uniden washington 8719 negative voltage on bias

[ira parker]

receive all modes just fine, pa section works as should. has weak signal to monitor radio all modes. checked freqs all is good. pre driver is good has proper volts and is switching. in am mode has 12.8 volts to final and driver in receive switching to tx voltage drops to -9 volts. unplugged tp7/tp8 voltage at pins on board is 12.8 tx or rx. even with tp7 and 8 unplugged finals still switch to negative volts? tested the pre driver , driver and final with transistor tester they checked good. could it be the varistor diode? can i bypass them to rule them out? tested c-454, c-191,c-162,c-146 all good within spec. tr-40 test good. any suggestions would be highly appreciated . thanks muddywaters.

 

[Handy Andy]

Be careful with the way you check voltage on an insulated case chassis like the 148 or Grant XL - Even Realistics and General it may explain the reason you're seeing negative.

The case is insulated from the foil board ground - the case is grounded as like the antenna would be, but the negative lead of the radio is foil board ground.

Double check the voltage readings with negative test lead to IF Can case shield, these are foil ground.

 

[ira parker]

Thanks Andy all voltage test are done to board ground, I've also found some negative voltage on tr 12 of the meter circuit s/rf side works no modulation movement at all i'll have to look into that to see if its common to the finals, but i haven't found out why its not pulling down the bias voltage to 6 volt on am. With no watts being generated i'm sure that its not seeing any amp draw to pull it down will have to dig into the switching network and check that its switching from ssb to am on the tx side. The schematic i have for reference voltages is very blurry and hard to read i have a nice clear schematic but it doesn't have the reference voltages on it, still looking for a clear copy with voltages. I get 0 watts output am and ssb but i can see a weak signal and modulation on monitor radio probing with coax jumper most likely just the pre driver. I think its c-183 on the meter circuit ( not looking at schematic at the moment) to pull for test purposes to take it out of the equation? But i'm older now so don't quote me on that lol, It's late for now so I'll have another go at tomorrow after work see what i can find, thanks for the input 73s for now Muddywaters.

 

[Handy Andy]

When there is no load, the bleed down resistor that 220Ω ohm one across the AM Regulator (that 1/2W Red - Red - Brown) the resistor pulls it back up. That is why you see no change (full voltage but no wattage) unless all the TP are in their posts - then you get nothing like it's a short somewhere.

The negative to ground - ok, as long as you are aware, there may be audio leaking thru the line to the TP's - rectified thru your meter - but if the AM Regulator is bad, you get readings and problem with troubleshooting like you're having now.

Been down that radio road - see if a new AM regulator can take care of the "0 volt" issue.

 

[ira parker]

I'm sure I have a 1419 some where have to dig them up, i just looked at the 1 in there now and its a sd1135 will have to look up the cross reference on that i know the original should be 1419 I've got some 945s for the companion part also. My schematic is blurry on the voltage reading for bce any chance off the top of your head where they should be, I think supply voltage on c, emitter switches to 8 volt ? It's been awhile don't pay any attention to the rust falling off the brain cells!!

 

[Holydvr]

I believe (been some time) that I had seen a few d1135’s in a few of my working radios.

 

[ira parker]

Sorry guys been under they weather a few days here. Update installed the new 1419 and 945 companion part voltages look good and steady. I've also found a clear copy of the voltage chart. So i went thru all the ic's and double checked voltages everything looks good there. Still losing voltage to the finals in tx all modes, Monitor radio will show a weak signal on am no modulation, will show weak signal with forward modulation on lsu/usb. retested predriver ,driver and final all show good, predriver is switching voltages are good. So i'll start checking everything in the driver/final section something has got to be shorting to ground on tx. Any pointers will be greatly appreciated. Thanks Muddywaters

 

[Handy Andy]

Did you ever check the CB/PA switch wiring?

Thinking something in the wiring might be affecting the power delivery to the AM regulator section you just rebuilt.

 

[S&W357]

My Uniden Washington was receiving fine. Transmit showed meters dropping low with & without amplifier.

Listening ears from other stations reported weak signal strength and weak audio.

Local repair guy found cold solder joints to be the culprit.

Seems to be AOK now.