so this was the first repair to clue me in to the ceramic package cold solder issue. i had been chasing a “bump its fixed” bad receive issue. every so often the SSB receive would drop down to no S meter reading and barely audible for even nearby 20 over signals. a good bump would fix it for ahile, then it would drop out again. after opening the case, resoldering any suspicious joints in the area, closing back up maybe two dozen times, i finally removed the shield over the DDS unit. pressing on the PCB around it seemed to have the greatest effect on the cutout.
thiere are two direct digital synthesiser units in the ts-570. they consist of an extinct custom chip F71022 and two ceramic pack precision resistor ladders. they are ic501 and 502 are the DDS, and CP500 CP501 CP502 CP503 are the four packs. my guess, the ceramic packs are only side soldered and heat expansion delta from the epoxy PCB cracks the joints. normally surface mount components do not get cold solder issues, its the hand soldered bigger parts and jacks usually. but these are like little ceramic circuit boards on top of the regular epoxy board and they dont stretch and contract the same. i reflowed cp500 and 501 that go with ic501 which gens the 8.83mhz carrier to the mixer and that cleaned it right up. pushing on the pcb no longer cut the SSB. these ceramic packs are in bunches of kenwood radios from 80’s onwards as they are very precision and probably temp stable too. tm-733 uses one for PL tone generator, ts-950sdx has same double DDS chips same packs, etc.
i know not best pic, but here is the dds unit w shield off. other dds is top left still under metal,shield. here looking at tx/rx board, serial jack top right, dds is big square chip in middle, and the resistor packs are to the left of the DDS with red lettering. i used a 1/64″ tip soldering iron, smeared flux on both sides of the packs, no solder, just touched each “notch” on the sides that is the contacts. no real legs or leads, just conductive paint goes from top of pack over the side edge. not sure if it continues under the pack. carefull not to scrape this paint off, and go in hot and fast and get out cuz the ceramic sucks
i post these in the hopes someone having similar problems will find these notes and will help them in some small way get their gear on the air again.
i was all excited about repairing the backlights on my new-to-me Kenwood TM-733 so put it on the air, and things looked good. then i turned on the CTCSS aka PL Tone and the transmitted audio got all garbled and bad distorted. Looking thru the schematic i find the same type of precision resistor pack i just re-flowed to fix a TS-570 is the heart of the PL tone generation circuit. sure enough, re-flowing the CP401 resistor pack and clean PL tones are output again. the r-pack is between the orange xtal and the large CPU. contacts are more on the side than on the bottom, 25 years of heat cycling and the ceramic moves not same as pcb.
so that fixed the distorted PL tones enough to hit the local repeater. good friends tell you the truth, so back on the bench after Ken gracefully told me “sounds like crap”. checking power output and meter doesnt move at any power setting. hmm, its obviously modulating and PL is clean enough to hit the local repeater? check on google about the final amplifier chip turns out, its not a chip, its a module. wut? i thought these things where epoxied inside?
nope, module, and the plastic cover pops right off to expose the main drive transistor, its pre-drive transistor, and a simple amplifier power chokes and feeds.
can you see the hairline crack in the choke line just off the tip of the probe?
no? neither can i. but the meter can see it probing from either side no contact. right in the middle of the bottom curvy trace (which would make a fun racetrack if i where an electron thus the inductance)
so some sloppy solder dragging and we have contact again. i say sloppy cuz the ceramic substrate ate the heat as i applied it and the solder kinda stayed right at the eutectic point. nope, no pic, im bareassed. but that fixed it, full 50w output and sounds clean so far. gonna have to get a proper service monitor or gen/spectrum unit all these radio projects….
heres a closer look at the 60 watt drive transistor. looks like an epoxy blob with a metal wire screen over perhaps a dozen small dies wired in parallel?
so the little 6volt lightbulbs on this awesome Kenwood TM-733radio burned out. made it 27 years ao thats pretty good. soldered in some white 3mm LEDS and changed the series resistors, boom, more readable than the orgiinal and way less current draw. next one ill try warm white, i kinda miss the warm glow of the original even if this has netter contrast.