I will keep you posted. Thanks Usually this problem has been easy. Always ends up being a diode. Not this time though.
A good way to look at this is...
Thru your S/RF meter - if it's nearly full scale and you have to turn it WAY DOWN to obtain a "natural" reading you've gotten before on the other 2510's you've worked on, then you know the problem is with the AGC portion of this AGC detector / Amp driver circuit.
Q118 simply lets Pin 14 send it's power into that 8.2K and 3.83V ZENER.
IF you don't have any voltage on that line - your AGC Pin 12 INPUT or Pin 3 of S/RF meter needs to be checked (refer to the TOO LOW reading versus TOO HIGH to be "natural").
TOO LOW means that the "detector" portion of the circuit is not sending power to Pin 3 thru those two diodes D117 or D118 (Multiplier design). It has nothing to turn AGC on with so you get full amplification.
IF that ZENER Blew - you'd have VERY HIGH S/RF meter action but would have some useabiltiy because the 8.2K resistor is allowing current to push down the PIN DIODE "window" thru C7. D178 is sending power of the IF stage amp Q104 - so there is some AGC happening there...Q104 is attenuated thru the difference in voltage drop across it's emitter resistor R23 a 680 ohm resistor - There is a cap C18 on the Emitter line - that if it is shorted - you can have all kinds of AGC happening and things look normal - but C18 isn't letting the Emitter even see that resistor - it's fully on.
If C7 shorted out, you'd have full scale no variability of meter all noise hiss stuff - shorted out cap drains the line of all control including the IF amp D178 Q104. Q118 can fail if left in this state though - D146 also will fail until shorted or blows open. Either case the circuit from C7 back to Q118 needs to be checked. You can use a 1K resistor across the 8.2K to see if S/RF meter Falls to check the status of the AGC line - if the Meter falls, then the issue is with that part of line - not the IF side.
The 1K across R2 trick only affects the PIN diodes
The IF AMP Q104 would not be affected...
MC301 PIN don't always go but they can blow open - then AGC works back at D178 and C7 just stays charged a real long time until it can drain back thru D146 into D178 mess at Q104. Again, the METER looks goofy because of this kind of response. It LAGS - because C7 is draining as D146 equalizes and R2 works against the input flow of current into the C7 cap. It holds this line high for a longer period of time - crashing into the noise floor until the cap loses enough charge into D178 and the Q104 IF amp opens again rushing noise into the receive.
It is my guess that the Pin 12 feedback loop from Pin 14 back to Pin 12 and Sense from Meter to Pin 12 is your main route to look into - that area takes two sections and METER action shows one way, while AGC action but no attenuation - means the Caps in the AGC Sense section, or beyond into Q118 is bad.
Clear as Mississippi Mud - right?
