[time-nuts] 10811 Repair - Voltage Check

[Dr Bruce Griffiths]

Dr Bruce Griffiths wrote:
> 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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>   
Jason

The crystal leads are supposed to be welded to the crystal base.
The blue wire looks OK but the green and red wires are suspect.
When welding wires one doesn't use solder dipped multistrand wires.
Maybe the red green wires do not make good electrical contact with the 
crystal pins.

Bruce