Cobra 2000 no modulation.

[brandon7861]

If the diode were open, there would still be voltage across it in circuit. In SSB, there is 13.8v feeding the 3.3k/1.5k divider putting 4.3v on the emitter of TR26. With R131 supplying current to the base of TR26 from the 8v source, there would still be about 5v at the base (4.3v+.7v Vbe). You need to lift a leg and test the Vf of the diode out of circuit.

If I misunderstood the schematic and one leg of VR12 is tied to the wiper and not floating, the 500Ω of the pot with the wiper not making contact could also cause this because 500Ω may be too much resistance allowing the base voltage to remain too high.

 

[slingslingbinks]

Hey Frankenstein.
Ohm checked R165 in circuit and it was 11M, Pulled it and out of circuit it measured 1.48K. In speck if it's a 10% resistor at 1,500 ohm's for R165,
Just for giggles i tried the radio with out R165, Still no AM transmit modulation, But SSB transit's and has transmit modulation, PA is choppy out the speaker,
Tomorrow i'll hunt down another 1.5K resistor and swap it out, Mabey get lucky and find a 1/2 ir 1 watt'r to replace it with.

 

[slingslingbinks]

Where exactly do you have the red probe as you are measuring?

Touching each end of the diode, and the ground of the voltmeter to the wire soldered between the xtal's.

 

[slingslingbinks]

Hey Brandon,
You said------ You need to lift a leg and test the Vf of the diode out of circuit.
With my gorilla finger's it would be easer for me to pull the diode and test it and if good or bad, Replace it with new since the lead's will be longer, But i'll pull it and test it in the am.

 

[brandon7861]

Hey Brandon,
You said------ You need to lift a leg and test the Vf of the diode out of circuit.
With my gorilla finger's it would be easer for me to pull the diode and test it and if good or bad, Replace it with new since the lead's will be longer, But i'll pull it and test it in the am.

Hold on. Before you do, what were the voltages you measured on each side to ground? Was the cathode of D64 closer to 4.3v or ground? If the diode is good, it should drop about .7v across it, if open, there will be all 4.3v across it (in ssb). If R165 is open, you will not get the 4.3v at the emitter of TR26, rather something much closer to the supply voltage.

 

[guitar_199]

This is getting interesting..... because you should be able to clip meter black to ground.... meter red to the anode of D64 ..... and see that voltage change from about .6 v (one diode drop) and go up as you rotate the pot.....

When the pot is at the bottom the cathode is grounded which means no more than one diode drp at the base of TR26..... 0.6 to 0.7. With a 500 ohm pot split with a 10k resistor it won't climb a lot.... but it should climb.

Can't wait to see how this turns out....

 

[brandon7861]

This is getting interesting..... because you should be able to clip meter black to ground.... meter red to the anode of D64 ..... and see that voltage change from about .6 v (one diode drop) and go up as you rotate the pot.....

Can't wait to see how this turns out....

Would it be .6V? The emitter of TR26 is not at ground, rather tied to the 13.8v rail via voltage divider. The divider R166/R165 cuts that down to 4.3v, One Vbe drop from there is 5v at the base (or anode of D64), so the anode should be around 5v and the cathode around 4.3v same as emitter.

 

[brandon7861]

I don't think R165 is open because removing it made a difference which wouldn't happen if it was actually 11MΩ as measured in circuit). The results you got when removing it make sense when the base bias of TR26 can flow into the lower potential collector (Edit that wouldn't happen i now see, but) TR26 would not be on in that case so TR25 would also not be on (its base high), which keeps TR24 turned on. With an out of circuit test on R165 showing 1.48kΩ, trust that R165 was good and leave it in.

 

[brandon7861]

I edited my above post

 

[guitar_199]

Would it be .6V? The emitter of TR26 is not at ground, rather tied to the 13.8v rail via voltage divider. The divider R166/R165 cuts that down to 4.3v, One Vbe drop from there is 5v at the base (or anode of D64), so the anode should be around 5v and the cathode around 4.3v same as emitter.

Hey Brandon! I don't know if you are still up......

here is my thinking... double check me and see what you think....

As I look at the circuit around TR26 I see the base bias leg as the bottom 6.48 v node... through R131 (10K) then through D46 then over to the AMC pot VR12 (500 ohms).

The first page of this PDF shows a minor redrawing of the TR26 AMC cicuit to match what I am thinking.......

The base bias is really kind of a "voltage divider" itself between R131 and VR12 but there is a fwd biased diode D46 in the lower half.

What I think this does is... when VR12 is ALL the way at the bottom... the cathode of D46 is grounded. So all you have is R131 and D46. All the divider can feed to th the base is a single diode drop (0.6 or 0.7). As you turn VR12 up that voltage will start to go up... raising the base...but still just setting a bias.

Where I am thinking the "action" comes from is actually the emitter.... raising up and down from the modulation.

*** for just a moment take a look at the second page ***

This is the AM REGULATOR and MODE SWITCH... which is what feeds that emitter voltage divider.

IN LSB & USB the mode switch connects straight to 13.8. This is because in SSB the final and driver get full DC power and the modulating RF comes from the balanced modulator ... through the XMIT Mixer ... and along the transmit strip.

IN AM though... the mode switch pin comes from the AM regulator which is biased such that it is at about half voltage.... or around 6.8 v. With no modulation... it will just be a 6.8 v steady level. When modulation is applied that 6.8 v is not modulated with the audio and driven down to the driver and final to act as their voltage supply. It is THIS signal that is applied to the R166/R165 divider.

What I am thinking is happening is that VR12 in the AMC control is set to provide a bias level at the base of TR26.... and the emitter is seeing the modulated 6.8 v coming from the mode switch and ... as the modulation increases past the bias set point on TR26..... the emitter's changes relative to the base cause TR26 to turn on and start pulling the collector downward....... to go over to TR25 and tell IT to turn TR24 on.... to start bringing the mic level down.

Take a look if you will.... and see what you think.......

I'm going to head for the horizontal right now.

This is fun... and I think we are starting to get somewhere now!!!!!!!

Have a good one....

 

[guitar_199]

Touching each end of the diode, and the ground of the voltmeter to the wire soldered between the xtal's.

SSB..... another one of my "clarification checks"........

If I am understanding everything to this point..........

IF..... TR24 and TR25 are installed....... and in the board.......

If you REMOVE TR26..... XMIT/PA audio is GOOD.

If you PUT TR26 back in..... XMIT/PA audio goes missing again.

Is THAT TRUE?????

Thanks!!!!
Bob

 

[brandon7861]

@guitar_199 I fell asleep, sorry. I think you are 100% correct. I didn't consider the extreme with the pot set to 0Ω. Nor did I consider the case where the base emitter junction is off. You are right.

What is the voltage at the base and emitter of TR26 in SSB?

 

[slingslingbinks]

Hold on. Before you do, what were the voltages you measured on each side to ground? Was the cathode of D64 closer to 4.3v or ground? If the diode is good, it should drop about .7v across it, if open, there will be all 4.3v across it (in ssb). If R165 is open, you will not get the 4.3v at the emitter of TR26, rather something much closer to the supply voltage.

Hey Brandon.
I installed a new 1.5K resistor in R165,
Checked voltages across D64.
In SSB the Anode side has 5.156 VDC and Cathode side has 4.884 VDC
At the Cathode of D64 with VR12 Full CW the voltage is 4..888 VDC and with VR12 Full CCW the voltage is 4.743 VDC.

The Voltages at the Base of TR26 in SSB are 5.155 VDC and The Emitter of TR26 is 4.526 VDC
Now disregard the message #33, I found my cheat sheet was wrong on the layout of the transistor. Please Bear with me that i try and find the correct location point's on the foil side of the board to do my volt check's. I found it's easer (For Me) to check them from the component side by moving the component a bit to access the pin's going into the board with a needle tip probe. When i did this in the picture in message #33 my cheat sheet on the lay out was incorrect. It made no since that the C voltage wasn't the same as the voltage as the top of R129

 

[slingslingbinks]

SSB..... another one of my "clarification checks"........

If I am understanding everything to this point..........

IF..... TR24 and TR25 are installed....... and in the board.......

If you REMOVE TR26..... XMIT/PA audio is GOOD.

If you PUT TR26 back in..... XMIT/PA audio goes missing again.

Is THAT TRUE?????

Thanks!!!!
Bob

Hey Bob.
With eather TR24 or TR25 removed i DO have transmit modulation in AM, SSB and PA.
With TR26 Removed i only have transmit modulation in SSB, No Transmit modulation in AM and PA is choppy out the speaker and no voice out the speaker.

 

[brandon7861]

Checked voltages across D64.
In SSB the Anode side has 5.156 VDC and Cathode side has 4.884 VDC
At the Cathode of D64 with VR12 Full CW the voltage is 4..888 VDC and with VR12 Full CCW the voltage is 4.743 VDC.

That puts only 0.272v across D64, and being it is silicon, that's too low. I think D64 turned into a resistor (or was accidentally replaced with a germanium diode like a 1N60).

As for there being such a high voltage on the cathode with VR12 fully CCW, either the fully CCW position is not 0Ω or that bugger got wore out and went high Ω's.
Pull D64, test it out of circuit, and while it is out, ohm across VR12 to verify its range of resistances.