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Post by Admin on May 7, 2021 23:10:31 GMT
I have a Metzo AM transmitter. It quit working a few years ago and has been in limbo. Trying to swap out some transistors, the traces lifted off the board and wasn't wanting to spend anymore time on it.
Fast forward to the other day, decided to give it another try. With no board layout and an iffy schematic I dove in.
Trial and error, I managed to figure it out and got it working.
Never did put out much signal so it's officially a Yardcaster. The good thing is it's off the to do pile.
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Post by Boomer on May 8, 2021 4:02:27 GMT
I have the Ebay cheapie knockoff version of the Metzo, and mine has the issue that I zapped one of the CMOS chips. It was static, but it shouldn't be, regular static electricity shouldn't zap a mounted chip, I might have reversed the power leads. One of the chips gets warm, and there's no signal.
No wait.. Looking at the Metzo schematic, it's not like my more simple Ebay one, it's more like the SSTRAN 3000, a lot like it in fact, with the differential transistor pair in the RF section.
Going by the manual, it's a low impedance output, 50 ohms. Here's what my manual says to do for more range:
-------------------------------------------------------------------
Aerial Tuner and Loading Coil
Most homebrew Antenna contain some reactance and have an impedance other than 50 ohms. A
device called an Antenna tuner (also known as an "ATU") will transform your Antenna system into a
non-reactive load of the proper impedance.
Since the actual wavelength at AM broadcast frequencies is over 950 feet, and the Metzo uses an
aerial length of only nine feet, you may want to experiment with a “loaded” Antenna (Aerial) for better
performance.
Using an 9 foot length of wire, a series inductance of about 300-400 µH is required to properly load the
circuit.
-----------------------------------------------------------------
Boomer
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Post by Admin on May 9, 2021 0:23:41 GMT
The Metzo I have has an ATU built-in. There are two sets of shorting pins arranged as a set of 3 and a set of 4. There are 3 combinations for tuning frequency bands: 500-800 kHz, 800-1200 kHz and 1000-1700 kHz.
There is another set of shorting pins to select internal ATU or use external tuning.
Using the wire antenna with the built-in ATU, the set of 3 and the set of 4 are both on position 1 for my 1500 kHz frequency.
Unfortunately, the schematic I have, although similar, is not identical to my Metzo. In particular it does not show the ATU.
The output stage is like the Wentzel circuit if you a familiar with that transmitter design.
External controls are volume, compression and limiter.
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Post by Boomer on May 11, 2021 11:04:37 GMT
Yes, I'm familiar with Wenzell from Techlib, I like that circuit and his writings about it, told with a touch of good dry humor. I guess Wenzell was the first to make the differential amp circuit popular as a transmitter final, then it seemed transmitter manufacturers saw its good merits and included it in their units, SSTRAN 3000, now I know about the Metzo, and a few other homebrew ones used it.
The diff amp was an improvement on the previous circuits that used a modulation transformer that might not have flat response, and LM386 modulator chips in circuits that might not be able to hit full modulation. After the diff amp came high efficiency FET finals with the SSTRAN 5000, Procaster and Rangemaster going to those. They're good because you can use a regular 12 volt supply, not 15 volts, because the FET final can get 100 milliwatts regulation power from 3-5 volts on it.
The schematic and instructions I have were possibly found right here in Part 15 Engineer's posts that I grabbed every one of when they went up! The schematic doesn't have parts values, and the output doesn't have much of an ATU, just a tapped coil and variable capacitor on the output. In the handbook I see it has settings for the internal ATU jumpers.
It's good to try and figure out, the Metzo should have the range of any other Part-15 AM transmitter, like the Talking House. You may be able to use some of the other circuits as guides to what voltages you should be seeing on the Metzo.
Boomer
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Post by Boomer on May 12, 2021 19:22:02 GMT
SourcesOriginal AM Micropower Transmitter (Wenzel) "The transmitter legally operates with "micro-power" and will not set any distance records but, unlike simpler designs, the frequency stays put and the fidelity is excellent." techlib.com/electronics/amxmit.htmSSTRAN AMT3000 Low Power AM Broadcast Radio Transmitter Kit Assembly and Operating Instructions, 2004. www.radiomanual.info/schemi/Surplus_Radioamateur/SStran_AMT-3000_user.pdfI know about one ALPB member who has built their own version of the AMT3000, so the schematic works. The diagram is on the last page of the manual. I haven't heard much about the Metzo, does anyone have information on the transmitter's origins or the company bio? Boomer
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