DTB "IF Filter Mod" for RCI 29xx radios?

[Mudfoot]

Exactly, which is why no shops are posting videos. They just stroke your back with fancy descriptions.

 

[The Mad Scientist]

M42duster it isn't just that they are just pulling your chain...That modification was intended to cure an entirely DIFFERENT problem that no one including the manufacturer could reproduce.

I have to say again as I have said before Image rejection and a lower noise floor are two completely different things.

Someone should post a complete explanation on a sticky and nail it to the top of this section so people can educate themselves as this seems to come up every so often.....:bdh:

Nevertheless, I am curious as to the perception of whatever when the radio comes back.

 

[buickid]

Mad Scientist: So what is this modification intended to resolve? It must do something, no?

 

[The Mad Scientist]

Mad Scientist: So what is this modification intended to resolve? It must do something, no?

:blink::blink::sad:

I think you should go back and re-read this thread. It has been mentioned more than several times.

*******IMAGE REJECTION********

 

[buickid]

:blink::blink::sad:

I think you should go back and re-read this thread. It has been mentioned more than several times.

*******IMAGE REJECTION********

Sure, I read that, no need to get aggressive. Hell I started this thread, I have every motivation to read the posts. But what does that mean to me? I know what white noise is, but how exactly does image rejection affect me? Sorry that I'm not an RF engineer. I tried to do some reading about image rejection, and understand what it "does" (passes only the secondary frequency that we want to create the heterodyning, since another frequency a similar distance away from the primary will also create the heterodyne on that frequency, to put it in a very non technical manner). Thats all quite plain to see, I have a pretty fair understanding of undergraduate level physics. But I don't quite grasp what that means in RF design, as an end result of going through all the rest of the radio's circuitry, coming out of the speaker. Surely it must have some noticeable effect on the receive, since its got a supposed solution (this mod). I understand its not the noise that it affects, but perhaps something else, like tone?

Thanks for taking the time to explain,
Trevor

 

[AudioShockwav]

It was intended to cure a "problem" that popped up during a test of the radio. I would guess the the open traces under the filter were capacitive coupling and allowing some unwanted RF to get past the filter.
Ranger could not reproduce the same problem.

It’s now time for the stock radio bench test. The first results are with the RF power control set to the full power clockwise rotation. The AM and FM output is at 9-Watts. The AM modulation swing is 22-Watts peak. The sideband power is 27-Watts PEP. Now the output readings with the RF power control in the low power counter-clockwise rotation. The AM/FM power dropped to 1-Watt. The AM modulation swing is 2.5-Watts PEP. The sideband power dropped to 4 –Watts PEP.

Testing the receiver indicated that the sensitivity was quite good on all modes. Even very weak signals on AM and sideband were cleaner than on the old 2950 and on most 10 Meter radios for that mater. Now satisfied with the sensitivity of the receiver, the selectivity was then checked. My crude method is to crank up the signal generator to full RF output, modulated to 100% with a 1KHz tone. This registers 30 dB on a calibrated “S” meter, or is equivalent to a signal of someone less than a ¼ mile away. On AM & FM, the radio performed much better than expected. The old 2950 didn’t do very well with this test. Sideband however, displayed a signal of approximately 4 bars on the LCD meter 200KHz on ether side of the center frequency before the strength started to decrease. This is to say, if someone was coming in at 30dB on channel 20, the signal from that transmission would still be received at four bars on channels 1 and 40.

Being puzzled and thinking this must be a defective radio; a second 2950DX was pulled from its box and tested. The results were identical. To confirm the test equipment was operating properly, two radios were tested, a Uniden Grant and a Galaxy 88. Both radios tested fine. Now the story gets more interesting, a call to JR at Ranger service didn’t resolve the issue. JR said they could not reproduce the results at their service lab. He went further to say the ARRL tested the RCI 2970DX and found no problem of this type. As concern grew, a decision to contact the author of the RCI 2970DX review in QST magazine was made. Contacting Wayne Irwin was a pleasant experience. After explaining that he wasn’t responsible for the lab testing, he offered to find what he could and reply. Wayne agreed the transmitter unwanted sideband figure of 39dB (50dB is a minimum figure one would expect) could be due to the design issue findings. Here is the response from Wayne:

Hi Again Bob,

I just checked with Joe Bottiglieri in the editorial office. It appears that the problem you found with the Ranger was not apparent in our lab tests. Since you have seen it in a couple different radios, he suggests that you consider submitting a little piece for possible publication in Hints and Kinks.

Again, many thanks for your feedback.

73,

Wayne K. Irwin, W1KI

Assistant to the ARRL VEC Manager

Tel: (860) 594-0305



The E-mail states the problem wasn’t apparent in their lab tests, but they don’t deny a potential problem. Looking at the receiver test results, a test for selectivity was done on FM, but no SSB selectivity test results were published. The assumption may have been made, that if FM were tight SSB would be better. I wrongly made this assumption myself on all the radios tested prior to this article. I will cover what I feel is a design flaw and what is needed to correct the potential problem in the next article, titled Image Rejection Modification.

I have also tried to contact Gordon West about his findings while reviewing the RCI 2970DX for Popular Communications. I haven't received a response yet.

Continuing the review with a modified IF stage showed the 2950DX to achieve excellent results on the sideband selectivity test. Compared to other 10 Meter radios, the RCI 2950DX performance was outstanding, and this one covers 24 MHz to 32 Mhz. Until now, radios selectivity suffered more as frequency coverage increased.

The Full review can be read here:
Review: RCI 2950DX Dual Band Amateur Radio

73
Jeff

 

[buickid]

So as I understand it, this only becomes a problem if there are very strong signals on adjacent frequencies.

 

[The Mad Scientist]

Yes you are correct, But acording to most the problem cannot be reproduced. I tried this modification almost ten years ago with no decernable difference. Thanks for posting a link to that thread Jeff, It explains it better than I could.

I must apoligize as I didn't mean to sound agressive, I didn't know that you tried to look into it. I guess I got so used to most people wanting to be spoonfed information rather than looking into it themselves. And I hate to see you waste your hard earned money on something that wont benefit you at all.

73's and good luck

Eric

 

[buickid]

Yes you are correct, But acording to most the problem cannot be reproduced. I tried this modification almost ten years ago with no decernable difference. Thanks for posting a link to that thread Jeff, It explains it better than I could.

I must apoligize as I didn't mean to sound agressive, I didn't know that you tried to look into it. I guess I got so used to most people wanting to be spoonfed information rather than looking into it themselves. And I hate to see you waste your hard earned money on something that wont benefit you at all.

73's and good luck

Eric

No worries Eric, I understand the frustration (I work part-time at a RadioShack... some people shouldn't be allowed near technology). :headbang I am a very inquisitive person, and try to read and understand as much as I can, and seek outside help only when I've done everything reasonably possible. Thanks for you help, I appreciate it.

Anyway, plot twist, Dave has offered to do the modification at no charge, so I'm only out the cost of shipping. No biggy, I'll do it in the name of science for WWDX! I'll be making that video when I get a chance, and get it sent out.

 

[loosecannon]

this is an interesting thread.
not sure how i missed it the first time around.

Mad Scientist has it right.
this IF mod is not about the noise floor, and wont make any difference in the sound of the receive with no signals present.

bascially, its like having two small antennas hanging off the bottom of the IF filter, causing some of the trash that "may" be present to be coupled through.

if you had someone close by, or with a really strong DX signal on a harmonically related freq. you might get some additional noise from them.

maybe a ham close by on 30 meters?

as for the noise floor issue, after discussing it with a couple of people who know way more about it than i ever will; the conclusion is that its because of a crappily designed local oscillator in the PLL section, and the only fix would be redesign the whole thing.
IE: not going to happen.

there is a way to make it less noticeable, and anyone who tunes up a galaxy/rci/ whatever style radio should know this.

its called the "antenna coil" and in the 2970DX its L8.
in other radios it may be a different number.

if this coil is tuned for max like you would any other coil during a receive alignment, you will have a high noise floor.

the fix is to do the RX alignment, peak L8 like normal, then remove the sig gen, and anything attached to the antenna jack on the radio.

now give L8 a bit of a turn clockwise and the noise should drop right off.
turn it just enough to drop the noise a bit, and you will notice that you have only lost a tiny bit of sensitivity. (not enough to make a difference IMO)

thats how i do it, and it was shown to me by a Galaxy service tech.
LC

 

[The Mad Scientist]

Thanks Loosecannon, This particular modification has come up alot on multible forums over the years.

I would also like to add this to your perfect explanation,

Before you look to adjust the antenna coil or L8 as listed on some radios, Please check BOTH VCO voltages Where the RCI2950DX is concerned. They are supposed to be set at 1v + or - .1

I have seen so many shops jack these voltages way up as a cheap and quick means of increasing the RX and causes an extreme amount of "white noise" as a result. Some shops still practice this leading some to beleive it is just part of the radio design. The RCI2950DX does have a small amount of white noise and is due to the higher noise rating of the SMD components used in manufacture and in my experence isn't worth writing home about.

I hope this helps as well.

 

[loosecannon]

mad sci,

i saw that you mentioned that previously, and i forgot to comment on it.

ive not heard about this before, but it does make sense.

ive never run into that before, but i just set them the way the service manual tells me to.
im just funny that way i guess.

thanks for the info.
LC

 

[Mudfoot]

No worries Eric, I understand the frustration (I work part-time at a RadioShack... some people shouldn't be allowed near technology). :headbang I am a very inquisitive person, and try to read and understand as much as I can, and seek outside help only when I've done everything reasonably possible. Thanks for you help, I appreciate it.

Anyway, plot twist, Dave has offered to do the modification at no charge, so I'm only out the cost of shipping. No biggy, I'll do it in the name of science for WWDX! I'll be making that video when I get a chance, and get it sent out.

Any updates?

 

[buickid]

It's on my list, little time, little money.

 

[Sarcasm]

Update: I received the radio I ordered and have been using it with good result, but I recently discovered the IF mod has not been done, upon looking at the warranty sticker (intact). Dave did offer a refund, but I want the mod done, so I'm sending it back. This offers a chance for me to make a before and after video, to see if it makes a difference. I'll take a before video of the radio listening to a dead channel, then have a short QSO with a local, then when the radio comes back, do it again under similar conditions.

So you bought a very nice expensive radio and it was never opened for the mods. I thought all his new radios came with an alignment in this price range. And being in Hawaii that's got to be killer for shipping. After reading the experts opinions did you end up sending it out yet? Or refund?