Cobra 148- Much Higher Performance Receive Setup Than Shottky Diode Receive Mod

When it comes to Cobra 148/2000GTL receive mods there seems to be only a few. The most notable able is the (1) 1st rf amp and (2)dioide swap and finally the (3)"Goldfinger" cap mod.

I purchased all the parts to do the 3 mods. As I stared at the parts I thought, "Wow, lots of parts...I hope I don't mess it up and have to put the covers back on and say I'm sorry..." At any rate, I proceeded to start replacing and after a short amount of time it was complete.

The performance was dismal at best. Kinda like a jedi mind trick..."Your radio receives better.." , not quite.

Almost the exact same feeling of putting louder exhaust pipes on a car makes it feel faster. I really did feel as if I had received a "Goldfinger", and not the radio.

Anyway, I sough out a "more better" approach and here is what I have built, installed and now currently run:

1. Crystal IF Filter (7.8Mhz)
2. Low Distortion/High Dynamic Range Detector

Link to Schematic of the Detector:
http://www.amwindow.org/tech/htm/lowdisdet.htm


The audio that pours from that radio is FANTASTIC. The IF filter really cuts the noise down and puts a leash on bleedover, it also adds a scoshi gain right in the 7.8mhz bandpass. Nice...

The new detector is really nice. When listening the high dynamic range is very apparent as the sound is full, warm and more natural sounding. This setup can demodulate up to 200% modulation and it really shows.

I recommend this setup up as a much higher performance alternative to the RX mods out there. The only "mod" I did was taking the output of the detector and running the audio through a second opamp, boosting to line-level then out to a much better audio amp and speakers. I don't want my precious recovered audio molested by half rate coupling/bandwidth limiting circuits/caps and the worst high-level audio amp (TA7222 ..blah..) Cobra could have used, ever.

I've also included an audio clip of the detector in action. My personal taste is he needs a bit more highs but over all I like it.

 

Thanks for your reply. Many CB radios aren't superb, whatsoever.

You talk about Uniden or Cobra as the designers of diode detectors but they aren't. Diode detection is a method of demodulation, a really cheap way of AM demodualtion at that. Lame diode detection has many drawbacks technically but it costs Uniden or Cobra very little in parts and manufacturing.

Let's investigate the drawbacks of simple diode detection. From this you can make an informed post instead of writing jibber jabber, gobbledygook or nonsense (as in nonsense poopie pants) !!!

1. Shottky or not all diodes have a forward voltage drop - that means your diode detector cannot hear what a precision diode hears when it comes to weak signals. There are conversations on my precision diode 148 that are not even there on my stock test 148 (or your stock radio either). I don't care about 2sc2999(e)'s or what ever or shottky diodes you choose, the performance upgrade is DISMAL at best. "Goldfingering" is a waste of time, money and parts and offers zero control after installation.

2. Diode detectors present a very low impedance on their input which DESTROYS selectivity. Precision diodes INCREASE selectivity due to their higher input impedance.

3. Diode detectors have a limited bandwidth with audio because they only spit out pulses of recovered audio where an R/C circuit is required to smooth out the waveform. A nasty condition pops up when the cap doesn't discharge quick enough or the cap is hasn't discharged enough when the next pulse(s) comes in - what do you think happens how do you think you ear will perceive that?

4. Simple diode detectors have a limitation of being accurate to about 130% positive modulation until mad distortion begins to occur. The precision diode detector can go well up to 200%. In other words, the Precision Diode detector requires less carrier to sideband ratio which not only helps in combating selective fading but with today's radios and the interest for hifi, modulation percentages can be up in the 150% area. That sounds like Sh%t on your radio but perfectly fine arm chair copy on mine.

5. Simple diode detectors have another nasty secret - they present frequency dependent varying loads at their input. What does that mean to you? It means that the overall frequency response of the diodes isn't flat and now the very section of you radio that is responsible for accurately reproducing the party you are speaking with, is coloring her/his tonality. You loose the subtlety of the persons voice and risk damaging the timber.

6. Simple diode detection is passive and has no feedback.

7. Simple diode detection, by it's very own virtue, has the most amount of drawbacks than any other demodulation method.


Seems to me Cobra/Uniden are really the butchers. They use the cheapest method ever to recover audio to make sure they can collect as much profit off you as possible. They know there are big modulation stations out there but choose willingly to not let you hear them properly. They choose to put profit in front of performance. Think about it, many of the high end HF radios (Kenwood, Icom etc, not Galaxy or "10 Meter" exports) do not have simple diode detectors but many use sync lock or precision detectors for AM. Why? Performance First (as Honda would say)

While your dioides hiss away with white noise, ignoring weak signals, and distorting audio, my detector is quiet, accurate, selective, sensitive and clearly showcases the mods in your receiver are useless, unnecessary (there is no "-" between un and necessary, just so you know) and benign.

Don't get me wrong, I think the 148 has a pretty good stock receiver but not in the way you think. See, I look at things like how good is the bandpass filtering, which type of circuits (Dual gate fet, Bipolar shut etc) are used for heterodyning, how is the AGC derived and in what sections does the AGC work with, does the radio use high or low side injection; These are some of the items I look at when deciding on what is a good 11 meter receiver or not so good. There is more to it than the volume knob or what you hear on your local cb home channel. Take a look for example at the TR-296. At first glance it appears to be a 148 clone, but upon further investigation, it's quite different. If I was on your train of thought, I would say "Thats a great receiver, just like the 148" which would be an incorrect statement.

I have a feeling that you took your radio to a local "tech" and she/he did the 2sc2999/shottky/goldfinger mod. This person told you how killer it was but when you brought the radio home it was no better - quite possibly worse. If you think an alignment and a transistor/diode swap is the gateway to better RX performance you should consider sending your radio to Snake head ( or whatever his name is ) to replace all the wires in the radio then whip out the golden screwdriver and tune 'er up good may be butcher in some new parts. Really , you think just because the parts fits there it's not a butcher job your wrong. A butcher job is when one unnecessarily swaps parts that have a negligible effect and overall the real debauchery is on your wallet and intelligence.

I posted my mod in the correct section of the forum; Mods. Please refrain from injecting comments about butchering in a section that is labeled for that.



Thank you, again for responding to my post...

 

I agree.

Really the point of the post is to show an alternative to the shottky diode setup. One that might cost a few cents more (maybe not if the parts are bought in china) but commands a respectable increase in performance. The precision diode is not the end all of mods but it's job is a very critical one.

You can also see that I run a crystal IF filter on my 148 as well. That coupled with the active detector is a really great setup with a difference that is as plain as sunlight.

I love chasing weak signal DX but don't have the funds to have super nice HF stuff, I have to use my nogin to even come close to a modern HF rig. I wanted that Kenwood AM sound so I converted my 2000 to low level AM. I want to hear better so I squeezed down the skirts in the band pass filter and opted for the better detector. I can't stand channel selectors so I ripped that out and put in DDS (AD9851).

Even my cheap sony world band receiver (not ham material) has sync lock for AM and it's great.

 

Greetings,

I've included the schematic for the crystal IF filter as an attachment. The PITA for this project is finding the crystals. I've harvested mine from a defunct 148's 7.8 mhz ssb filter. After installing the unit I simply adjusted the bandpass filter (gave it the golden screwdriver treatment) with an o'scope but it's just as easy to use your ear - it's pretty sensitive too!!

The 5532 is a really good audio opamp, I use one after a FET mic preamp in my D104, great tone. For the precision detector, please use an opamp with a fast slew rate. I use a TLO82 which is quite fast and performs flawlessly, however, I am unable to comment on the 5532 as I've never used one in this circuit; always a TLO series.

Initially I used vector board to build both circuits but have long since made PCB's as you can see in the pictures in the first post of this thread. The point is you can use vector and the circuits will perform satisfactorily.

The filter I have hooked up to a 12 volts in the radio while the detector is running on the RX switched 8 volts. Also, if you don't use the second opamp don't forget to terminate it properly or you could end up with extra noise in your precious audio.

On a last note, the filter works just as well with 10.695mhz crystals for radio's that use that freq as an IF. Installation of the filter must be outside the NB!!

Great to see others who like to tinker!!!

 

@Flatlander: You can use any small signal diodes you have. I've used 1n4148, 1n60p, it doesn't really matter. The secret to the diodes is use ones that have the closest voltage drop. Use your dvm in the "diode" mode and find three that are closest. I found when purchasing diodes it's best find them of the same reel. The good news is most diodes are well behaved and have characteristics which are very close to published specs.

@711 : Please do not remove the SSB fliter. The IF filter circuit is inserted within the bandpass filter. On the 148/2000 chassis please remove c47 (a 2pf ceramic coupling cap) and insert the filter there. C48 is an unacceptable insertion point as it is inside the NB loop. Also make sure to get the input and output correct. Although it does no harm to the circuit, it makes the receive really weak (I'm embarrassed to say this happened to me and drove me crazy for about 20 min!!!)

@711: I would love to see you work/ideas with the 5532 on the D104. I have a FET amp wired as a triode with a 10meg input impedance. You may already have the knowledge of the phase reversal with this config. Using the inverting input of the 5532 restores the phase. It only needed a gain of about 3 or 4 and not only fixed the phase problem but improved the S/N ratio as well. I use 15 volts on the FET biased for class "A" operation and a +/- 15 voltage for the 5532. This setup dramatically changes the frequency response of the MC320 element.

 

@711: I think my explanation was flawed. The crystal IF filter is inserted at c47( as stated above) and the precision diode detector replaces the AM rectifier diodes in the 455khz area.

Please have a look at the pictures I uploaded in the first post of this thread. You can see the IF filter in the right center (7.8 Mhz area) of the pic and you can see the diode detector in the upper left region (455khz area) . These mods won't destroy the rest of your radio.

 

@Robb

Greetings,

The filter takes virtually no test equipment. Really it either works or it doesn't.

For the diode detector a simple pc sound card o'scope can be used which are free on the internet and a if you have a soldiering iron already - you can make a probe with parts on your bench!

I'm not trying to be an a$$ here but your statement that people don't have access to test equpt. , IMHO, is incorrect.

It doesn't take a Textronics 465B to play with 455khz down to audio. Honestly pc based o'scope are good enough, and work pretty good actually - How do I know -- I've done it, I've used it. When you don't have money to buy everything, one needs to investigate every avenue of free or low cost "stuff". You may balk and claim free pc based o'scopes are garbage and are limited in bandwidth but the real question is "Will it do the job, will it allow me to complete my project with reasonable results?" and, unequivocally, in this case the answer is yes.

Solidering irons can also build RF Volt meters, for the 7.8 mhz filter all one really needs is relative readings and an RF volt meter is perfect, it's cheap, available, super easy to build and accurate enough. It also finds many. many other uses around the radio from housekeeping, repairing to aligning - literally a plethora of uses.

With no disrespect intended to you, Robb, I believe that it is your train of thought that is flawed. Your not questioning every specification, your not looking at things from more than one angle, and not utilizing what life has give us from millions of years of evolution - your noggin. I find it far more debilitating to not think outside the box than to have a lack of test equipment. There is almost always a solution - Sure not every time but more times than is truly apparent.

If you do have criticism, I would happy to hear it. I don't take it personally and with that information, I can make changes to make my mods for the better which I share with anyone who wishes to endeavor to persevere in radio.

Inexpensive or free doesn't always mean less performance.

I don't know what a video would do? No one gave me a video so I'm not sure how to make said video, although I am open to hearing how you would do it and perhaps with that guidance I may be able to create something of value to others. But in all reality, I have extremely limited video skills and a not so good camera. Here's sample of my Oscar winning movie:


I did, however, love the pun. Made me laugh, I though "Darn, I wish I thought of that".

Thank you for responding to my post

 

Well, it may give him something else to do other than swap wires. If he does take it claim at his own, and builds it correctly, it will only be beneficial to his clients.

It might be strange, however, for someone who primarily does "nothing work" to some how magically come up with a product that is way beyond his "normal" work.

Just sitting here, I can only image what it would be called. "I'll put that ear package in there too, and I guarantee you' what. This radio will hear 3 to 1 better." said the truck stop tech.

So, yea - Ear Package - I love it!!