I have a chance to bargain buy several military spec input transformers that the owner says were made for similar use as the best of the old UTC input and matching transformers. Thats all he knows about them and no documents come up about them. How would someone go about reverse engineering an input transformer? Start with an ohmeter I suppose just to try to get a grouping of the coils and pins and ratios. Then maybe hook up an LCR meter to read inductance of coils. Then maybe go to the signal generator and scope to know more about bandwidth and quality? Then finally put music through it? Is it possible to experimentally "back into" what the original schematic and specification data sheet would have been? How to do it?
Pictures will be nice, one of each type, showing name plate and terminals.
Use a small 3 to 6V AC source if needed later to find the winding ratios.
As there will possibly be high voltages, take proper safety measures..
Use a small 3 to 6V AC source if needed later to find the winding ratios.
As there will possibly be high voltages, take proper safety measures..
Last edited:
Electro Networks was a military contractor apparently, no documentation is known.
Understood yes high voltages, and flyback voltages/pulses that can fry your signal generator! I've already permanently damaged a newer digital function generator testing unloaded input transformers. But my old 1970's Leader analog signal generator handles all that abuse just fine, it actually makes cleaner waves than the Rigol function generator I cooked.
Understood yes high voltages, and flyback voltages/pulses that can fry your signal generator! I've already permanently damaged a newer digital function generator testing unloaded input transformers. But my old 1970's Leader analog signal generator handles all that abuse just fine, it actually makes cleaner waves than the Rigol function generator I cooked.
These may be specially made for military radios, radar, sights and so on. May not be useful in audio.
Wait for other members to comment.
Wait for other members to comment.
You'll be able to figure out the turns ratio approximately from measuring DCRs. DCR will scale roughly proportional to inductance and no. of turns goes as sqrt(inductance). With turns ratio in hand you'll then know if high voltages are a likelihood.
DCR, inductance, and turns ratio are easy tests. But other parameters may be more difficult to determine.
If inductance is high (e.g. >100 H), but DC resistance is low (a few hundreds of Ohms), it is likely that a transformer has ungapped high permeability core. Such transformer may have good broadband capability, but will not allow even small DC.
If there is a center-tapped winding, one can judge on winding symmetry by DC resistance. When both halves have close DC resistances, symmetry should be good. Same applies to separate windings of equal AC voltage.
Terminals may have indication of transformer connections, like B, P, F, G, ground. If there are numbers, lower numbers usually correspond to primary, and higher numbers to secondary. From ratios and numbers it may be possible to guess what kind of transformer it is: input, output, mixing, etc.
Low frequency performance is largely determined by inductance. Knowing inductance, it is easy to calculate -3 dB low frequency point at a given source impedance. Maximum low frequency level can be roughly estimated by transformer weight and size.
A good test for high frequency performance is observing square wave on a scope, with secondary unloaded. Frequency of ransformer ringing on crests and troughs, indicating self-resonance, can be determined from the known fundamental frequency. Self-resonance frequency sets the limit of what can be expected of HF response.
Hope your transformer is something useful. Many military transformers were purposely designed with limited bandwidth (about 300 - 3,000 Hz), which improved selectivity of reception without compromising intelligibility of speech.
If inductance is high (e.g. >100 H), but DC resistance is low (a few hundreds of Ohms), it is likely that a transformer has ungapped high permeability core. Such transformer may have good broadband capability, but will not allow even small DC.
If there is a center-tapped winding, one can judge on winding symmetry by DC resistance. When both halves have close DC resistances, symmetry should be good. Same applies to separate windings of equal AC voltage.
Terminals may have indication of transformer connections, like B, P, F, G, ground. If there are numbers, lower numbers usually correspond to primary, and higher numbers to secondary. From ratios and numbers it may be possible to guess what kind of transformer it is: input, output, mixing, etc.
Low frequency performance is largely determined by inductance. Knowing inductance, it is easy to calculate -3 dB low frequency point at a given source impedance. Maximum low frequency level can be roughly estimated by transformer weight and size.
A good test for high frequency performance is observing square wave on a scope, with secondary unloaded. Frequency of ransformer ringing on crests and troughs, indicating self-resonance, can be determined from the known fundamental frequency. Self-resonance frequency sets the limit of what can be expected of HF response.
Hope your transformer is something useful. Many military transformers were purposely designed with limited bandwidth (about 300 - 3,000 Hz), which improved selectivity of reception without compromising intelligibility of speech.
Last edited:
I would be carefull with DCR measurements. Keep the test current very (I mean VERY) low to avoid magnetizing the core.
It would be better to measure the ratio with an ac signal.
Jan
It would be better to measure the ratio with an ac signal.
Jan
Refer to https://www.diyaudio.com/community/...nknown-transformer.391894/page-3#post-7174583
Post #41. Roughly same question.
Post #41. Roughly same question.
Just how much current are you putting through your digital meters? If you did accidentally magnetize a core, wouldn't it get de-magnetized by the audio signal?