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N3umw
| | Posted on Monday, January 20, 2003 - 8:46 am: | 
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I have a new Galaxy DX-949. I've replaced the power amp (Q54) with a 2SB817. This is supposed to allow greater voltage stability for the entire radio. I've also put the unit on the bench and realigned the TX, PLL, and set the modulation.
When I first turn the radio on and my car is at 36F, the sideband signals are so far off that the stock clarifier doesn't have the range to pull them in. I'm not sure but I think the transmitted signal is off by almost the same amount. After 15 minutes (and the car has warmed up), the signals have drifted until the clarifier is set to 12 o'clock.
I realize that the radio uses a crystal and not a TCXO, but is this much drift acceptable? If not, what can I do to correct it? |
Ss8541
| | Posted on Monday, January 20, 2003 - 10:48 pm: |
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yes this is normal when working in 30degree weather. there isn't much that you can do. but you can solder all the xtal tuning cans together. this keeps them from expanding/contracting toward and away from each other during temperature swings, therefore helping to keep the frequency more stable. it also helps with vibration. will it make a huge difference, no, but it helps.
the clarifier in these are 'fine' clarifiers with about +/- 1.5kc of swing. so at 36degrees, you may just run out of 'play' in the clarifier as you have experienced.
the 2sb817 -ONLY- adds better stability for the modulation area/tx voltage reg. the rest of the radio is unaffected(90%). this is a common misconception that i see all over the net. it is also a common misconception that this modification really benefits the cb version/s of radios using this type of modulation scheme. they pull too little current due to their 'choked' output for this modification to be of any real benefit. |
2600
| | Posted on Tuesday, January 21, 2003 - 1:03 am: |
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SS8541 is right on. The more range your clarifier knob has, the more it will drift with temperature. A 40-channel CB that has the clarifier "unlocked" ONLY, and covers under 5 kHz end-to-end will nearly always be more temperature-stable.
A friend made his Galaxy Saturn more stable a few years ago with an odd trick. He cannibalized a junk camcorder for a weird part that is used to keep the spinning drum warm when the 'corder is used in low temperatures. It was a small blob with two wires that contained BOTH a small heating element and the thermostat. He soldered the crystal/trimmer coils together (as suggested above) and stuck this to the side of them. It required 5 Volts DC and would keep the crystal and the trimmer coils warm while the radio was off. He added a 5-volt regulator chip that ran off the unswitched part of the base-station power supply. Doing this in a mobile would create a steady current draw, probably more than a dashboard clock. Might be too much for a car that sits too long between trips.
I asked him for a part number, something I could use to buy some of these, but alas, it was unmarked. Great idea, hard to copy.
73 |
N3umw
| | Posted on Tuesday, January 21, 2003 - 1:35 pm: |
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2600, The weird part with two leads was probably a thermistor. It looks like a lead (Pb) coin with two leads just soldered to the sides.
The Down East Microwave group (HAM) has a method of stabilising a crystal LO by desoldering (removing) one lead of a 60 degree C PTC (thermistor) and then resoldering that side of
the thermistor flat to the crystal's body and soldering the remaining lead to a voltage source. You then insulate the two components with cotton balls and/or expanded polystyrene that is cut to fit.
I guess the same thing would work in a CB if you first soldered all of the crystal cases together. Although using two or three PTCs might be better.
The PTC acts as a resistor and generates heat (resistance) until it reaches 60 C (140 F). As most radios work at less than 60 C (even on a hot day in a car), this keeps the crystal at a constant 60 C temperature preventing drift. The foam insulation helps to stabilize the crystal temperature by blocking outside variations.
As 60 C is much warmer than the radio's normal operating temperature, you would probably need to realign the rig after performing this mod.
I might try this just to get my rig on frequency faster. By the time my rig warms up, I'm home! |
Freebird
| | Posted on Wednesday, January 22, 2003 - 8:10 pm: |
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you may want to try a cobra 148gtlits below zero here where i am and this radio is right on freq when i turn it on.just my 2 cents. |
Ss8541
| | Posted on Thursday, January 23, 2003 - 12:59 am: |
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freebird, you better keep that 148gtl. if it is right on the money in ssb mode(or any mode for that matter) at 0 degrees, it is 1 in a million. |
Freebird
| | Posted on Thursday, January 23, 2003 - 4:37 pm: |
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yup i will keep it i dont know why but when i turn it on in my car at zero degrees its right on the money.guess its just a good radio.oh well om off to talk to skip on my base..good luck!  |
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