[time-nuts] 10811 Repair - Voltage Check

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jan 23 16:35:34 EST 2007


Dr Bruce Griffiths wrote:
> 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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>
>   
Jason

An alternative to the inductors is to substitute a known good 10MHz 
crystal with an ESR less than 50 ohms, you can always pad the ESR to 50 
ohms with a small resistor.
This crystal need not have high stability or frequency accuracy 
(although it should be within 100ppm or so of 10MHz with a 30pF load) 
and a fundamental crystal can be used, it is only used to  test the 
oscillator  circuit.

Bruce



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