23-04-2012, 12:33 PM
Personally, I like direct coupled transistor amplifiers.
It's funny - I had an uncle who was largely responsible for starting me off on electronics - not least by giving me a soldering iron for my 5th birthday! He was an RF person, and worked for Thorn (latterly Thompson) in Enfield, and I believe he had a hand in the designing the tuner in the TX10 and others. He has a callsign, and built all sorts of weird and wonderful RF things. But despite all this, he really didn't understand anything to do with audio! Well, that's what he said - I reckon he was being modest.
I could never get my head around this, because the stuff he did was well beyond my comprehension - still is, really. Audio, to me at least, is much, much easier. Like you Rob, he cited the DC-coupled nature of everything as the problem - yet DC-coupled amplifiers are used in many walks of life beyond audio - video circuits and oscilloscopes, for example... And I know that he designed and built video gear. Still, I guess some of us are just "wired" differently...
Sorry to ramble!
I don't think that a fault in T1 could damage anything. After all, under conditions of overdrive, T1 could be driven into saturation or total cut-off, and if being overdriven resulted in damage, it would be a pretty poor design! Actually, a common fault-finding strategy is to short-circuit base and emitter (to turn it off) or s/c collector and emitter - I do that with a pair of tweezers while observing the midpoint voltage on a 'scope.
The biggest cause of problems is normally a faulty output transistor. The initial failure could be caused by random component failure (maybe tin whiskers!), overheating or over-current. What happens next depends on the design and a degree of luck, but let's assume that one transistor has a collector-emitter short. The midpoint will jump to the rail or ground, depending on whether the upper or lower transistor when s/c - but the rest of the amplifier will see this, and via negative feedback will attempt to bring the midpoint back down, and it does this by driving current into the base of the working output transistor. What happens next depends - the other transistor might pass enough current to bring down the power supply and no further harm will result - apart from perhaps flattening the batteries! On the other hand, if the power supply is a "beefy" one, it will "win", and the other output transistor will fail through excess collector current or overheating...
In terms of further consequential damage, if this was a big hi-fi amplifier with no output capacitor and +/-50 volt rails, your loudspeakers will be somewhat unwell after all this! Decent amplifiers have a protect circuit that disconnects the loudspeakers via a relay should DC offsets appear on the output.
Problems can propagate back down the amplifier too. What tends to happen is that the junctions in the output devices become a great big blob of metal, meaning that all 3 terminals get connected together. So the rails appear at the bases of the transistors - and thinking about it, this is probably how T3 bought it. Oh, and have a look at R14/R15, because these probably cooked during the "event". T2 should have survived, but you never know...
Again, sorry to ramble, and sorry if I'm stating the obvious here - but hopefully all of this reinforces my advice to use a current-limited PSU
I reckon that Joe needs a break from this set, so adding a current limiter to his PSU would be a nice little diversion
Oh, and regarding the transistors, I know that the manual mentions LFK4 - something that I hadn't heard of before, but the good folk at UKVRRR came up trumps: http://www.vintage-radio.net/forum/showt...hp?t=57721
But...
I've looked some photos of mine, and they use an OC71 for the bias generator, not an AC127. R13 connects to the junction R11/R12 from memory (can't see in the photos)
All good fun
Mark
It's funny - I had an uncle who was largely responsible for starting me off on electronics - not least by giving me a soldering iron for my 5th birthday! He was an RF person, and worked for Thorn (latterly Thompson) in Enfield, and I believe he had a hand in the designing the tuner in the TX10 and others. He has a callsign, and built all sorts of weird and wonderful RF things. But despite all this, he really didn't understand anything to do with audio! Well, that's what he said - I reckon he was being modest.
I could never get my head around this, because the stuff he did was well beyond my comprehension - still is, really. Audio, to me at least, is much, much easier. Like you Rob, he cited the DC-coupled nature of everything as the problem - yet DC-coupled amplifiers are used in many walks of life beyond audio - video circuits and oscilloscopes, for example... And I know that he designed and built video gear. Still, I guess some of us are just "wired" differently...
Sorry to ramble!
I don't think that a fault in T1 could damage anything. After all, under conditions of overdrive, T1 could be driven into saturation or total cut-off, and if being overdriven resulted in damage, it would be a pretty poor design! Actually, a common fault-finding strategy is to short-circuit base and emitter (to turn it off) or s/c collector and emitter - I do that with a pair of tweezers while observing the midpoint voltage on a 'scope.
The biggest cause of problems is normally a faulty output transistor. The initial failure could be caused by random component failure (maybe tin whiskers!), overheating or over-current. What happens next depends on the design and a degree of luck, but let's assume that one transistor has a collector-emitter short. The midpoint will jump to the rail or ground, depending on whether the upper or lower transistor when s/c - but the rest of the amplifier will see this, and via negative feedback will attempt to bring the midpoint back down, and it does this by driving current into the base of the working output transistor. What happens next depends - the other transistor might pass enough current to bring down the power supply and no further harm will result - apart from perhaps flattening the batteries! On the other hand, if the power supply is a "beefy" one, it will "win", and the other output transistor will fail through excess collector current or overheating...
In terms of further consequential damage, if this was a big hi-fi amplifier with no output capacitor and +/-50 volt rails, your loudspeakers will be somewhat unwell after all this! Decent amplifiers have a protect circuit that disconnects the loudspeakers via a relay should DC offsets appear on the output.
Problems can propagate back down the amplifier too. What tends to happen is that the junctions in the output devices become a great big blob of metal, meaning that all 3 terminals get connected together. So the rails appear at the bases of the transistors - and thinking about it, this is probably how T3 bought it. Oh, and have a look at R14/R15, because these probably cooked during the "event". T2 should have survived, but you never know...
Again, sorry to ramble, and sorry if I'm stating the obvious here - but hopefully all of this reinforces my advice to use a current-limited PSU
I reckon that Joe needs a break from this set, so adding a current limiter to his PSU would be a nice little diversion
Oh, and regarding the transistors, I know that the manual mentions LFK4 - something that I hadn't heard of before, but the good folk at UKVRRR came up trumps: http://www.vintage-radio.net/forum/showt...hp?t=57721
But...
Quote:One thing to check; with at least 2 examples that I've looked at, the biasing arrangements around T3 were different to the manual. Rather than the NPN AC127, there was a PNP something or other... As a result, R13 was connected to a different part of the circuit.
I've looked some photos of mine, and they use an OC71 for the bias generator, not an AC127. R13 connects to the junction R11/R12 from memory (can't see in the photos)
All good fun
Mark
