[time-nuts] 10811 Repair - Voltage Check

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jan 23 16:21:35 EST 2007


Jason Rabel wrote:
> Following the manual I started probing all the points that had AC & DC
> voltage specs listed.
>
> Q1 & Q3 DC voltage is okay.
> I could not measure any AC voltage for Q1, and Q3 was a lot lower than the
> spec.
>
> CR5 was supposed to be -1.5V DC, but I was only measuring like +.5V?
>
> The C3/R3 point on the bottom (where blue wire is connected) was also a LOT
> lower than the AC spec. It says 1-4V p-p, but I was only in the millivolts.
>
> Red wire coming out of the oscillator I was measuring *maybe* 150-200mV p-p.
>
> BTW, found the replaceable parts page, it starts on page 32.
>
> I'm going to go ahead and measure Q3,Q4,Q5, and Q9 while I have it open. But
> I'm still as stumped as I was before. :(
>
>
> Jason
>
>
>
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>   
Jason

If the AC input to the AGC detector is very low the detector diodes will 
be forward biased via R5+R6+R7 and the dc voltage at CR5 anode will be 
will be slightly less than Q1's base voltage.
The symptoms all point to very low crystal current, maybe the crystal is 
defective.
Its probably time to find a couple of inductors with values of 10uH and 
12uH and substitute these for the crystal as specified in the service 
manual paragraph 8-95. The critical properties of these inductors are

1) Inductor self resonance occurs well above 10MHz.

2) Inductor ESR at 10MHz is around 50 ohms so that it is similar to that 
of the crystal.

You can always use a high Q inductor and add a series resistor if 
necessary.
If the ESR isnt similar to that of the crystal Q1 will be biased at a 
much lower current than when a good crystal is present.

Bruce



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