Those of you not into technical stuff might find this boring, but I share it here for my fellow tech/engineer types who might enjoy reading of the troubleshooting and repair of a transmitter.
I recently picked up a couple of AM-30 transmitters on eBay. One was in near-mint condition, super clean inside and out. It was as if it had never been used except there was a lead connected to the input terminals. Perhaps it was operated once to test, and then reserved as a backup/spare? Anyhow, I connected it to a dummy load and an audio source and powered it up. It put out its rated power and sounds good. SCORE!
The other AM-30 was a different story. I considered that it might have a problem from the photos as the AC line fuse was missing, but I took a chance anyway.
Right off the bat it looked like someone had replaced the voltage regulator transistor. Upon closer investigation, I saw that the insulating washer (which isolates the transistor tab, which is connected to the collector and is at power supply potential, from the chassis) had been omitted. Perhaps it would be that simple? Nope. Further inspection revealed that the wires for the emitter and base had been crossed. The previous 'technician' (who didn't know how to properly mount the device) apparently was trying to install it as shown in the schematic, which, BTW, is incorrect. In fact, having three leads, there are exactly three ways to connect a transistor. There is the correct way, which is not too difficult for someone familiar with basic power supply circuits to determine. There is the incorrect way shown in the schematic. And this one was connected in the remaining, also incorrect way.
Surprisingly, the transistor survived the incorrect wiring unscathed, but there remained a dead short in the power supply. That turned out to be a shorted bridge rectifier. I just happened to have one around.
With the bridge replaced, it no longer blew the fuse. Power supply voltages seemed reasonable, and it made RF. As I slowly increased the power, though, the power supply voltage began to sag and I began to notice quite a bit of hum, which got worse as I increased the power. At about half-power the audio began to get severely distorted.
Have you guessed yet? It was the main electrolytic filter capacitor for the 30-volt supply. Possibly just dry from age, or maybe stressed from the shorted bridge (the safety vent looks like it may have let some of the Magic Smoke escape at least once).
In any case, I sumped it with a good one and now the voltage holds steady and teh signal is clean -- free of hum and distortion, even a full power. YAY!
Still has a minor niggle or two - the on-board meter works for power output, but not for audio level. That needs looked into, but even if unrepairable is only a minor inconvenience.
I'd have probably been disappointed had I paid the asking price, but for what I actually paid via "Make Offer" I guess I did OK, and it's always rewarding to successfully repair something and return it to useful service.
Tomorrow I'll connect up one of the 'new' (to me) transmitters and TCU-30 coupler to see how the range and performance compare to my Radio Systems TR20/CP15
It's refreshing to hear from someone who obviously knows what he is doing. This is very helpful to everyone here.
How about some specifics about your experience with distance coverage with AM carrier current? That would be very helpful for those who might want to venture into that realm.
It's only been a few days since you received this treasure find, and already you have taken control and gotten past the fixup stage! Excellent!
I absolutely enjoyed reading about the steps taken to solve the troubles.
Please blog the heck out of this, keep us up to date.
I will be slowly catching up with my two broken LPB CC transmitters, and your example will prod me on.
I have to come up with better words
to say for things like this.
Anyway - yeah - this is exciting. I'm
sure we will all want to follow this.
Bruce, DOGRADIO
I'd be delighted to share my experience. Where? Perhaps the 'blog' section of this forum? I notice there isn't a 'carrier current' board - that might be cool.
Am I the only active carrier-current station on this forum? It seems that even while I am still in learning mode myself I am becoming the carrier-current 'guru' around here.
What a difference 140 KHz makes ....
I'll go into a bit more detail when I have a few more minutes to write, but here's the Reader's Digest version:
I connected up the newly repaired LPB transmitter and an LPB coupler. This is on 610 KHz. I got about double the range that I was getting with my Radio Systems gear or 750 KHz. While the LPB is 30 watts and the RS is only 20, a 50% increase in power wouldn't seem to account for a doubling of range.
Long story shorter -- I remembered I had a crystal for the Radio Systems for 600 KHz, so I popped that in, and tried the Radio Systems transmitter and coupler on 600 and got nearly the same range I got with the LPB on 610. Slightly weaker signal at the farthest points; about what I'd expect for the 33% less power the RS puts out.
So... that proved that the biggest gain was a result of using the lower frequency! And the signal is well-behaved, as a carrier-current signal should. Beyond the near field of the transmitter and wiring closest thereto, I found that as I move away from the power line, the signal drops off sharply, becoming unusable somewhere between 50-100 feet from the line.
So for the time being, I'm now on 600 with the Radio Systems transmitter/coupler, if only because they are more gracefully installed, though I'll likely switch to the LPB for the extra bump in power and the fact that it has a larger heatsink and therefore runs cooler than the Radio Systems, and maybe I'll be able to run it without a fan.
There has been plenty of good talk over the past several years about Carrier Current.
My station was active for about a year, but is off now because of illness in the transmitter.
The guru a few years ago who helped me make it all work was RFBurns, a guy who knows carrier current right gown to the size bolts used to hold the equipment together. He was my guest on three editions of The Low Power Hour, talking at length about carrier current. You can scroll around to find those shows at
RFBurns is now the regional engineer for a chain of licensed stations all over Wyoming, but he is helping me now get my new 5Watt LPB AM-5 up and running. I will be sharing what I learn as part of MrNatural AZ's blog.
Another guy with many CC experiences is Bruce DOGRADIO, who is still working on designing his own coupler.
PhilB asks about range. I had been getting up to 1/2 mile on 750KHz. Now, on 600, I can get up to a full mile along the power lines. I imagine if it was only a single straight run it might go much further, but I notice that at each point where the line branches off or where a transformer/house connects, it drops off just a tiny bit. OTOH, there are some oddities that I attribute to teh "smoke and mirrors" of radio propogation, such as the slight increase experienced at each branch or transformer connection as I pass that actual point, and the occasional dead spot along the line, or tiny pockets of coverage that seem to mysteriously appear well outside the "normal" contour.
I've mentioned this before, but it's
interesting, so I'll just mention it again.
I knew my signal was really inside the
power line when I was at my neighbor's
house about 15 houses down the street.
I had a battery powered radio (unfortunately)
- the only reception of my signal was on one
side of the livingroom, where the power line
came into the house. I wish the radio could
have been plugged into the wall AC. I would
have found out more about how the signal
really travelled in the house wiring.
Bruce, DOGRADIO
A week ago i was browsing Ebay and saw a very nice LPB coupler that had been modified with a heavy duty ac power cord and a 3 prong plug in place of the hard wire directly into a breaker box. I never thought of that.
Now i wish i had the green backs to buy that coupler along with a lpb transmitter.
Anyway, gave me something to consider.
I am guessing that the 3-prong plug seen by rock95seven was a 220VAC type plug, and I did that when first installing my LPB carrier current system.
I plugged into an existing 220VAC circuit that had served a laundry dryer machine.
The result was poor, BUT when I wired directly into the AC panel the result was just as bad.
In fact, my system refused to work well going into the hots, feeding both legs of the 220.
I ended up with a neutral fed hookup, which has worked very well.
In the future I will try a different coupler into the 220V hots to try and learn why the first hookup worked so badly.
By "badly" I mean that the SWR (standing wave ratio) could not be reduced into the safe area.
I noticed some people who have had
sucess with carrier current have fed the
neutral and connected the ground lead to a stake
buried in the ground outside.
Let me see if I can say this correctly.
That means there is a significant potential
difference between EARTH ground and neutral, so
the transmitted energy feeds the line effectively.
But if one feeds the nuetral and then just
connects to the power line ground, then there
is a different situation. (I think?) I think the
potential difference between nuetral and
ground is not as significant. The nuetral and
ground are next to each other, and the
electromagnetic field is coupling between
both wires. In house wiring (and up on the
powerlines too?) there isn't much potential difference.
And also - these wires are not shielded.
So this doesn't work as well as the
first approach with the earth ground
stake being connected to transmitted
ground.
Well, I'm just putting it out there.
Bruce, DOGRADIO
In the case of the neutral injection method of carrier current, the "official" instruction sheet printed from LPB specifies having a separate self-installed ground rod, so I think you are probably right MICROdog.
Yet, the standard method of delivering into the HOTS with the ground-side to the neutral side is considered the ORIGINAL way of doing it, and is expected to tune up with a coupler so that range is improved and standing-wave reduced.
However, given our own small and local experience, we note that all the literature says that results are extremely different from one case to another because of how electric lines are all different lengths and have different loads... there is no such thing as a "perfect" electrical line situation.
In the attempt to be able to adapt to such a wide range of conditions the professional couplers have a very wide-range of settings. But when NONE of the available settings achieve a decent tuning we are stuck with a problem.
In most areas the neutral and ground are tied together in the breaker box. This is called bonding. Ideally there is no voltage difference between earth ground and the ground pin of an outlet. There can be a voltage difference between the neutral and ground pin due to load current in the neutral.
Using neutral injection with an isolated ground rod provides an RF impedance between the two (neutral and earth ground) which allows the RF signal to develop. Otherwise, using the AC ground and neutral would present more of a short circuit to the RF and little of the signal would develop.
I was hoping I would hear from you.
And CARL, At first I didn't see your post,
I appreciate yours also!
Very best wishes.
Bruce, DOGRADIO