142 low transmit

[Hawkeye351]

Got an old Cobra 142 GTL on the desk today.

Issue:
Low transmit in all modes.

This radio belongs to a local that I talk to on a daily basis. He talked on this radio for about 6 months after he got it back from me. I just recapped it and aligned it. It was doing around 18w in all modes and AM would key from 0 to 7w and talked great. Well, he locked his key down one night (all night long) playing music. The next day he noticed only about 3w max out of it. So I got it back thinking it was just the final or driver.

Driver tested as 17 HFe
Final tested as 28 HFe
Pre-driver tested as 37 HFe
Xmit mixer showing no output

Replaced xmit mixer, I then had output from mixer.

I have a good 27.205 signal on the pre-driver when keyed up.

I have an ever better 27.205 signal on the driver when keyed.

I have an unstable low signal on the final when keyed, won't stabilize on frequency.

Figured the driver wasn't amplifying so I replaced it with one that has an HFe of 29.

Getting only 9.5w max on all modes now.

One thing I noticed:
The driver and final bias VR adjustment values don't correspond with the schematic for the 142. This one had a 1k ohm in each bias spot. The schematic calls for a 500 ohm at the driver and a 5000 ohm for the final.
The driver adjustment was turned completely counter clockwise (shows as 170ma), the final adjustment is set at 2.03a (yes amp). If I adjust either one away from their spots to adjust the bias the way it should be the power drops out completely. Oh, and there is a 2.2k ohm resistor coming off the wiper of the final bias VR over to the solder pad to the right of it on the bottom side.

I replaced those two bias VR's with what the schematic calls for and I still have to have the driver all the way counter clockwise and final all the way clockwise (170ma driver, 2.03a final).

Signal on the base of the final still unstable and not locking on freq.

Collectors of pre-driver, driver, final, Passthrough are all correct.

Is 29 too low an HFe for a 2166?
Is 28 too low an HFe for a 1969?

Any ideas on the low output?

 

[doffo]

Unsure if its related, but maybe the voltage regulator is giving it grief?
Cobra 140/142 GTL service bulletin

 

[Hawkeye351]

I'll check that, thank you.
I did check voltage from power supply, it's showing 14v. Lights or nothing dims when keyed up.
Radio receives great.

I'll check that regulator though...

 

[brandon7861]

The Uniden Washington I have uses 1k pots where the schematic shows 500 and 5k, so I think thats normal.

 

[brandon7861]

I had a 2166 in my junk box. Not sure how healthy it is, but its reading 73 Hfe. I think those the transistors are on their way out being that low. Its not uncommon for transistor gain to drop when abused..

I recommend a bias check before installing new ones, and a smokeless smoke test. Don't forget that the final in series with its bias transistor will show the same forward drop as the MV13YH dual junction diode. The diode needs to have a leg lifted to test it so you don't see the two transistor junctions measuring across an open diode.

 

[SuperLid]

Got an old Cobra 142 GTL on the desk today.

Issue:
Low transmit in all modes.

This radio belongs to a local that I talk to on a daily basis. He talked on this radio for about 6 months after he got it back from me. I just recapped it and aligned it. It was doing around 18w in all modes and AM would key from 0 to 7w and talked great. Well, he locked his key down one night (all night long) playing music. The next day he noticed only about 3w max out of it. So I got it back thinking it was just the final or driver.

Driver tested as 17 HFe
Final tested as 28 HFe
Pre-driver tested as 37 HFe
Xmit mixer showing no output

Replaced xmit mixer, I then had output from mixer.

I have a good 27.205 signal on the pre-driver when keyed up.

I have an ever better 27.205 signal on the driver when keyed.

I have an unstable low signal on the final when keyed, won't stabilize on frequency.

Figured the driver wasn't amplifying so I replaced it with one that has an HFe of 29.

Getting only 9.5w max on all modes now.

One thing I noticed:
The driver and final bias VR adjustment values don't correspond with the schematic for the 142. This one had a 1k ohm in each bias spot. The schematic calls for a 500 ohm at the driver and a 5000 ohm for the final.
The driver adjustment was turned completely counter clockwise (shows as 170ma), the final adjustment is set at 2.03a (yes amp). If I adjust either one away from their spots to adjust the bias the way it should be the power drops out completely. Oh, and there is a 2.2k ohm resistor coming off the wiper of the final bias VR over to the solder pad to the right of it on the bottom side.

I replaced those two bias VR's with what the schematic calls for and I still have to have the driver all the way counter clockwise and final all the way clockwise (170ma driver, 2.03a final).

Signal on the base of the final still unstable and not locking on freq.

Collectors of pre-driver, driver, final, Passthrough are all correct.

Is 29 too low an HFe for a 2166?
Is 28 too low an HFe for a 1969?

Any ideas on the low output?

Driver gain seems low. Typically higher than final when good.

 

[Hawkeye351]

I see on the datasheet for both,

2166
Minimum - 35
Typical - 70
Maximum - 180

1969
Minimum - 10
Typical - 50
Maximum - 180

So, the driver being at 29 is past the minimum point and the final at 28 is way below the typical point, veering on the minimum point.

Got a used 2166 at 67 HFe and a NOS Mitsubishi 1969 at 51 HFe. Gonna throw these in and go from there.

I'll give an update later.

 

[nomadradio]

A long key down could have compromised C148 and/or C149. All the final's RF current goes through C149. Makes it a candidate for heat damage.

73

 

[KneeBiter]

Got an old Cobra 142 GTL on the desk today.

Issue:
Low transmit in all modes.

This radio belongs to a local that I talk to on a daily basis. He talked on this radio for about 6 months after he got it back from me. I just recapped it and aligned it. It was doing around 18w in all modes and AM would key from 0 to 7w and talked great. Well, he locked his key down one night (all night long) playing music. The next day he noticed only about 3w max out of it. So I got it back thinking it was just the final or driver.

Driver tested as 17 HFe
Final tested as 28 HFe
Pre-driver tested as 37 HFe
Xmit mixer showing no output

Replaced xmit mixer, I then had output from mixer.

I have a good 27.205 signal on the pre-driver when keyed up.

I have an ever better 27.205 signal on the driver when keyed.

I have an unstable low signal on the final when keyed, won't stabilize on frequency.

Figured the driver wasn't amplifying so I replaced it with one that has an HFe of 29.

Getting only 9.5w max on all modes now.

One thing I noticed:
The driver and final bias VR adjustment values don't correspond with the schematic for the 142. This one had a 1k ohm in each bias spot. The schematic calls for a 500 ohm at the driver and a 5000 ohm for the final.
The driver adjustment was turned completely counter clockwise (shows as 170ma), the final adjustment is set at 2.03a (yes amp). If I adjust either one away from their spots to adjust the bias the way it should be the power drops out completely. Oh, and there is a 2.2k ohm resistor coming off the wiper of the final bias VR over to the solder pad to the right of it on the bottom side.

I replaced those two bias VR's with what the schematic calls for and I still have to have the driver all the way counter clockwise and final all the way clockwise (170ma driver, 2.03a final).

Signal on the base of the final still unstable and not locking on freq.

Collectors of pre-driver, driver, final, Passthrough are all correct.

Is 29 too low an HFe for a 2166?
Is 28 too low an HFe for a 1969?

Any ideas on the low output?

When I was young way way back in time we had another kid who loved throwing carriers. Thought he was a DJ. One day I offered to "peak" his radio. Once I got it I proceeded to tune the rf down to barely anything. I jumped resistors to make the meter peg when keyed. Let all the others on the ch know to play along like he was killing us. It was great. His signal was so low we all had background music and could all talk right over him. So my question. Why? If he throws carriers and blew it up why fix it for him?

 

[Hawkeye351]

He fell asleep with the key locked down. I've already spoken with him about doing this again. He now understands the damage it causes, not just to the radio but on the airwaves also.

And, I look at it this way,
He's never done me any wrong, never caused me any trouble, always offers to pay me, and my locals know the only way to get me upset at them is to use my work as a weapon against me. They know I'll stop working on their stuff if they disrespect me by using my radio work against me.

He used to be an alcoholic trouble maker, but he's changed a lot in the past two year, stopped drinking as much, calmed down on the arguing and now enjoys a decent conversation. Although he still does ignorant stuff at times but we all do stupid crap at times, he's changing, he's learning.