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Crystal filter question

brandon7861

Loose Wire
Nov 28, 2018
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1,287
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I am repairing a Cobra 148GTL (1996 Malaysia, same board as Texas Ranger 296GK, board EPT014810Z). The crystal filter is bad. I have signal going in, nothing coming out. The problem is that I only have one junk radio with a 7.8MHz crystal filter, and that is from the Teaberry Stalker II board I gave up on earlier this year because it was so nasty.

Assuming it is good, can I use the crystal filter from the teaberry in the 148GTL? I am not sure if there are subtle differences that will end up biting me..

Thanks!!!
 

Well, it won't fit, it is physically too large, so to even try, I need to make a support to raise the replacement crystal filter 3/4" above the board to clear the other components. I figured on using twisted pair to connect it to the board.

I have a vague memory of a signal integrity lecture I saw long ago where (I think) it was stated for a mismatched conductor to not exhibit a signal reflection of consequence, the impedance discontinuity needs to be under 1/20th of a quarter wavelength long, or something like that. 3/4" is well under that at 7.8MHz, so I'm going to hope this works better than it will look.
 
I know, it will sit too high to put the cover on. This cobra board is going to be put inside the Stalker II chassis and be used as a base radio.

This whole project is charity. I made the mistake of suggesting to someone an ebay radio, and that radio had apparently been soaked and put away wet for years and years, So, instead of trying to fix the stalker board, which was totally shot, I decided on the transplant to try to make up for my bad purchase suggestion.

The owner has always wanted a Cobra 2000, and being this board is close to what is in the Cobra 2000, I figured I would call it the Stalker 2000 when it is finished. I'm not making a single dollar on this, so it is what it is I guess.

Edit: the Teaberry filter number is UF-047A
 
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I know nothing about the Teaberry filter, but if the impedance and center frequency is the same, you can remote the filter with coax without any consequence. If the impedance is not the same, you will have a problem.
 
I know nothing about the Teaberry filter, but if the impedance and center frequency is the same, you can remote the filter with coax without any consequence. If the impedance is not the same, you will have a problem.
In my limited research, it seems these crystal filters (not homemade ones) have input and output impedances around 800Ω. I do not know that for sure, but the resistances around the circuit would suggest it is higher than 50Ω. Assuming a several hundred ohm impedance, I fear that a 50Ω coax of any length would be a far greater mismatch than a twisted pair or even a single wire running along side the new bracket (ground). In either case, I am hoping that the minimal length of the added run avoids these issues all together.

As for the center frequency, I assume they are the same. Although the service manual for the TR-296GK (what this board number corresponds to) doesn't list the 7.8MHz frequencies in the tuning procedure (I think they use an up-mixed signal down the line to adjust that oscillator), the service manual for the 148GTL does list them (and those match the ones for the stalker).
 
Alignment went smoothly. The only issue I had was the harmonic filter. I ended up adding a 20pF cap across the 27pF thats in it and that let me find a dip when adjusting the trap. I'm talking to my friend (thats getting it when its done) on it right now, he says it sounds great. Tomorrow it becomes a base unit.
 
A Cobra 2000 without the wood, extra meter, power supply, clock/counter and added knobs.

73
Theres a tiny amount of fake wood grain on the stalker two. If I count ganged knobs as two, the cobra 2000 has 9, this has 8. It does, however, have the missing tone control in the form of a switch, which could probably be converted to a pot if I has somewhere to put it (maybe a third ganged pot?). The stalker has two meters. The missing clock will remain missing.

I might have to make a custom wood face for this thing. The stalker face is in bad shape anyway.
 
Why not add a Frequency Counter without the clock .......... :ROFLMAO:
Maybe use a DS3231 RTC clock module to provide a 1PPS signal to a SN74LV8154 which in turn feeds a MCP23017, so that frequency and time can be read by an Arduino or Pi zero that can then drive a display for both? Or is that just a bit too far out there?
 
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Well, I must have either screwed up in my assumptions or this filter is a little out of whack too. The numbers in the manuals match, even the VCO frequencies, so I assume the filters started out the same, but maybe it drifted a little.

I feel like an idiot calling the alignment done before zooming in and looking closer. After seeing the harmonic problem fixed and things on the SA looking good zoomed out, I took a quick look at the waveform using the scope as I put 200 to 2.5kHz into the mic. Why I didn't use the SA, no clue. Intensity didn't drop much at all. Sounded good on the air too, great reports, even called the guy on the other end to hear for myself. All seemed GREAT.

All was not great, he could hear me on the other sideband. Not as loud, but I was in there.. Checking with the SA, I now see that 1kHz into either sideband produces the desired signal, but also the same signal in the other sideband that is 20dB down. If I put 1kHz in, say on 27.205 USB, I get the 27.206, but also 27.204 at -20dB. Works the same the other way. The rolloff in the other band begins at about 200Hz and doesnt disappear until about 1.5kHz. 20dB is less than 4 s-units difference, thats not acceptable. My washington does not do that.
 
And I don't think it is cross talk between the extended crystal filter wires either as it does roll off fast. If the signal was coupling between my extended wires, it wouldn't roll off like that.
 

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