[Central-Electronics] Central-Electronics Digest, Vol 87, Issue 4

[john]

50 Hz seems mighty fussy for a 55 year old transmitter. I think you might be expecting more performance than the radio is capable of giving.  These rigs were built in an era when the other station was using separate TX/RX and a small amount of knob twisting was to be expected.

Seems mighty stable for the era , actually!

73
John K%MO

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>    1. 20A drift: new one just like the old one (k6jek)
> 
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> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Fri, 23 Sep 2011 16:59:30 -0700
> From: k6jek <k6jek at comcast.net>
> Subject: [Central-Electronics] 20A drift: new one just like the old
> 	one
> To: central-electronics at mailman.qth.net
> Message-ID: <B91F1DF6-9B39-4BFD-A787-8209121303BD at comcast.net>
> Content-Type: text/plain; charset=utf-8
> 
> I upgraded my very early 20a to the newer 6U8 oscillator circuit from the old 12AU7 circuit that was only used for three months.  Alas, the new circuit isn't much better than the old one for drift but at least it doesn't have the 50 Hz pull problem.  
> 
> Next I suppose I'll try some temperature compensating caps although with the old circuit, just putting a small fan on the outside left blowing in to the lower vent made a big difference, enough that my enthusiasm for fooling with caps is diminished.  
> 
> What was the 50 Hz pull problem?  If the CARRIER knob was turned beyond 8, the oscillator was pulled 50 Hz.  I would do just that to join a QSO -- switch to a dummy load, turn the carrier up, zero beat the QSO.  Pretty soon some honest soul would say "Jon you're 50 Hz high" (or maybe it was low). I would check by using the same procedure.  I'd be dead on.  They'd say, no 50 Hz high. How could that be?  It was making me crazy. I thought the crystal was jumping.  No the oscillator was being pulled by the carrier insertion.  No wonder they junked that circuit after three months.
> 
> I didn't figure out the mechanism of the oscillator pull.  Instead I just rebuilt to the 1955 schematic.  No more pull.
> 
> I am disappointed this version is no better than the last for drift.  I'm not surprised since others with newer transmitters also noticed the 9 MHz drift. Still I had hope.  At the same time that CE changed the oscillator, they put 33 pF N750 caps across the RF phase shift coils which must help the stability of the phase shift. They did not put any temperature compensation in the oscillator itself, however.
> 
> Some numbers.  After 5 minute warm-up measured every 10 minutes.  Regulated 115 VAC, 75? F controlled room temperature.  Drift in the last ten minutes
> 
> Time	Old	New
> 10		-9	-29
> 20		-46	-53
> 30		-46	-45
> 40		-38	-37
> 50		-32	-25
> 60		-26	-16
> 70		-21	-13
> 80		-17	-10
> 90		-12	-8
> 100		-10	-6
> 110		-9	-6
> 120		-5	-2	
> 
> 
> Cheers,
> 
> Jon
> 
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> End of Central-Electronics Digest, Vol 87, Issue 4
> **************************************************