[Elecraft] low audio on K2 s/n 3641 (was RE: PLL ReferenceOscillator Range Test)

[Chris Kantarjiev]

>> So I started on the Receiver Signal Tracing in my manual. Everything
>> was more or less correct until I got to the crystal filter:
>> 
>> 4. Post-Mixer Amp Output. Expected 2.2Vrms. Got 3.02Vrms
>> 
>> 5. -5dB Pad Output. Expected 1.40Vrms. Got 3.04Vrms
>> 
>> 6. Crystal Filter Output. Expected 0.35Vrms. Actual 0.094Vrms.
>> 
>> I stopped there, because this seems very wrong.

DolfinDon suggested:

> If you are losing the signal out of the crystal filter check each crystal 
> pin to ground with an ohm meter. You should get a high reading not a short. 
> Sometimes if too much solder is used the solder will flow up under the 
> crystal shorting it to the ground plain on the top of the board under the 
> crystal. If one of them is shorted you will need to remove that crystal and 
> remove the excess solder.

Nope, it's not a shorted crystal; all pins measure 'infinite' 
resistance to ground.

I measured each pin with the RF probe. Here's what I get, which
doesn't make a lot of sense to me (this is viewed from the bottom
of the board):

        3.16v (in)

X7      1.1     3.16
X8      1.0     1.1
X9      0.58    1.0
X10     0.78    0.58
X11     0.48    0.78

These are "rough" measurements - neither the radio nor the signal
generator had been warmed up when I took them just before going to
work, but the relative values are about right.

There's no drop at the -5dB pad because I have removed a KNB2 and
jumpered 1-6 on the connector; the pad itself is 'gone'.

Should there really be such a big drop across any of the crystals? And
why would the voltage go back up? 

(And should I be concerned that the crystals are marked 4.9136-S instead
of 4915-20?)

I notice, now that I look at them again, that the output measurement
is not very far from the 0.35V that's expected. There's a diode (D34)
between the last crystal pin and the jumper W3, perhaps that's the problem?

73, 
chris