President Washington. Low AM/SSB power. Help?!

[doffo]

Hello,

After recapping the radio, the radio works great on receive and can hear stations no problem. The issue is when you transmit, it is very low AM power, down to under a watt. SSB Never sees passed the same. To be sure, the Driver Bias you can adjust, and set it to the 40mA, but the Final bias? Adjusting the pot makes no changes to the milliAmps... Unsure what it means or something else may be taken out.

Power is closer to 1 watt at channel 40, but channel 1 the power basically disappears.

Again, you can pick up stations and everyone is heard just fine.
Any help would be appreciated on what might be going on. I double checked that I didnt accidentally reversed an electrolytic cap, and all seem ok.

 

[BC Coyote]

It sounds like the driver is making power, but the final isn't.
Is there voltage getting to the final during transmit ?

 

[doffo]

On AM, the meter on the radio itself does back swing a little on AM when you talk, but on SSB it goes forward about quarter of the way.

It sounds like the driver is making power, but the final isn't.
Is there voltage getting to the final during transmit ?

Just got back in from the garage a moment ago.

I should of pulled the final out and tested it. Seems dead. Should of known as they had the AM power pot fully cranked out. Doh!

Replacing the final now. Thank you for your input as you already had it narrowed fast. There is bias, but literally over 110 mA. No matter how you adjusted, it kept the same and slowly went down and settled around 80ish?

Good news is I found my 50v Vishera Tantalum!! So will be able to replace that as well.

 

[doffo]

It sounds like the driver is making power, but the final isn't.
Is there voltage getting to the final during transmit ?

Back inside... Bleh.. New final didnt do it. Behaves the same.

Even with the Tantalum C179 replaced, it behaves the same. The milliamps is now up to 150 mA, and adjusting the pot does little to no effect.

 

[doffo]

Done for the night. Checked L43, L44, R179 and R199 and are ok.

 

[BC Coyote]

What is the voltage on the final during transmit ? Is it the same in AM as in SSB ?
Did you accidently create a solder bridge/blob somewhere in the power circuits feeding the final stage when you were recapping ?

 

[nomadradio]

VR16, the final transistor's bias control is famous for getting blown out when a final fails. Turning it tends not to show much difference after it gets damaged. It is such a common risk, we change VR16 unless it looks new, just to prevent aggravation later.

D51 is also a risk, along with D66 and D67. Those two fly above the pc board on the solder side.

73

 

[doffo]

What is the voltage on the final during transmit ? Is it the same in AM as in SSB ?
Did you accidently create a solder bridge/blob somewhere in the power circuits feeding the final stage when you were recapping ?

I was getting 150 mA during transmit on USB with the mic gain all the way down. I will check on AM when I can.

 

[doffo]

VR16, the final transistor's bias control is famous for getting blown out when a final fails. Turning it tends not to show much difference after it gets damaged. It is such a common risk, we change VR16 unless it looks new, just to prevent aggravation later.

D51 is also a risk, along with D66 and D67. Those two fly above the pc board on the solder side.

73

Appreciate it. Will have a quick look at those diodes and I may go ahead and replace VR16 for a peace of mind.

 

[nomadradio]

AM transmit won't tell us anything useful from that measurement.

VR16 controls the transistor's collector current with *NO* drive power. The "NO" part is important, since the carrier drive fed to the final in AM mode disrupts this measurement totally. VR16 controls how much heat the final transistor will have to generate to keep sideband audio clean. Multiply 60 mA times 13.8 and you get 0.828 Watts of zero-signal "idle" heat. Set this current reading too low, and the SSB audio gets scratchy. Too high and the final overheats. Also not good.

73

 

[doffo]

AM transmit won't tell us anything useful from that measurement.

VR16 controls the transistor's collector current with *NO* drive power. The "NO" part is important, since the carrier drive fed to the final in AM mode disrupts this measurement totally. VR16 controls how much heat the final transistor will have to generate to keep sideband audio clean. Multiply 60 mA times 13.8 and you get 0.828 Watts of zero-signal "idle" heat. Set this current reading too low, and the SSB audio gets scratchy. Too high and the final overheats. Also not good.

73

I am a bit too tired to find where D66 and D67 was. I just seen the two that are screwed onto the driver/final, but that one is D51.

It still did the 150 mA and climbing and though it starts out at a lower value when you key up, it climbs... This was on Channel 19 USB. It got much worse on CH. 40 and was much higher.

The Driver Bias stays at the 40 mA I set it to.

Already 12:30AM here. I am off to bed lol. Thanks for the help so far.

 

[nomadradio]

, it climbs... T

D51 is toast. It will kill as many final transistors as you allow it it. Can be replaced in a pinch with a 1N4148, but some resistor values might have to be tweaked if VR16 turns down, but not enough. Can't remember this substitute pushing VR16 to the other direction, where it won't turn up far enough.

73

 

[doffo]

D51 is toast. It will kill as many final transistors as you allow it it. Can be replaced in a pinch with a 1N4148, but some resistor values might have to be tweaked if VR16 turns down, but not enough. Can't remember this substitute pushing VR16 to the other direction, where it won't turn up far enough.

73

I will give it a go.. A shame it probably took out the final when even I would key it for a second at a time.. So with D51 toast, is there a reason why they mount it up on the transistor screw? I do have some 4148 diodes, so for a temp solution I can do that... I will report back to you about it maybe Sunday evening. Today Gotta go visit some family. Appreciate your input. btw, where is D66 and D67? I am blind apparently and couldnt locate em lol. Just curious.
With D51 being toast, what is the reason where channel 1 makes no power, but then channel 40 it made just about a watt?

 

[nomadradio]

As a rule they are "flying" above the solder side of the pcb. The white screen print on the component side of the board does not show them. Uniden added them as a band-aid 'update' to the radio after the first production runs were sold.

73

 

[brandon7861]

So with D51 toast, is there a reason why they mount it up on the transistor screw?

I believe that it is sometimes mounted on the transistor screw so it is the same temperature as the transistor. As the transistor heats up, it's Vbe drops a little. If the bias does not change in response, the transistor conducts more and the operating point changes. If the diode is on the board rather than on the transistor, it still helps make the adjustment less sensitive by limiting the adjustment range, but it doesn't do much for temperature stability. With the diode on the transistor, it's Vf can change in proportion to the transistor Vbe as it heats up, and that allows the bias to track the final's temperature change automatically, assuming the change in Vf and Vbe with temperature is close to the same between the parts.