[R-390] WG: Capehart made R-390A Serial #312 -- SOLVED

[Jim Whartenby]

KarstenWhat do you mean by "the counter is accurate"?  What is the precision of your frequency counter's time base?I would think that you would need a time base that is several orders of magnitude more precise then the oscillators that you are trying to measure.  In other words, it is "difficult" to measure to 1% with a "ruler" that is accurate to 5%.JimLogic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Friday, April 11, 2025 at 04:40:36 AM CDT, Karsten Simon via R-390 <r-390 at mailman.qth.net> wrote:   

 SOLVED: counter input Z selector set to 1 MegOhm. Counter input is BNC
female. Connected counter input to J415 (the crystal oscillator output) via
a very short (20 cm) RG178 patch cable. Counter display now reads 32.996234
MHz -> 3rd harmonic. Jaques‘ hint (bold, below) got me on the right track.
Tnx Jaques!
Not solved yet: 3rd harmonic is 3766 Hz low (33,000.000 -32,996.234=3,766).
All other frequencies are also low. Ovens turned off. Deviation is even
worse with ovens on. Counter is accurate, have checked that. Will make a
list of all bands 0…32.

Very helpful list here. I really appreciate.

Karsten  

_____________________________________________
Von: Karsten Simon [mailto:karstenpg.simon at web.de] 
Gesendet: Donnerstag, 10. April 2025 23:02
An: 'Jacques Fortin'
Betreff: AW: [R-390] Capehart made R-390A Serial #312


Hello Jaques,

maybe You point in the right direction. Your questions:
*    how do you probe the oscillator output ?
    Connect frequency counter input to J415 on Oscillator Subchassis.
Connection is by 3 ft (1 meter) RG58. Counter input is switchable either 1
Megohm or 50 Ohm. No difference 50 Ohm <> 1 Meg.
*    this will add to the C431H in the V401 circuit - Maybe the cause? I
will try a shorter piece RG58 (1 ft) tomorrow.
*    31 MHz works ok.
*    Counter is being used to confirm that all Xtal frequencies work ok.
*    Will study Y2K-R3 6.2.9 again tomorrow.
73 + many tnx for your thoughts
Karsten  

-----Ursprüngliche Nachricht-----
Von: Jacques Fortin [mailto:jacques.f at videotron.ca] 
Gesendet: Donnerstag, 10. April 2025 20:42
An: 'Karsten Simon'; r-390 at mailman.qth.net
Betreff: RE: [R-390] Capehart made R-390A Serial #312
Wichtigkeit: Hoch

Hello Karsten,

Can you explain how do you probe the oscillator output ?
It is a connection to your frequency meter that use any coaxial cable ?
What is the input impedance of the frequency meter ?

Sorry to ask, but maybe you did not figure the implications: if any
additional capacitance is introduced in the 2nd crystal oscillator
structure, this will add to the C431H in the V401 circuit, then probably
preventing the oscillator to work on the 3rd harmonic of the crystal.
BTW, to answer a question asked by Larry Haney, the 31MHz band probably
works OK and that is because the stage works in doubler in that band (2 x 17
MHz).

Are you trying to "tune" the Xtal outputs to the "right" frequency by using
the frequency meter, or did you use it just to confirm that the oscillator
is working on all ranges during the alignment process ?
>From the Y2K-R3 alignment instructions, nothing specifies to do so.
The "trimmer adjustment"  instructions in the section 6.2.9 should be
followed without connecting anything, except the 2nd mixer, to the 2nd
crystal oscillator output !

73, Jacques, VE2JFE in Montreal

I am currently working on realignment: checking and tuning Crystal
Oscillator subchassis. Frequencies measured with an accurate  8-digit
digital counter. Everything appears to be ok. One exception: oscillator
output on band 30 is 22 MHz (2nd harmonic), should be 33 MHz (3rd harmonic).
Active crystal is Y403 (11 MHz). Trimmer capacitor C431H appears to be ok.
Output (pin 3 of T401) is in the range of 3 to 5 volts and can be adjusted
by turning C431H. Band 8 (11 MHz osc. output) is ok. Band 19 (22 MHz osc.
output = 2nd harmonic) is ok. Any ideas why band 30 output is 2nd harmonic
and not 3rd?
Regards
Karsten
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