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- Опубліковано 31 жов 2013
- This is part two of the repair video for the Yaesu FT-7800 dual band transceiver. For part one, see the link below this text. The faulty PIN diodes turned out to be discontinued. A viewer of the part 1 video suggested that the successor radio (FT-7900) is very similar and uses a different part number for these PIN diodes. Further investigation revealed that this part of the circuit design is essentially identical, so I ordered the diodes used in the 7900. This video shows the replacement process and bench check for 2 meter transmit operation. I deeply apologize for the lousy camera angle and video work at about the 9 minute mark.
Part 1:
• #116: Repair Log, Part...
To learn more about PIN Diodes, see this tutorial video:
• #118: Basics of PIN di... - Наука та технологія
I enjoyed this repair. You have the same style as Peter at TRX Bench. You use the block diagram and schematic to explain what you are doing. GREAT job.
Excellent tutorial, we all have different routes to fault finding, I probably would have looked at the rf on the PA device first suspecting the PIN switching, a current meter on the power supply may have given an early clue that most of the VHF tx path was working. Your tutorial though is very good and methodical, excellent video quality too. I think Yaesu pushed their luck a bit using the parallel diodes. Most of my transceivers have the power turned down a bit, it is amazing what it does for reliability.
I have this same radio right in front of me. It was my first base station and it works great. Great to see the thought process behind this repair as well as seeing inside it. Thanks Alan!
Thanks again Alan for all the interesting videos you share with us. Did watch this again.. (over the years maybe this is the third time watching it). It is wery interesting the combination of you talking and showing both the block-diagr., schematics, and the physical fault-finding on this radio. //Have a nice day across the pond there. Br, from Norway.
Looks like the pin diodes are something we will have to deal with for the future. Thanks for the videos on pin diodes and this repair video. Very good job!
Superb video, I have the 7900 and watching you makes it look far simpler than it is. A job well done. G7RSA 73s
Nicely done and beautifully explained. Thanks for taking the time to put this together.
Alan, you have a gift for technical gab & articulation. Thanks for sharing this.
Great repair and fantastic guidance through the though processes of the repair. Thank you.
Very impressed at your handiwork, keeping the soldering iron, tweezers, and hands out of the way of the camera (for almost all of the video, anyway)!
This was an awesome video. As an automotive technician, I see one thing in common that makes repairs quick and easy and that would be the 'service manual'. Those would be important to have when diagnosing and repairing problems.
Very nice set of videos, love the description of the process with the practical application!
you are such a pro! nice to see someone that knows what their doing!
Very nice circuit tracing examples. Nice schematics, that always pays off, when trying to fix anything. I would have to believe this a know fault in the FT-7800 among Yaesu owners since it failed out of nowhere for no reason.
Hi Alan, very enjoyable and educational videos, thank you for your time.
Nice video, Good teacher Today I changed two 1N4148 through hole diodes on a freezer control board. These diodes were serving as clamp diodes each across its own relay. The freezer is up and running on a trial period to see if it goes through the defrost periods properly. If it works 15 cents and a little knowledge saved $600. I was going to replace them with 1N4002 diodes since the old ones didn't seem to be up to the stress and maybe with a zener in tow, but I chickened out.
Another Great Job. Always look forward to your videos
Great explanation of how to use a PIN diode as a switch.
This is an excellent tutorial. Thanks for sharing.
I was eagerly looking forward to this video. You have nice set of hot tweezers. I certainly look forward to a lecture on switching diodes and pin diodes. Actually i am experimenting with 1n400x series myself. It would be very nice to explain how to build a pre-amp pin-diode switch with this technology; something based on a MAR-X or any low noise GAAS-fet and how to protect them for giving up in blue smoke with pin-switching.
A big thanks for all your nice work!
Best repair video I've seen in long time... Feel free to add some more !! Not like Dave Jones who seems to be doomed on his repair videos !! ;-)
Lots of fun to follow you along to success!
Excellent repair. I always like repair videos. Well done.
Very interesting, thanks for taking the time to upload .
Another great video. The trouble shooying technique was educational and you made surface mount soldering look easy. 73 Drew
Very instructive. Thank you for these great videos.
Cool video. I've got a Yaesu VX-7R that I've been told suffers similar issues. I've got all the tech docs for the radio so when it goes I'll be able to fix it.
Great! video, looking forward for more videos and learning more thanks Allen!
Boy, that Jerry sure is lucky to have a friend like you 😜. Nice video
First time watching. Great detailed video
Good work and very informative. Subscribed, and looking forward to watching all the other interesting-looking videos. Thanks for the channel and effort.
Wow!! good Job on the repair and explaining while you work..wish I have that kind of skill level.
Very nice ! Congratulations and 73 !
After this I think I could fix my lawnmower. (smile ) Didn't understand any of it, way way over my head, but very interesting just the same. I watched both vids and they were pretty cool. There sure is alot going on under the hood.
Great vid Alan, I would also like to see some stuff on PIN diodes....
Very nice. I have a similar issue but different is not trasmmiting 70cm. However the brand is QYT KT-7900D... QuadBand. I found the same problem. I found the Squematic for KT7800 and QYT told me I can use it in reference this new KT7900d unit....
Excellent video, as always. I am very much hoping to see a tutorial on PIN diodes sometime. I perfectly understand the concepts of regular diodes (e.g. silicon diodes), but I'm completely unfamiliar with PIN diodes. I am very curious about what characterizes these diodes in comparison to other ones.
Sometimes a failure in the DC drive circuitry of a PIN diode used to switch RF power is the cause of the failure. Insufficient DC bias puts the diode into a resistive portion of its operating region, rather than the saturated minimum-resistance region. (It acts like an attenuator.) The diode then dissipates substantial RF, overheats, and fails, and similarly a new replacement diode will also eventually fail. So, it's important to also check DC bias when a RF power-switching PIN diode fails. (A symptom of insufficient DC bias is slightly reduced RF power output from the radio, and/or a slight drop in RF power output after a few seconds.)
Great, it was fun here also. Never have worked on surface mount so it was helpful. You made it look simple, thanks. W4GSM
Great informative video thanks for posting
Good and useful video
Very informative video thank you. M3KQW in the UK.
Thank you for sharing your knowledge
Nice work.
Weren't you concerned about the RF power going directly into your scope when probing on the output of the final? You sampled the output power on the antenna line but probed directly inside the radio. Is a 10X probe sufficient to protect your scope front end in a case like this?
Would probing for gate/base drive signals be a good way to diagnose an audio amplifier?
Great video, mine doesnt key up at all, no sound .... can you fix mine?
Where did you find your service manual?
Great tutorial on diagnostics. My yeasu 8900 keys up repeaters but no audio output on 2m. Going to try 70cm see if it works. Really enjoy your channel!
73 KC1CEG
Thanks for taking the time to finish the part 2 video and showing the outcome of the repair. Good Job! Any chance of info as to the part number and where to order from? It's been a few years so no worries if you can't find the info I'll have to do some digging around in the FT-7900 Service manual for the diode numbers. Also I'm wondering how much voltage was on the final RF transistor and how you didn't destroy your oscilloscope when testing it? I once measured a 30 watt radio that has over 1000Volts on the finals and my oscilloscope is rated for 400Volts max.
I obtained the part number from the FT-7900 service manual, and ordered the replacement parts directly from Yaesu Parts Department in California.
Wonderful troubleshooting very good tech using your info to repair tyt th9800 vhf norf output problem oks a lot like yeasu products
Another gem Alan. When you installed the replacement diodes you chose to use a conventional iron. Any reason why the hot tweezers could or should not be used? Does soldering one end at a time open up the possiblity of creating mechanical stress onto the component after the solder has cooled? Noisy fan on that rig.
Thanks again for all the videos and guidance.
Great Video!
Excellent video. My first question is where did you buy the Pin Diodes. Mouser or Digikey?
Love the watt meter I have the same one
Very very interesting video. I had never heard of pin diodes before. Are they specifically designed for RF applications? I would love to see a quick video on them if you have the time and interest! Thanks!
With your help I may have figured out what's wrong with my FT7800. I checked the diodes in the VHF receive line and found one that didn't show anything with diode check. This is D1015 an antenna switching diode. All others measure about .7 volts. This one reads nothing. I am now trying to find a source for some. Thanks again and I will let you know how it goes.
When I got the radio it was transmitting on VHF and UHF and receiving on UHF only. It turned out to be D1015. I was able to buy the part from Yaesu for about 30 cents and the radio works great now.
Super job as usual Alan...thanks! 73 - Dino KL0S
Superb!
Great video!! Thank you for sharing.
I do have one question, it is my understanding that you must be very careful not to exert any pressure on a component while desoldering until you get reflow on all sides due to the fact that if one side reflows before the others you run the risk of lifting a pad. What are your thoughts on this? Thanks again --KD7YCX
Yes, you are absolutely right.
Great vid, more like this please :)
Great Video. Thanks for sharing. 73s Charlie..
Quick question: do we need to clean the flux apart from the aesthetics?
Those discoloring inductors are worrisome, possible design flaw, or improper using that radio. Good video.
nice vid, only one question, why clean up the flux? nobody will see that anyway
LOVE THIS VIDEO THANKS
where did u get the parts from and how much I need to do the same thing to my 7800 the power is dropping off on uhf and vhf tnks mike in Canada.
I got the parts through Yaesu's service department. I don't remember the cost. Shipping was likely more than the part cost.
Excellent video, Thanks for this, I found it very helpful.
73s
G4GPY
do you have the link to were to order the pin diodes?
+Christopher Ritchie (Adventurous) I ordered them directly from Yaesu - I believe I called the number on the Yaesu website.
wow a very unknown brand, Litec Corporation. They just make a bunch of RF diodes, nothing more. The part in the 7900 (L709) seems to be obsolete as well but has a successor (L8104) it seems. Nice repair.
Nice video. Thanks
Great vid,, see little of this type of thing, and others said the same thing. I used to do a lot of this and used a beckman dvm with a beckman rf probe as it would handle the high voltages you see sometimes, and also used a special low level torque wrench, about 6 pounds if I recall, for transistors. Were the pin diodes in a balanced circuit involving a transformer or split loading coil ? Do you think that these diodes are involved in some sort of protection system ? If you mentioned what you thought they were used for I missed that, just that they were a part of the low pass filter system. Considering the work you did the camera work was perfect. Realize this was an old video . Was subscribed but will definitely be investigating your work after seeing this which was forwarded to me by another technician.
These PIN diodes were in the transmit signal path, and would be biased on for 2m transmit, and off for 440.
@@w2aew Oh, so they are switching in and out matching components maybe. Nice.
@@californiakayaker No, actually switching the entire RF path.
@@w2aew Will be looking at my 8900 to see if they were used in it.
I'm sorry if I missed it, I just scanned the comments. Do you have a part number and a source for those diodes? I just purchased a FT-7800 on ebay and the guy said it works perfect on UHF but is very weak transmit on VHF and no receive on VHF. I don't have anywhere near the equipment you have to troubleshoot, but I'm hoping it is one of them PIN diodes. Thanks
From your description, it doesn't sound like these diodes are the problem. In my case, VHF reception was fine. Only VHF transmit was affected. In your case, it sounds like both VHF Tx and Rx are affected. So, I would look into a more common element. Of course, these PIN diodes are part of the T/R switching in the VHF path. You can test for basic functionality with a DMM. The ones that I replaced were sourced from Yaesu directly, as I indicated in the video. Yaesu p/n G207562.
Ok thanks for the reply and sorry I did miss that from the video.
Curious to know if you further tested removed diodes. Why would both fail? Any thoughts on failure cause? I consider in repair if another component/circuit led to failure as replacing broken part may be futile if cause for failure is elsewhere. So as in medicine always get thorough patient history.
The removed diodes were open. Research indicated that this is a common failure mode for this rig. I suspect the failure is at least partly due to a little bit of poor design practice. It's generally a poor idea to put diodes in parallel because if they aren't well matched, the current isn't split evenly. Thus, one devices gets stressed more and can fail early, which leaves the other to carry the full load which causes it to fail. Everything looked normal with the bias, so I suspect that poor matching was the ultimate failure cause.
*****
Interesting. So one failed and then the other stressed out :) Would you recommend then, in this case, to use a single diode but one rated slightly higher than the maximum expected forward biased current? I do assume tho that in a repair situation such as this, staying within manufacturer's original design is a safer approach.
k1mgy That's what I suspect happened. As you may know, forward voltage of a silicon junction decreases as temperature increases. Thus, if there are a pair in parallel, the one with slightly lower junction voltage will take more current, thus dissipating more power, thus heating up more and lowering the voltage. Basically, you have the potential for thermal runaway. This is why you always see small ballast resistors in series with the emitters of paralleled NPN pass transistors in high-power linear power supplies. In this case, it probably wouldn't be severe, but you can get some of the same effect, and including series balancing resistors would be undesirable. I generally assume that the manufacturer has evaluated the alternatives for their design, so usually decide to follow what they're originally designed. In this case, they've used this exact same arrangment in at least 3 different rigs that I've looked at - the FT7800R, the 7900R and the 8800 - so I figured they probably considered the alternatives. I don't know if they did any "matching" of the devices during manufacture.
Hello, can you please tell me what type PIN diodes you used?
I have the 8800 with a sort of same ( no tx on uhf ) thank you
I don’t remember the part number but they were the PIN diodes from the 7900, ordered directly from Yaesu.
That’s the ( my) problem, I can’t find the type in the schematic. It drives me crazy.
@@ronnypostma85 The replacements were part number L709CER, and I ordered them directly from Yaesu in California.
@@w2aew thank you so much. I’m pleased with ya! I (we) have a daily round with Netherlands amateurs. Looking forward to talk to you some days 👍
From PD0NBJ
Great job. I love watching your video. 73's kc2ddx
Good job can I send you one for repairing
What is number of Parallel PIN Diode for Yeasu FT 8800?
I don't know, you'll have to check the Service manual.
very much appreciated
Whats the partnumber on the diodes?
The ones I used were from Yaesu p/n G207562
me gusta el trabajo limpio y pulcro de usted mister. muchas gracias por compartir sus videos
Thank you for your nice comments!
siempre veo sus tutoriales mister mas por el manejo del osciloscopio tektronix tengo el model.465m y un digital de 60megas hice el circuito pulpo de usted y si sales unas formas graficas muchas gracias mister .
I really want a pair of those hot tweezers haha handy
I've had some water soluble fluxes actually corrode my solder joins and pcb. So I'd say yes, unless you're using a no-clean flux (which is what I've switched to and still clean using Isopropyl like Alan)
Do you work on icom 706m2g
I've only worked on one in the past. I did a repair on a damaged connector and installed some optional filters. I only work on my own radios, or those from close friends.
bravo!
I liked the video. If anyone is interested in SMD soldering, it is great fun, and easier than some scaremongerers want to make you believe. There are quite a few beginner kits available if you want to give it a try. Spend half a day watching tutorials on UA-cam, and you're good to go.
Hi Alan, i have found your Video because since yesterday i have a Problem with my ft-8800. The Hardware ist still the same as the 7800. The Problem is, that i don't receive anything. After a VHF Fieldday i don't receive anything but very strong signals in the near of the TRX. I have two Antennas on my car. One monoband for 2 meter and one monoband for 10 meter. The distance of the two antennas is under 1 Meter. The TX workes fine. If D1123 on the Parts Layout is broken (Its the Absorber), does the TX work? I think i had too much power on the 10 meter antenna and so the RX from the ft-8800 died. :-( I hope you can help me. 73 de DO4ZA
I dig the hot tweezers
Soldering that has got to be tricky, especially since if you slip you could burn the enamel off those inductors.
👍👍👍👍👍
More repair videos please, highly interesting, and as always presented with panache :)
2E0ILY
Very Nice Vid Thank You VE3RQB
5:10 tense! :O
Nice video, Thanks ! .... VE2TIH
THANKS FOR THE VIDEO. SUPER.
GREETINGS XE1UYZ