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$3 For Better DX-959 Stability?

Robb,
I have worked on a DX2547 that belonged to a friend and I reduced the frequency drift by 7-10x. It is a very long story but I recognize exactly where in the process you are because I was there myself. The radios frequency is basically temperature dependent, to put it another way. I didn't know what all the talk was over the years on the internet about "drift" since I never owned a Galaxy, and the SSB rig I've used the most over the last 21 years is a Cobra 146 (if it drifts, I can't tell whatsoever--clarifier never ever moves). Anyway, after experiments and measurements it was obvious: ambient temp up=frequency down. I went through many drawing board ideas and thoughts but the answer was in my 1995 Handbook. Also, you'll see it in different web pages usually by hams trying to stabilize an old rig or some homebrew vfo they built. Google: "A Look at VFO Drift in the MFJ QRP-Cub." Also have a look at WA0ITP's book "Crystal Sets to Sideband" chapter 10, which is online in .pdf at the qrparci.org website.
In a nutshell, what I found was that the drift is most likely being caused by a change in permeability of the slugs in the tuning coils. This was found experimentally after reading of the possibility. At any rate, the solution is far more important than the cause of the problem. Galaxy, IMO, put SOME temp compensation in the radio with the UJ caps but not enough. Why did they stop where they did? Because: 1.the first thing everyone does when they open the box is unlock the clarifier. 2.charlie only requires .002% freq. accuracy (that's 540Hz at 27MHz, yuk!). 3. you never get more in a cb than absolutely necessary, and that's industry wide historical practice. Therefore, I INCREASED the compensation by replacing caps that drifted more than the uj's in order to counteract the inherent drift in the radio. It took a very long time to arrive here but: I replaced C130 with a 22pf P3K(n1500) and replaced C281 with 3x 33pf caps: 1@ uj (n750) and 2 @ R3L (n2200). That last one gave a "custom" TC and they were installed in the original location, plus in the unused j28 & j29 positions. The drift went from approx. 400-450Hz between 65-90F down to approx. 40Hz over that temp range. The owner finally got it back and was tickled to death over it. The caps came from Mouser, about $2 actually. I never got around to writing up what I did, so when I saw your posts (and heard you on the radio discussing it!) I wanted to share it with you. There's a bit more so email me. Out of time for now. 73 and good luck.

Awesome! Or as Bart Simpson would say,"Cowabunga!". I'd love to share this info with DTB and hear his reaction.
By the way, they've got big sounding fireworks up here in Tennessee. I'm up here for the weekend. No wimpy fizzing puny stuff here. I had to look out my hotel window to see if my SUV blew up or something. :pop:
 
What I did follows. However, Mouser has obsoleted one of the caps used. It was an AVX brand 22pF P3K 100V cap and had standard size leads that fit right in the board. However, they still sell a 22pF P3K with larger 20 gauge leads, larger spacing, and with a slight re-drill of the board should work. See attached drawing. First, remove C281. It is a 100pF UJ(n750). Replace it with a 33pF U2J. It is available at Mouser in a 1KV pkg that fits fine in the radio. Next (refer to drawing) prepare a pair of 33pF R3L caps (Mouser) as shown. These are 3KV size caps; can't find anything smaller, however, the fat 20 gauge (0.032") leads fit into the holes on the board where J28 and J29 would be. If your radio has connectors here, but not used, remove them. Otherwise vacuum out the solder. If they are in use for some other option or something, you'll have to find another way to do this. The SG pad of J28 conveniently leads right back to the top side of C281 (how convenient!). This is where we will parallel our 3x 33pF caps to get 99pF. Bend the right lead of C281b just below the body of the cap out to the right so it lays flat against the pc board and hook it around the left lead of C281c and solder. You'll have to get the spacing right first if you do this off the board. The other two pads lead to ground which is perfect. Solder everything in place, recheck the SG joints to make sure the first one didn't reflow with the last one. You now have a 99pF combo at around n1700 temperature coefficient. Next, pull C130. For a quick reduction of drift down to about 90HZ from 65-90F replace it with another 33pF U2J cap. Otherwise, to install the 22pF P3K you'll have to open the holes a bit with a small drill about 0.036" to 0.040". Then bend the leads to fit and solder. This probably isn't the only way, you could just tack it on to some smaller leads passing through the holes. This combo should give you <50HZ drift from 65-90F. What we are doing is adding "drift" in the opposite direction to counteract the inherent drift in the radio. It is the easiest solution. The closer you get the better they cancel each other out. Other combo's of caps may work, maybe better, but this is what I found available that worked well. More doors open when you "hack" the caps in with the larger leads. Of course, realign the radio, particularly the masters L20-22. Much more to follow. So many details, so little time, and I'm sure many questions will come. I will gladly answer them but it may take a few days as time is scarce. 73 and have fun!
 

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Inquietude.... I know its an old post and I was gonna do the other suggested cap mods years ago by Robb and decided to try something today because it was driving me nuts for so long having to clarify every minute for 20 minutes till the radio warmed up, so I just did your 4 cap mod and it works incredible. I love it thanks for the tip.
 
After all of this time, I think that Xit3's ideas were definitely inspiring. I did not try Inquietude's mods; but they would be interesting to look into. But Xit13's mod concerning the varactor diode change to a early Uniden varactor and two cap changes were by far the best way to go!

Still would like to hear from anyone that has anything to add . . .

Seems that this radio - and others built along the same design/style - are still a problem that cannot be fully made right due to the poor design. This radios true fault is in the design off of the drawing board; and only a few things are only marginally helpful to change that situation.
 
Who's Xit3 and what are the mods, kind of lost me there, which isn't hard to do...

Jay in the Great Mojave Desert
 
Who's Xit3 and what are the mods, kind of lost me there, which isn't hard to do...

Jay in the Great Mojave Desert

Xit13' is the short form for 'Exitthirteen'
Sure you know him, he's been here a long time, as you have been too - Jay.

The 959 Galaxy radio use the SVC251 varactor diode. Changing that diode out for the early Uniden type varactor limits the range of the slide. Which means that there is also less variance/'drift' in temperature changes. The cap changes Xit3 made also keep that part of the circuit a bit more stable as well. There is R113 IIRC, that changes the amount of clarifier slide. Limiting the range of the clarifier also has reduces the amount of drift the radio will have.
 
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Hello Robb: Wow great info here and great thread. Thanks.

Yeah limiting or reducing the clarifier frequency is a great call, don't need to slide down a bunch of Kc's anymore.

Jay in the Great Mojave Desert
 
I had an RCI2950 from 1991 to about 1998 then it was stolen. I first noticed dimming on transmit so I changed and upgraded the voltage regulator which helped but did not solve the problem. Then I found that the crystal filter was bad. That solved the problem.

So the problem was the crystal filter all along but the voltage regulator was also bad and on it's way out.

My point is that everything affects everything and often more than one thing is the issue.

Clearly your modifications helped but did not solve the issue. I would suspect the parts you did not change are the ones that are to blame. I suspect the crystals. I used a wheat bulb on some radio's to create a max shift ghetto crystal oven to warm the crystals up and then keep them at temp in mobile use. I would see temp's as low as -28F in the winter in my car/truck and it could take some time for things to heat up and even then they could fluctuate a lot over the course of a day.

Sometimes controlling the operating conditions artificaly is more effective then attempting to fix the problem properly with higher quality components. The jelly bean parts are not usually the problem it is the non-jelly bean parts that are often the real bottle neck for reliability and consistency.
 

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