[R-390] Update to 3 Mc issue with R390A

[Ken Harpur]

Thanks to Don and Bruce,

Very useful information that I have filed away for future reference. I will be digging one of the R-390As out of storage in the next few weeks to start a full restoration. If I can get it finished by the end of winter (August/September at my QTH) I'll be very happy...Then on to the next one!

Best Regards
Ken


On 24/03/2011, at 6:58 AM, 2002tii wrote:

> Ken wrote:
> 
>> I find this very interesting...I have an R-390(non-A) that exhibits
>> the same behaviour in the crystal calibrator module...
>> 
>> Turn it on and it's good for about 30 seconds or so then
>> dies a slow death. I had initially thought it may have been a cap
>> or resistor that's gone out of spec. The idea of crud getting in
>> there and causing mischief hadn't occured to me.
> 
> It's certainly possible, but the most common cause of flaky operation 
> of the calibration oscillator is drift of the multivibrator.  The 390 
> and 390A both synchronize an astable (free-running) multivibrator to 
> a crystal reference (1 MHz in the case of the 390, 200 kHz in the 
> case of the 390A).  If the multivibrator's free-running frequency 
> drifts too far, the "kick" from the crystal oscillator is mis-timed 
> and fights rather than assists the multivibrator, which damps the 
> oscillation.  In both the 390 and 390A, the multivibrator should be 
> able to lock throughout the (small) pulling range of the crystal oscillator.
> 
> In both radios, if there is no output from the crystal oscillator, 
> the calibrator will generally be running but far off frequency.
> 
> On the 390, check for 1 MHz output (couple a 10x probe through 10-20 
> pF to the plate of V901).  If that is OK, try adjusting C912.  If 
> that doesn't work, suspect drift in the components around the 
> multivibrator, V902, or V902 itself.
> 
> On the 390A, check for 200 kHz output (couple a 10x probe through 
> 10-20 pF to the plate of V205A).  If that is OK, suspect drift in the 
> components around the multivibrator, V206 (there is no trimmer on the 
> multivibrator free-running frequency on a 390A), or V206 itself.
> 
> Best regards,
> 
> Don
> 
> 
> Copyright (c) 2011.  Not for redistribution
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