delara newsDelaware Amateur Radio Association, Delaware OH VOL 37 NUMBER 1
Craig MIller W8CR
Don’t Give Up on an Old Friend Part II
As you may recall, last month’s article left you in suspense regarding the attempted repair of the Regulator Board for my (old) Yaesu FT1000MP MKV transceiver. Well the parts came in from Mouser, so I delicately, with my new surface mount soldering station, replaced the DC-DC chip. No negative 9 volts. I continued to replace the capacitors involved in the circuit, still nothing. Boy am I stuck. I was sure I found the culprit.I removed each resistor (tiny little buggers) and tested their values – all fine. I needed to learn how this chip works, so onto Google to search for some information describing how these DC-DC converters function. I stumbled upon a document published by a chip maker that described how these components work. It turns out these little guys are very cool. They operate at very high frequencies, 100 kHz, one pin generates a sawtooth signal another senses current, a very complicated yet elegant devices. Depending how you arrange the surround components, you can convert a high voltage to a lower one, a low one to a higher value, or create a negative voltage. Pretty interesting gizmo.Three critical parts that make this thing hum are a capacitor, a diode and an inductor. I replaced the caps already, even the diode but blew off the inductor(s), what could ever go wrong with an inductor? An inductor is nothing more than a coil of wire. This board has two little inductors about the size of resistors. I measured the continuity and they showed about 15 ohms resistance, that sounds about right (I think). I give up.I found a ham in Bulgaria of all places, that had a spare Regulator Board, cheap. I ordered it. A few weeks later it showed up. I quickly replaced the bad board with the good one and the radio came to life! Yippee!!!!. Still scratching my head, I pulled the board out and compared component by component and also oscilloscope traces. The good board is showing a way different sawtooth pattern than the bad one. Further comparisons of components, guess what?One of the tiny inductors, that I totally blew off as a potential point of failure, had a partial short. I pulled the part and the old board came to life, generating the missing -9VDC. Whiskey Tango Foxtrot! A 15 cent part brought this great radio to the verge of abandonment. Wow – I was a happy camper! I ran the radio through its paces – everything works…except….no transmit power. OH CRAP!!!I’m sure I bumped a connector or broke a little wire, so back into the rig I dive again. I trace the RF signal from one stage to the next, using my old Tektronix oscilloscope. Moving on to the power amp module. With a little probe, I’m looking at the signal at a pre-amp transistor then disaster happened. The probe, while the radio was transmitting a signals accidentally slipped off the collector lead and briefly shorted with the emitter lead. ZAP!!! The power supply kicks off, I hear a relay in the radio click a few times – everything dies.My heart sank 10 miles. I think I just fried everything. I disconnect the separate power supply from the radio and turn it back on, it came to life – that’s good. Plug it into the radio – ZAP. Power supply trips – everything dead.Long story short, I sent a surge into the two power MOSFETs which damaged them causing a dead short from the 30 VDC power supply. Nice. Fortunately, I isolated the high voltage from the power amp, the rig came to life again – close call.I found a spare power amp module on eBay and replaced the bad one. The rig now is back alive but still no power out. I’m just about done with this thing. Maybe it’s time I cut my losses and play Taps while I bury it behind the garage next to the broken Leg Lamp. Poking around, I find a loose ribbon cable that wasn’t seated properly in a connector. Clamped it down, held my breath, turned on the rig, pushed the PTT and (play the victory music) IT WORKS!!! Full power out, receiver sounds great. There is a God!Time to button my baby up. Knowing I’d lose track of what screws went where, I labeled an old ice cube tray with the location the hardware came from during disassembly month ago:All is well in the world. What a way to wrap up the old year and start the new. I’ll repair the bad boards so I’ll have spares on hand if needed in the future. It was a risk tackling something this complex without the proper training, knowledge and tools but boy was this a learning experience. I’m not sure I want to take on another project like this anytime soon but it sure was worth shedding a few tears to get my old friend back on the air.