Ohms readings on speaker output transformer - Printable Version
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Ohms readings on speaker output transformer - Buzz - 10-20-2013
Hello:
I've got a farm radio RCA 24BT-1. All caps and most resistors have been replaced. The audio sounds a little distorted and weak, especially on a close local station. I replaced alll 4 tubes, and there was no difference.
The speaker and transformer look to have been replaced as wires are spliced and taped. The schematic says there should be 440 Ohms across the transformer input. Mine reads 49 Ohms... Would that little difference effect the audio quality?
Thanks to any that reply
Buzz
RE: Ohms readings on speaker output transformer - Radioroslyn - 10-20-2013
Hard to say don't get the resistance of the output transformer confused with the output impedance.The ohm meter won't tell you what you want to know. How's the alignment?? How are you powering it?? Are your voltages what they should be?? The voltages tend to be a bit more critical especially the filament. They also don't play LOUD. The output is only good for maybe 150mw or so undistorted.
Terry
RE: Ohms readings on speaker output transformer - Buzz - 10-21-2013
The alignment was done according to specs. This is being powered by a battery eliminator kit from Antique Electronic Supply. The "A" voltage for the filaments is spot on @ 1.5 V. The 90 V "B" voltages Measures 87 V.
How to you measure impedance? or can you?
Buzz
RE: Ohms readings on speaker output transformer - morzh - 10-21-2013
Buzz
What sch tells you is probably indeed the resistance which is 140 Ohm. Yours is 49 Ohm. This is OK. In this case the sch does not rely on your DC primary resistance to set the working point.
You should worry about the total impedance of the primary, as Terry suggested. The impedance is a complex thing but in first approximation you could consider it to be your voice coil impedance reflected via the transformer which is simply multiplying it by the square of the turn ratio.
For example if your voice coil is 0.7 Ohm and your turn ratio is 100 then your reflected impedance is 0.7x10,000 which is 7kOhm.
Potentially (not necessarily in your case) the impedance mismatch may cause:
1. Loss of power (the impedance should be matched for the optimal transfer of power from the tube to the voice coil, this is in fact why we use the output transformers - to match impedance).
2. If the impedance is badly out of range the working zone of the tube may slide to the non-linear zone, hence distortion. Again, may or may not. There is range where the device is linear and it is not that narrow that a step left or right results in distortion.
3. And of course, there are inductive part that coupled with other elements (parasitics or intentionally used capacitances etc, plus the output impedance of the tube) affects the gain on various frequencies, so you may have less of some and more of some others. This is in small radios is not that important as the quality of the sound in the first place is not, well, exactly Hi-Fi, plus it can be corrected somewhat with tone control.
the easiest thing is first to look at your audio signal quality using an audio gen and a scope (or simply look at the received sound with the scope - see if it is distorted - clipped, clipped unevenly, etc), and see where (at what stage) it gets the distortion.
RE: Ohms readings on speaker output transformer - Radioroslyn - 10-21-2013
So Buzz That's why why I didn't answer the Impedance question. I'm no rocket scientist but I think Mike IS.
Instead of using a scope you could use a signal tracer and hear the distortion but it maybe a bit easier to see on a scope. Don't know if you have a scope. Signal tracers are a bit cheaper and simple to use.
GL
Terry
RE: Ohms readings on speaker output transformer - exray - 10-21-2013
I'm reading the schematic as 440 ohms. They put these resistances on schematics as a seat-of-the-pants reference for checking the transformer. You don't need to know the impedance unless you are searching for a replacement and then that info can generally be extracted from a tube manual.
Any possibility you are off by x10 reading the wrong scale of your meter? Something is definitely not right if you are only measuring 49 ohms. I would suspect a bad transformer if that's truly the case.
RE: Ohms readings on speaker output transformer - Buzz - 10-21-2013
You're right exray,, its 440 ohms on the schematic.. The 140 ohms was a typo. I've since corrected it.
My meter is fine, tried on a VTVM too.. 49 or 50 ohms. This radio uses a 3Q5GT tube. The voice coil measures 3.3 ohms.
Maybe someone put in a speaker and transformer that didn't match..I dont know..
I know this is a cheap radio, but the case is perfect, so I don't mind if I spend a few bucks to get it to play at its best. Any ideas on where to get a speaker and transformer set, or maybe just output transformer..
Dont know what to do...any suggestions?
Buzz
RE: Ohms readings on speaker output transformer - morzh - 10-21-2013
It is rarely that a transformer will develop a case of "1/10-resistanseism", it is likely to be open or have a turn short (one or a few turn inside develop a short across which defeats the transformer physics, at the same time your DC resistance measurement may look OK).
So it is a good chance that like you said someone put a replacement inside.
SIgnal tracer is not a bad idea either. It will allow you to pinpoint the stage where the distortion occurs. Of course scope allows to see WHY it occurs. But then a tube stage is a simple one so once you know where it is, it is only so many things that could go wrong with it.
RE: Ohms readings on speaker output transformer - Buzz - 10-21-2013
Hi morzh;
I dont have a signal tracer , but I have a signal generator. Would that help? LOL prob not.
Don't have the funds to purchase expensive equipment like a scope, although I've used them during my working years,but no expert on them.
Just wish i had some old speakers and output transformers lying around to alligator clip together and test.
Buzz
RE: Ohms readings on speaker output transformer - codefox1 - 10-21-2013
I'm sure you can scrounge or borrow a known good tube output transformer and speaker to hook up and see if this is the problem. Might play a little softer if the impedence is not a match, but won't harm anything.