There has been a lot of confusion concerning the bias settings on the 858ssb chassis. The Sams for the Cobras and the Presidents Washington and Grant, as well as the Cobra service manuals, call for a driver bias of 40 ma, and a final bias of 70 ma. The Sams for the Realistics, Robyns, Teaberrys, and the Presidents Adams and Madison
(plus the Realistic service manuals) specify 40 ma and 15 ma respectively. The reason for this difference is that Dynascan (Cobra) and President placed their 858 orders with Uniden well ahead of the other marketters. Robyn and Teaberry waited until 3 months later to place their order. Tandy waited until June of ‘77 to place their order to give dealers time to sell existing stock, and to have the new rigs available for the ‘77 Christmas sales season.
In the intervening time, Cobra and President were overwhelmed with warranty claims for blown drivers and finals. Uniden suspended production, set out to determine the problem and devise a solution. It turned out that the original driver/final circuit design had too much power potential. When these rigs were set to the FCC limits of 4 watts AM and 12 watts pep SSB, they would cruise along all day no problem. But when the screwdriver jockeys
in the field cranked up the supply voltage, peaked out the transmitter, and clipped all of the limiters, these sets blew drivers like crazy. The final transistor, which ran pretty hot anyway with the specified 70 ma bias current, started smoking from excessive keydown times.
Uniden responded by redesigning the driver/final circuit for less output potential, and specified a new final bias
setting of 15ma. They issued field service bulletins 1208 and 1208A which specified the necessary circuit changes, and sent the bulletins to their authorized service centers. The Sams for the Cobras and the President Grant and Washington were published before the service bulletin, and show the original circuitry and the 70 ma final bias
spec. On the other hand, the Sams for the Realistics, Robyns, Teaberrys, Pres. Adams and Madison (as well as the
Realistic service manuals) were published after the service bulletin and show the revised circuit and the new 15 ma bias spec.
BTW, the service bulletins are shown on page 5 of Sams #135.
Many techs are unaware of the service bulletins and are setting the bias wrong. If you have any doubt .....
http://www.cbtricks.com/radios/cobra/138xlr/graphics/cobra_138xlr_139xlr_tb1208a.pdf
All of this , word for word from @unit_399 . I guarantee it is the gospel
(plus the Realistic service manuals) specify 40 ma and 15 ma respectively. The reason for this difference is that Dynascan (Cobra) and President placed their 858 orders with Uniden well ahead of the other marketters. Robyn and Teaberry waited until 3 months later to place their order. Tandy waited until June of ‘77 to place their order to give dealers time to sell existing stock, and to have the new rigs available for the ‘77 Christmas sales season.
In the intervening time, Cobra and President were overwhelmed with warranty claims for blown drivers and finals. Uniden suspended production, set out to determine the problem and devise a solution. It turned out that the original driver/final circuit design had too much power potential. When these rigs were set to the FCC limits of 4 watts AM and 12 watts pep SSB, they would cruise along all day no problem. But when the screwdriver jockeys
in the field cranked up the supply voltage, peaked out the transmitter, and clipped all of the limiters, these sets blew drivers like crazy. The final transistor, which ran pretty hot anyway with the specified 70 ma bias current, started smoking from excessive keydown times.
Uniden responded by redesigning the driver/final circuit for less output potential, and specified a new final bias
setting of 15ma. They issued field service bulletins 1208 and 1208A which specified the necessary circuit changes, and sent the bulletins to their authorized service centers. The Sams for the Cobras and the President Grant and Washington were published before the service bulletin, and show the original circuitry and the 70 ma final bias
spec. On the other hand, the Sams for the Realistics, Robyns, Teaberrys, Pres. Adams and Madison (as well as the
Realistic service manuals) were published after the service bulletin and show the revised circuit and the new 15 ma bias spec.
BTW, the service bulletins are shown on page 5 of Sams #135.
Many techs are unaware of the service bulletins and are setting the bias wrong. If you have any doubt .....
http://www.cbtricks.com/radios/cobra/138xlr/graphics/cobra_138xlr_139xlr_tb1208a.pdf
All of this , word for word from @unit_399 . I guarantee it is the gospel