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

[doffo]

Just got home a bit ago. Great to visit family.

So.. I desoldered the legs of the diode, and it checked out ok on the final side... To be sure, I desoldered the diode on the driver side, and it checked out exactly the same... :/ Both working and not letting anything through when you reverse the probes.

This guy.... on R180.... 56k resistor according to the thing... But man, its a big ugly guy!!!?? lol... compared to the rest of the resistors.. This one is reading a direct short, and no matter the ohms settings, its reading 0.00... I tested another resistor I had in the drawer to be sure my DMM wasnt messed up, and it was reading the ohms of the other resistor just fine... Is R180 the culprit?

 

[NZ8N]

56 Ohms?? Or .56 Ohms. It looks like an Omega after the 56, the K might be a temperature rating.

 

[doffo]

56 Ohms?? Or .56 Ohms. It looks like an Omega after the 56, the K might be a temperature rating.

Ooops, good catch.. Yea, an Omega after the 56. so should it of read 56 ohms?

 

[NZ8N]

This is a uPD858 chassis or MB8719 ?

Edit: I found a schematic for Cobra 138XLR, which is also a uPD858. R180 is listed as a 56 Ohm at 2w resistor.

 

[doffo]

This is a uPD858 chassis or MB8719 ?

Edit: I found a schematic for Cobra 138XLR, which is also a uPD858. R180 is listed as a 56 Ohm at 2w resistor.

858 chassis. You were right. Yea, the thing read 56 ohms.. its fine...

Coming to a loss on what is causing the havoc. Wish I woulda gave it a clean and a test prior to a recap to be sure it wasnt something I did. I will have to double check every capacitor I did. The whole radio receives at least, but transmit is a whole different ball park lol. Only other thing I havent done is swap the driver out.

 

[brandon parker]

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.

Sounds like you have some swr issues or something. If it was only the bias you wouldn't have that much variation between channels. But swr would sure cause that. But as far as bias.... You need to start fresh. Remove you tp jumper and check your finals. If you cannot adjust the finals themselves using the pots then there is a deeper problem. Most likely it had final failure on its way to you "probably immediately before coming to you " and that final failure cascaded and took out other vital components. Are these bit finals of mosfets? Either way it's Imperative that you have proper bias, as the finals will act as switches and not amplifiers as they are intended in this apication. If you have trouble feel free to reply and I'll be happy to help my friend!

 

[doffo]

Sounds like you have some swr issues or something. If it was only the bias you wouldn't have that much variation between channels. But swr would sure cause that. But as far as bias.... You need to start fresh. Remove you tp jumper and check your finals. If you cannot adjust the finals themselves using the pots then there is a deeper problem. Most likely it had final failure on its way to you "probably immediately before coming to you " and that final failure cascaded and took out other vital components. Are these bit finals of mosfets? Either way it's Imperative that you have proper bias, as the finals will act as switches and not amplifiers as they are intended in this apication. If you have trouble feel free to reply and I'll be happy to help my friend!

I appreciate it. The radio sits up really high in the garage's shelf so I don't touch the radio for awhile. From what I was able to do at the time, bias checked out fine and the driver/final checked out good. Even put in another set just to be sure. Diodes were ok on both sides and nothing shorted. There were parts of the board that got really hot on some of the resistors. I checked the diodes in the area, but none were shorted out. I will attack the radio some other time. For now I got other radios to use. I will get back to you once I get the itch to have another look. Thanks again.