Galaxy 2547 Freq Response Question

[The Bruce]

First, I understand that CBs are set for a certain amount of frequency response on the transmit side (around 2800 Hz of bandwidth, IIRC on the Galaxy 2547).

My question is this: Is there a pot on the radio that adjusts where on the frequency band that 2800 Hz is set? The reason I ask is because my 2547 tends to (nearly) drop "S" "F" and "C" sounds when I transmit (high treble sounds), while (slightly) emphasizing the bass parts of my voice.

I'm using a studio microphone, in-line with a mixer, so I can monitor the audio going into the radio, via headphones. It's fine. I've played with the levels (mic gain on the radio vs. levels on the mixer) and the treble-response remains the same; the higher frequency sounds are virtually inaudible, compared to virtually every radio I've heard out there.

Again, is there a pot on the radio that adjusts which portion of that 2800 Hz is filtered out? :headbang

 

[RatsoW8]

How does the radio sound without the studio mic and mixer? You're asking a question that is a bit ambiguous and hard to answer without actually hearing the actual transmitted audio. Can you post a recording of what you should like on the air?

Unless you are over driving the audio section of the radio to distortion you should be able to hear the consonant sounds, which are independent of audio frequency. In other words the F, C and S sounds are more timbre than pitch (frequency). You can use those consonants at any audio frequency. I think what you are seeing is the result of distortion on voice peaks on those hard consonant sounds..

 

[The Bruce]

I'll try it out tonight without the studio mic/mixer and let you know.

 

[The Bruce]

Another Question,

The specs for my radio call for a 500 Ohm microphone.

I'm running my audio gear through a 600 Ohm connection (via an iBox) into my CB. Should I switch out the resister on my iBox, or is the difference in impedance negligible?

 

[Robb]

Just finished doing the 'Goldfinger RX/TX Audio Mod' to a Uniden/President Grant radio. You have to remove and replace 12 caps with different values - basically. Six caps for RX and six for TX; took about an hour or so. This mod was intended for the Cobra 148GTL. However, the balance modulator circuit and receive circuit are identical to the Grant - so I tried it and it worked just fine. Made a very cool difference. It won't change the width of the filter parameters; but it will make the most of the available bandwidth the filter has. I imagine this mod will work with any radio that uses the same balanced modulator and receive circuit. Compare the 148 schematic to your radio's balanced modulator/receive circuit and see.

http://www.deepsouthradios.com/forum/showthread.php?7305-Goldfinger-Hifi-mod-Thread

 

[unit248]

I was under the impression that the Cobra 148 didn't use the balanced modulator for AM transmit modulation (only for SSB).

Am I wrong? Confused? Misinformed? (I know some ham rigs generate AM that way, but I thought for CBs it was different.)

Also, I have a question for The Bruce: how are you monitoring your transmitted audio? If you're using another CB rig as a receiver, then the highs in your audio may be getting clipped a little there too. The limiting factor is usually the filter in the receiver, which is only 6Khz wide. That's total: you end up with only 3Khz on either side of the carrier, so you're limited to 3Khz of RX audio response no matter how wide your transmitted signal is. (And that's not including whatever roll-off may be present in the audio amplifier circuits too.)

Also, I would check how high you have the carrier turned up on the transmitter. To start with, take your maximum peak output power rating and divide that by 4, then set the AM carrier level there. If you set it higher than that, you may end up sounding distorted. (It looks like the Galaxy 2547 isn't a high power rig so that may not be an issue, but we don't know if you have any extra 'fire in the wire.')

-Bill

 

[The Bruce]

I was using my mobile to monitor my base (basically my son talking into the base while I drive around). I wasn't picking up distortion of any kind, and the audio's sounds clean. It's just that "S" sounds are fairly inaudible. With the locals around here (mainly running Cobras), their "S" sounds are very distinctly there (unless they were running a stock mic).

I'm currently waiting to get my radio back from DTB Radio. Turns out one of the finals was bad and that certainly could have been a contributing factor to the problem (but we'll see).

At the end of the day, I'll like wind up doing the caps replacement (on TX and RX) anyway, since it's been on my list of things to do (along with a channel mod).

 

[Robb]

Just compared the 2547 schematic to the 148GTL, and they are NOT the same. Didn't have time to do it the other day. So, the Goldfinger mod will not work with your radio. However, if you can find this 'Goldfinger' tech/dude he may know how to modify your radio for a better RX/TX audio response in the 2547.

Cobra 148GTL schematic:
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf

Galaxy 2547 schematic:
http://www.cbtricks.com/radios/galaxy/dx2547/graphics/Dx-2547_sch_main.pdf

If you find the IC marked 'AN612' on the schematic; that is the balanced modulator IC.
The 2547 circuit to the right of it is NOT the same as the 148.

 

[Robb]

This might be what you need.
The 2547 uses the same radio chassis as the Galaxy 959 IIRC.

Hi Fi mod for ept6900 series board

Thanks to 'Deep South Radio Forum' and 'Goldfinger' . . .

I would NOT remove the AM Limiter as suggested. Your audio won't be clean and could complicate using an amp if you are going to do that. Especially if you are running or going to run a HiFi-type mic. It would ruin TX on SSB as well.

Another thing that would help immensely is to get the TX coils re-tuned using your best mic to achieve the best/proper impedance match.

 

[The Bruce]

This might be what you need.
The 2547 uses the same radio chassis as the Galaxy 959 IIRC.

Hi Fi mod for ept6900 series board

Thanks to 'Deep South Radio Forum' and 'Goldfinger' . . .

I would NOT remove the AM Limiter as suggested - though. Your audio won't be clean and could complicate using an amp if you are going to do that. Especially if you are running or going to run a HiFi-type mic. It would ruin TX on SSB.

Another thing that would help immensely is to get the TX coils re-tuned using your best mic to achieve the best/proper impedance match.

I appreciate all of the helpful advice. I'm not sure what you're talking about on that last part (scratching head). Could you elaborate? Are you talking about inside the iBox or inside the radio? :blink:

 

[Robb]

From 'CB Tricks':
Galaxy Radios DX2547 Service Manual,


SSB TX Power:
Set radio to CH 20,
Mike Gain max
USB TX mode.
AF signal 30 mV, 1 KHz to microphone.

Connect RF power meter to antenna jack. AF SG to microphone.

L40, L42, L43, L44 - Adjust for max. power with min. spurious emission

L40, L42 - Balance Power Between CH 1 and CH 40.

http://www.freesound.org/people/klangfabrik/sounds/28636/

Each different mic that is used for a radio presents a different impedance to a radio. This situation will affect the state of the tune of the TX coils, freq response, and the radio's TX output to some degree. SO, it is always best to tune the TX coils with the mic you intend to use. I use the above method when using a mic and mic preamp and a computer with that .wav file on LSB, an external watt meter, and then into a 50 ohm/100 watt dummy load. Perhaps my method isn't exactly done 'by the book'; but it should get you very close to the mark. If you don't have the experience nor understand the concept; then it is better to bring your setup to a local radio tech that knows what he is doing and have him do it for you.

BTW - I am not a pro nor a tech; just an Amateur having fun and getting the desired results . . .

 

[The Bruce]

I'll be getting my radio back from DTB Radio on Wednesday, so I'll listen to the radio and decide where to go from there.

The only part of the above procedure that might be a little tricky for me is figuring out whether or not the radio is receiving 30 mV of input -- or is that really important? Is the goal simply to tune the coils for max power? Ultimately my biggest problem would be knowing whether or not I'm putting out any spurious emission (I don't own a scope).