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Old uniden Grant PC-196aa board

ExitThirteen,
I believe Robalo is reverting this unit back to the original (but flawed) design per unit_399's instruction. Dangerous area to play in.

Robalo said:
After the HIGH POWER MODIFICATION FOR UNIDEN 858 SSB CHASSIS
whats the driver and final ma measurements please. ca not find cobra 139xlr service manual .
Click to expand...

73
David
 
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Recheck your resistance FROM your Test Point, to the BASE lead of the FINAL.

What is your OHMIC value (should have little to none)

Do the same for your Driver - Test POINT to BASE lead - OHMIC check...

We're doing a simple continuity check but we need to rule out a bad connection.
 
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Here's my thoughts...

if you introduce a higher ohmic value - say Buffer resistor - in an effort to protect the Final - you add a resistive element across the Diode - affecting the mA drive - but if the Diode is reverse biased - then you are seeing only the ohmic result of when the Base turns on...the diode conducts one way, but the Transistor and Diode paring are back to back...the Diode is simply not conducting due to the polarity of it's connection in the bias circuit...
 
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No problem E-13 I just reverse the service 1208A to see the myth if those radios swing to plus 20 watts and just find out it the same. It not worked it just for save the final
 
Yeah, the radio isn't going to swing more because of more bias current, only thing increasing bias current does it change the class in which the final operates, up to a certain amount of current. Each final will bias differently, and will have their own "soft cap" on bias current, depending on whether it's a C1307, a C1969, or a C2312. It won't make the final produce any extra output, as you discovered. All of my D858 chassis that have a C1307 final in them have had the bulletin done, and are set at 20mA. All of those radios produce in excess of 20W PEP on SSB @ 20mA.

That "myth" is definitely just that... a myth. :LOL:


~Cheers~
 
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ExitThirteen,
I had a handful of C2312 "pulls" for my 139XLR (best measured an Hfe of 35) so I ordered a couple new ones.Waiting on delivery. I will also be changing TR401, D406 and TR302 as there was a little more heat on these parts than I was comfortable with! What are your thoughts on remote mounting TR401 to the rear panel on a heat sink?Hopefully this week it will be done!

73
David
 
ExitThirteen,
I haven't though about that but may! Keeping all of the heat away from any other components was my line of thinking. Somewhere around my mess, I found an aluminum heat sink for a TO-3 device and thought as a rear panel modification, I could live with that modification as the rest of the unit appears completely stock and as built from the factory. (Rationalizing in my head the debauchery I have seen on these great radios!)

Thanks. I will let you know how it turns out!

73
David
 
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Dmans,

Oops. I got confused on TR401, that's the pass regulator transistor for the power supply, yes? If so, leave that one where it's at, it's not hurting anything there. If you are concerned with cooling, you can replace it with a TIP35C and it will run a little cooler, just make sure to use a ceramic insulator. I thought TR401 was a TO-220 case transistor, so that was my fault. I'm so scatter-brained today. Sorry! :confused:


~Cheers~
 
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ExitThirteen,
I may very well use a TIP35C there. That transistors capabilities compared to the 2SD188 should allow me to leave the rear panel stock!

Thanks for the heads up! Yet another reason this forum is my only Social Media participation!!!

73
David
 
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Anytime an 858SSb driver/final circuit is returned to its original form, the driver and final bias settings should be 40ma and 70 ma respectively. The point is to maintain the FCC limits of 4 watts deadkey on AM, and 12 watts PEP on SSB. People maxing out the original sets is what caused them to have problems. The design wasn't "flawed", the users were. Stick to the FCC limits, and these rigs will be loud, clear, and RELIABLE.

BTW, Sue Nguyen sets the final bias on her 858 rigs at 100ma. I argued with her about that, as I thought 100ma was way too much. But she was adamant and maintained that it was best. Sue is THE BEST TECH I have ever known, and I respect her opinions. But in this case, I will stick to 70ma.

- 399
 
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Something not right if a pick and tune the radio on SSB I get about 18 watts, on AM drop to 1.5 watts. Now if I pick and tune on Am I get 4 watts swing 18 watts,and SSB drop to 12 watts what’s wrong whit this issue.
 
Technically, bias current should be set based on the beta (gain) of the transistor, and not by a set number. Now obviously, in a mass production setting, this was impossible to do... but as a rule, that's the proper method. But if replacing the driver, final, or both, that's the way it should be done. If you're a purist. :LOL:

But, I'm not that much of a stickler about this, unless I know I am installing a transistor that is very different than the stock transistor (installing a C1969 in place of a C1307 or vise versa).

Unit_399,

You are 100% correct that 100mA is too high for a C1307... that is far beyond what is needed to bias the transistor into a linear region for use on SSB, and all it will do is produce excess heat in the process. 60-70mA puts it well into AB territory.


~Cheers~
 
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