Hello, I have an M175 that I just replaced the MRF455 with Ma-COM 455s(matched received from reputable source in SD). It is acting peculiar. The SWR is only good 1.2/3 on low. . Up in the 3 area on Med and High. Also any modulation on med or high results in the keying relay clicking. My SWR meter is after the M175. I have tried 6ft and 8ft jumpers between the radio and assistant with similiar results. My dead key is set between 2-3W. It runs another 2 455 and 2 1446 just fine(no swr issues). I have read the other posts(high swr with amp on) I understand my amp tune could be wonky. Most likely is for that matter. Do not know how to experiment with that tune but would guess the var cap in there has something to do with it. Some of the information is a little confusing. Going to check all resistors etc. Just checking if anyone else has experienced this? Thank you.
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M175 High SWR
- Thread starter 2315 Robert
- Start date
Messenger M175 like this?
Has the amp ever worked properly since you've owned it?
The other MRF455 transistors died and that's why you've installed the Macom 455's?
I'm not sure about this, but I've seen these online and they usually have SD1446's installed. Wish I could tell you more, maybe the box needs to be re-tuned as you've stated. Different parts, different characteristics?
Has the amp ever worked properly since you've owned it?
The other MRF455 transistors died and that's why you've installed the Macom 455's?
I'm not sure about this, but I've seen these online and they usually have SD1446's installed. Wish I could tell you more, maybe the box needs to be re-tuned as you've stated. Different parts, different characteristics?
Correct. I have a new pair of 1446's that I though I was going to need to for White Tornado 225 that I got off ebay. I could just got for the swap in the name of science or hope that someone give me some info on how to tune the box to the new transistors.
I am not sure. I thought it was a Messenger but does not come up on the list on CB Tricks.
Same one as in the video. I have checked all of the resistors and diodes and they all check good. I have one diode that is below the board that shows continuity but think it is normal as all off my boxes have one that shows this way.
I would try adding a negative-feedback network to each transistor. A 100-ohm 2 Watt resistor in series with a 0.1uf capacitor will frequently calm down a twitchy amplifier built with high-gain transistors like MRF455. The series-pair of parts goes from each transistor collector to the base on the same transistor.
This will change the amplifier's input impedance, tends to lower it. Check the input-side SWR with a meter between the radio and the amplifier while it's keyed and producing power. There is usually a capacitor in parallel with the input transformer's primary. Trimming the value of this cap will frequently fix the problem and get the input-side SWR back below 2 to 1. A trimmer cap can be wired in place of the fixed cap, and used to zero in on the optimum capacitance value. You can then remove it, measure the value and install a fixed part close to that value, or just leave the trimmer where it is.
This method has worked for us numerous times.
73
This will change the amplifier's input impedance, tends to lower it. Check the input-side SWR with a meter between the radio and the amplifier while it's keyed and producing power. There is usually a capacitor in parallel with the input transformer's primary. Trimming the value of this cap will frequently fix the problem and get the input-side SWR back below 2 to 1. A trimmer cap can be wired in place of the fixed cap, and used to zero in on the optimum capacitance value. You can then remove it, measure the value and install a fixed part close to that value, or just leave the trimmer where it is.
This method has worked for us numerous times.
73
Thank you for information. I will try that. I even have an MC250 with the cap and resistor setup and was curious why they were in there. They come off of T1 with .01 cap to the resistor soldered between the transistors T1 and T2 with the 250pf cap on it. I looked for pictures of another MC250 with this setup to no avail. I saw a popular youtube person that had done work on an MC250 and had great video of the inside of the box without the resistor and cap setup. I have a great electronics store nearby so it should be no problemo getting a trimmer cap. Thank again for the information. 73's.
Nomadradio, I made it to the store today and asked for the variable/trimmer. Had the box with me and they told me I should adjust from the onboard. Am I incorrect that the variable on the board is just to equalize the transistors. Not the input tune. Want my stuff to be clean and non-spurious. Trying like the crazy to get a spectrum analyzer through auction.(non-ebay). Putting the swr meter between makes everything below 1.8. Our last radio shop(cb shop I could tell at least they used a signal generator to tune) within a 100 miles is for sale even though they were doing great business. They fixed a radio that I had problems with forever(cold solder joint that existed for 20 years). I bought that radio from Bill Good in Ontario, CA many a year ago. Just time for Jerry to retire. I have enjoyed radio my whole life but, now I need to get better educated primarily that my 15 year old son is interested and has more questions that I can answer. My ocilloscope lives in his room. Any more you can give is appreciated and thank you for the information.
The trimmer cap is to replace the fixed capacitor, usually a small silver mica found on the primary of the input transformer. That would be the wires that pass through the holes. One end of the wire wound inside the input transformer is grounded, the other one is fed the radio drive from the relay, and maybe also a high/low switch. The silver mica cap is wired in parallel with this winding. It's usually between 75 and 150 pf. A good starting value for the trimmer cap would have a max capacitance of around 200 pf. The optimum capacitance value is the one that achieves the minimum input-side SWR, Removing the stock silver-mica cap and wiring the trimmer in its place allows you to set the trimmer for lowest input SWR. Should help, just can't predict how much before trying it.
73
73
Copy, thank for that I will go try to find and adjustable ceramic in that range tomorrow. Just to make sure the variable cap on board is to equalize the transistors?
Equalize?
In what way?
The capacitor in parallel with the input transformer's primary winding affects both transistors equally, since it's in the circuit upstream from where the push-pull drive power is split off into two transistors.
So, if something is supposed to equalize them, it would have to be downstream from the input side of the input transformer, right?
I'm taking the word "equalize" in the sense of the two transistors being somehow not equal, one to the other.
What that capacitor does is to manipulate the amplifier's input impedance. Never remember referring to that as equalizing. Since it's in the circuit before the drive is split between the two transistors it should affect both of them equally. If they are out of balance somehow, it won't affect that.
73
In what way?
The capacitor in parallel with the input transformer's primary winding affects both transistors equally, since it's in the circuit upstream from where the push-pull drive power is split off into two transistors.
So, if something is supposed to equalize them, it would have to be downstream from the input side of the input transformer, right?
I'm taking the word "equalize" in the sense of the two transistors being somehow not equal, one to the other.
What that capacitor does is to manipulate the amplifier's input impedance. Never remember referring to that as equalizing. Since it's in the circuit before the drive is split between the two transistors it should affect both of them equally. If they are out of balance somehow, it won't affect that.
73
Thank you. That was stated perfectly. I have read somewhere that it what was onboard trimmer was for.Now I know.
nomadradio Thank you again. So by using the information that you gave me I got it down to 1.5 on the other side of the box. With the onboard trimmer even better. So in the name of science I put the 1446's in and removed cap/resistor, and trimmer I had put in. I had near the same results as the 455's. So this time I only used to on board adjust. It is down to 1.2/3 on L/M/H. I know now that if I had got a trimmer in the 50pf range could I could have had the 455's the same without touching the onboard(as if it did not exist). I an waiting for a meter that has reflect but I can tell things are good by temp, nothing showing up on 54 mhz, and the way my thief on coax frequency counter is behaving. I appreciate all you help.
Robert, I am glad you are back in business. I have one question though, how are you taking standing wave ratio measurements without a meter that reads reflect? Do you have access to a facility or a friend with gear?!