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Magnum 257hp Garbage
- Thread starter pkmak12
- Start date
The RF power control on this and many other "10 meter" rigs is not an RF power control at all. It's a voltage control for the AM modulator supplying DC to the finals.
During AM, CW, or FM this control will effect the power output by adjusting the final voltage to 6 volts or less. Since the finals are biased on only when 12 volts are applied to them, they run class C in all modes accept SSB.
This change in bias in accomplished simply by changing the supply voltage in SSB. That is the only mode where the final runs linear and has 12 volts applied. You cannot lower this voltage and still be biased for SSB without readjusting the bias level.
That is why your power control does nothing on SSB and they tell you to use the mic gain as a substitute. The control is bypassed so 12 volts feeds the finals and the forward bias works at this voltage. The reason it's OK to run this stage class C on AM is because the finals are high level modulated.
During AM, CW, or FM this control will effect the power output by adjusting the final voltage to 6 volts or less. Since the finals are biased on only when 12 volts are applied to them, they run class C in all modes accept SSB.
This change in bias in accomplished simply by changing the supply voltage in SSB. That is the only mode where the final runs linear and has 12 volts applied. You cannot lower this voltage and still be biased for SSB without readjusting the bias level.
That is why your power control does nothing on SSB and they tell you to use the mic gain as a substitute. The control is bypassed so 12 volts feeds the finals and the forward bias works at this voltage. The reason it's OK to run this stage class C on AM is because the finals are high level modulated.
So shockwave, when running an amp in ssb mode you would need to adjust the mic gain to accommodate the total output as there is no carrier for ssb is this correct? I don't want to burn anything up. Thanks for any and all information. God bless.
Well... that is what the instructions say. The problem with that is the mic gain is also no RF power control. Reducing the mic gain is definitely not the way to reduce output power if you need a specific level of low drive to feed an amp.
The reason is it's completely unreliable as far as maintaining this lower power level. One loud sound in the mic and the transmitter could easily rise to full power and over drive an amplifier.
In order to accurately control SSB output power, we must be addressing the peak power level, not the average audio level. That is best done by adjusting the ALC control. Lots of "10 meter" radio users hack out the modulation limiter and that often disables the SSB ALC too.
I read something recently about issues that people were having with the 257HP radios. Now I do not run an amp so what I read might not apply to everyone but somewhere is a notice from Magnum to run the power control all the way up and adjust the output your looking for with the mic gain. I agree with what you say and driving an amp the way you want but really if your going to use a 257 for a radio to run an amp why not just get the non HP version that does like 20 watts and not have to worry about over driving an amp
I thought I read that the non hp radio's were old stock, meaning that they were not the latest version with all the bugs fixed in them?? is magnum still making NEW non hp 257's??
I'm not 100% sure but I think you are correct. Not that the 257 non HP had bugs so to say but there were a lot of 257Hp radios that did have bugs to work out. They did make, at one time, a blue face 257 non HP radio, I had one and worked great, with very loud audio on SSB with the stock mic. Some of the early 257 radios were, from what I have read, made in a different plant and did work very well. There are also a few spin offs from that manufacturing place, like the Road Pro, Radio Shack 485, Albreght 485, some were single final units but did work great, staying on frequency, good audio, just output was about 15 to 20 watts on SSB. I think the most reliable of the 257HP radios were in the old green and white box with the stickers on them talking about 80 watts. That was the best working HP that I had, when they switched to the newer box that is when the issues started
I have a magna force 350hd is why I ask. It all is true there should be no carrier on ssb and thus the rf power can be run wide open and not throw a 15 watt carrier. This has been my experience anyway, you should see 0 watts on key up on ssb and full modulation when speaking. I may be way off base here but I haven't had any issues thus far. Just want to make sure I'm doing it right. And just to clarify the magnaforce 350hd is a high drive box, on am I use a 2 watt dead key and let it swing into the amp, amp never even gets warms as the max input is 120 watts pep, I think closer to 100watts, it has 2 toshiba 2879's in it and it is a b biased amp. Never had any of the neighbors complain so I don't think I'm bleeding over. Like I said I just want to make sure I am found it the right way. I don't really use the amp very often, just to talk dx occasionally. Thanks for all the input and god bless.