[Racal] RA 6790/GM Problem

[Bob Camp]

Hi

If you suspect the varicap, the check is pretty easy. Just short it  
out. The frequency should go to the low side of the BFO  range *real*  
fast. A more elegant is to jump across it with a 220 pf capacitor. It  
should also take the VCXO to the low side of the tune range.

Bob
KB8TQ


On Jun 18, 2007, at 8:30 AM, Carcia, Francis A HS wrote:

> I had a similar problem with a RA6830 BFO and found a bad varactor in
> the vco. If the frequency is high check the VCO control voltage. If it
> is low the problem is in the oscillator if it is high the problem  
> is in
> the pll. Frank wa1gfz
>
> -----Original Message-----
> From: racal-bounces at mailman.qth.net
> [mailto:racal-bounces at mailman.qth.net] On Behalf Of Bob Camp
> Sent: Sunday, June 17, 2007 8:25 AM
> To: I Z 6 E G K
> Cc: racal at mailman.qth.net
> Subject: Re: [Racal] RA 6790/GM Problem
>
> Hi
>
> If the VCO is running 10% high it is no longer running under crystal
> control. The first thing to suspect is the crystal. Grab any crystal
> you can find that is close to 22.75 MHz and hold it in the circuit.
> If the oscillator locks to the frequency of the new crystal then
> that's the problem.
>
> Finding out what the specifications are on the correct crystal may be
> difficult. That will make ordering a replacement a bit tough. It may
> be easier to find a replacement board !!
>
> Bob
> KB8TQ
>
>
>
> On Jun 17, 2007, at 8:00 AM, I Z 6 E G K wrote:
>
>> Hello all,
>> I'm experiencing an Out Of Lock condition with the BFO.
>> Byte test always displays 04 error code.
>> On the A8 board, in the area of 22.75 MHz BFO VCO, I can adjust L4
>> for a
>> voltage reading on TP8 within specs (between 4.5 and 7 Volts DC)
>> but the
>> frequency measured on J4 (BFO Output) slightly changes and it's far
>> from
>> requested kHz 455. It's abt kHz 500.
>>
>> I understand that the Divided-by-50 frequency is abt MHz 25 and not
>> MHz
>> 22.75.
>> Any idea ?
>>
>> Mny tnx and 73
>> Pierluigi IZ6EGK
>>
>>
>>
>>
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>
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