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Old school president Grant DX PC 999 board
- Thread starter Staybolt
- Start date
http://www.angelfire.com/electronic/cbdoctor/npcrc360014b.html
C208 on PC-999 board.
C208 on PC-999 board.
OK....I just showed where it's hooked up. No need to be so harsh about it.
What does the resistor do for the modification?
I didn't mean to be harsh, sorry. Resistor sets negative peak compression level. Without does not work as intended.
It is crucial for NPC-RC modification. Many people in the past did that mod wrong, blaming that it does not work etc...
Mike
It is crucial for NPC-RC modification. Many people in the past did that mod wrong, blaming that it does not work etc...
Mike
If you play with the various resistor values you can use, you'll see how the mod takes your "pre-set" carrier voltage level and use the resistor as a means to "Drain the tank" of a reservoir of power to allow the envelope to go up and down along the AM carrier power - Pre-set line (where you set your carrier).
You have already played with the AM Carrier Power control - you'll see where the meter you measure this envelope power tends to swing more when you have lower carrier power than when you turn it up . There is a level or limit as to how much carrier DC power is effective as for AM symmetry - fully modulated signal - before you begin to see this swing start to drop in power - telling you you've gone too far positive or too high in your DC Bias.
If you turn down your carrier - you "appear to" get more FORWARD swing out of it due to the "equilibrium" of the circuit stabilizes it's poor or reservoir once you turn the carrier power trimmer below a set value of DC volts you set at your Test points.
What you're seeing It's not "Forward" per-se- but you have changed how the reservoir attains and holds the power from a more of a symmetric wave into more asymmetric appearance. It's because you've shifted the "Center point" or equilibrium of the carrier to envelope to more envelope than carrier - shifting how the carrier is perceived to now follow more of the envelope is a power swing than as a "steady power level" - it still is, just you're audio peaks have taken over the meter reading than the carrier present.
That is a way of explaining it - but if you try different resistor values as well as where you place them;
In series with, or parallel one with the diode...you affect the way power flows in and out of the Gain Cell part of the circuit that controls envelope. It (the envelope) lags behind carrier power - it is for the AM regulator to compress audio onto the DC Bias.
As you can see, a resistor in series with a capacitor is used in the capacity of a power reservoir or gain cell as a method to apply power to the audio as an envelope control - and a bandwidth component - it only will allow a set range of audio frequencies to be imposed on the Carrier DC bias - a capacitor across the resistor tapped at the output - offers a hi-frequency filter to help control another portion of the audio spectrum that passes into the DC Bias going into the Driver and Final
Like this...
You just have to remember the "pre-set" power level back at the AM carrier power - sets a THRESHOLD level of where Carrier to Audio ratios are meant to be set.
So say, a PC-122 or even a Galaxy - both use AM regulators and need to have their TP AM Carrier power pre-set for reference before you even begin the tune-up process - you need it to keep the AM power symmetrical - you usually set this to an average of 1/2 of the power supply rail - or about 5.6 volts - which is close the 13.8V typical Automotive input from a running engine - minus the inherited voltage drop across the Main Pass Regulator transistor PNP of about 1.2 volts or so - leaving you about 11~12 volts working in your typical AM regulator output. (you have to consider the power choke and all associated wiring connection points as hits adding to this voltage drop.
This is why the 5.6K and 560 ohm values show up - to keep amplifier ratios to maintain the 1:4 ratios of carrier to envelope power.
So when you play with the values and use the diode - you affect the way the amplifier controls the power and it's gain too.
The diode allows the power to drain away from the pool or reservoir faster than the radio can replace it. Draining or flattening out, the AM Regulators ability to swing power below your set carrier power level.
The Resistor helps offset how quickly this power is drained away thru the diode - it limits it's ability to "dump" the Gain Cell cap into the AM Bias line (output to Final and Driver) from being too quick and distorting the audio envelope.
This maybe more than you need to know, but I'm posting this to help others later on, when someone else wants to know more about the NPC mod. It's a LEGACY issue - so others can pick up where this left off...
You have already played with the AM Carrier Power control - you'll see where the meter you measure this envelope power tends to swing more when you have lower carrier power than when you turn it up . There is a level or limit as to how much carrier DC power is effective as for AM symmetry - fully modulated signal - before you begin to see this swing start to drop in power - telling you you've gone too far positive or too high in your DC Bias.
If you turn down your carrier - you "appear to" get more FORWARD swing out of it due to the "equilibrium" of the circuit stabilizes it's poor or reservoir once you turn the carrier power trimmer below a set value of DC volts you set at your Test points.
What you're seeing It's not "Forward" per-se- but you have changed how the reservoir attains and holds the power from a more of a symmetric wave into more asymmetric appearance. It's because you've shifted the "Center point" or equilibrium of the carrier to envelope to more envelope than carrier - shifting how the carrier is perceived to now follow more of the envelope is a power swing than as a "steady power level" - it still is, just you're audio peaks have taken over the meter reading than the carrier present.
That is a way of explaining it - but if you try different resistor values as well as where you place them;
In series with, or parallel one with the diode...you affect the way power flows in and out of the Gain Cell part of the circuit that controls envelope. It (the envelope) lags behind carrier power - it is for the AM regulator to compress audio onto the DC Bias.
As you can see, a resistor in series with a capacitor is used in the capacity of a power reservoir or gain cell as a method to apply power to the audio as an envelope control - and a bandwidth component - it only will allow a set range of audio frequencies to be imposed on the Carrier DC bias - a capacitor across the resistor tapped at the output - offers a hi-frequency filter to help control another portion of the audio spectrum that passes into the DC Bias going into the Driver and Final
Like this...
You just have to remember the "pre-set" power level back at the AM carrier power - sets a THRESHOLD level of where Carrier to Audio ratios are meant to be set.
So say, a PC-122 or even a Galaxy - both use AM regulators and need to have their TP AM Carrier power pre-set for reference before you even begin the tune-up process - you need it to keep the AM power symmetrical - you usually set this to an average of 1/2 of the power supply rail - or about 5.6 volts - which is close the 13.8V typical Automotive input from a running engine - minus the inherited voltage drop across the Main Pass Regulator transistor PNP of about 1.2 volts or so - leaving you about 11~12 volts working in your typical AM regulator output. (you have to consider the power choke and all associated wiring connection points as hits adding to this voltage drop.
This is why the 5.6K and 560 ohm values show up - to keep amplifier ratios to maintain the 1:4 ratios of carrier to envelope power.
So when you play with the values and use the diode - you affect the way the amplifier controls the power and it's gain too.
The diode allows the power to drain away from the pool or reservoir faster than the radio can replace it. Draining or flattening out, the AM Regulators ability to swing power below your set carrier power level.
The Resistor helps offset how quickly this power is drained away thru the diode - it limits it's ability to "dump" the Gain Cell cap into the AM Bias line (output to Final and Driver) from being too quick and distorting the audio envelope.
This maybe more than you need to know, but I'm posting this to help others later on, when someone else wants to know more about the NPC mod. It's a LEGACY issue - so others can pick up where this left off...
999,
I would imagine that your scope pattern didn't look to good with only the diode.
can you describe what it looked like, or post a pic of it?
im just curious.
best way to do the NPC mod is to start with a trimmer resistor, say 1K in series with the diode.
set the modulation to 100% before doing the mod.
once you put the mod in, adjust the trimmer to close up those flat lines between the positive peaks and make a nice looking envelope.
then remove the mod, and measure the resistance of the trimmer.
replace it with the closest standard value resistor and you're good to go.
for the galaxy/ ranger chassis, it usually ends up being around a 390 ohm resistor.
not sure what it would be for this chassis.
also, you can "upgrade" the AM regulator in these older radios to match what Galaxy/ranger did.
these old radios use a TO-92 and a TO-220, and if you want you can use a TO-220 and a TO-247 like the "big" radios do.
is it necessary?
no, but hey why not.
LC
I would imagine that your scope pattern didn't look to good with only the diode.
can you describe what it looked like, or post a pic of it?
im just curious.
best way to do the NPC mod is to start with a trimmer resistor, say 1K in series with the diode.
set the modulation to 100% before doing the mod.
once you put the mod in, adjust the trimmer to close up those flat lines between the positive peaks and make a nice looking envelope.
then remove the mod, and measure the resistance of the trimmer.
replace it with the closest standard value resistor and you're good to go.
for the galaxy/ ranger chassis, it usually ends up being around a 390 ohm resistor.
not sure what it would be for this chassis.
also, you can "upgrade" the AM regulator in these older radios to match what Galaxy/ranger did.
these old radios use a TO-92 and a TO-220, and if you want you can use a TO-220 and a TO-247 like the "big" radios do.
is it necessary?
no, but hey why not.
LC
You helped me with doing mine like this. Should I crack it open and add a resistor in line?
I put a trim pot on my TR 936 and it worked great for dialing it in on the scope.
You kept the 560 ohm...
(properly done - My reason for the Big Thank You)
Makes this a "FAIR FIGHT"...
Keep us posted! - PLEASE!
- No one is chatting at the moment.
