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148GTL final bias COLD VS WARM

KD2GOE

KD2GOE

W9WDX Amateur Radio Club Member
May 30, 2013
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Buffalo
Hi guys it is Me again (y)


I got a cobra 148 GTL (Philippines 1992) I tested it out in the house talking to some friends USB, it was just off freq so I fixed that and the cut limiter's R131 and put the slug back in L38 and pushed coil L3 back together and set the Driver & Final bias to 30/50 "was 45/100 :unsure:". now it is talking Great!

BUT! when it is cold like 30° in the car my friends on USB told me "it sounds like crap" and "it sounds a little rough around the edges" which hurt my feelings as it would to any one.. but as my car got up to temp i was told it sounds awesome!

Now I am thinking I am dealing with a biasing problem today I took the radio out side and let it chill out for 2 hrs... rushed it back in plugged every thing in and the bias was 28/28??? :unsure: that would be why I sound like crap on SSB..

when adjusting the Trim pot it is not linear and jumps up then down so I am expecting that is the root problem. but not ruling it out.
I am going to do one more test and look at the resistance on the trim pot jumps around.. is there any other thing you guys think it can be if the trim pot is fine? the temp sensing diode?
 

All parts in the radio should be OK down to that temperature and even below that to a certain degree. Doesn't mean that they will all fall within those specs. In your case, it could be that a couple of parts or even one that aren't into spec. Could also be a poor solder joint that is made even worse with the lowering of temps.

Are people saying that your audio is distorting; or are you just way off freq?
 
Robb said:
All parts in the radio should be OK down to that temperature and even below that to a certain degree. Doesn't mean that they will all fall within those specs. In your case, it could be that a couple of parts or even one that aren't into spec. Could also be a poor solder joint that is made even worse with the lowering of temps.

Are people saying that your audio is distorting; or are you just way off freq?
Click to expand...

they are saying i sound distorted.. till it warms up.. as for freq they say i am fine..

as it warms up the bias slowly creeps up from 28mA to 55mA
is there a way to test that temp diode?

i had a lot of caps left over from when i redid my RCI-2990 so i went and recaped the radio it did not help...

i am going to put an order for a few 1K trim pots what are the tol of the stock ones 20 or 30%?
9C1Driver said:
It just needs to warm up. I would restore R131, better cleaner audio that way.
Click to expand...

i did put R131 back in...
 
Last edited:
mkube396 said:
they are saying i sound distorted.. till it warms up.. as for freq they say i am fine..

as it warms up the bias slowly creeps up from 28mA to 55mA
is there a way to test that temp diode?

i had a lot of caps left over from when i redid my RCI-2990 so i went and recaped the radio it did not help...

i am going to put an order for a few 1K trim pots what are the tol of the stock ones 20 or 30%?


i did put R131 back in...
Click to expand...
I like to take out old caps and then test them after the re-capping is done.
In older radios, you would be surprised how many of them are marginal and some are outright close to shorting.
But it could be bad solder joints, dirty VRs, even some transistors and diodes can get funny if temps get too high or low.
 
I found a 1k trim pot in a cobra 29 i have it fixed the adjustment so there is no dead spots now...

i put it back out in the cold and same thing final bias was low at 30mA and was creeping up... To add it is not steady when up to room temp set at 50mA it will creep up to 55mA in 30 sec with it keyed down. the driver will only move up .3 of a mA
is it normal for it to creep a little?
i all so touched up all the solder joints..

the only things that come to mined is the temp biasing diode and TR37... is there a way i can test them?

the job of the biasing diode is to drop the mA's when the final gets to hot?
 
The value of some components changes with heat. This is what you are seeing.

the only things that come to mined is the temp biasing diode
Click to expand...

I'd be really surprised if they put temperature compensation into a CB. I suspect that isn't its function at all.
 
Re-cap the entire radio since its 21yrs old. There's 2 limiters in 148's one for AM and one for SSB. Im guessing you clipped the wrong one. If its a nice radio dont use it in your vehicle till spring, lol.
 
mkube396 said:
I found a 1k trim pot in a cobra 29 i have it fixed the adjustment so there is no dead spots now...

i put it back out in the cold and same thing final bias was low at 30mA and was creeping up... To add it is not steady when up to room temp set at 50mA it will creep up to 55mA in 30 sec with it keyed down. the driver will only move up .3 of a mA
is it normal for it to creep a little?
i all so touched up all the solder joints..

the only things that come to mined is the temp biasing diode and TR37... is there a way i can test them?

the job of the biasing diode is to drop the mA's when the final gets to hot?
Click to expand...

By replacing it, if you have a parts radio to steal from.
Or, if you have a can of freeze spray testing the diode in an indoor environment after the radio has warmed up. This method of using freeze spray to isolate/find a bad part is pretty much the way it is done if/when there is a part with a thermal issue.

http://www.walmart.com/ip/Max-Pro-FR-Blow-Off-Freeze-Spray/21576411

M0GVZ said:
I'd be really surprised if they put temperature compensation into a CB. I suspect that isn't its function at all.
Click to expand...
The bias diode here in question is used for an AB biased output. That is the way most bipolar transistor CB/SSB radios are built - BTW. Can't say that holds true for the MOSFET designed radios, but those radios do have FCC approval as 10m radios.
 
Last edited:
Robb said:
M0GVZ said:The bias diode here in question is used for an AB biased output. That is the way most bipolar transistor CB/SSB radios are built - BTW. Can't say that holds true for the MOSFET designed radios, but those radios do have FCC approval as 10m radios.
Click to expand...


Yeah I get the feeling that he thought it was to compensate for ambient temperature.
 
Z-MAN 369 said:
Re-cap the entire radio since its 21yrs old. There's 2 limiters in 148's one for AM and one for SSB. Im guessing you clipped the wrong one. If its a nice radio dont use it in your vehicle till spring, lol.
Click to expand...

I did I posted that in Post #5


Robb said:
By replacing it, if you have a parts radio to steal from.
Or, if you have a can of freeze spray testing the diode in an indoor environment after the radio has warmed up. This method of using freeze spray to isolate/find a bad part is pretty much the way it is done if/when there is a part with a thermal issue.

Max Pro FR Blow Off Freeze Spray: Computers : Walmart.com
.
Click to expand...




Yes I did order a can of freeze spray on line yesterday (y) when it gets in ill give it a blast... and report back what happens....




M0GVZ said:
Yeah I get the feeling that he thought it was to compensate for ambient temperature.
Click to expand...

nope I do understand what the diode is there for.... and being attached to the chase it can be affected by ambient temperature
 

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