Brand new Magnum 257 out of alignment.

[Night Ranger]

I just picked up a brand new Magnum 257 from a well known CB dealer. Right out of the box it was mis-aligned. The first was the bias on the final transistors was set to class C. The un-mistakable crackling sideband audio was a dead give away. I adjusted the bias setting and cleaned all the crackle out of the audio. Easy fix.

LSB had a carrier on transmit. Re-aligned the carrier balance and took care of that.

Next the IF shift for lower sideband transmit is off. I'm not referring to the transmit frequency. I'm referring to the audio bandwidth on transmit. The USB transmit sounds nice, but the LSB transmit audio is too muddy (lacks high end frequency response). I looked through the schematic and service manual, but I'm not quit sure where to set the IF shift for lower sideband transmit. On other similar radios I have seen a single adjustment that affected both upper and lower sideband. Basically you set it for a compromise between the two. On ham radios like the Drake TR-7 there is an adjustment for each mode.

The schematic is available here;

www.cbtricks.com/radios/rflimited/magnum_257/graphics/magnum_257_sch.pdf

and the HTX-10 service manual (very similar to the Magnum 257) is available here;

rogertango.com/pdf/htx-10-service-manual.pdf

Any help would be appreciated. I do have a nice Protek B-818 multi-function frequency counter.

Thank you,

Night Ranger

[Sandbagger]

Jan 28, 2009 20:29:44 GMT -5 Night Ranger said:

I just picked up a brand new Magnum 257 from a well known CB dealer. Right out of the box it was mis-aligned. The first was the bias on the final transistors was set to class C. The un-mistakable crackling sideband audio was a dead give away. I adjusted the bias setting and cleaned all the crackle out of the audio. Easy fix.

LSB had a carrier on transmit. Re-aligned the carrier balance and took care of that.

Next the IF shift for lower sideband transmit is off. I'm not referring to the transmit frequency. I'm referring to the audio bandwidth on transmit. The USB transmit sounds nice, but the LSB transmit audio is too muddy (lacks high end frequency response). I looked through the schematic and service manual, but I'm not quit sure where to set the IF shift for lower sideband transmit. On other similar radios I have seen a single adjustment that affected both upper and lower sideband. Basically you set it for a compromise between the two. One ham radios like the Drake TR-7 there is an adjustment for each mode.

The schematic is available here;

www.cbtricks.com/radios/rflimited/magnum_257/graphics/magnum_257_sch.pdf

and the HTX-10 service manual (very similar to the Magnum 257) is available here;

rogertango.com/pdf/htx-10-service-manual.pdf

Any help would be appreciated. I do have a nice Protek B-818 multi-function frequency counter.

Thank you,

Night Ranger

That is a function of the carrier oscillator adjustment, the 10.69x frequency oscillator. It is usually set for center freq on AM (10.695) and usually a + or - 1.5 Khz offset for the 2 sideband modes. The offset in frequency is corrected by the mixer oscillators (4.5 Mhz, most likely) which correct the offset back to center frequency.

The carrier oscillator should be set so that the crystal filter takes out the opposite sideband and your audio bandwidth falls within the (typically) 3 khz bandwidth of the filter. If the carrier oscillator frequency is set too low (on LSB, or high on USB), you'll roll off the high end of your audio. Conversely, if it's set too high, your audio will sound "thin" or tinny".

The normal procedure sets the oscillator frequency by a freq counter, but that assumes that the crystal filter is exactly centered at 10.695 Mhz. But sometimes there is a little shift, so it's probably best to set it up using a variable audio frequency generator on the mic input to see that the power falls off uniformly on each end of your desired audio bandwidth (300 - 3500 hz). Once you set this correctly, you can use the offset oscillator adjustment to bring your carrier frequency back to center.

[Night Ranger]

Update on my new Magnum 257. I took my Cobra 148 GTL out of my mobile, and I've been giving the Magnum 257 a try. After the minor fixes I had to do the radio sounds nice on transmit. I taped it on my home base while I was out riding around on the mobile.

On receive the Cobra 148 GTL re-issue has alot quieter receive than the Magnum 257. I'm not saying the Magnum has a bad receive. It is just noisier than the Cobra 148 GTL re-issue. The selectivity is pretty good for a 10 meter/CB export. It has better selectivity than most CB radios I have tried.

I still prefer the way the Radio Shack HTX-10 tunes around 10 meters verses the Magnum 257. The Radio Shack HTX-10 can cover the entire 10 meter band in 1 kHz steps including the scan. The Magnum can only scan in 10 kHz steps. You can manually tune in 1 kHz steps with the clarifier, but it only rotates in a 10 kHz window (+5 up and + 5 down).