Cobra 2000 power out and bias.

[Brian Gosper]

I am having trouble with biasing of the driver and final. I can’t turn driver down below about 75 ma.and then it drifts about to about 95 ma when keyed . And I can get the final down to about 75-80 at the lowest but then as I hold down the key, it will slowly drift higher to well over 100 before I unkey. And there is no RF output at all despite that. Voltages appear normal at the driver and final. What the heck!! Help! Why do they both drift higher?

 

[sp5it]

You measure in USB, no mod?

 

[Brian Gosper]

Yep just confirmed. And again there is no RF power out on ssb or when I switched it to AM. .

also rechecked voltages at the driver and final. In AM TX the voltage across the center leg of the driver is only 2.6 V.(which curiously is the same as AM TX from the bias transistor). Other 2 are ok (0.0 and 0.6) Sam’s says should be 5.6. Ok in SSB. And final is ok.

backing way up the line, voltages at the base and emitter of Tr41 and 42 are about half of what it should be.

So lost.

 

[nomadradio]

Ahhhh. Where to start?

no RF power out on ssb or when I switched it to AM. .

The radio shows no output power on AM or SSB?

Add to this the bias-current reading is too high and rises the longer the mike is keyed?

Makes it sound like a toasted driver and final both.

Not something we see all that often.

Readings at TR41 and TR42 have no meaning when sideband is selected. Those two transistors are the AM (only) modulator and carrier control.

Does sound as if it's time to test the driver and final.

73

 

[Handy Andy]

I am having trouble with biasing of the driver and final. I can’t turn driver down below about 75 ma.and then it drifts about to about 95 ma when keyed . And I can get the final down to about 75-80 at the lowest but then as I hold down the key, it will slowly drift higher to well over 100 before I unkey. And there is no RF output at all despite that. Voltages appear normal at the driver and final. What the heck!! Help! Why do they both drift higher?

If you use MOSFET's that requires a different type of biasing method.

IF you're using Bipolar (2166 - 1969 E&C) then you may have the Bias diodes installed backwards.

Banded, Cathode end - routes to boards' FOIL (not chassis) Ground.

Verify your Bias Diodes ground is the same ground as your Battery (POWER) negative ( - ) return

 

[Brian G]

If you use MOSFET's that requires a different type of biasing method.

IF you're using Bipolar (2166 - 1969 E&C) then you may have the Bias diodes installed backwards.

Banded, Cathode end - routes to boards' FOIL (not chassis) Ground.

Verify your Bias Diodes ground is the same ground as your Battery (POWER) negative ( - ) return

Getting Current reading on the final even without keying the mike! About 20 Ma on SSB down to 18 when keyed. AM about 4 ma then -2.0 when keyed! For the driver, I get about 90 ma on ssb and over 180 ma on AM when keyed. These are the lowest settings I can get

But to follow up on your reply...
The driver and finals are original replacement types C1969 final and C1306 driver. The bias diodes are the stock ones also. I did take them out of another radio so I suppose there a tiny chance they are installed backwards. I do have supposed replacements for them that look like regular diodes but I haven’t tried them because I wasn’t sure how to mount or install them. If you happen to have a nice closeup pic of the bias diodes and which leads run where that might help me confirm. Which side is the side that has that colored dot?

 

[Handy Andy]

I'll get a clear shot of the XYL's- give me a few minutes.

​

 

[Brian G]

I'll get a clear shot of the XYL's- give me a few minutes.

View attachment 43897​

Thanks. That is the way I had them in. I had an ECG601 which is supposed to be a sub for D50. I put it in for a laugh. Can get the SSB bias on the driver down to about 75-84 ma but no lower. It drifts up and down as you keep it keyed. On AM it is much higher-around 180-190 ma. Wouldn’t it normally be higher on SSB??

can’t crank it down to normal either way.

I also checked the voltages again on the driver with that replacement diode in. Normal on SSB. On AM the center leg is supposed to be 5.6v on transmit. It’s half that at 2.8.

How does Tr37 fit in? Checked voltage there too.
AM: collector: 8 volts. Base 1.4 Emitter 2.82
SSB: base same. Collector 10 v and the emitter is 11.75 volts.
Other than the base, those all seem to off vs Sam’s. And it looks like that 2.82 is the same as that center leg of the driver. which feeds which?

Not sure if that is a possible part of the trouble or if the weird readings there are a symptom of somewhere else? Chicken or the egg.

 

[Handy Andy]

The 2.82 is from the AM Regulator TR41/TR42 mess...separate issue.

(Well, could be related, it's turned on by the Base ( see below) of one of the Driver or Final or both to pull down DC - it's on when it shouldn't)

TR37 - Emitter leg is supposed to be your BIAS output voltage to the BASE of the 1969 at about 0.7 volts - better check for shorts.

​

The purpose of TR37 is a current driver - allows for more current regulation using just the bias trim pot and the Diode forward biased to ground from the Base of TR37 - uses less current to trim - allowing the trim pot to be smaller and used to bias the Base of TR37 thru it's diode to supply more deep current loads thru the Emitter leg from the TX 8 volt for the Base of the 1969 (which original design was a 1307 - current muncher when switched on)

 

[Brian G]

The 2.82 is from the AM Regulator TR41/TR42 mess...separate issue.

(Well, could be related, it's turned on by the Base ( see below) of one of the Driver or Final or both to pull down DC - it's on when it shouldn't)

TR37 - Emitter leg is supposed to be your BIAS output voltage to the BASE of the 1969 at about 0.7 volts - better check for shorts.

View attachment 43900​

The purpose of TR37 is a current driver - allows for more current regulation using just the bias trim pot and the Diode forward biased to ground from the Base of TR37 - uses less current to trim - allowing the trim pot to be smaller and used to bias the Base of TR37 thru it's diode to supply more deep current loads thru the Emitter leg from the TX 8 volt for the Base of the 1969 (which original design was a 1307 - current muncher when switched on)

wow what a disaster. No idea where to start lol. Why is that Tr37 emitter 2.8 and not 0.7 first question.