30-06-2015, 07:11 AM
(This post was last modified: 30-06-2015, 07:24 AM by Radio Fixer.)
Well it to me would be another box to have to find room for Nick. But its always nice to build something.
'Old timers' just use a signal generator and a scope for audio transformers. It could also claim to be a better way of doing it as its easy to check frequency response at the same time. Something that can be forgotten when doing this is to include a suitable resistor to mimic the Ra of the valve driving the transformer. Without it the low frequency response will look fantastic. But with it the amount of primary inductance will show up and the response tail off as the frequency from the generator goes down. My scope even has a dB scale for easy measure of the 3dB down points if comparing transformers.
For mains transformers I simply hook them up to the supply and measure the unloaded windings (even the primary taps can be checked if they are easy to get at). The primary current should be minimal when doing this, if not its an indicator of shorted turns. In rare occasion I might load the secondary windings with resistors, if its an unknown transformer that I intend to use in something. If it runs cool at the sort of currents I might need then it could do the job. Normally of course it will be out of a radio and would just go back there and see what happens. If I can keep my fingers pressed firmly on it after its been on an hour then OK, if not its over 50C and I would do something about it. Its becoming usual to me to have to add metal clad resistors (bolted to chassis underneath and out of sight if possible) in the primary as the mains voltages are running here in the high 240s. Of course the Test Set would be safer for the less experienced and measuring anything directly connected to the mains then care is a must. An isolation transformer can be used.
Have fun with your build. If you don't have them yet then look out for a quality scope and a couple of generators (an audio and an RF are vey useful with FM on the latter if you think you will work on those radios a lot. For the odd occasion than an AM genie can be made to do the job).
Gary
'Old timers' just use a signal generator and a scope for audio transformers. It could also claim to be a better way of doing it as its easy to check frequency response at the same time. Something that can be forgotten when doing this is to include a suitable resistor to mimic the Ra of the valve driving the transformer. Without it the low frequency response will look fantastic. But with it the amount of primary inductance will show up and the response tail off as the frequency from the generator goes down. My scope even has a dB scale for easy measure of the 3dB down points if comparing transformers.
For mains transformers I simply hook them up to the supply and measure the unloaded windings (even the primary taps can be checked if they are easy to get at). The primary current should be minimal when doing this, if not its an indicator of shorted turns. In rare occasion I might load the secondary windings with resistors, if its an unknown transformer that I intend to use in something. If it runs cool at the sort of currents I might need then it could do the job. Normally of course it will be out of a radio and would just go back there and see what happens. If I can keep my fingers pressed firmly on it after its been on an hour then OK, if not its over 50C and I would do something about it. Its becoming usual to me to have to add metal clad resistors (bolted to chassis underneath and out of sight if possible) in the primary as the mains voltages are running here in the high 240s. Of course the Test Set would be safer for the less experienced and measuring anything directly connected to the mains then care is a must. An isolation transformer can be used.
Have fun with your build. If you don't have them yet then look out for a quality scope and a couple of generators (an audio and an RF are vey useful with FM on the latter if you think you will work on those radios a lot. For the odd occasion than an AM genie can be made to do the job).
Gary
