>> >> >> I finally got a chance to take a look at Steve's PTO. As I suspected the >> frequency jumping is caused by the large (main) tubular cap. I don't know >> of anything special about that cap, it's marked as an NP0, so I just stuck a >> Dur-Mica in it's place. I'm sure Drake had a reason for using that type of >> cap, but darned if I can figure out what it was. Maybe purchasing got a >> good buy on a box of them! :-) I measured the cap I removed as 92.2 pF, >> the schematic specifies 82 pF, and the color code reads 93 pF, so who >> knows.... I selected the Dur-Mica from a bunch of 90 pF ones, and got a >> 92.3 to replace the original, so hopefully it won't destroy the linearity. >> Considering that Drake later went to a pair of 45 pF caps and then a 45 and >> 35 pF pair, the 'good buy' idea gains credence!! I haven't made any checks >> of linearity yet, and I assume the select at test caps in parallel are used >> to 'fine tune' for the particular coil. >>
Hi Garey, Isn't that interesting...? That small rascal of a capacitor can be the cause of so many woes to these Drake PTOs, either in and of themselves as in Steve's case, or because of bad solder joints in mine. I am happy that you've confirmed for posterity what's been ailing Steve's unit... As for mine here, well, since I re-soldered the connections of that white tubular capacitor, my travails seem to have disappeared---and I've given the Twins here some pretty extensive "road work" to confirm my "fix" here, too. You know, Garey, it occurred to me: if linearity of the PTO has changed dramatically since installing a new cap, what if you were to replace it with a smaller value unit---say 80-pfd.---and paralleled that with a small air trimmer of some 15-pfd., or so? Space inside the PTO can would pose a challenge, but I'm sure it could be done if executed carefully with the right components. If linearity suffered, one could always set the air trimmer to a slightly different value, & try again. It would be a tedious process, alright, but it might have possibilities. Indeed, one might even be able to mount the trimmer inside the PTO can & access it's setting by way of a small drilled hole in the can itself. An insulated tool would have to be used to re-set it, of course. Once the sweet spot has been achieved, the hole could simply be covered over with an over-sized piece of foil & taped to the can itself with masking tape... To try & adjust the frequency WITHOUT the can in place, the PTO frequency---in my T-4X at any rate---will be some 100+-KHz HIGHER than "normal". Just a thought. Anyway, that's certainly good news---thanks so much for sharing your findings with the group. ~73~ de Eddy VE3CUI - VE3XZ _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
