04-08-2020, 12:28 PM
One thing that I don't think has been mentioned yet: AF11x
That Mullard module has 3 of them in there - an AF114 and a pair of AF115s usually, but variations are possible.
As we don't appear to have figured out whether this is an AF or RF/IF fault, it is possible that an intermittent tin whisker could be a contender, especially as physical shock from the wavechange switches makes a difference. Also, whiskers can cause leaking - as opposed to dead shorts - which can cause all sorts of bizarre symptoms. I usually eliminate those first, and many extremely obscure faults have been cleared that way.
My first move would have been a 'scope or powered speaker across the volume control to decide where the fault actually was. Certainly, knowing how bad these PCBs are, I wouldn't have chosen to disturb the caps until I had good reason to be suspicious of them, such as actual measurements of in-circuit conditions. They should be Philips blue axial types. Very, very occasionally these can fail dead short, but if that doesn't happen, they seem to plod on forever in cool-running transistor radios. I always power transistor radios from a current-limited bench PSU, which makes it easy to keep an eye on the operating current and avoids collateral damage from shorted caps (I'm sure that would have been the case here, but it's surprising how many repairers don't, so it's a point worth mentioning).
Personally, I also wouldn't replace the Lockfits until I've got in-circuit measurements that confirm their guilt. In my experience, their normal failure mode is noise - if this is in a part of the circuit where it doesn't matter, then you might never know about it. Complete failures happen, and probably more so than the equivalent BC108 or BC548, but later samples were much better in my experience. Obviously, that might be different for TV sets where the operating conditions and applications are different, but I've almost no experience there - but in transistor radios, they are definitely "innocent until proven guilty" IMHO.
Another thing that hasn't been mentioned: headphone socket. They normally respond to a bit of switch cleaner. It's easy to forget about these, especially when it feels like the fault is more complicated, but it could well be choosing to join in with the "fun"
Just one final thought: there's a chance the IF module was built with AF12x transistors. Certainly, the LP1164 used in the R707 moved to those late in production (identifiable by "02" on the label). I don't know offhand if the LP1171 got the same treatment, but if you open up the module and find factory-fit AF12x types fitted, then that could be a useful data point. And if so, a photo of the labels on the case would be useful to see if they identified it in the same way as the LP1164. But if present, we'd need to be sure that they weren't fitted during a previous repair before deciding.
Good luck!
Mark
That Mullard module has 3 of them in there - an AF114 and a pair of AF115s usually, but variations are possible.
As we don't appear to have figured out whether this is an AF or RF/IF fault, it is possible that an intermittent tin whisker could be a contender, especially as physical shock from the wavechange switches makes a difference. Also, whiskers can cause leaking - as opposed to dead shorts - which can cause all sorts of bizarre symptoms. I usually eliminate those first, and many extremely obscure faults have been cleared that way.
My first move would have been a 'scope or powered speaker across the volume control to decide where the fault actually was. Certainly, knowing how bad these PCBs are, I wouldn't have chosen to disturb the caps until I had good reason to be suspicious of them, such as actual measurements of in-circuit conditions. They should be Philips blue axial types. Very, very occasionally these can fail dead short, but if that doesn't happen, they seem to plod on forever in cool-running transistor radios. I always power transistor radios from a current-limited bench PSU, which makes it easy to keep an eye on the operating current and avoids collateral damage from shorted caps (I'm sure that would have been the case here, but it's surprising how many repairers don't, so it's a point worth mentioning).
Personally, I also wouldn't replace the Lockfits until I've got in-circuit measurements that confirm their guilt. In my experience, their normal failure mode is noise - if this is in a part of the circuit where it doesn't matter, then you might never know about it. Complete failures happen, and probably more so than the equivalent BC108 or BC548, but later samples were much better in my experience. Obviously, that might be different for TV sets where the operating conditions and applications are different, but I've almost no experience there - but in transistor radios, they are definitely "innocent until proven guilty" IMHO.
Another thing that hasn't been mentioned: headphone socket. They normally respond to a bit of switch cleaner. It's easy to forget about these, especially when it feels like the fault is more complicated, but it could well be choosing to join in with the "fun"
Just one final thought: there's a chance the IF module was built with AF12x transistors. Certainly, the LP1164 used in the R707 moved to those late in production (identifiable by "02" on the label). I don't know offhand if the LP1171 got the same treatment, but if you open up the module and find factory-fit AF12x types fitted, then that could be a useful data point. And if so, a photo of the labels on the case would be useful to see if they identified it in the same way as the LP1164. But if present, we'd need to be sure that they weren't fitted during a previous repair before deciding.
Good luck!
Mark
