Thanks for reading the thread Al, and for your comments. Yes, those little computer amp/speakers are excellent in that role, and though it's easy to derride them due to their ludicrous ouptut power claims (50Watts RMS etc!) their reproduction is actually quite repectable. I've got a few which I've picked up at car boot sales for a pound or so, for which you get a pair of reasonable speakers, an amp, and if you're lucky, the 'wall wart' PSU to power them. As you say, a DC blocking cap is essential because if you inadevertently squirt 150V or so into them from say a valve anode, they don't like it up 'em!
As to signal injectors, I've attached a couple I made long ago - one in a Woolworth's icing syringe on a piece of perfboard in the late 1950s in my late teens. It's still going strong as the scope trace shows! I made the other in the late 1980s, built into a felt-tipped marker case with the few components soldered together 'ugly style' so they'd fit into the case, along with a small battery. There are scores of circuits for injectors on 'tinternet. I do have a soft spot for the Vellman K7000 injector/tracer kit as it's so small and neat, and the PCB is self-supporting behind the front panel with the two on-board volume controls.
Over the years, I've found the injector/tracer technique by far the quickest and easiest way to trace faults. I think the quickest fault I traced was on a dead Bush TR82C for a neighbour. A signal injected at the volume control slider showed that Audio amp stage worked fine, and there were tunable signals up to the IF stage at IFT3. It had to be the detector diode and it was. In the TR82, the diode lurks inside the IFT3 can, which I asume was the Bush designer's little bit of mischeivous fun to hide it there as there's ample space for the diode to be out in the open on the chassis. They were having a laugh at our expense, surely! Two minutes to find the fault, ten minutes to replace the OAA90 diode. Without an injector/tracer I could have been faffing around for ages, checking voltage readings and generally poking around at random, suspecting transistors and so on.
As to signal injectors, I've attached a couple I made long ago - one in a Woolworth's icing syringe on a piece of perfboard in the late 1950s in my late teens. It's still going strong as the scope trace shows! I made the other in the late 1980s, built into a felt-tipped marker case with the few components soldered together 'ugly style' so they'd fit into the case, along with a small battery. There are scores of circuits for injectors on 'tinternet. I do have a soft spot for the Vellman K7000 injector/tracer kit as it's so small and neat, and the PCB is self-supporting behind the front panel with the two on-board volume controls.
Over the years, I've found the injector/tracer technique by far the quickest and easiest way to trace faults. I think the quickest fault I traced was on a dead Bush TR82C for a neighbour. A signal injected at the volume control slider showed that Audio amp stage worked fine, and there were tunable signals up to the IF stage at IFT3. It had to be the detector diode and it was. In the TR82, the diode lurks inside the IFT3 can, which I asume was the Bush designer's little bit of mischeivous fun to hide it there as there's ample space for the diode to be out in the open on the chassis. They were having a laugh at our expense, surely! Two minutes to find the fault, ten minutes to replace the OAA90 diode. Without an injector/tracer I could have been faffing around for ages, checking voltage readings and generally poking around at random, suspecting transistors and so on.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
