Cobra 142GTL No Transmit or Receive

[brandon7861]

My question was about the base voltages that time. Being you don't have a current meter to check bias levels, I just wanted to make sure you had about a diode drop across them. Sometimes bad transistors can have an open collector which wouldn't affect the collector voltage, so that was just one last test on the transistors.

You are going to need a current meter for the TX alignment anyhow, so lets stop here until you get those fuses. I think the low power out might be a can out of whack, and to go down that part of the alignment, we need to set/verify bias current.

 

[CSA1863]

Sorry was distracted by the cat and didn't thoroughly read your post.

Base Voltages - TR 39 USB & LSB TX 0.67v TR 41 USB & LSB TX 0.84v

Couldn't find any fuses locally. Should have them tomorrow.

Thanks!

 

[brandon7861]

The base voltage on TR41 looks a little high to me, there may be a problem there. Then current meter will tell us.

 

[CSA1863]

Got my fuses.

With the Meter set to 200 mA positive lead clipped to TP8 and negative lead inserted into the connector of the green wire leading to the 2SC1306 driver I'm reading 56mA

When testing TP7 to the 2SC1969 final I am reading 0

 

[brandon7861]

Got my fuses.

With the Meter set to 200 mA positive lead clipped to TP8 and negative lead inserted into the connector of the green wire leading to the 2SC1306 driver I'm reading 56mA

When testing TP7 to the 2SC1969 final I am reading 0

Bad final. I was figuring that with such a high base voltage. A working transistor would have pulled that down a bit. You will have to find a replacement.

56mA on the driver is a little bit high too, you will want to back that down to what the service manual says.

Check that 10ohm resistor at the final base too, just to be sure that isn't open. I doubt it, but maybe.

 

[CSA1863]

Once I saw there was no current to the final I assumed it was bad. I will check the resistor to be sure.

I have a few 2sc1969 transistors so if the resistor is good I will replace the transistor and get the bias set on both the driver and final.

 

[CSA1863]

Resistor checked 10 Ohm. At first I thought it was bad as it had continuity but so did the couple of new 10 Ohm resistors I checked. I realized that there wasn't enough resistance for the multi meter's continuity check. The Final failed the transistor check.

I replaced the final. Set the driver bias to 35mA. When setting the final bias VR9 is turned all the way down and its 53mA. Thinking something is off some where. Also VR9 was probably 3/4 of the way up. I turned it down before attempting to set the bias.

 

[brandon7861]

Check TR40.

 

[CSA1863]

TR40 USB TX emitter 0.61 collector 8.22 base 1.16

TR40 AM TX emitter .78 collector 3.23 base 1.13

 

[brandon7861]

D47 is two diodes in one. If that went bad, VR9 would be providing more current to the base of TR40 than it should. I think you should check D47, but you will have to lift a leg to do it. If you don't lift a leg, your meter will still see two diode drops, the b-e junctions of TR40 and TR41. The readings would be very similar, so it is imperative the measurement is on the D47 alone.

That is the one strapped to the heat sync.

 

[brandon7861]

My guess is that D47 went bad causing a spike in current to the base of TR40 which turned on the final too hard. Thats probably what smoked the final.

The function of that dual diode is two-fold. It tracks the temperature and adjusts the bias to compensate for the increased heat, and it provides two diode drops instead of one because the b-e junction of the buffer TR40 drops half of it.

Odds of finding a replacement are low unless you have a donor radio. There are dual diode SMD packages available, but they likely have different thermal curves, so they wouldn't track perfectly with the transistors temp, but something is better than nothing. I can try to find the part numbers if you want to order them. Some people put two 4148's in series there. I would be tempted to use two SMD transistors as diodes.

 

[brandon7861]

Once you get that sorted out, the next mystery will be why you only have 3.23v on the collector of TR40 in AM TX. Should be the same as in SSB. Either you measured wrong or something specific to AM is loading down the 8v TX rail. I'm getting ahead of myself though, one step at a time.

Good catch btw, turning down VR9. When finals go, I often try a new one without testing these bias parts because my bench supply is limited to 2A and it trips really easy telling me there is an issue. I need to get in the habit of having people check those parts before installing the final since not everyone has such a limited supply. Nomad thankfully caught my error in another thread about the 458 and said something where I missed it. I almost owed you a final!

 

[CSA1863]

Looks like the mv-13yh diode is no where to be found. I will remove the leads from the board and test the diode this evening and let you know what I find.

 

[brandon7861]

I will remove the leads from the board and test the diode this evening

Removing just one will do, no need to remove both. If you are careful, you can remove it from the diode board instead. The diode is the little 3-legged transistor-looking piece in the middle. Removing one of the green wires and testing at that board would be easiest.

 

[brandon7861]

I am going to take one of the diodes out of my washington when I get home from work tonight and take some forward voltage to forward current measurements and get on digikey and buy all the SMD diode arrays that fit that curve and test each one to see which stabilizes the final current the best. If I find a suitable part, I will report my findings.

I'm getting to have too many irons in the fire. My probe parts finally arrived too, so thats another weekend, but this is one of those parts I'd bet a whole lot of people would want a viable replacement for. We need to know lol.

The MMBD4148SE is two 4148's in series in one SMD package that looks just like the one in the radio, but there are so many options, I cannot recommend any without testing them. I am kind of excited to dig into this one!