Hi Guys, This is going to be a rather L-O-N-G post, so if you have no interest in the "...inner workings" of any of the PTOs in your older Drake equipment, please feel free to hit the delete button at any time! Hi Hi
As you may recall from my previous posts here on the Reflector, my T-4X transmitter (1967 vintage, serial #12779-G) suddenly developed an intermittent "worbble", or "jump", in its frequency. I went down the list of the usual suspects, i.e. I cleaned all of the controls, all of the tube pins / socket receptors, the guide shafts of the PTO...I even soldered-in an extra ground strap from the PTO "guide fork" to the side enclosure, as was specified in an old Drake service bulletin (thanks, Garey!). The worbble persisted. I next received the suggestion that the zener diode regulating the voltage of the PTO transistor was probably "going south" on me (thanks, David), and that the varying control voltage from the zener in its death throes was, in turn, varying the frequency. Made sense to me, so last week a friend of mine kindly dug into his junk box, & is mailing me a replacement zener even as I type this... To-day I removed the PTO shield can (an exercise that becomes easier with practice, believe me!) in preparation of the arrival of the zener diode...and with the cover temporarily gone, I decided to do some last minute "...poking & prodding" of the PTO components with a bamboo chopstick(!), to see if I might be able to duplicate the warbbling condition, at will. By placing your hand anywhere near the coil assembly itself, you'll effect an immediatre change in frequency---so I kept my hand capacitance at bay as much as possible, utilizing the length of the chopstick. As expected, the monitored frequency of the PTO changed slightly, as I poked & prodded the different wires & parts of the PTO---but when I applied even the gentlest of pressure on one side only of C89---a 3000-pfd. maroon-coloured silver mica cap in the oscillator tank---the exact same warbble & instability of before started anew! I then pressed the other side of this same capacitor, and the signal stabilized. Hmmmm...sounded like a bad / "aged" solder joint of sorts to me, so I applied heat to both points on the board contacting the capacitor's leads, causing the solder therein to re-flow. I then attempted to again induce the instability of before, but could only make the PTO frequency change slightly in frequency, much as was done before by applying pressure to the other side... Problem solved---I think (fingers crossed!). If I experience any more "warbbling" in the future, I'll simply wholesale replace C89. Out of curiousity (didn't that kill a cat once...?!), I pondered how I would have even seen the zener in the PTO, let alone replaced it (the components are upside down on the board, facing the chassis!)---so I proceeded to "...boldly go where few Hams have gone before," i.e. I completely dis-assembled the "electronics" of the PTO from its mechanical dial. This was certainly intimidating at first, I will admit, but not all that difficult. Besides, with everything exposed, it afforded me an excellent opportunity to clean-up (literally!) in there. How's it done...? First, with the shield cover removed, unscrew the four stainless steel(?) screws securing the vertical coil support to the dial assembly (you'll see that these 4 screws go through brass bushings). This will physically free the board & coil assembly, but in order to completely remove it, you have to (1) unsolder the white wire connected to the ground lug on atop the chassis & at the side, (2) unsolder the shielded wire coming up from under the chassis away from one of the front solder lugs of the board / coil assembly, and, (3) unsolder the white & red striped wire that comes up from below the chassis, to another lug on the board coil assembly... VOILA! This allows you to slip the assembly off of the brass shaft/guide coming from the mechanical dial assembly, exposing everything that works in & out of the coil that needs cleaning with isopropyl rubbing alcohol. NOTE: remember to draw a sketch, or some other visual reminder, so that you can successfully reverse this procedure when you put everything back together again! With all said & done, the calibration of my dial here on my T-4X seems no worse for wear because of this exercise, but do heed this warning: with the PTO shield cover removed, the frequency GOES UP SOME 100-KHz! I struggled to "spot" my frequency on the receiver beside the rig during its testing, and discovered---only after considerable trial & error!---that the shield removal has that great an effect upon its output. With the PTO shield cover back in place, dial read-out is normal again... So there you have it, a (hopefully) successful, resolution to a hitherto very vexxing issue! Proof yet again that these old rigs are a joy to both operate, as well as to work on... Long live Drake...! ~73~ de Eddy VE3CUI - VE3XZ _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
