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Thanks :) I found the 959 radio here http://www.cbtricks.com/radios/galaxy/dx959/graphics/dx959_sch.pdf As LC says this one is very similar to my mk2 148GTL-DX but I spotted they have included a very basic upgrade to try and offset the noise seen in my hissy 148 video. If you look...

Hi Bob You have confused me slightly because I don't think the Stalker will be as affected by the AGC issue on FM. This is because the Stalker (and the mk1 Cobra 148GTL-DX) don't use any AGC on the second mixer or 455kHz IF stages. The S meter gets driven via the AM/SSB path so the AGC at...

Here's a link to the video Cobra 148GTL-DX AGC HISS - YouTube I assume this is the white noise you are referring to? If the AGC system had a better design then the radio would sound almost as good as it does with no AGC at an S9 signal because the S/N would be preserved better. FM...

I'll try and knock up a quick youtube video of what a Cobra 148GTL-DX sounds like on an S9 signal on FM. You will all be familiar with the hissy noise you get on the carrier. Now an S9 on the meter is approx -73dBm and this is a big fat signal. Uber signal to noise ratio yet the 148...

Hi LC In terms of white noise there's two main issues at work here. There is the obvious 'noise above thermal noise' due to the system noise figure and this limits sensitivity. However, with today's noisier bands I think an 8dB system noise figure should be fine. But there is another issue...

Hi George I do like TM1 and it's one of the few forums where everybody seems to get along and can have a laugh etc. There are a few stubborn myths and beliefs on there and some obsessive collectors and I've learned not to be too harsh about certain radios because it can upset some of the...

Hi George I saw a few of your old posts on TM1 and it seems a shame you were banned? Note to Robb: My (UK) callsign G0HZU dates to around 1987. I initially passed the RAE at college in the early 1980s as a spotty student and got the G0 call a few years later. Getting back to the...

Here's the analyser plot of 2SC1674L gain vs 2SC2999D gain. The scale is 1dB per division. It isn't really worth explaining which trace is which because they are so close. This really isn't a surprise because the gain of this common base amplifier is defined by the ratio of input...

Hi LC Yes, one of the things to do is alter the coupling cap. However, this does reduce the signal level so there is a tradeoff here. To counter this the gain of the first stage can be increased by altering the bias point. However, this needs care to avoid instability and there's other mods...

That's quite a comprehensive setup! In terms of improving the Cobra 148GTL-DX for receive sensitivity I think there are only two realistic methods (besides an external preamp) One is to make the sensitivity more consistent across all 200 channels of an expanded radio. i.e. try and get 8dB...

I've not used a grant but I think there's several versions. The one that is common here in the UK is very similar to my PB010 mk2 148GTL-DX in terms of the receiver circuits :)

I can offer some more thoughts about the front end design of the radio if this is of any interest. If you look beyond the first RF amplifier and BPF coils you arrive at the first mixer. This is usually an active mixer and will have a certain conversion gain (depends on the type of mixer)...

Hi Wa4gch Your tests show a huge 5dB difference in sensitivity between the two transistors and I think something surely isn't right there. I have actually built the front end of a 148 receiver on a separate PCB and tested it for noise figure. i.e. I tested the input amplifier stage. I...

Oh no, I've just ordered 2x 2SC2999 to do a comparison on :laugh: Seriously though I don't expect to find a significant difference. The common base amplifier used in the 148GTL-DX has fairly predictable performance at 27MHz and the gain is fairly well defined by the components around the...

Hi Robb I really appreciate your reply :) I guess we all share a common cause here in that early Uniden CBs are not as sensitive as they could be. Over here in the UK the SSB CB of choice was always the Cobra148GTL-DX and it's the radio I have the most fondness for. It often gets modded...

It's late here but I had a go at fitting some Schottky diodes to a 148GTL-DX on the AM detector. I don't have any 1N6263 diodes but I did find some old HP 2800 diodes and these are very similar. The results are quite interesting and there's probably something for everyone here :) First...

Exactly. That's very well put :) I'm actually trying to help people like Robb spot the BS but he really wants to keep up with the beliefs and reject any attempt at scientific analysis. His post #95 sounds a bit like someone defending a religion based on numbers of followers. They can't all be...

For the noise blanker mod as claimed by the OP, yes. The claim above is utter BS. He's effectively claiming it will make the signal several dB louder. The signal you receive doesn't even go through the noise blanker path so how can the diodes make it louder? Total BS. This is so wrong...

But you already have speculated that because it's in text above for all to see? BTW I do have several CBs here. eg the mk2 Cobra 148GTL-DX export model. I'm also aware of what contributes to the noise floor of the 148 receiver because I've owned a 148 since I was a student and I've tested...

One more time on the noise blanker diodes... The noise blanker detector typically uses unbiased germanium diodes. An unbiased germanium diode detector will still work reasonably well for tiny signals. However, swapping these diodes for Schottky may well actually make the detector worse...