29-03-2013, 04:59 PM
I've seen a lot of these. Never had this exact fault with a Hunter before though...
The output transistors can indeed suffer from tin whiskers. It's much less common than the AF11X transistors, but I'm seeing it more and more these days...
It's just possible that an intermittent collector-base short exists at T5. It's also just possible that a short to case hasn't been noticed because of poor contact between the heat sink and ground. Not sure if that's very likely in all honesty.
Joe is right about the 5-pin connector, but this tends to cause oscillation and other unrelated problems. Nonetheless, it's worth checking and cleaning it (and the solder joints).
An essential piece of information is missing: when in fault condition, what is the current consumption?
The transistor lineup in the RP38A never changed. OK, the service manual says that T1 and T2 should be BC108, but I've only ever seen BC148. Funnily enough, the original RP38 uses BC148 for T1. When replacing BC148s, I use BC108.
The Lockfit transistors are a little bit unreliable, but they don't tend to fail intermittently in my experience; rather, they go noisy or low gain. How much this matters depends on where the transistors are in the circuit. Complete failures are pretty rare IME.
I've had the symptoms you describe with the A205 used in the Sovereign II and Herald. But that's normally caused by a faulty AC128 in the VAS position - that's T4 in that and the RP38A. (By VAS, I mean voltage amplifier stage. Many people incorrectly call that the "driver").
In my experience, I've found the electrolytic capacitors in these to be extremely reliable. On the older RP18 amp, leaking capacitors can cause all sorts of problems. But this leakage isn't intermittent.
I think I'd start by replacing T1, 2, and 3 with modern equivalents. BC548 and BC558 would be good, but almost literally anything would do. If the fault persists, at least you can reinstall them, and apologise for suspecting them. Then I'd try replacing T5 and T6 - obviously these are harder to source, which is why I wouldn't try them first.
Have a good look at the underside of the PCB for battery leakage damage. I've had some where the solder has gone strange, and needed to be cleaned off and replaced - I presume that was caused by battery leakage, but perhaps it was that "special" solder that Philips used to use
Sorry that was all a bit of an unstructured brain-dump, but hopefully helps...
Cheers,
Mark
The output transistors can indeed suffer from tin whiskers. It's much less common than the AF11X transistors, but I'm seeing it more and more these days...
It's just possible that an intermittent collector-base short exists at T5. It's also just possible that a short to case hasn't been noticed because of poor contact between the heat sink and ground. Not sure if that's very likely in all honesty.
Joe is right about the 5-pin connector, but this tends to cause oscillation and other unrelated problems. Nonetheless, it's worth checking and cleaning it (and the solder joints).
An essential piece of information is missing: when in fault condition, what is the current consumption?
The transistor lineup in the RP38A never changed. OK, the service manual says that T1 and T2 should be BC108, but I've only ever seen BC148. Funnily enough, the original RP38 uses BC148 for T1. When replacing BC148s, I use BC108.
The Lockfit transistors are a little bit unreliable, but they don't tend to fail intermittently in my experience; rather, they go noisy or low gain. How much this matters depends on where the transistors are in the circuit. Complete failures are pretty rare IME.
I've had the symptoms you describe with the A205 used in the Sovereign II and Herald. But that's normally caused by a faulty AC128 in the VAS position - that's T4 in that and the RP38A. (By VAS, I mean voltage amplifier stage. Many people incorrectly call that the "driver").
In my experience, I've found the electrolytic capacitors in these to be extremely reliable. On the older RP18 amp, leaking capacitors can cause all sorts of problems. But this leakage isn't intermittent.
I think I'd start by replacing T1, 2, and 3 with modern equivalents. BC548 and BC558 would be good, but almost literally anything would do. If the fault persists, at least you can reinstall them, and apologise for suspecting them. Then I'd try replacing T5 and T6 - obviously these are harder to source, which is why I wouldn't try them first.
Have a good look at the underside of the PCB for battery leakage damage. I've had some where the solder has gone strange, and needed to be cleaned off and replaced - I presume that was caused by battery leakage, but perhaps it was that "special" solder that Philips used to use
Sorry that was all a bit of an unstructured brain-dump, but hopefully helps...
Cheers,
Mark
