RCI 2950DX strange TX meter readings

[Lkaskel]

Hi Everyone,
I have an op with a 2950DX and mostly uses it on AM. When he 1st keys up the meter will display 4 bars and display 3-4 bars of red on modulation. After about 30 seconds of use (not continuous) the meter will no longer show modulation. It wil contiinue to show the carrier power. The radio's output does continue to function normally. Any thoughts as to why the meter will not show modulation?

Thanks!!

 

[Lkaskel]

As an update, I broke out the trusty Knight VTVM and measured the input to VR10 (TX meter adjustment) to compare it to the wattage output of the radio. On an AM carrier I read 4.2vdc at VR10 and the voltage dropped to 3.3vdc over 15 seconds while the carrier remained steady. While I do not have a schematic for this board (EPT695016A) other 2950 schematics seem to show that the TX meter is driven from the gate of the driver. What on earth could cause the voltage to VR10 to drop like that when the carrier is solid?

 

[nomadradio]

VR10 is fed from D89 and D90, set up as a half-wave voltage doubler, fed from C218, a one-half pf cap that is fed from the output of the final transistors.

Don't have the 2950DX schemo handy, but the 2995DX uses the same circuit board.

https://cbtricks.org/radios/rci/rci_2995dx/graphics/rci_2995_dx_sch.pdf

Either those diodes or C218 would seem the logical culprits. Unless the trimpot itself is hosed.

73

 

[Lkaskel]

VR10 is fed from D89 and D90, set up as a half-wave voltage doubler, fed from C218, a one-half pf cap that is fed from the output of the final transistors.

Don't have the 2950DX schemo handy, but the 2995DX uses the same circuit board.

https://cbtricks.org/radios/rci/rci_2995dx/graphics/rci_2995_dx_sch.pdf

Either those diodes or C218 would seem the logical culprits. Unless the trimpot itself is hosed.

73

Thanks LC!!

This is a clip from the 69VHP that I was referencing.



I'll have to trace it out on the board to figure out which version I have.

 

[nomadradio]

Well, that's a horse of a different color.

That one sets the bias voltage on the final transistors. It's set by breaking the DC circuit to the final drain circuit and inserting a current meter. Old rule was 60 mA per transistor. I've been told that 20 mA is safer.

73

 

[Lkaskel]

Well, that's a horse of a different color.

That one sets the bias voltage on the final transistors. It's set by breaking the DC circuit to the final drain circuit and inserting a current meter. Old rule was 60 mA per transistor. I've been told that 20 mA is safer.

73

UGH!!!! I need to get my head above water!!! That's better than saying I need to remove it from.......

So, I think that schematic is wrong by labeling VR10 in that location. In the alignment guide VR10 is for TX Meter. On that schematic the TX meter adjustment is labeled VR9. I should have known better and slowed down!!!

I did pull the capacitor. It's a 1uF 50V tantalum 1206 case. I put a through hole electrolytic in it's place and all is well! All I have on hand are 1uF 16V tants 1206 case. Interestingly enough earlier versions of the 2950 used 1uF 16V in this spot. I wonder why they bumped it to 50V? Was it a manufacturing conveinence of does it really need a 50V. Hmmmm. When the radio keys it's hitting that circuit with 4.6vdc and goes to almost 6vcd on 100% modulation.

Thanks LC so much for your help!!! Sometimes we are our worst challenge