Can anyone give me any help on setting the bias on this thing? The AM regulator and finals have been replaced. I looked at the magnum 257 and it doesn't seem the same. Any Ideas?
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Optima MK3 Bias Adjustment?
- Thread starter 338_MtRushmore
- Start date
@338_MtRushmore - if that was you on the dxChat last night ...
When you need to tune a radio like that look for those trim pot but also there is some other things to look for to help you as you adjust the bias...
When you set your gate, it looks like you also a Drive linearity control on the far left side. I call this a linearity control because it affects how the Gate and RF drive levels mix together - If you set Gate too high, weak RF may not be able to be properly amplified, while too much RF drive will cause a "hot gate" problem and they can latch the parts on until they cool off - which won't happen in these radios - they are Audio Bias drive enabled all the time.
When you need to tune a radio like that look for those trim pot but also there is some other things to look for to help you as you adjust the bias...
Look carefully...
So when you adjust for Bias, remove those jumpers to set your Gate Trigger Voltage - you can also use those headers as a Current meter shunt tap point for checking the mA draw of the two.
When you set your gate, it looks like you also a Drive linearity control on the far left side. I call this a linearity control because it affects how the Gate and RF drive levels mix together - If you set Gate too high, weak RF may not be able to be properly amplified, while too much RF drive will cause a "hot gate" problem and they can latch the parts on until they cool off - which won't happen in these radios - they are Audio Bias drive enabled all the time.
- I noticed that the far left control was "hard over" not a good sign for SSB user or AM/FM as well. Can be a signal that tells me the reasons the driver and final are popping is from excessive drive.
- The Upper right control is carrier and SSB power (an AMC/ALC envelope control)
- The thing has got quite a strip for power envelope - there are two pre-drive stages onto the Driver and Final so linearity and drive levels are important to keep in check for the Driver and Final don't blow up.
For Educational Purposes Only...
Poster cannot accept responsibility for errors
omissions, oversights, undersights and missing dentures
in using this information.
Proceed at your own Peril!
Poster cannot accept responsibility for errors
omissions, oversights, undersights and missing dentures
in using this information.
Proceed at your own Peril!
It is more than likely a NFB to help remove spurs showing up in the Scope or Spectrum Analyzer. (Flat topping) In the SSB modes, the ALC or AMC is supposed to keep things in check, but on a board like that, they may have had to install some type of attenuator as a design fix to keep SSB modes and AM/FM modes (when you defeat one type of limiter - say AMC) and their limiters from not working. ALC may be ok, but if AM and FM limiters are defeated or turned off (down) - the games afoot, and things can go wrong.
For some operators they used that trimmer as a means to obtain swing by lowering AM power and then turning the attenuator to it's highest resistance - to make it more sensitive to swinging modulation envelope - that's fine but it can get them into trouble when the radio starts to eat a final or two - then they know something is wrong.
It's just not a good idea to see pots "hard over" on any radio.
It is one of the first things techs look for in the problems with operation of the radio - is to see what got turned up or off on the board - that is a big clue to the damages that may have happened.
As I went over that board, there was "Fresh" versus the "OEM" soldering. So the radio has had work done to it - so it's not a big deal but if you follow the traces to that pot, you'll see they arrive from the Gate side (the output coupling cap - to the gate - there are two - follow from that, back to the Pre-driver in the other shielded box
You can't NFB a MOS device without some hardship - so this one looks to act more like an attenuator to lessen the input signal - much like a volume control - on the gate of the Driver. This is more for the dynamics this output section exhibits - you did say this has gone thru some Finals before - well, there ya' go. Too much input on one of the gates she will propagate that onto it's output into the next stage. The end result can be a cascade failure.
It's why I call the effect "Hot gate" it's is much like how an amp gets run too high in Class AB as in pushed More in to Class A than Class C - they will conduct for a longer cycle and if not enough time is left for that input to cool off. You can have a thermal runaway issue and lose the part from excessive power and heat dissipation.
For some operators they used that trimmer as a means to obtain swing by lowering AM power and then turning the attenuator to it's highest resistance - to make it more sensitive to swinging modulation envelope - that's fine but it can get them into trouble when the radio starts to eat a final or two - then they know something is wrong.
It's just not a good idea to see pots "hard over" on any radio.
It is one of the first things techs look for in the problems with operation of the radio - is to see what got turned up or off on the board - that is a big clue to the damages that may have happened.
As I went over that board, there was "Fresh" versus the "OEM" soldering. So the radio has had work done to it - so it's not a big deal but if you follow the traces to that pot, you'll see they arrive from the Gate side (the output coupling cap - to the gate - there are two - follow from that, back to the Pre-driver in the other shielded box
You can't NFB a MOS device without some hardship - so this one looks to act more like an attenuator to lessen the input signal - much like a volume control - on the gate of the Driver. This is more for the dynamics this output section exhibits - you did say this has gone thru some Finals before - well, there ya' go. Too much input on one of the gates she will propagate that onto it's output into the next stage. The end result can be a cascade failure.
It's why I call the effect "Hot gate" it's is much like how an amp gets run too high in Class AB as in pushed More in to Class A than Class C - they will conduct for a longer cycle and if not enough time is left for that input to cool off. You can have a thermal runaway issue and lose the part from excessive power and heat dissipation.
Is there any other likely problems I can check for? I'm not too keen on messing with factory settings just because they aren't centered.
- No one is chatting at the moment.
-
-
-
dxBot:63Sprint has left the room.
-
dxBot:kennyjames 0151 has left the room.
-