages. AM IF and FM RF end are both courtesy of modules - presumably Mullard, but it's been a while since I've looked in one - but happily the service data contains details of those. In the examples I've seen, the caps were not Philips axials - they were the usual "suspect on sight" types, like TCC Elkomold, etc. It was fairly quickly replaced by the R707. The R600 carried on in parallel with the R707 for a while, but was replaced by the R606 and later R606MB. Early R600s had the mesh grille, but most IME had the flat sheet type, with the black stripes painted on (though that paint isn't very resilient in my experience, so I'm always very cautious when cleaning them). Both grille types dent very easily. If you want to see a proper grille design, look to the extruded aluminium used by Hacker in the '70s - I dread to think how expensive that must have been!
I've got 3 examples of the R700 in storage, and should be able to make 1 decent one from them. Let me know if you need any parts from an R700 if/when you tackle one.
Sometimes, the whiskers can be measured from case to electrodes (has to be more than one, as the can isn't in electrical contact with anything in this position) - a simple voltage measurement of the can relative to earth can also be useful. But the heat and handling from de-soldering can be enough to temporarily clear the whiskers, meaning the transistor works perfectly upon test, but of course a replacement cures the fault. That's also confusing, but you can sometimes find light leakage (>100k) from case to one electrode, which confirms the presence of whiskers in some form.
Zapping the whiskers is a cure that'll last for decades if done properly. Of course, some won't want to do that; that's fine, no need to get into that here. But even if you don't re-use the transistor, it does at least prove that whiskers were the root cause...
I don't know how many times I've seen this exact fault with the Hacker circuit, but at least a dozen. But I've had a couple of instances where it wasn't whiskers - it was a genuine fault with the junction leaking very heavily. Not surprisingly, the leakage amount was highly temperature dependant, and it didn't change at all after zapping. In circuit, it worked fine when very cold, or when blasted with freezer. The Hacker circuit runs this transistor at about 4 times the power level than the Roberts - they even fit a small heat sink - so the time constant with that fault was pretty short. It's possible that the variations you're seeing in the static test are temperature related - that's easy to investigate should you be curious.
I've found that the Peak Atlas transistor tester is a lot better with germanium than the Chinese testers. Although occasionally - perhaps one time in 20 - the Chinese tester gives a sensible result when the Peak Atlas did not. I'm glad I have both, but use the Peak most. I got mine from Maplin for only £20.
Taking all the TO1 and TO7 transistors together, I'd say that actual junction failure is very rare indeed - certainly in my experience of transistor radios. Ignoring cases of incorrect battery polarity killing output transistors and similar abuse, I can probably count on one hand the number of dead junctions. I've even had germanium transistors with broken encapsulations that were working just fine. But I have had countless apparent junction failures that went away completely when the whiskers were treated - even though the measured resistance of the whiskers to might have been hundreds of kΩ, which could easily have been dismissed as insignificant. I've "zapped" hundreds of them now, and never damaged one in the process, and as I haven't had a reoccurrence ever, I'm confident in giving a 5 year warranty on whisker treatments (I give the customer the choice, and most of the time they prefer originality, though I might steer them towards replacement if there's lots of labour involved in getting to them - e.g. Roberts R707). Some time ago it got to the point where if I have any RF/IF fault at all in a radio using TO7 transistors, I remove and zap before doing anything else. Apart from a quick check to leakage to case, I don't bother testing before or after - I just zap and put them back in. Offhand, I can only think of one occasion where that caught me out because of a "real" junction fault - there must have been some others, but I really can't think of any. Luckily, that was in a Hacker where the transistor was very easy to access.
Yes, the R700 came first, as per RM.org and various brochures, etc. You can tell it's an older design - all germanium, for a start. It's perhaps the only Roberts portable to have Hacker-style separate AM and FM IFs stages. AM IF and FM RF end are both courtesy of modules - presumably Mullard, but it's been a while since I've looked in one - but happily the service data contains details of those. In the examples I've seen, the caps were not Philips axials - they were the usual "suspect on sight" types, like TCC Elkomold, etc. It was fairly quickly replaced by the R707. The R600 carried on in parallel with the R707 for a while, but was replaced by the R606 and later R606MB. Early R600s had the mesh grille, but most IME had the flat sheet type, with the black stripes painted on (though that paint isn't very resilient in my experience, so I'm always very cautious when cleaning them). Both grille types dent very easily. If you want to see a proper grille design, look to the extruded aluminium used by Hacker in the '70s - I dread to think how expensive that must have been!
I've got 3 examples of the R700 in storage, and should be able to make 1 decent one from them. Let me know if you need any parts from an R700 if/when you tackle one.
