Cobra 2K issues...

[Hawkeye351]

Ok guys/gals...
Got a Cobra 2000 GTL on the desk.

Issues are:
1. No transmit whatsoever.
2. Constant static in speaker, even with volume turned all the way down.

When turned on, with mic connected and volume all the way down, there is still a lot of static in the speaker but you can't hear local stations. When the volume is turned up, then you can hear the local stations but that static is still present.

No sign of transmit at all.
Checked collector voltage on both the driver and final, along with collector of passthrough regulator, all seems normal. Removed driver, final, pre-driver and pre-pre-driver, tested all with no issues. Replaced the 2 bias electrolytes. Replaced the choke next to the driver bias VR. Replaced bias VR itself. Checked and replaced C95. Checked R108 (98.6 ohms). Replaced C18 (value was well below range). Checked over solder joints. Everything looks good. Final bias adjusts fine, but I get nothing from the driver bias.
Final bias - 70ma (adjusts smoothly)
Driver bias - 0ma (will not change)

Any ideas?

 

[SuperLid]

Ok guys/gals...
Got a Cobra 2000 GTL on the desk.

Issues are:
1. No transmit whatsoever.
2. Constant static in speaker, even with volume turned all the way down.

When turned on, with mic connected and volume all the way down, there is still a lot of static in the speaker but you can't hear local stations. When the volume is turned up, then you can hear the local stations but that static is still present.

No sign of transmit at all.
Checked collector voltage on both the driver and final, along with collector of passthrough regulator, all seems normal. Removed driver, final, pre-driver and pre-pre-driver, tested all with no issues. Replaced the 2 bias electrolytes. Replaced the choke next to the driver bias VR. Replaced bias VR itself. Checked and replaced C95. Checked R108 (98.6 ohms). Replaced C18 (value was well below range). Checked over solder joints. Everything looks good. Final bias adjusts fine, but I get nothing from the driver bias.
Final bias - 70ma (adjusts smoothly)
Driver bias - 0ma (will not change)

Any ideas?

Does the radio switch into the transmit mode? Or just stay in RX when PTT is engaged?

 

[Hawkeye351]

Forgot to mention that.
Yes, it switches to transmit according to indicator. Transmit indicator lights up when I key up the mic.

 

[SuperLid]

Forgot to mention that.
Yes, it switches to transmit according to indicator. Transmit indicator lights up when I key up the mic.

 

[SuperLid]

Check TX and RX voltages on IC4

 

[Hawkeye351]

MB3756
Bottom of radio, front facing me, starting with pin farthest from front:

Receive
1 - 8.27
2 - 13.55
3 - 8.28
4 - 0
5 - 13.55
6 - 8.29
7 - 0.01
8 - 0.48

Transmit
1 - 8.27
2 - 13.54
3 - 8.28
4 - 0
5 - 0
6 - 0.21
7 - 0
8 - 8.29

Got collector voltage going to collectors of driver and final. Bias for final will adjust from 0ma up to 100ma (even higher if needed). Bias for driver stays at 0ma no matter where you turn the driver bias VR. Replaced driver bias VR with another one, same result. Replaced bias cap (C214) with new one, same result.

 

[SuperLid]

IC4 voltages look good. Check bias diode (D50).

 

[Hawkeye351]

Driver diode (mounted to chassis beside driver):
544 one way
1614 the other way

Final diode (mounted to chassis beside final):
919 one way
1126 the other way

I did check for voltage on the driver bias VR feed leg. When keyed up it's around 3.44v, when unkeyed it's around 12v. So, the driver bias VR is getting voltage, just no current being taken off that voltage. Like the driver isn't demanding any current at all.

The driver tested great in the tester, as did the final, pre-driver and pre-pre-driver.

 

[Hawkeye351]

Story on how this occurred:

The owner stated that he replaced all the caps himself, then realigned the radio completely. He said it was doing 2w swinging to 8w. Only a few days later, he was in his garage messing around (not in the radio, lol) while it was turned on. He noticed smoke coming from the driver/final section so he immediately unplugged the power. He stated he ordered a new 2166 and 1969 off eBay, yikes. He replaced them and hasn't had power since.

The final and driver don't look fake, they tested great.

 

[SuperLid]

Story on how this occurred:

The owner stated that he replaced all the caps himself, then realigned the radio completely. He said it was doing 2w swinging to 8w. Only a few days later, he was in his garage messing around (not in the radio, lol) while it was turned on. He noticed smoke coming from the driver/final section so he immediately unplugged the power. He stated he ordered a new 2166 and 1969 off eBay, yikes. He replaced them and hasn't had power since.

The final and driver don't look fake, they tested

 

[SuperLid]

Post a photo of the final and driver. eBay transistors are ALL FAKE! Unless pulled from a working radio, I would not trust the parts.

 

[Hawkeye351]

Sure

 

[Hawkeye351]

I just noticed, I put in a 502 (5000 ohm) in the driver bias VR instead of the 102 (1000 ohm). My bad.

I'd still get some kinda ma draw even with the 5k in place wouldn't I?

Gonna swap that out and get back with y'all in the morning.

Thank you for taking time out to help, it's greatly appreciated.

 

[nomadradio]

Time to find out what voltage is on each bias diode while keyed in SSB mode.

D50 the driver's bias diode should have around 6/10 of a Volt on it while keyed. This is the normal forward-bias voltage of a silicon diode.

D49 on the final has two diodes in series inside it. This doubles the normal 6/10 of a Volt to around 1.2 or so. Feeding this into the base of TR37 will now drop around 6/10 of a Volt feeding out of the emitter of TR37, getting you the desired 6/10 feeding the base circuit of the final transistor.

If the voltage reading is low, next I would check C153 to see that it's not reversed and shorting internally. This would shut down final bias.

Likewise, a low reading on D50 would point me to C214, to see if it's reversed and shorting to ground.

Re capping a radio is not for the faint of heart. Even a pro will get one electrolytic reversed from time to time. Changing all the electrolytics in a radio has a statistical risk, no matter how careful you are to get the polarity right on every one.

73

 

[Hawkeye351]

Ok, did some voltage measurements today.

TR41 (passthrough reg)

Receive:
Base - 5.7v
Collector - 13.55v
Emitter - 12.92v

Transmit:
Base - 5.44v
Collector - 13.55v
Emitter - 4.87v

Pre-Driver (TR39)
Receive:
Base - .09v
Collector - .48v
Emitter - 0v

Transmit:
Base - .75v
Collector - 8.31v
Emitter - .02v

TR37
Receive:
Base - .48v
Collector - 6.35v
Emitter - .23v

Transmit:
Base - 1.27v
Collector - 7.51v
Emitter - .68v

Driver (2166)
Receive:
Base - .42v
Collector - 12.93v
Emitter - 0v

Transmit:
Base - .61v
Collector - 4.87v
Emitter - 0v

Final (1969)
Receive:
Base - .23v
Collector - 12.93v
Emitter - 0v

Transmit:
Base - .67v
Collector - 4.87v
Emitter - 0v

I noticed that TR39 (pre-driver) doesn't seem right. What are y'all's thoughts?

Measurements taken in AM mode only.