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uniden washington
- Thread starter kaos513
- Start date
Really sounds as if there's no bias on the driver and final. If there were, they would draw the test-point voltages down the normal six-Volt range when you key the mike.
But when the final and driver do not draw any collector current, the voltage coming out of the AM modulator transistor can 'drift' upwards from any audio-voltage spike or voice audio. With no load to ground, the voltage tends to rise, but not fall.
The normal current that the final and driver will draw when THEY have a drive signal on AM will pull the test-point voltage down to its normal level.
Just reinforces my suspicion that the carrier is not being delivered to the driver transistor.
73
But when the final and driver do not draw any collector current, the voltage coming out of the AM modulator transistor can 'drift' upwards from any audio-voltage spike or voice audio. With no load to ground, the voltage tends to rise, but not fall.
The normal current that the final and driver will draw when THEY have a drive signal on AM will pull the test-point voltage down to its normal level.
Just reinforces my suspicion that the carrier is not being delivered to the driver transistor.
73
I concur with Nomad - the 220 ohm resistor is what pulls up your no-load condition.
Since there is no signal to turn any sort of bias on, either Driver is done driving or the Pre-driver has issues.
The R185 (OR R195) that 220ohm resistor is across the power rail to the output side of the 1419 (Regulator) transistor. That is what makes you see the 13.8VDC.
So, in a no-load condition - the 220 ohm resistor doesn't have that oomph to pull down any voltage if there is little current draw. Now, for the sake of simplicity, the 220 ohm resistor helps the 1419 by acting as a ballast or swamp resistor - it handles part of the current so the 1419 doesn't have to work as linearly like an amp and switch at the same time - as later AM regulator systems do which use 1012 instead.
Now the fun this about this is, you can do the trick they do with that 220 ohm 1W resistor - on (across actually) the Collector/Emitter of the 2SA1012 used in such radios as PC-122 or the TRC-465 - and lower your heating bill that way - for the resistor will draw amperage across it as well as the 2SA1012. This works well for SINGLE FINAL OUTPUT radios such as listed here. Sure you get AM envelope and it's dynamics which will heat up that rear corner - but the load is shared when in steady state condition. They, as in both, dynamic change of impedance the 2SA1012 exhibits on the load seen by it versus that 220ohm fixed - is far greater than the sum of the resistive effect the 220ohm resistor plays on the output of the AM regulator. But the load is shared across a continuously variable impedance the 2SA1012 is, and the Fixed resistance the 220 ohm offers. But when in AM it can provide a load share that reduces the linear loss and heat dissipation the 2SA1012 suffers in heat production - extending it's life.
Done with care though - this does allow the battery voltage access to the Final and Driver and can damage them if you decide to "Volt up" the radio.
Since there is no signal to turn any sort of bias on, either Driver is done driving or the Pre-driver has issues.
The R185 (OR R195) that 220ohm resistor is across the power rail to the output side of the 1419 (Regulator) transistor. That is what makes you see the 13.8VDC.
So, in a no-load condition - the 220 ohm resistor doesn't have that oomph to pull down any voltage if there is little current draw. Now, for the sake of simplicity, the 220 ohm resistor helps the 1419 by acting as a ballast or swamp resistor - it handles part of the current so the 1419 doesn't have to work as linearly like an amp and switch at the same time - as later AM regulator systems do which use 1012 instead.
Now the fun this about this is, you can do the trick they do with that 220 ohm 1W resistor - on (across actually) the Collector/Emitter of the 2SA1012 used in such radios as PC-122 or the TRC-465 - and lower your heating bill that way - for the resistor will draw amperage across it as well as the 2SA1012. This works well for SINGLE FINAL OUTPUT radios such as listed here. Sure you get AM envelope and it's dynamics which will heat up that rear corner - but the load is shared when in steady state condition. They, as in both, dynamic change of impedance the 2SA1012 exhibits on the load seen by it versus that 220ohm fixed - is far greater than the sum of the resistive effect the 220ohm resistor plays on the output of the AM regulator. But the load is shared across a continuously variable impedance the 2SA1012 is, and the Fixed resistance the 220 ohm offers. But when in AM it can provide a load share that reduces the linear loss and heat dissipation the 2SA1012 suffers in heat production - extending it's life.
Done with care though - this does allow the battery voltage access to the Final and Driver and can damage them if you decide to "Volt up" the radio.
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Ok hear me out, don't have to do anything just consider then decide for yourself.
Thanks for the board, got a pic or two so they may or may not help...
Ok, looking over what you have, - you may have lost the Driver - why? Take a look ...
R185 is part of the Negative Feedback loop for the Driver - it's to reduce an artifact of a Bi-polar driven with Wide open gain in a Class better (not greater - just Better - per duration of drive per cycle - a.k.a. - AB) The Driver is in an open loop condition if R185 or its cap brother the 0.0047uF is damaged.
What can occur is a runaway issue and is why I've been more bent lately about using lower or less Bias and resetting mA draw on these Bipolars a little lower - just to keep the Duty Cycle and AB class from getting pushed into Class A. How? by the lack of RF control or bypass.
Ok I won't post that circuit - just go up until you see the full TX Strip graphic post and review that as you work thru this one - I'm only addressing an issue of what I've been seeing with heavy drive AM and Modulation problems that others tends to mod the chassis to get out of them.
When you DO NOT BUFFER the Bias Feedline - you are taking a big risk in allowing small levels of both RF and Audio form a re-rectification of themselves as BIAS voltage and power showing up on the BIAS circuit This skews the mA bias into a HIGHER bias or STRONGER push into a different Class - like CLASS A operation.
Some would say GREAT! Thats' what it is supposed to do...well ok you can be the captain of your own ship and ask yourself later why it sank - when you drive a part into Class A, sure it takes on more power to conduction angles are longer duration (Zero to the 360) but you also have the issue of Bipolar tolerance for high drive levels on the Base region - in Class AB you can get away with a lot - but not in Class A - you burn that baby up in no time - just set it to factory specs in mA draw and yell for a few minutes into the mike with the Limiter clipped - then check temperature. You may need to buy Marshmallows - it's your dime this time.
In most cases this is moot.
But in some, this heavy modulation and audio envelope can re-rectify into a bias condition going into and above 250mA - folks that's 1/4 of an Amp - ok? Do the math yourself - watch the mA draw on your meter as you discuss the finer points of AUDIEEO on the limiter clipped Hi-Po miracle you got open and on the operating table. That is gonna' make a Final or Driver go GRAND FINALE in style - as in mid-sentence - no carrier - dead, done, out the door, dead-as-a-doornail - it's is now a DOORSTOP due to this re-recitification - so BUFFER your BIAS lines! Shield them from RF feedback or at least restrict the power flow so you can recover those parts without having to CRY to RF Parts for more and your pocketbook too...
Wanted to show you something else too - so others may know.
IF you have issues with STABILITY or warble - look for and fix the L501 power feed to the
PLL - it may have a resistor on it - that's ok, just remove the resistor - install a jumper as you see fit.
IF the radio is oftentimes used in SSB and you've got it modded for the customer - then the MB3756 will need all the help it can get in regulation and capacity reserve. So if the customer isn't complaining - it's not needed - don't have to do it - but if it warbles - look for it and repair it - you just may find it helps smooth out the jitters because the PLL can track it's own internal reference and not get starved out in the process.
Ok, This is about that R185 and the AM Regulator thingy - to make a long story short, do what you want but - DO NOT REMOVE R185 if you don't have to - why? You'll see when it comes to the statements above and what you'll wind up with a bunch of dead parts from it if you're not careful. I would add a part like R185 (220 Ohm, 1W) onto the Collector and Emitter of the AM Regulator so you can sleep better at night.- it'll run the AM Regulator Cooler than without it - its' help to shed some of the power loading by acting as a Parallel power source of fixed resistance - meaning it will have a fixed draw but helps the Regulation by providing a optional path the power does not have to jump thru a junction or two and have to deal with the losses in both linearity and heat for power dissipation.
Ok, rant off - have fun, enjoy your stay...Be sure to sign our guest book and grab a coffee and doughnut and mingle with the crowd for a while. But, we know you do have a life to live so get out there fix this Gosh darn thing and go home so you can enjoy it - Life I mean - well, the radio too if it's yours.
Thanks for the board, got a pic or two so they may or may not help...
Ok, looking over what you have, - you may have lost the Driver - why? Take a look ...
R185 is part of the Negative Feedback loop for the Driver - it's to reduce an artifact of a Bi-polar driven with Wide open gain in a Class better (not greater - just Better - per duration of drive per cycle - a.k.a. - AB) The Driver is in an open loop condition if R185 or its cap brother the 0.0047uF is damaged.
What can occur is a runaway issue and is why I've been more bent lately about using lower or less Bias and resetting mA draw on these Bipolars a little lower - just to keep the Duty Cycle and AB class from getting pushed into Class A. How? by the lack of RF control or bypass.
Ok I won't post that circuit - just go up until you see the full TX Strip graphic post and review that as you work thru this one - I'm only addressing an issue of what I've been seeing with heavy drive AM and Modulation problems that others tends to mod the chassis to get out of them.
When you DO NOT BUFFER the Bias Feedline - you are taking a big risk in allowing small levels of both RF and Audio form a re-rectification of themselves as BIAS voltage and power showing up on the BIAS circuit This skews the mA bias into a HIGHER bias or STRONGER push into a different Class - like CLASS A operation.
Some would say GREAT! Thats' what it is supposed to do...well ok you can be the captain of your own ship and ask yourself later why it sank - when you drive a part into Class A, sure it takes on more power to conduction angles are longer duration (Zero to the 360) but you also have the issue of Bipolar tolerance for high drive levels on the Base region - in Class AB you can get away with a lot - but not in Class A - you burn that baby up in no time - just set it to factory specs in mA draw and yell for a few minutes into the mike with the Limiter clipped - then check temperature. You may need to buy Marshmallows - it's your dime this time.
In most cases this is moot.
But in some, this heavy modulation and audio envelope can re-rectify into a bias condition going into and above 250mA - folks that's 1/4 of an Amp - ok? Do the math yourself - watch the mA draw on your meter as you discuss the finer points of AUDIEEO on the limiter clipped Hi-Po miracle you got open and on the operating table. That is gonna' make a Final or Driver go GRAND FINALE in style - as in mid-sentence - no carrier - dead, done, out the door, dead-as-a-doornail - it's is now a DOORSTOP due to this re-recitification - so BUFFER your BIAS lines! Shield them from RF feedback or at least restrict the power flow so you can recover those parts without having to CRY to RF Parts for more and your pocketbook too...
Wanted to show you something else too - so others may know.
IF you have issues with STABILITY or warble - look for and fix the L501 power feed to the
PLL - it may have a resistor on it - that's ok, just remove the resistor - install a jumper as you see fit.
IF the radio is oftentimes used in SSB and you've got it modded for the customer - then the MB3756 will need all the help it can get in regulation and capacity reserve. So if the customer isn't complaining - it's not needed - don't have to do it - but if it warbles - look for it and repair it - you just may find it helps smooth out the jitters because the PLL can track it's own internal reference and not get starved out in the process.
Ok, This is about that R185 and the AM Regulator thingy - to make a long story short, do what you want but - DO NOT REMOVE R185 if you don't have to - why? You'll see when it comes to the statements above and what you'll wind up with a bunch of dead parts from it if you're not careful. I would add a part like R185 (220 Ohm, 1W) onto the Collector and Emitter of the AM Regulator so you can sleep better at night.- it'll run the AM Regulator Cooler than without it - its' help to shed some of the power loading by acting as a Parallel power source of fixed resistance - meaning it will have a fixed draw but helps the Regulation by providing a optional path the power does not have to jump thru a junction or two and have to deal with the losses in both linearity and heat for power dissipation.
Ok, rant off - have fun, enjoy your stay...Be sure to sign our guest book and grab a coffee and doughnut and mingle with the crowd for a while. But, we know you do have a life to live so get out there fix this Gosh darn thing and go home so you can enjoy it - Life I mean - well, the radio too if it's yours.
Sounds like an unrelated issue from the original problem.
Sounds like a dirty volume control, or it is worn out.
Squirt some DeOxit in the volume pot and work it and see if it doesn't take care of it.
Sounds like a dirty volume control, or it is worn out.
Squirt some DeOxit in the volume pot and work it and see if it doesn't take care of it.
And recheck for other mods you may have missed, you didn't show the bottom of the board so I can only hope it's something related to the age as in a bad cap passing current thru keeping a stage engaged.
Not I don't see it...
PLL seems to be "shimmed" possibly by fixed or there's something more exotic on the Foil side...
PLL seems to be "shimmed" possibly by fixed or there's something more exotic on the Foil side...
