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Older first generation 148GTL can not adjust driver bias.

Low_Boy

Low_Boy

Sr. Member
Jan 21, 2010
1,934
1,199
173
Rochester N.Y.
Probably one of the first 148GTL's. Galvanized inside of covers. White power jack. NEC1306 Driver.
Bias turned all the way down 80 ma on the driver. and shows frequency on SSB. the only way I can get the driver bias to 25 ma is to remove the 5K pot VR9 and install a 47K. I then Can get the bias where it belongs but the radio only puts out a few watts. Also carrier balance does not get rid of this problem.
R185 good and correct
R209 good and correct
C200 good and correct
C156 good and correct
C153 good and correct
L42 good and correct
L41 good and correct
1306 driver good
bias diode good.
All electrolytic's changed.
Also I do not not know what this is for but newer Taiwan 148 GTL's have a D510 prevision. this radio does not have that spot but there was a diode soldered to the underside of the board where it would go. There is no difference with the diode there or removed. The radio I have with the D510 prevision does not use a diode? What isn that diode supposed to do?
What am I missing.?
 

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Your diode may be soldered in wrong...(backwards)

When you can't adjust BIAS - the Diode may be installed in reverse - so it doesn't share the load current - but also you may want to prevent blowing the DRIVER by putting in a shunt resistor across the diodes mounting points to help prevent that problem from happening again.

See below...
Cobra148.png

The Driver doesn't come with a load sharing resistor - so you may want to put one in to help with problems further down the road when it comes to flaky operation.
 
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Low_Boy said:
... I do not... know what this is for but newer Taiwan 148 GTL's have a D510 prevision. this radio does not have that spot but there was a diode soldered to the underside of the board where it would go. There is no difference with the diode there or removed. The radio I have with the D510 prevision does not use a diode? What isn that diode supposed to do?
What am I missing.?
Click to expand...

Diode 510 is a provision for a POLARTIY protection diode. Note the "violet" colored wire - is that not the Test Point soldered in direct?

Then the D510 is for the prevention of reverse polarity when others whom may need to jumper the final...
 
Bias diode is soldered in correctly. When I get home I will try the diode and resistor and see what that does. Also the newer radio I used for the picture has a broken off leg on one of the disc caps between the final and driver. The problem radio also sounds terrible with the bias that far out.
 
Problems with bias usually are bad diode (can be ok on meter), bad bias pot or bad transistor you are setting bias to.
Diode is easiest to replace, take new 1n4148 ant try.
Mike
 
Then when you get time - check the Bias VOLTAGE too.

Not just the Current...

If voltage drops linearly and is adjustable - then you have an open line to the BIAS diode - the transistor is adjusting this mA reading as a linear switch.
  • That ability to adjust the BIAS voltage by trimming the BIAS pot, means the Bias diode is either out of circuit, blown or reversed.
It is my fear the Bias Diode may be soldered to CHASSIS ground, not Foil ground. Chassis is insulated from the main Negative lead on these older boards. So check to make sure the diode is soldered to FOIL ground...not Chassis - for if you can tap the DVM lead onto one side and then check it back to Chassis (the CASE ground back there) you need to find the FOIL ground side of this...

Again part of the problem with those older hand drawn boards versus the newer robotized ones that are "square edge right angle trace routings" is where the Boards mounting tabs at the Back panel rivets - are not FOIL ground they are CASE CHASSIS ground.
 
I will check all this. The bias diode tested good but may be bad. The Pot VR. Checked good also. I will see if bias voltage drops also. I will keep up to date.
 
Well with bias diode removed 350ma. With pot turned all the way down. The good is voltage does not change. The very bad. Only with power cord plugged in chassis and board ground have continuity.
 
Fixed this radio is very clean and unfolded except for clarified unlock. Everytime I plug it in I am hearing a click like a relay. Turns out to be IC 5 mixer. Somewhere along the line someone soldered pin 9 and 10 together bridged them. I unsoldered that and have no more board to ground continuity. Can adjust driver bias. Radio sounds good now. Mixer still clicks like a relay when plugged in with power on. What is all that about? Why would anyone jump pin 9 and 10 lol?
 
They are mounted back near the final and driver why not on them is a question for someone better than me. Heat sink compound for better heat transfer
 

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