The title of this thread maybe should be "Things Are Not Always as They Appear" but that would cover about everything and this thread is about an audio chain hum problem which was a bit tricky to track.
The situation was that hum appeared in the AM and FM audio chains which are separate but can be switched together to simulcast. Each has a NanoCompressor immediately before the transmitter to limit modulation to the proper levels and normally the FM chain is fed by the cable box to housecast music for private use and the AM is fed by the computer. It was noted that when the two chains were connected together so the source was the computer then noticeable hum appeared in both channels. When operated separately, the hum was only in the FM signal.
The thing which made this tricky to diagnose was that the hum was in both AM and FM when operated together which would lead one to suspect the hum source was common to both so the search began with the computer audio output. By using an audio amplifier to signal trace it was found that the hum was present at the computer audio output but only when connected to the transmitters and it was also observed that the hum source was somewhere in the FM chain and was feeding back into the AM chain and the computer output signal.
More patching and testing indicated the hum source was the INPUT to the NanoComressor on the FM side and this was conducted back toward the computer source and into the AM chain.
The NanoCompressor hood was popped and some basic troubleshooting was performed. The unit is powered by a 9.5 VAC transformer and the unit contains voltage doubler circuits which boost this to +/- 17 VDC and these are then regulated down to +/- 15 VDC.
A voltage doubler circuit uses a series connected capacitor and these get pounded fairly hard with each cycle of the AC and thus are prone to fail. These capacitors were replaced and the hum was gone.
It was noted that the 17 VDC regulated down to 15 VDC doesn't give the regulators much head room to eliminate ripple on the raw 17 VDC so the supply transformer was replace with a 10.5 VAC unit in case the line voltage happens to be low.
What was surprising (at least to me) was that the hum was fed back from the input of the compressor to the other inputs which is usually not the case. Generally, this type of failure affects only the output of the audio device but not in this case.
The hum is now inaudible and the case is closed.
Neil
Radio8Z sets a good example about not assuming what the hum is before exploring further, which is what I've been doing with the C.Crane FM Toy Transmitter.
The Crane is used for my audio editing, but the audio channel has a chronic hum which I've always presumed to be a ground loop. But is that what it is?
Since I don't think Radio8Z will be here inside the Internet Building anytime soon, I need to tackle this myself.
It's time to do what he did, and track the hum from point A all the way to point Z, to find out once and for all if it really is a ground loop.
If it turns out to be a ground loop, there are corrective measures that can be taken.
If we find the ground-loop is trapped inside the Crane device, which could be the case, getting these apart takes Chinese mysticism and Houdini, since paranoid patent security tries every trick to keep the lowly consumer out of his own micro-transmitter. The case could get cracked and require gluing.
Those kind of concerns didn't stop Radio8Z from finding his hum, so I need to get strong and do the job.
Starting next week.
Aha! The Radio8Z method of depth checking is already paying off.
If I had been right about the hum in my C.Crane FM transmitter being an audio ground loop, then turning the input level control all the way off should close the hum path and bring silence. Agreed? But it doesn't change the hum at all, which means the hum is inside the transmitter!
Theory No. 2 has just popped in my head, and it is ....... possibly defective power supply!
To test Theory No. 2 I will search for a safe replacement supply and be back to report the result.
A quick test would be to put some batteries in the C.Crane and see what happens. If the hum goes away, put an isolation transformer between the C.Crane and your audio source and reconnect your external power supply. If it comes back, it's the wall wart..
Good find, Neil..
Carl wrote: "...If I had been right about the hum in my C.Crane FM transmitter being an audio ground loop, then turning the input level control all the way off should close the hum path and bring silence..."
If the source of the hum is in the audio chain and is other than a ground loop then this test will zero out the hum so this doesn't differentiate between the ground loop and another audio chain source.
A very common source of hum in an FM signal is the RF getting into the power supply and/or the transmitter circuitry. I have ALWAYS had hum with all of my FM transmitters unless they are in a metal box with a ferrite clamp-on around the power cord. This hum is usually, but not always, present when the audio cable is unplugged so this is something to try. This hum would still be present with the tx. audio input gain set to zero.
Let us know what you find so we can learn.
Neil
The good tips on trouble-shooting are excellent, and I will follow them as I proceed.
To 12V Man, wow, ya, BATTERIES! Obviously. I actually never thought of that. I can do that tonight.
And Neil, you have triggered a deja vu feeling by mentioning wrapping the power wire around a ferrite clamp-on, because right next to the C.Crane Transmitter sits a receiver for my wireless microphone which has a ferrite clamp on the power cord, let me explain why.
While we were designing the Big Talker Shortwave Transmitter, I had it on the air in this same room with a dipole antenna reaching the length of the house, front to back, right up under the ceiling. But when I used my wireless mic, at 184.3mHz, I could hear everything being broadcast at 13.560mHz from the Big Talker.
The simple solution was the ferrite on the power cable. It blocked the RFI completely!
More later.