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Newbie homebrew qrp ....update
Regarding the 3-watt transmitter I'm building (and having trouble
with), here's an update. I removed the lowpass filter and loosened the wiring, and for a while it seemed to help. The transistor didn't get as hot. But the output stayed the same (less than 1 watt). But then I noticed, as I've noticed before, that the voltage measured at the dummy load doesn't always drop when the key is released. And the transistor stays hot even after the key activity has stopped. I did as one poster suggested. I removed the antenna from my receiver, tuned around while transmitting, and discovered the following: (Crystal is 7110 kHz) at 3554 kHz, signal is S8 at 7110 kHz, signal is S6 at 14218 kHz, signal is S6 at 21326 kHz, signal is S7 at 28433 kHz, signal is S5 Even I can see that this isn't good. And the emitter of Q2 connects to the same trace as everything else that's grounded in the circuit, and it's not a very big trace. I'm probably going to rebuild this rig, without the circuit board. Many thanks to all who gave me advice on this. Any further comments are also much appreciated. I put the schematic on my website. The design calls for 28 VDC for a 5-watt output, but I am using 12, which is also ok according to the article. http://www.kcnet.com/~oyster/transmi...ansmitter.html -John Sandin KC0QWE Remove the "T" to respond by e-mail |
In article ,
says... I did as one poster suggested. I removed the antenna from my receiver, tuned around while transmitting, and discovered the following: (Crystal is 7110 kHz) at 3554 kHz, signal is S8 at 7110 kHz, signal is S6 at 14218 kHz, signal is S6 at 21326 kHz, signal is S7 at 28433 kHz, signal is S5 I'm probably going to rebuild this rig, without the circuit board. Many thanks to all who gave me advice on this. Any further comments are also much appreciated. Build it "ugly style" on an unbroken sheet of copper-clad board, and try a ferrite bead or two in the base and/or emitter leads of the transistors. The FAA and your local cellular-phone company will thank you. :) -- jm ------------------------------------------------------ http://www.qsl.net/ke5fx Note: My E-mail address has been altered to avoid spam ------------------------------------------------------ |
John Miles ) writes:
In article , says... I did as one poster suggested. I removed the antenna from my receiver, tuned around while transmitting, and discovered the following: (Crystal is 7110 kHz) at 3554 kHz, signal is S8 at 7110 kHz, signal is S6 at 14218 kHz, signal is S6 at 21326 kHz, signal is S7 at 28433 kHz, signal is S5 I'm probably going to rebuild this rig, without the circuit board. Many thanks to all who gave me advice on this. Any further comments are also much appreciated. Build it "ugly style" on an unbroken sheet of copper-clad board, and try a ferrite bead or two in the base and/or emitter leads of the transistors. The FAA and your local cellular-phone company will thank you. :) -- jm I didn't look at the pictures, but someone said something about long leads to coils and maybe variables. Sometimes it makes sense to make the circuit board used for the "ugly construction" the front panel, or at least mount it flush with the front panel. Then, all the controls can be mounted on the circuit board, near where they are connected and with the actual shaft coming out the circuit board, yet they can be accessed from the front panel. Or mount the controls 90 degrees to the front panel, but again have them mounted on the circuit board so the leads are short. Michael VE2BVW |
John Miles ) writes:
In article , says... I did as one poster suggested. I removed the antenna from my receiver, tuned around while transmitting, and discovered the following: (Crystal is 7110 kHz) at 3554 kHz, signal is S8 at 7110 kHz, signal is S6 at 14218 kHz, signal is S6 at 21326 kHz, signal is S7 at 28433 kHz, signal is S5 I'm probably going to rebuild this rig, without the circuit board. Many thanks to all who gave me advice on this. Any further comments are also much appreciated. Build it "ugly style" on an unbroken sheet of copper-clad board, and try a ferrite bead or two in the base and/or emitter leads of the transistors. The FAA and your local cellular-phone company will thank you. :) -- jm I didn't look at the pictures, but someone said something about long leads to coils and maybe variables. Sometimes it makes sense to make the circuit board used for the "ugly construction" the front panel, or at least mount it flush with the front panel. Then, all the controls can be mounted on the circuit board, near where they are connected and with the actual shaft coming out the circuit board, yet they can be accessed from the front panel. Or mount the controls 90 degrees to the front panel, but again have them mounted on the circuit board so the leads are short. Michael VE2BVW |
John Sandin wrote:
I removed the lowpass filter and loosened the wiring, and for a while it seemed to help. The transistor didn't get as hot. But the output stayed the same (less than 1 watt). But then I noticed, as I've noticed before, that the voltage measured at the dummy load doesn't always drop when the key is released. And the transistor stays hot even after the key activity has stopped. This seems to me to be a sure sign the amplifier transistor is oscillating on its own. In this circuit the code key doesn't interrupt the amplifier - if it's oscillating on its own, it will do so whether the key is closed or not. What *frequency* is it oscillating at? Good question. (if I were you I'd try to get my hands on a 51-ohm or 47-ohm 2-watt resistor and use it in place of the antenna until you get the oscillation in the amplifier fixed. Unfortunately, simply replacing the antenna with the resistor *might* fix the oscillation! The antenna is essentially part of the circuit..) I have my suspicion that the fact the leads for L1/L2 (the coils between the two transistors) and L3/L4 (the coils between the amplifier and the antenna) are long and close together may be your problem. You really need wires in RF circuits to be as short as possible. Maybe remove the connectors from the back of the box, put the coil sockets there, and put the *connectors* on top? I suspect the RF on the leads for L3 is being coupled into the leads for L2. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
John Sandin wrote:
I removed the lowpass filter and loosened the wiring, and for a while it seemed to help. The transistor didn't get as hot. But the output stayed the same (less than 1 watt). But then I noticed, as I've noticed before, that the voltage measured at the dummy load doesn't always drop when the key is released. And the transistor stays hot even after the key activity has stopped. This seems to me to be a sure sign the amplifier transistor is oscillating on its own. In this circuit the code key doesn't interrupt the amplifier - if it's oscillating on its own, it will do so whether the key is closed or not. What *frequency* is it oscillating at? Good question. (if I were you I'd try to get my hands on a 51-ohm or 47-ohm 2-watt resistor and use it in place of the antenna until you get the oscillation in the amplifier fixed. Unfortunately, simply replacing the antenna with the resistor *might* fix the oscillation! The antenna is essentially part of the circuit..) I have my suspicion that the fact the leads for L1/L2 (the coils between the two transistors) and L3/L4 (the coils between the amplifier and the antenna) are long and close together may be your problem. You really need wires in RF circuits to be as short as possible. Maybe remove the connectors from the back of the box, put the coil sockets there, and put the *connectors* on top? I suspect the RF on the leads for L3 is being coupled into the leads for L2. -- Doug Smith W9WI Pleasant View (Nashville), TN EM66 http://www.w9wi.com |
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Thanks again to all. I'm going to take charge of this problem first
of all by rebuilding the project, using direct wiring, using the layout recommended by QST, June 1967. Using this method, I can fit everything into the just the lid section of my existing box and I won't have to remake the tube socket holes in a new chassis. Based on what everyone has said, I ought to get rid of all that loose wiring anyway, prior to making any tests with a scope, which I don't yet have. I want the thing to be well-constructed, to eliminate all that unpredictability we've been talking about. I will also use only my 50 ohm dummy load for testing until everything's as good as I can make it. On 8 Oct 2003 05:40:24 -0700, (Ashhar Farhan) wrote: (John Sandin) wrote in message ... Regarding the 3-watt transmitter I'm building (and having trouble with), here's an update. ... john, a receiver will probably report all the harmonics of the crystal. so that is not really a big deal. almost all the harmonics will also sound pretty powerful and equal to each other in the HF receiver because the recevier's AGC will boost the lowering outputs to a higher level. the output at 3.5mhz can be easily explained by a receiver shielding deficiency. what i would really suggest to you is to TAKE CHARGE. i mean, figure out exactly what is going and how. do you have access to an oscilloscope? a single oscilloscope trace is really worth a thousand blind trails. failing which, i suggest that you should first make yourself some test instruments. i would highly recommend a wavemeter. it requires about 5 components (a couple more if you want to build a GDO-cum-wavemeter). A single coil should cover most of the HF band of interest to you. then ... rebuild the transmitter from scratch, keep everything at 50 ohms impedance. refer to EMRFD for guidlines. then ... don't push too much gain in each stage. It is actually easier to debug and build a transmitter with more stages having lower gain than trying to push maximum out of fewer components. finally ... shoot for about 7 watts of output. An 80 cent IRF510 should easily give you that much. use the wavemeter and the a power meter to measure the harmonic outputs. But frankly even if you are pushing out 50% of the energy in fundamentals, then political correctness aside, you should be able to make contacts. QRP of single watters are best left to old war horse like W7EL. it is tough for a novice to get through on a single watt being rock-bound. i would also suggest that you hook up with a ham down the block to be able to monitor your signal. then another across the city ... then another across the state... - farhan -John Sandin KC0QWE Remove the "T" to respond by e-mail |
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