Sorry if wrong forum, looking for help in restoring rx of Yaesu FT-77
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| Feb 24th 2012, 16:34 | |
KE4OFWTotal Topics: 0 Total Posts: 0 |
Hi All, Sorry to have my first message be a plea for help but that's what it is. I have been licensed since HS but have not been really that active for quite a while, I got the itch again and picked up a used FT-77 that seemed to be in decent operating condition and I think it probably was until I started trying to use it! First Impressions: Applied power, connected up through an MFJ tuner with a random wire of about 30ft give or take and just adjusted things for the highest signal levels and could receive with no issue across all of the bands I/etc. All things seemed to work, meter, etc etc. Bad/stupid operator time: So after a few days of just listening only I wanted to check out the transmit, and I wanted to see if I could get that random wire to tune up at all just to see if it looked like the various modes were going to work. I had planned on trying to tune it up, then work on soldering a new mic while I let the rig idle/continue warming up. So after doing, maybe 5 or so quick keydowns (no more than 2 seconds in duration each) in cw mode on 20M to see if I could get the random wire tuned down at all it appeared that my receive had gone silent. This is when I began kicking myself and wondering if I had just been stupid and asking for it with trying to tune at all with just a short piece of random wire, or if my grounding wasn't right or something and had allowed some stray RF straight back into the rig and had blown out the front end or something. With dummy load in place, it does appear to still transmit at a healthy near 100 watts. Wannabe Hack opens the case and warms up the iron: So I began to google around, and I got a hold of the operating manual as well as the full schematics, but I didn't have any luck looking for anything that may have been a "service manual' to maybe walk through various troubleshooting yes/no decision charts for various issues. I did manage to find this FT message board, and a message where someone mentioned no receive and there was mention made of the two 2.2OHM resistors on the REG board one that was in line with the RX 13.5 leg off of the board and one that was inline with the TX 13.5 leg off of the board. These resistors seemed to test ok to me but since I had some parts available I went ahead and swapped out the one that was inline with the path that ends up going towards the RX 13.5 line. Let me also back up and say this real quick, with the rig powered on, and NOT keyed there was no voltage on the RX8 and RX13.5 lines of the reg board where I am thinking that there should be when the key is not down. While probing around on that reg board while trying different key up/down scenarios to see which changes I could observe I evidently made something else go poof and then noticed that now the S Meter Lamp was off and the freq display was kinda going nuts like someone was just going kinda crazy with the main tuning knob. I thought at this point I had really sealed the deal and totally botched the rig up. I took a step back, left it for the evening and on the next evening realized that the other 2.2 ohm on the reg board in line with the TX had gotten all out of spec. I replaced this resistor and I was back to the first/original non-working state.. which is.. everything powers up.. smeter lamp on.. s meter needle fully to the left.. no movement/signal present and no noise from the speaker in any mode or band selection. freq display was fine and moved as expected with movement of the main tuning knob. From more reading around I have also checked the little fuse/lamp that seems to be there to protect the front end but it is intact and hand has continuity. I even bypassed it temporarily to be sure but no change in the problem condition. Most recent observation: Last night I was messing around with trying to fully understand how the transistor switching works on that reg board but I am still not 100% there. I can gather that with the presence or lack of voltage on the TRX pin of the regboard (which I have probed and seems to be around 11 volts off key and then 0 when the key is closed) is the main trigger to make the changes in the flow/switching of those transistors. One thing I did do just to see what changes would take place, was that I actually disconnected the TRX line from the regboard. When I did this, I then got some power on my RX 13.5 line, and on the RX8 line (but it does seem a little high like maybe 10-11 volts). Also with the TRX line disconnected I observed that I could hear static when I switched over to the FM mode (please note I do NOT have the FM module installed). Between the sidetone heard and the static now in the FM switch position I think that safely rules out the AF amp right? Is there anyone out there that can steer me in the right direction of what I could maybe try to focus on next? In the meantime I have gone ahead and ordered enough stuff to pretty much rebuild the reg board because if I am going to go through the trouble to label and remove all of its connections for a full inspection I am going to go ahead and swap out the pieces on it.. unless someone thinks otherwise. I'll attach the REGboard portion of the schematic that I cropped/enlarged out of the manual if that can let any gurus out there give me some clues. Summary: FT77, powers up fine No receive audio/static present on any band/any mode. No movement of S-meter needle. When unkeyed there is no voltage present on the rx8 and rx13.5 lines. Disconnected TRX line from reg board and observed voltages present on RX8 and rx13.5 (but rx8 seemed a bit high like 11volts maybe, rx13.5 is right on). With TRX line disconnected I can hear static when mode switch is put to FM (note I do NOT have the FM module installed). Protection Lamp/Fuse tests fine, bypassed for the heck of it for short time and no change. Hoping maybe all of the problems are with the regboard perhaps and not elsewhere. Thanks in advanced for any input.. and sorry for the long post, but I figured the more info the better for this type of help request. -Jamie KE4OFW Lou, KY |
| Feb 24th 2012, 16:52 | |
KE4OFWTotal Topics: 0 Total Posts: 0 |
I have cut just the portion of the schematic out for the reg board, as well as some tech for the manual that explains the logic for the switching on the board. Apologies in advance if this is frowned upon, go ahead and delete the posting or let me know and I will be glad to. Thanks, Jamie http://dl.dropbox.com/u/9835955/ft-77_regboard_schematic.png http://dl.dropbox.com/u/9835955/ft-77-reg-board-operation.png |
| Feb 29th 2012, 18:38 | |
KE4OFWTotal Topics: 0 Total Posts: 0 |
I've not had much luck interpreting some of these things. Learning as I go. In the meantime, I decided to look from the outside coming in to the radio.. as far as RF. I am going to try to check out that first TR relay that signal goes through as it comes in via the coax/etc. I can see it moving when the rig is powered up or down but I am sure which way it should be by default or even if I see movement in the relay if that would still mean there is an issue with the contacts or something else. It is a bit hard to get to but that was the next easiest I could think off where a signal could be getting kept out of the rest of the box. Thanks, Jamie |
| Feb 29th 2012, 19:39 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
Tuning for maximum signal, surprisingly enough, isn't a good way to tune a transmatch for receive, no matter how logical it sounds. The problem is that a receiver of that vintage typically has a MOSFET front end, which has a very high input impedance, as opposed to the transmitter, which is designed to work into 50 ohms. There are typically three approaches that I use for fixing equipment. The first is to make sure that the oscillators all work--quite a bit of circuitry is devoted to the various oscillators and it is quite easy to look for them with a receiver or spectrum analyzer without much effort. Since the transmit works, we can assume that they work. The second approach is signal tracing--put in a signal and see if it passes through the various stages as expect. This should be relatively straightforward, since signals disappear entirely. The third approach, measuring DC voltage levels, works if you have a good service manual that tells you what the voltages should be--but you do have to watch for circuit revisions--don't blindly trust the service manual if things don't make sense. But, if you have a bad capacitor or relay, as is often the case with a missing signal, this technique won't find it. The first order of business would be to restore the 13.5 and 8.5 volt RX voltages--which may be a bit of a challenge since it appears that it is in the control circuitry, and not the easy fix of the pass transistors, which appear to be working. Zack Lau W1VT ARRL Senior Lab Engineer |
| Mar 2nd 2012, 19:24 | |
KE4OFWTotal Topics: 0 Total Posts: 0 |
Zack, Thank you so much for taking the time to read and reply to my question. I really appreciate it. Quickly, on the topic of the transmatch/tuner. I was merely following the directions of this old MFJ versa tuner. Which I think basically states that you you start out with your antenna and tuner match knobs set at about 3.5 then you adjust your inductor selector until you get the most noise/signal. Then you go about adjusting the antenna and tuner match for best swr. Just making sure I shouldn't be ignoring the manual here. I've also heard that its never a bad idea to open up these MFJ tuners to fix any loose connections and cold solder joints which seems to be an issue on many of these. I will attempt to see what I can handle as far as the signal tracing. Should be a good learning experience for me. I've also since found one capacitor on one of the boards that appears to maybe have burst. I thought it was some kind of glue but i dont think I see any other ones similar to this. I may just replace it with a similarly rated new cap from my parts bin just to rule that out. Thanks again! -Jamie KE4OFW |
| Mar 2nd 2012, 23:32 | |
W1VTSuper Moderator Total Topics: 0 Total Posts: 0 |
The idea of using the receiver to peak the transmatch works much better if the receiver has a similar impedance to the transmitter. One way of doing this is to switch in a resistive attenuator--but this only works if you have enough noise to compensate for the reduced receiver sensitivity. Grounding can be an issue--folks often don't realize that half the antenna is the ground system--so you really need a pretty substantial ground system to keep the ground "cold" for RF. Zack Lau W1VT ARRL Senior Lab Engineer |
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