Hey. Gang. I'm working on a Uniden Washington MB8719 version. The AM power is 4 watts & with audio swings to 5 watts. There is an issue with the SSB output power being excessivly high. It's 18 watts with a whistle. I thought the ALC potentiometer VR7 was defective so I replaced it, no change. I was looking at the schematic & service manual & see that TR25 is the ALC detector so I replaced it. It then drives TR 31 & 32 to reduce modulation thus SSB rf output so I replaced those two transistors also but no change. If I reduce the modulation via the mic gain on the radio face it reduces the SSB rf output but there is obviously an issue that causing VR7 to have no effect. Any ideas where I should look next. ? Any help would be greatly appreciated.
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Uniden Washington excessive ssb output.
- Thread starter dxing.bruce
- Start date
Only thing that comes to mind is the voltage regulator IC is probably shot? Though that one usually is a high AM carrier or little to no receive on the audio. The AM regulator transistor on those boards were not the greatest but I never had one personally go out on me. I did buy up 10 of them though the NTE counterpart. Aside the possibility that capacitors can be an issue, it might take a bit to narrow down what is happening.
Give this a look and it would be ideal to do the service bulletin even if the voltage IC is ok. http://hammadeparts.jivetones.com/A...ematics/COBRA/cobra_142gtl_tech_note_1226.pdf
Robb on here is the person who I read the information from years ago when I went in to fix my few 142 GTL radios.
Give this a look and it would be ideal to do the service bulletin even if the voltage IC is ok. http://hammadeparts.jivetones.com/A...ematics/COBRA/cobra_142gtl_tech_note_1226.pdf
Robb on here is the person who I read the information from years ago when I went in to fix my few 142 GTL radios.
From Robb's post:
Service Bulletin #1226 calls for a 18.2v zener diode; but a more common 18v zener works just fine. Done this to a few radios. It is done to save the voltage regulator from failure. The other diode in this mod is the SR1K2; just use a common 1N4001.
Service Bulletin #1226 calls for a 18.2v zener diode; but a more common 18v zener works just fine. Done this to a few radios. It is done to save the voltage regulator from failure. The other diode in this mod is the SR1K2; just use a common 1N4001.
We run into this from time to time. My swiss-army knife for an ALC fault is a 'scope. Not sure how to troubleshoot it with just a meter. Never needed to learn how.
You should have just under 8 Volts DC on the collector of TR25 with the mike keyed on SSB. Turning the ALC trimpot VR7 should get some downward spikes on TR25 collector when you modulate. If not, the problem is connected to TR25's immediate vicinity.
If you get those spikes, next place to look is TR31. I would guess that maybe one of TR31's three legs has been clipped. Just the same, you should get around 6/10 of a Volt on TR31 collector when you modulate, and VR7 at one end. Just can't remember which end of travel on VR7.
Keep an eye out for a clipped resistor in the vicinity of TR25 and TR31. Don't know which one. We don't do that trick.
73
You should have just under 8 Volts DC on the collector of TR25 with the mike keyed on SSB. Turning the ALC trimpot VR7 should get some downward spikes on TR25 collector when you modulate. If not, the problem is connected to TR25's immediate vicinity.
If you get those spikes, next place to look is TR31. I would guess that maybe one of TR31's three legs has been clipped. Just the same, you should get around 6/10 of a Volt on TR31 collector when you modulate, and VR7 at one end. Just can't remember which end of travel on VR7.
Keep an eye out for a clipped resistor in the vicinity of TR25 and TR31. Don't know which one. We don't do that trick.
73
Got it fixed. Turns out C89 a 0.01 uf film capacitor was shorting out TR32. There was a dead shot across this transistor & when I removed C89 the short stopped. I replaced C89 with a new capacitor & the ALC now work. Thanks for all the help guys.
Good stuff. When you can and have the parts, do the service bulletin to your radio.
Least for a peace of mind that you did it.- No one is chatting at the moment.
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