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Repair video: Yaesu FT-7800 Dual Band VHF/UHF FM Transceiver - part 1...

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w2aew:
A friend asked me to look at his Yaesu FT-7800 dual band FM transceiver (ham radio) which stopped transmitting on 2 meters. I put together this video showing the troubleshooting procedure I took to uncover the problem, which appears to be a pair of PIN diodes in the final filter stage. One person commented in the video that these PIN diodes might be obsolete. (Torex XB15A709A0HR). Checking Torex's website confirms that these diodes are discontinued.  I did find some from a Hong Kong supplier.  However, I'll look for some local (US) sources before ordering from overseas.  The datasheet for the original is a bit sparse, so not a lot of data to go on for looking for a suitable replacement.  Anyway, here's the "Part 1" video, which shows the troubleshooting process I went through.   Hopefully, I'll be able to locate replacement PIN-diodes and finish this repair...


Here's the link to my "repair log" video:

AG6QR:
Interesting video, as always.  I have that radio's successor, the FT-7900.  For curiosity's sake, I did a bit of searching and found that the corresponding diodes are L709CER in the 7900.  Yaesu probably ran into the same problem about the original diodes being unavailable and found the substitute.  I'm not sure if those are direct drop-in replacements, or if they made some changes around the bias network or something, but if your searches for the original part comes up empty, that might be something to pursue.

ElectroIrradiator:
Nice video. I might suggest having a chat with your friend about the circumstances surrounding the death of the PIN diodes. When they die, it can be due to a loose/arching connection in the antenna system. If the failure happened while using his regular antenna, then the repair may not last for long, unless any antenna problems gets fixed before putting the radio back in service.

What kind of probe are you using in the video? Not sure I'd dare throwing 100Vpp @ 144 MHz at even a passive probe... ???

Flump:
watched this this morning
nice vid, i have to do something similar
with an old hf kenwood of a friends that
has no output

w2aew:

--- Quote from: ElectroIrradiator on October 27, 2013, 05:31:42 pm ---Nice video. I might suggest having a chat with your friend about the circumstances surrounding the death of the PIN diodes. When they die, it can be due to a loose/arching connection in the antenna system. If the failure happened while using his regular antenna, then the repair may not last for long, unless any antenna problems gets fixed before putting the radio back in service.

What kind of probe are you using in the video? Not sure I'd dare throwing 100Vpp @ 144 MHz at even a passive probe... ???

--- End quote ---

He was just operating normally.  I'll have him check for an intermittent in the antenna. I'm suspecting the failure was due to the fact that these diodes were in parallel with no provision to evenly split the current. One probably took most current, failed, and left the other to die. Just my opinion.

The probe was a Tek P6139A. Rated for 400Vrms at low frequencies, deranged to 50Vrms at VHF and low UHF, so it was OK.

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