25-07-2011, 08:07 AM
High on my list would be a simple signal injector/tracer, a capacitor tester/reformer, and an ESR meter, all of which are easy and enjoyable home-brew projects, on which circuits and contructional details abound.
When I get a dead set, after all the usual preliminary tests, I use an injector/tracer to inject a signal into the wiper of the volume control. Good volume of the signal out of the speaker suggests that the audio stages are working. I then apply the tracer to the wiper, and no signals there suggests the problems are in the IF/RF stages, so I work back towards the earlier stages checking for signals at the grids of the valves (or base of transistors) until the faulty stage is located. If there's no sound out of the speaker when a signal is injected into the volume control wiper, but when the tracer is applied to the wiper, you get good tunable signals, that means the RF/IF stages are working, but the audio stages are not.
On a transistor, if you have a sigal on the base of a transistor, but not on the collector, or at the input to a detector diode, but not on the output, they are suspect. On transistor radios which have earpiece sockets, as most do, they are high on the list of suspects. If the contacts are tarnished and open circuit, due to having been sprained by the insertion of an earpiece jack, the set will appear dead. A check on the input with a signal tracer may reveal loud signals, but no signals on the other side of the socket. I wonder how many restoers have seen AFxxx transistors, and assumed that they're duff, yanking them out, only to find that they're fine. After hours of frustration, if they wever do get the set working, they'll find that the earpeice socket was the culprit, and could have been found with a signal tracer in two minutes.
There's a bit more to it than that, but not much more.
Lots of stuff on 't'internet about signal tracing, inluding on youtube. Just google 'radio fault-finding+signal tracing' and see what pops up.
I do have a valve tester - a Taylor 45D which I bought for a tenner at a radio rally about 20 years ago after it had been ignored all day, but that was when such things were seen as obsolete junk, and prior to e-bay sending prices into the stratosphere. They're interesting, indeed fascinating, to use, and for many, a 'must-have' item, but they're never ever going to recoup their high initial costs for most owners. Scopes too are interesting bits of gear, but again, for restoring radios have very limited use, and many owners seem to want to buy complex expensive scopes such as ageing Textronics, which have a capacity for measurement beyond the capabilties of their owners. More use on TVs where investigating waveforms is useful, if not essential, but not a lot of use on radios.
There are of course many more scopes around than there are vlave testers, so comparatively speaking, scopes are good value for money. I've been amazed recently, to discover that basic signal generators appear to be fetching eye-watering prices on internet - £150 - £250. Again, for what we are about, a basic one can be built for under £20.00, for what little use it will have. I don't often find in practice that the cores of tunnig or oscilator coils or IFTs have been messed about with, and most seem to stay on tune for the life of the radio.
Just my off-the-cuff thoughts.
David
When I get a dead set, after all the usual preliminary tests, I use an injector/tracer to inject a signal into the wiper of the volume control. Good volume of the signal out of the speaker suggests that the audio stages are working. I then apply the tracer to the wiper, and no signals there suggests the problems are in the IF/RF stages, so I work back towards the earlier stages checking for signals at the grids of the valves (or base of transistors) until the faulty stage is located. If there's no sound out of the speaker when a signal is injected into the volume control wiper, but when the tracer is applied to the wiper, you get good tunable signals, that means the RF/IF stages are working, but the audio stages are not.
On a transistor, if you have a sigal on the base of a transistor, but not on the collector, or at the input to a detector diode, but not on the output, they are suspect. On transistor radios which have earpiece sockets, as most do, they are high on the list of suspects. If the contacts are tarnished and open circuit, due to having been sprained by the insertion of an earpiece jack, the set will appear dead. A check on the input with a signal tracer may reveal loud signals, but no signals on the other side of the socket. I wonder how many restoers have seen AFxxx transistors, and assumed that they're duff, yanking them out, only to find that they're fine. After hours of frustration, if they wever do get the set working, they'll find that the earpeice socket was the culprit, and could have been found with a signal tracer in two minutes.
There's a bit more to it than that, but not much more.
Lots of stuff on 't'internet about signal tracing, inluding on youtube. Just google 'radio fault-finding+signal tracing' and see what pops up.
I do have a valve tester - a Taylor 45D which I bought for a tenner at a radio rally about 20 years ago after it had been ignored all day, but that was when such things were seen as obsolete junk, and prior to e-bay sending prices into the stratosphere. They're interesting, indeed fascinating, to use, and for many, a 'must-have' item, but they're never ever going to recoup their high initial costs for most owners. Scopes too are interesting bits of gear, but again, for restoring radios have very limited use, and many owners seem to want to buy complex expensive scopes such as ageing Textronics, which have a capacity for measurement beyond the capabilties of their owners. More use on TVs where investigating waveforms is useful, if not essential, but not a lot of use on radios.
There are of course many more scopes around than there are vlave testers, so comparatively speaking, scopes are good value for money. I've been amazed recently, to discover that basic signal generators appear to be fetching eye-watering prices on internet - £150 - £250. Again, for what we are about, a basic one can be built for under £20.00, for what little use it will have. I don't often find in practice that the cores of tunnig or oscilator coils or IFTs have been messed about with, and most seem to stay on tune for the life of the radio.
Just my off-the-cuff thoughts.
David
