07 April 2012

The ailing ALC of the venerable FT-ONE - a gooey story

I recently got to restore an FT-ONE to full working order.   This particular FT-ONE brought back memories from 25 years ago, as it used to belong to a good friend in whose shack I drooled all over it, as it was the "Yaesu flagship" of the mid-eighties. That friend sadly passed away a few years ago, and the rig, after staying idle in storage for some years, was bought by another friend who sent it to me to shape it up, as it was quite unusable.
After fixing several problems (among them xtal oscillators that didn't want to go where they were supposed to and also an obnoxious general case of acute screwdriveritis the rig had gone through) and proceeding to full alignment, I stumbled across a most peculiar problem: The ALC was acting up. Although I could balance the directional coupler on 28 MHz (as indicated in the service manual), and then I could complete the ALC alignment transmitting into a dummy load, after a few hours, when first switched on and put to TX, the rig had a very low RF output (10 - 20W) and showed a large SWR indication for no apparent reason (transmitting into a dummy load). If I went on transmitting a carrier for a minute or so, the power would slowly come up, but even then it never reached more than 60 W or so on 1.85 MHz, and various other levels on the other bands - also, the ALC indication was fluctuating for no apparent reason.
Fig. 1 The gooey stuff!
I thoroughly checked the ALC board, suspecting a bad germanium diode - all checked out OK, as did all the transistors and trimpots on that board. Nothing there. I started suspecting some other part of the ALC feedback loop, but then my brain - in a rather rare epiphany - took in what my eyes had been seeing all along. Take a look at Fig. 1, click to enlarge.
Do you see the (kindly provided) red arrow? It points to a blob of some kind of gooey glue material intended to keep the directional coupler  toroid in place. I grabbed my ohm meter probes and placed them on two spots on the goo blob. Bingo! The goo was conductive! I read an indication of about 100 Kohm with the probes spaced a few millimeters. So the conductive goo was "leaking" RF current into the reflected power detector (D01), fooling the ALC circuit into believing that a serious SWR existed, so it promptly proceeded to reduce the output power (Yaesu calls this the APF function). Observe in the picture that the goo covers both the TX line (the bare thick wire through the toroid) and the winding ends on the right (where the enamel insulation had been removed).

Fig. 2 Off with the goo!
How to fix? Simple! Take a look at Fig. 2 (the "after" picture!). Using a thick needle, I scraped off as much of the brown goo as I could. I almost took it all off, it's hardened and breaks off in little chunks. I was careful not to scrape off the enamel insulation from the toroid windings.
After removing the goo, everything returned to normal. The ALC alignment went extra smoothly, and the power output is steady and OK on all bands. The high SWR protection now works perfectly.
This nasty goo has crossed my path for the second time: An old TS-440 that would impulsively unlock its PLL had plenty of the same goo on the VCO components. The resulting inter-component leakage was causing the PLL to unlock. It seems that with age this stuff oxidises and becomes conductive, upsetting sensitive circuits like VCOs and directional couplers. I wonder what brand name that goo was!!