Cobra Cam 88 Modulation Mod

[oldgeezer]

I been reading about increasing the dismal Cobra Cam 88 modulation. Some talk about mods, but no one actually said what the mod is for some ungodly reason. These are great sounding radios and I believe the Cobra Cam 88 has a more sensitive receiver vs the Asian tube radio that shares same chassis marketed under Robyn, Gemtronics, Colt, Robyn, Teaberry, etc.

The Cobra Cam 88 has a larger modulation transformer vs the Asian chassis CB radios. Although the Cobra Cam 88 has a wimpier 12AQ5 vs the 6BQ5 audio/modulator tube in the Asian radios, it is really about how much voltage is available at the audio/modulator tube, not the tube itself. So, I believe the Cobra Cam 88 will ultimately produce more forward modulation swing with maintaining audio clarity vs those Asian chassis radios.

Anyways, simply cut the resistor out that is wired at the bottom of the Dyna Boost switch. Dyna Boost is good for stock radios, not good for peak modulation due to diodes that clip modulation peaks with the switch on. With the resistor removed and Dyna Boost switch in off position the radio will now dead key 4 watts and swing to 10 watts. I know 16-18 watts is better, but at 10 watts peak it will now swing using a small two tube amp with 6JU6 from 40 to 100 watts. A great audio boost is the Tuner +3 microphone. It has an audio compressor built in for higher average modulation like that Dyna-Boost switch & sounds fantastic too.

The mod beats the hell out of the original modulation swinging the 6JU6 amp from 40 watts to 60 watts. The larger watt swing will now get you heard on skip. You can get a larger amp and swing from 200 to 400 watts with this radio. You can now keep this radio as a real player on the CB channels. Have fun.

[Sandbagger]

Jul 25, 2017 21:42:23 GMT -5 oldgeezer said:

I been reading about increasing the dismal Cobra Cam 88 modulation. Some talk about mods, but no one actually said what the mod is for some ungodly reason. These are great sounding radios and I believe the Cobra Cam 88 has a more sensitive receiver vs the Asian tube radio that shares same chassis marketed under Robyn, Gemtronics, Colt, Robyn, etc. Plus the Cobra Cam 88 has a larger modulation transformer vs those Asian radios. Although the Cobra Cam 88 has a wimpier 12AQ5 vs the 6BQ5 audio tube in the Asian radios, it is really about how much voltage is available, not the tube itself. So, I believe the Cobra Cam 88 will ultimately produce more forward modulation swing with audio clarity vs those Asian chassis radios.

Anyways, simply cut the resistor out that is wired at the bottom of the Dyna Boost switch. Dyna Boost is good for stock radios, not good for peak modulation due to diodes that clip modulation peaks with the switch on. With the resistor removed and Dyna Boost switch in off position the radio will now dead key 4 watts and swing to 10 watts. I know 16-18 watts is better, but at 10 watts peak it will now swing using a small two tube amp with 6JU6 from 40 to 100 watts. The beats the hell out of the original modulation swinging the small amp from 40 watts to 60 watts. The larger watt swing will now get you heard on skip. You can get a larger amp and swing from 200 to 400 watts with this radio. You can now keep this radio as a real player on the CB channels. Have fun.

While the Cam 88 is similar to the chassis made by Panasonic, there are some major circuit differences. Also, the 6/12AQ5 tubes are rated for similar power output as the 6BQ5, so there really isn't much of a power advantage one way or the other. You can play around with biasing and try to get a little more power out of it, but I suspect that the gains would be minimal for the effort required. The other item worthy of note is that the Panasonic chassis radios as well as the Cam 88 use a modulation scheme known as Heising Modulation . This scheme does not use a conventional modulation transformer, which makes it simple to design, but it has limitations in performance.

These radios were designed back when the FCC power limit was 5 watts INPUT, which meant an output of about 3 watts of power. At 3 watts of power, you can achieve 100% modulation (12 watts PEP), but it will be somewhat distorted. Your best bet to maximize "swing" in one of these tube rigs, is to back the carrier power down (usually by increasing the value of the resistor in series with the plate supply voltage, or the screen grid resistor), that way the ratio of modulation to carrier power increases, and you can achieve a higher % of modulation.

Here's a primer on Heising modulation:

www.w8ji.com/Heising%20modulation.htm

[oldgeezer]

Never seen schematic. I thought the audio transformer shared a standard modulation design. Apparently the Cobra Cam 88 has a cheap modulation system, but hey- it sounds great.