858PLL final bias current low

[liquidh8]

I have a president washington, 858PLL, I was going thru the alignment procedures, and I set the driver bias to 40mA, but when I go to do the final, I only get 30mA.

The radio seems to talk fine and puts out 22-23 watts on SSB, and about 20-21 on AM, but it is a 1979-80 radio and I just bought it and am going thru it, this has me a bit stumped.:headbang:headbang

Any suggestions?

Thanks guys!!

 

[Robb]

Have you worked on many radios/done bias adjustments before?

Never worked on the 868 type Washington before myself.
What were the bias set before you adjusted them?
If anything, I would put them back to where they were in the mean time.

 

[liquidh8]

Yes, I have. The driver bias wasn't but 3-4 mA off, the spec says +/- 10. But the final is right around 33mA, It won't go any higher, I can get it to zero, but no higher. The spec says 70mA +/- 10, this is the 1307 final.


But I dunno why it is off, I left it the, but I would like to know what the deal is.:headbang

 

[Robb]

I can't find a schematic for the 858 chassis.

 

[liquidh8]

I have a link for one, but when I downloaded it I have nothing. Though I think the cobra 139xlr is similar, using the 858Pll

http://www.cbtricks.com/radios/cobra/139xlr/graphics/cobra139xlr_sch.pdf

 

[Robb]

I have a link for one, but when I downloaded it I have nothing. Though I think the cobra 139xlr is similar, using the 858Pll

http://www.cbtricks.com/radios/cobra/139xlr/graphics/cobra139xlr_sch.pdf

Exitthirteen - a forum member here - has a lot of experience with the 858 chassis radios. He may well know the answer to the question. You might give him a PM if he doesn't see this thread first.


Here is the pdf for the 139 - here below

 

[liquidh8]

Ok thank you! I'll leave the thread thru tomorrow to see if anyone else reads it and has some suggestions. I am about to recap the radio, but would like to figure this out first.

 

[mackmobile43]

Check the power resistor in that circuit as it's value maybe wrong or degraded due to it's age.

 

[ExitThirteen]

Ok, Robb sent me here, I might be able to help (altho it's been quite awhile since I've had to check bias current on a bi-polar final on one of these old D858 AM/SSB chassis... everything I do now are mosfets on these).

Got a question, are you checking bias current in AM or SSB? To set bias current, make sure you're in one of the SSB modes, USB is generally most common.

By where you would set your bias current, you'll see (most generally) a white wire that can unplug off the board. Unplug that white wire on the final side, (there will be two white wires, one for the driver, one for the final) and check the pin that the wire plugs onto for 13.8V with a voltmeter. We'll start there. Tell me what you get.

~Cheers~

 

[liquidh8]

Check the power resistor in that circuit as it's value maybe wrong or degraded due to it's age.

Which one is that.

Sorry for the delay in responding, I never subscibed to my own thread, lol

 

[liquidh8]

Ok, Robb sent me here, I might be able to help (altho it's been quite awhile since I've had to check bias current on a bi-polar final on one of these old D858 AM/SSB chassis... everything I do now are mosfets on these).

Got a question, are you checking bias current in AM or SSB? To set bias current, make sure you're in one of the SSB modes, USB is generally most common.

By where you would set your bias current, you'll see (most generally) a white wire that can unplug off the board. Unplug that white wire on the final side, (there will be two white wires, one for the driver, one for the final) and check the pin that the wire plugs onto for 13.8V with a voltmeter. We'll start there. Tell me what you get.

~Cheers~

USB I am checking it in. I'll go and check that voltage.

I also developed a new issue with it, but that is in another thread.

 

[liquidh8]

I have 13.8 on both the test points for the driver and final.

 

[liquidh8]

revisiting this thread, I just found some service bulletins for the 139xlr, it has you remove a few parts because the final and or driver keep blowing, my driver blew like 3 times. After reading, the bulletin says 15mA for the final, 40 mA for the driver. I think this mod was done to the president since a few parts seemed missing, but I don't think the whole mod was done to it. Anyone have some insite?

 

[liquidh8]

bump, anything? I can't get this thing to sound good, I have to undo the cobra service bulletin. I have the schematic for the Washington, but the radio doesn't have r204, a 10 ohm 2 watt, instead there is a 47 ohm from base to ground of the final.

 

[Mk121]

Bumping this thread, because I am experiencing same issue original poster had. Driver bias sets fine but final bias won't go higher than 25ma or so. Maybe my radio is one that had the factory 15ma final bias? I have to look into that service bulliten.