Cobra 139XLR or Uniden Washington??

[2NC995]

Apparently this post was for almost nothing. both sold overnight.

At least I gained knowledge. Which is good.

Washingtons come up all the time. I recently bought two. Both reasonably priced. One is hacked to high heavens with half-assed mods and it’s inside looks like it spent a year in a saltwater marsh....and other an unmolested gem.

You really never can tell what you’re going to get.

 

[ExitThirteen]

Both are great radios, but between the two, the 139XLR gets the nod... it's hard to top the venerable D858 SSB chassis. I run a Realistic TRC-458 myself, which is the same basic chassis as the 139XLR, and it's been amazing. The Uniden Washington would be a slightly better performer on AM, but if you plan on running SSB most of the time, the 139XLR will have the edge over the Uniden Washington.


~Cheers~

 

[Dmans]

ExitThirteen,
Couldn’t agree more. My 139 is currently down with a bad final so have to work my back up rig. TS440. (Though I do have about 6 other back ups I could rotate in)
Once the 139 is back going it will be centered on the desktop!

73
David

 

[ExitThirteen]

Dmans,

If that radio has had the service bulletin done that involves installing a 47 ohm resistor between the base and emitter of the final, change it from a 47 ohm to a 22 ohm, 1/2 watt. It allows far easier adjustment of the bias current, and it helps with stability. If the radio does not have the resistor installed at all, do install a 22 ohm there. I did that with all of my D858 radios, and it helped me dial in the bias current much easier.

Also, if replacing with a C1307 final, 15-20mA is all you need. If running a C1969 or C2312, 50mA is ideal.


~Cheers~

 

[Dmans]

I will probably go with a 2312 (I have a few) and will set the bias conservatively. (I have plenty of outboard power. If I can get 10 clean watts out on SSB, I am happy.)Best I can tell the driver and final are original to this unit. It has had the factory service bulletin performed (47 ohm on b/e of final) I will try a 22 ohm and see I fit dials in better. I did change all of the VR's in this unit to multi-turn VR's. That alone has made it easier to align. I have had many radios in the last 40 or so years, but this one will never leave my possession!
I do have a mint 138XLR I may experiment with. I don't run any mobile radios but own about 20 of them.

73
David

 

[ExitThirteen]

My TRC-458 currently runs the C1306/C1969 pair. At the time, I didn't have any extra C1307's, (I now have 10+ good C1307 used pulls) so I installed a C1969. Works great, no issues. I installed a 22 ohm resistor at the final, and a 39 ohm resistor at the driver. Bias currents are set at 40mA for the driver, 50mA for the final. Radio has performed flawless since. I got the radio with a failed final and a messed up bias circuit that was putting over 1 amp (yes, you read that right) of bias current to the final. The 22 ohm resistor across the final fixed it 100%. I also noticed that the driver bias current would "float" between 25-50mA, so I installed the 39 ohm resistor across the driver. That fixed the floating issue 100%. Let me be clear, the floating issue was not the driver thermally tracking, but actually floating up and down with no modulation on SSB.


~Cheers~

 

[Dmans]

Would that be from some instability in the bias circuit? Glad to know the 39 ohm resistor solved it.
I was having problems setting the bias on my final. It was all over the place and would not stay steady at all. Trying to adjust with the VR caused me to chase the 15Ma target and never could get it set. After installing the mutli-turn pot I was able to nail it down at 15Ma but had no output from it. I guess 43 years took it's toll.

If I can get some of these other projects completed, I will get back to it. Been on a TS-440 with dots on the display for about a month (as time permits) that has caused me to lose some hair!
That may come sooner rather than later as my work schedule just went from 65-70 hours a week to 40. One good side benefit to the coronavirus issue for me.

Thanks for the info. I may need to bother you in the near future for more if I can't get it straight.

73
David

 

[ExitThirteen]

Feel free to PM me if you need any help. I'd be glad to assist. It's never a bother!

As far as the floating goes, I'm not sure if it's directly caused by any instability within the circuit itself, but I WLL say that the bias current range is WAY too large. On almost every D858 chassis I've ever worked on, the bias circuit would allow me to adjust bias current from 0-400mA+. That range is far too large. By installing the lower value resistors, that tightens the range on the bias current adjustments considerably, in the neighborhood of 0-100mA. This allows easier adjustment, while increasing stability.


~Cheers~

 

[Dmans]

the bias circuit would allow me to adjust bias current from 0-400mA+. That range is far too large.

That explains a lot! When you have a 300 ohm pot that turns about 190 degrees that is 43 years old...........no wonder!

73
David

 

[ExitThirteen]

Well, to be fair, the original bias circuit didn't really have any range limit set resistors in place, that's why the adjustment range is so large. That's what the 22 and 39 ohm resistors do, is they set the range limit of the 300 ohm adjustment pots. It's not so much the fault of the 300 ohm adjustment pots themselves, as much as it was just poor design of the bias circuit in general.


~Cheers~

 

[Dmans]

I had to do something similar to my TR-296 AKA "The Turd" to get it set. Chased it off and on for months (as time permitted). After adding the resistor (don't remember the value) it set just fine and talks great. So it sits on the shelf and watches me like all the rest of them. Ha!

Do you think with a good final transistor the 47 ohm at the b-e junction will allow proper bias setting?

This regular paying job of mine is getting in the way of my hobbies. May be time to examine my priorities!

73
David

 

[ExitThirteen]

For the D858 chassis, 22-27 ohms will be the sweet spot for the final in most cases. For the driver, 33-39 ohms tends to be ideal. 47 ohms on the final still tends to make the range a bit too large. It certainly helps decrease the range, but it's usually not enough, especially if the final is the original C1307, since the bias current on that final should be in the 15-20mA area.


~Cheers~

 

[CB590]

Just saw this one. Has a Uniden Plate (where there is sometimes a President or Washington) on the speaker, and Washington under the AM/LSB/USB indicator area. Kinda different IMO. Has extra channels. $170+35

Says 4-8 AM and 12 on SSB

 

[w9cll]

Much later model and honestly not worth what they are asking. It's not a bad rig but I feel you can find it cheaper than that.

 

[Shadetree Mechanic]

For the D858 chassis, 22-27 ohms will be the sweet spot for the final in most cases. For the driver, 33-39 ohms tends to be ideal. 47 ohms on the final still tends to make the range a bit too large. It certainly helps decrease the range, but it's usually not enough, especially if the final is the original C1307, since the bias current on that final should be in the 15-20mA area.


~Cheers~

Bump, great information. I will be checking this out. I now have a Madison, a 139, and a Robyn. All 858 style, thanks!