Yaesu FT-757 and FC-757AT Matching Tuner Restoration - Page 1
Forum section: electronics / rf-microwave

author: xrunner
author_rank: Super Contributor
timestamp: December 18, 2016, 12:16:40 am
content: A ham friend had this system in storage for about 15 years. Said the radio didn't xmit and the tuner was in an unknown state. I offered to take a look at it.Both units were very dirty. The radio seemed to receive OK, so I took it apart and expected to find blown finals but upon troubleshooting I found that there was no input signal to the final amp board. Turned out that all it was, was a loose connection on the other end of the little coax cable feeding the final stage. The burned out lamps were replaced with LEDs. After a complete cleaning and a full service alignment, it works great now.The tuner was another issue. The nylon shaft couplers were cracked causing the capacitors to slip, where the hex screws were, and had to be fixed using small stainless steel hose clamps as no suitable replacement couplers could be found. The metering system (FWD power and REF power) did not work, because it was found that the schottky diodes (5) were all defective in the rf detection coupler. I ordered the correct diodes NTE583 (substitute for 1ss101), but for the time being, I tacked in 1N914s and that part of the tuner works OK for now.However, the automatic tuning feature did not work either, and this was caused by a defective JFET, so that had to be replaced as well.Also, typical of these older units the lamps were burned out and were retrofitted with LEDs. And the memory backup battery was no good, so tacked in a new battery holder and put in a CR2032. The tuner can work with the radio, as it can track the band switching via a cable, but the original cable was missing, so I ordered the two types of connectors needed to make up one - a 9 pin molex and an 8 pin JST type connector. Now the automatic bandswitching works also.I think the owner will be happy with the results.

author: The_Penguin
author_rank: Regular Contributor
timestamp: December 18, 2016, 02:03:39 pm
content: Nice job, looking good.

author: xrunner
author_rank: Super Contributor
timestamp: December 18, 2016, 05:28:30 pm
content: Quote from: The_Penguin on December 18, 2016, 02:03:39 pmThanks, Here's what I had to do to fix the capacitor coupling (pic attached). I'd rather have found a proper flexible shaft coupler but all the ones I found were all metal and unfortunately the capacitor shaft cannot be electrically connected to the gear shaft (which is grounded). Anyways it works so can't complain too much.

author: SeanB
author_rank: Super Contributor
timestamp: December 18, 2016, 06:10:31 pm
content: Another method is to lubricate the bushes, so they turn freely, and use some large diameter self adhesive lined heatshrink and shrink it on the acetal couipler. Self amalgamating silicone rubber tape also works for low friction drive.

author: TheSteve
author_rank: Unknown
timestamp: December 18, 2016, 09:34:16 pm
content: Nicely done, thank you for sharing your efforts. I've always liked the 757 series from Yaesu, when I was younger it was one of those radios I always dreamed of having. Fast forward a bunch of years and I've had one, did a little work to it and resold it just so can say I had one.

author: xrunner
author_rank: Super Contributor
timestamp: December 18, 2016, 11:08:00 pm
content: Quote from: SeanB on December 18, 2016, 06:10:31 pmThat's a possibility too. Do you happen to have any ideas on a source of plastic flexible shaft couplings? The shaft dia. is 6 mm. All I could find on Ebay and a few other places were metal ones (and most were too long also). But there must be many suppliers out there.

author: SeanB
author_rank: Super Contributor
timestamp: December 19, 2016, 07:07:08 pm
content: Not really, I would just whip them out of some 6mm ID nylon air hose, using a crimp ferrule at each end. Cheap, flexible and lasts a long time. for really high torque I would use some nylon rod of appropriate diameter, and drill it out to size, then put some grubscrews into it, and then make a flex joint by cutting through the tube partially in the mid section.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 19, 2016, 07:30:54 pm
content: I still have the full set: transceiver, tuner and power supply. I got the transceiver cheap because it was broken, fixed it and found a deal for the tuner and power supply. Has not been turned on in years so I guess I should sell it, but I don't think it's going to make a lot of money.

author: xrunner
author_rank: Super Contributor
timestamp: December 20, 2016, 03:01:40 am
content: Quote from: PA0PBZ on December 19, 2016, 07:30:54 pmThat's cool. I recently saw the tuner go for I think about ~$300 on Ebay. The radio as well seems to go for ~$300 - $400. But ... if you have a sentimental attachment to the units you have might as well keep 'em.

author: cncjerry
author_rank: Unknown
timestamp: December 27, 2016, 07:49:09 pm
content: Check small parts.com which is now Amazon for the couplers

author: xrunner
author_rank: Super Contributor
timestamp: December 27, 2016, 11:04:58 pm
content: Quote from: cncjerry on December 27, 2016, 07:49:09 pmOK I'll check it out.

author: cdev
author_rank: Super Contributor
timestamp: December 27, 2016, 11:59:23 pm
content: xrunner, aren't those couplers supposed to insulate? If I were you I would try to replace the metal hose clamps with the original couplers or at least all nylon (insulator, with the original spacing) If it is an ATU its basically a tank circuit, you could possibly get arcing? I don't know, just speculating. It may work fine with nine out of ten antennas and then on the tenth it arcs and zap, blows out the final amplifier. Its my understanding that solid state finals can be destroyed in an instant. They are often fragile, those cute, small, solid state HF rigs!

author: xrunner
author_rank: Super Contributor
timestamp: December 28, 2016, 12:58:49 am
content: Quote from: cdev on December 27, 2016, 11:59:23 pmYes. The other side connects to a metal shaft off a potentiometer and if electrically connected would make the tuner non-functional.QuoteI already tried to source the correct size in a nylon coupler and none could be found (I did see some plastic ones on Amazon after you mentioned it and that may still be a possibility). The original couplers are not available. The metal clamps do not cause an issue because they do not touch any other metal - they only simply put pressure on the coupler to hold it tight they neither touch the capacitor side nor the pot side, and nothing to speak of couples through them, this is evidenced by the tuner working properly at this time. It's no worse than it was in that respect anyway because the metal shafts of the potentiometer and capacitor were already within 1/4" inside the coupler.QuoteThere won't be any issues like that because the tuner will not be handling more than 150W. It's been tested at 120W and SWR of about 5 and it works just fine now.

author: cdev
author_rank: Super Contributor
timestamp: December 28, 2016, 01:20:39 am
content: I am sorry, I hit send, as I often do, before reading the whole thread, when I did I realized you probably know a lot more than I do, and - so I kind of made a fool of myself. Its a pretty little rig! Looks like an ideal station for a busy person who doesn't want their ham setup to take up too much space but still be full featured.

author: xrunner
author_rank: Super Contributor
timestamp: December 28, 2016, 01:48:17 am
content: Quote from: cdev on December 28, 2016, 01:20:39 amWell I don't think you did, but in any case I've made a fool of myself plenty more times. It's all in my post history - I could delete it but leaving it there makes me consider what I write more carefully.

author: twistedxknights
author_rank: Newbie
timestamp: January 04, 2017, 10:04:39 pm
content: nice to see the radio repaired.. wanted one of these 20+ years ago when I was just starting to learn about ham radio. wish i could find a one for a reasonable price when I have the funds..

author: xrunner
author_rank: Super Contributor
timestamp: January 06, 2017, 02:10:52 am
content: Quote from: twistedxknights on January 04, 2017, 10:04:39 pmThe guy actually hadtwoof these radios and the tuner in storage for all those years. The first one worked OK, but was very dirty and had to have the reference oscillator tweaked, but otherwise was fine. He has since loaned that one to another ham who uses it to work digital modes such as JT65.