Yaesu FT-450 Poor Rx sensitivity above 10 MHz - Fixed (but....)

Discussion in 'Radio Circuits, Repair & Performance' started by G1OJS, Jun 13, 2019.

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  1. G1OJS

    G1OJS XML Subscriber QRZ Page

    A bit of a story here with the conclusion being:
    • The "Mute Rx" transistor had failed short circuit and reduced high band Rx performance but didn't kill it completely - so if you suspect yours, it might be worth checking.
    • Trust your instincts if you buy one second hand from a dealer & you suspect this, and ask for a meaningful (i.e. vs another FT450) a/b test at the least.
    So I've had my FT-450 for over a year now. When I first got it (second hand from a dealer) I found it very quiet on the higher bands but had nothing to compare it to and it was my first transceiver at my new QTH after a 6 year break from the hobby, so I rationalised it as being lucky to live in a QRM-free zone. I did take it back to the shop and compare with another receiver on one antenna, and was convinced it was OK. Any differences I put down to the other transceiver being superior and it was hard to tell anyway because of the high QRM around the shop (so band noise beat Rx noise on all bands and "S-meters aren't calibrated are they?").

    However, now that the higher bands have been opening up again the nagging feeling came back & I verified that band noise wouldn't beat receiver noise on 28MHz. Then I read this https://www.wirehead.be/index.php/2018/10/10/yaesu-ft-450-deaf-on-higher-bands/. I decided to measure those two protection diodes and found them to be failed open circuit, not short. So a fault, but not one that would cause poor sensitivity.

    A bit more poking around revealed that Q2005 - the "Mute Rx" transistor - had failed short circuit between base and collector. I carefully removed it from the board and, yep, higher bands are now normal and much more lively with noise and signals on average maybe 4 S-points above where they were. I replaced Q2005 with a spare RF signal transistor from my stock.

    So - it's fixed. However:
    • The two diodes are still open circuit and I guess I should replace them - though they've been like it for over a year. On the other hand the Rx is not now protected by the shorted Q2005.
    • My repair isn't pretty as I used a TO92 case transistor with legs bent carefully to line up with where the SOT-23 SM pads are.
    • I wonder what would happen if I simply removed this transistor? I know it mutes the Rx input on Tx, but for what purpose given there is also a Tx/Rx relay? Is it needed to mop up stray signal leaking through the relay? (Similar question - what happens if my ugly repair fails e.g. emitter / collector soldering goes open circuit?)
    I'm posting this as I said in case anyone else suspects their Rx on the higher bands, and I wonder what people would do next:
    • Leave it - it works
    • Replace the o/c diodes
    • Replace the o/c diodes and attempt a better repair by buying a genuine replacement Q2005?
    • Try to find a spare RF-IF board and play at the above knowing I can swap the board if it goes wrong?
    • Buy another rig, sell this one with this story in the advert?
    All of the options are part of the fun of the hobby!
     
    Last edited: Jun 13, 2019
  2. G4COE

    G4COE Ham Member QRZ Page

    Not looked at the SirKit, just wondering if a static discharge had got to it via the antenna or possibly RF from another transmitter say? The two diodes, may either be protection diodes, pin diodes or even tuning diodes.

    Reason I posted was I thought it odd for the radio to still work with the diodes opened circuit, that made me wonder. Have a shufty here at HIFI ssb for spares clicking on the logo in the central window.

    Dave
     
  3. G1OJS

    G1OJS XML Subscriber QRZ Page

    Thanks Dave, the circuit diagram shows that they are definitely protection diodes anti parallel from signal path to ground - so o/c will simply allow signal through. They are labelled "Protect" :).

    But yes, one of the main reasons for posting the story is that the effect of the Q2005 short, whilst not subtle, is not blindingly obvious either if it exists when you acquire the radio (I'd have noticed if it appeared after I did something!).

    Will check out HIFI ssb - I came across a few sources for the transistor whilst googling to verify the pin-out etc.

    Something definitely happened to the rig before I got it, don't know if static would be enough to blow *both* diodes and short Q2005 - especially as static would tend to be either + or - so would go for one diode not two - maybe it was a quick burst of RF not enough to physically fry the transistor as seen in other stories.
     
  4. G1OJS

    G1OJS XML Subscriber QRZ Page

    Update - thanks to super-speedy delivery of replacement Q2005 from this eBayer https://www.ebay.co.uk/usr/bijkker I now have replaced my temporary fix with the real thing. No noticeable change in performance, just satisfaction. I still need to replace the protection diodes.

    Thought I'd post some pics:

    IMG_0253.JPG last.jpg

    Temporary repair on the left, final repair on the right. It's still not pretty, but it feels better having the proper component there. Before I tried TX, I applied some positive bias to the base and verified that the receive mutes. Also checked continuity of the three solder joints.

    Whilst I had the lid off, I took some pics of the blown protection diodes (left) and, for comparison, the same diodes in another location (right). The internal heat damage is obvious in the protection diodes! I'd say that was RF, not static!


    diodes.jpg ScanImage011.jpg
     
  5. G1OJS

    G1OJS XML Subscriber QRZ Page

    ...and the diodes have arrived:

    Left to right - blown diodes in place, diodes removed, new diodes in.

    Again, not as pretty as the brand new board, but functionally back to factory condition & not bad for a first attempt at repairing SMD boards!

    Capture5.JPG Capture6.JPG Capture7.JPG
     

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