[R-390] Second LO and bad xtals

[Bob Camp]

Hi

Actually an “old school” approach with something like an LM frequency meter works quite well for checking the crystals. 

Some math:

10 MHz off by 15 KHz => 1,500 ppm error. At that frequency, a crystal that ages as much as 100 ppm over it’s life is rare. Shock and vibration on one that meets the specs would be well under 10 ppm. Maximum pull on the same crystal (if the load was off) is likely 400 to 600 ppm. The normal crystals in these locations are not vacuum sealed, a seal break would not cause a significant shift. 

Something really strange happened to those crystals. Things like cracks in the blank can cause these sort of shifts.

Bob

On Sep 26, 2014, at 7:00 PM, Craig Heaton <hamfish at efn.org> wrote:

> Hello Alan & All, 
> 
> Been watching this thread and the AM Forum with interest. 
> 
> Here goes: Test point E402 and the grid leak voltages in the Y2K manual
> suggest a value between -4 to -11 VDC. Guessing here that those measurements
> were made with a USM-116 (HP-410 B or C) with an input Z of 100 megaohm or
> so. 
> 
> Considering most of our newer test equipment is lucky to have an input Z of
> 10 megaohms; would it be better simple to use a frequency counter to check
> for correct frequencies per setting of MC knob??????? Perhaps using a pocket
> size freq counter with a telescoping antenna held next to V401 would be a
> better indication? Lacking a correct frequency, then check for voltage at
> the pins of V401 or consider which crystal might be toast??
> 
> Craig, 
> 
> -----Original Message-----
> From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Alan Victor
> Sent: Friday, September 26, 2014 2:29 PM
> To: r-390 at mailman.qth.net
> Subject: [R-390] Second LO and bad xtals
> 
> In an earlier post I noted a bad 9 MHz crystal, off frequency by as much as
> 15 kHz at injection. Indeed, I found 4 other bad crystals for the 10, 6, 17
> and 23 MHz band. In all cases 13-15 KHz error. The net of this, if you see
> issues in obtaining PTO end points as well sensitivity, check those crystals
> in the second LO sub chassis. Either the heater or some significant
> vibration over the years took a number of these units out.
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