[Boatanchors] Lysco 600 VFO

[Bill Stewart]

Hi Drew,
Tnx for the suggestions. In the Lysco, the osc and buffer tubes (both 6AG7s and get real hot) are 1 1/2 inch or so away from the vfo variable, which has the temp. comp. cap attached to it (comp. cap located between the tubes and variable). Not a real good design.

Today I did something I shud'a done at the beginning...I swapped the two 6AG7s...and about 90% of the chirp problem went away...a big change. The vfo still wobbled around a bit which reminded me of a problem I had with a Johnson vfo that developed a freq jump problem. The fix was to change the key. I have several J-44 keys, which uses a small leaf spring for tension and contact rather than a coiled spring such as used on a J-38. Several of these J-44 keys have caused freq jump due to a high resistance developing at the contact point on the leaf spring. After changing to a different key, the wobble went away. Now the main problem is with a little drift, no doubt caused by the heat problem. It really gets hot in the cabinet and will probably attach a muffin fan somewhere on the cabinet to suck out some heat. I may also look at moving the temp. comp. cap to the opposite side of the variable, away from the 6AG7s.

Thanks again for your input.
73 de Bill K4JYS 
 
----- Original Message -----
From: "Drew P." <drewrailleur807 at yahoo.com>
To: boatanchors at mailman.qth.net
Sent: Sunday, June 13, 2010 2:26:21 PM GMT -05:00 US/Canada Eastern
Subject: Re: [Boatanchors] Lysco 600 VFO



On the Lysco 600 VFO, Bill Stewart wrote:

[snipped]

"I vaguely remember a review of the Lysco in either CQ or QST and as I remember, no mention was made of the drift, which I now think is normal...of course, Lysco was an advertiser. I have noticed that C4, the osc. temp. comp. cap is very close to the 6AG7 buffer tube, which gets very hot. I wonder if this might cause some drift...maybe only warm-up drift, tho."

It would likely be of great benefit to reduce heat input to the oscillator temperature compensation capacitor and to any of the other frequency-determining components.  The modifications required could include improving ventilation, adding heat shields, and relocating hot components where feasible.

One experience of mine in this regard was with my Utica 650 6 meter AM transceiver.  The external accessory VFO, the model V-650, drifted like crazy and the drift cycle would repeat every time I would key the microphone.  Given the nature of 6 meter AM QSO's in this area and of AM QSO's in general, the long-winded "old buzzard" style of transmission, the 20 KHz drift was quite a problem. I had to use a frequency counter while transmitting and keep one hand on the VFO knob.

The Utica V-650 VFO is a 4 inch cube having beautiful shiny chrome plating on the cabinet and little else to commend it.  On the 4" x 4" chassis is the VFO tube, tuning capacitor, inductor, 0A2 voltage regulator tube and a 10 watt dropping resistor for the VR tube, all in close proximity to each other. Particularly troubling was the dropping resistor, dropping the radio's 380V B+ dowm to 150V for the 0A2 and getting way too hot to touch, right next to the VFO inductor.  This design ensures thermal cycling with resultant drift on each and every key-down.

My fix?  I mounted a terminal strip on top of the chassis and relocated the dropping resistor there, away from its original home below chassis. The effect was dramatic; the drift almost disappeared.  

And shortly thereafter, a friend gave me an 8.4 MHz crystal so I could be rock-bound and then the whole VFO issue became moot.

Though employing measures to reduce heating to frequency-determining components from external sources will not address internal (RF induced) heating, it may be that in your Lysco as was in my Utica, that external heat sources constitute the primary culprit.  You might find success in drift reduction in your Lysco with something as simple as strategic mounting of a computer fan. 

Drew


      
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