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- Thread starter Robalo
- Start date
@Robalo - there is something on the board, that doesn't look right.
The only reason this is coming up is because I ran into another radio using a Push-Pull design that uses a PRESET resistance to work the limiter...
The variable is some small in value they use a preset value instead.
So why I mentioning it?
Because I see this...
It's over by D205 - which I think is part of the "Mic amp Module" in numbering.
It needs to "multiply" that AC audio into a working DC voltage value to push peaks into the limiter - this looks wrong for some reason...like a jumper to defeat something. Only it was done wrong.
The only reason this is coming up is because I ran into another radio using a Push-Pull design that uses a PRESET resistance to work the limiter...
The variable is some small in value they use a preset value instead.
So why I mentioning it?
Because I see this...
It's over by D205 - which I think is part of the "Mic amp Module" in numbering.
It needs to "multiply" that AC audio into a working DC voltage value to push peaks into the limiter - this looks wrong for some reason...like a jumper to defeat something. Only it was done wrong.
Ok, thanks, then this may help you more...
For whomever may need this, they used to use a FIXED value resistor to make the MOD% pre-set - you have to locate the resistor to change to affix the value to offset the "bias feed side" Your pull up resistor into the circuit so the when you use the other half of the resistor divider (your Pull Down) those negative peaks source thru the cap and resistor and change the amplification gain of Q12 - see above...
How I see it, the NEGATIVE bias diode needs to be either removed, or "shorted" out - to obtain the full gain you want in the MOD Limiter - which you can't remove - because Q12's Emitter is offset it's load resistor thru this circuit affecting the gain of the audio amp - you need Q12 in there because the Squelch amp as well as DETECTED audio pipes into this area to be amplified.
For whomever may need this, they used to use a FIXED value resistor to make the MOD% pre-set - you have to locate the resistor to change to affix the value to offset the "bias feed side" Your pull up resistor into the circuit so the when you use the other half of the resistor divider (your Pull Down) those negative peaks source thru the cap and resistor and change the amplification gain of Q12 - see above...
How I see it, the NEGATIVE bias diode needs to be either removed, or "shorted" out - to obtain the full gain you want in the MOD Limiter - which you can't remove - because Q12's Emitter is offset it's load resistor thru this circuit affecting the gain of the audio amp - you need Q12 in there because the Squelch amp as well as DETECTED audio pipes into this area to be amplified.
Now that you uncovered that pot, I see it's labeled VR 205. Modulation adjustment states VR 206. To be clear, are you saying VR 205 changed the modulation somewhat or did it remain the same? If it did make a change, I suspect it is actually the modulation pot. Sometimes they change board labeling when the manufacture updates to a high board version. If it made no change, I'd look for VR 206 to see what it does or if it actually exists. If that fails I would look at Andy's clip or short diode suggestion. The The Sam's book would help a great deal as to its exact location.
Sunbulls you right it was vr206 40-60 % mod just talking,1k with signal gen 100 %
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. What? What is the purpose off that part ? It is actin like fuse and turn off the radio in case of short
No, I was being facetious - just kidding.
Sigh...
But on a more serious note...
To see a thru-hole diode, mounted - but missing it's "pellet" and the glass or epoxy completely gone - makes you wonder if the radio suffered something catastrophic - like a lightning strike close by the antenna or a power supply issue.
You don't see a fully mounted, soldered in part, missing the "Guts" and it's envelope to contain it - very often.
You do seem to get some really wild, if not - unique - problems with radios - Robalo
And by appearance, the way the formed leads holding it, are still there, means there was a part there, but not now...
Sigh...
But on a more serious note...
To see a thru-hole diode, mounted - but missing it's "pellet" and the glass or epoxy completely gone - makes you wonder if the radio suffered something catastrophic - like a lightning strike close by the antenna or a power supply issue.
You don't see a fully mounted, soldered in part, missing the "Guts" and it's envelope to contain it - very often.
You do seem to get some really wild, if not - unique - problems with radios - Robalo
And by appearance, the way the formed leads holding it, are still there, means there was a part there, but not now...