Restoring a FT-101EX

Discussion in '"Boat Anchor" & Classic Equipment' started by KE4CQ, Apr 13, 2019.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    There may be other +6V supplies in the rig, but the +6V Reg line feeding the VFO is only generated on the regulator board. If you're seeing +6V at the VFO it almost has to be on pin 13 of the PB1547 board. You might check that again and also check to see if the +6V supply is reaching the VFO mode select switch which is where it goes before it runs to the VFO itself.

    Don't get hung up in the voltages on pins 8 and 10 of the regulator board. Those feed the Clarifier (RIT) circuitry and in the PB1547 version of the regulator board they're derived from the +13.5V supply as opposed to deriving it from the +6V regulated supply as they did on the earlier 1314 version of the regulator board.

    It's likely that your PB1547B board does have circuit changes relative to the base PB1547 board so don't get too hung up in specific components and reverse engineer the schematic if you have to, but the +6V regulator should be similar to what's shown in the service manual for the base version of the board, a regulator IC and pass transistor that sets the +6V bus voltage.

    When tracking funny supply problems, it sometimes helps to break the existing supply circuit open and inject a known good lab supply. For instance you could lift the side of R12 that connects to the pass transistor and pin 1 of the regulator IC and inject a known good +6V from a lab supply with the negative lead of the lab supply connected to chassis ground. See if your receive warbles go away when you do that. It does a few things, it tells you if oscillations or ripple on the +6V supply is causing your receive distortion problems and it also validates the +6V from the regulator is actually reaching the VFO as it should. Of course putting a scope probe on pin 1 of the regulator IC may be a faster way to figure out if the +6V supply has issues.
     
  2. KE4CQ

    KE4CQ Premium Subscriber QRZ Page

    Thank you for your insight into my project!
    I had a few minutes this morning and looked at the VFO again. I now see that the input that I was measuring at 6v was coming from the clarifier. The other input into the VFO was hidden behind a large insulated wire and so I did not realize I had another input! So, I measured that one and came up with 11.5v. I verified it by moving the VFO select switch - on and off.

    Good idea on injecting 6v into the circuit. I may do that this evening. I have some new caps coming and plan to go ahead and replace those. Last night I removed Q2 and tested it with my Fluke multimeter in diode mode. All checks show good.

    From the schematic, it looks like the 6 volts should come out of pin 8 on Q1 which is an 8 pin IC (uPC141C). I have found some of those IC's on eBay and will go ahead and purchase one.

    My GOOD reg board from my FT-101E put into the EX is all good so I would think that my problem is probably on just the reg board - hopefully:)


    Kevin
    KE4CQ
     
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It sounds like you're on the right track.

    The regulator output is on pin 2 and drives the base of the pass transistor. The pass transistor is there to supply more current than IC can handle directly, IOW the IC alone is a low power device and can't source all that much current so Q2 is included to handle the heavy lifting on voltage regulation but Q1 (the IC) controls the voltage on the base of Q2 which also controls the voltage at the collector of Q2 which is your +6V regulated bus.

    I'd check all the passive components around Q1 before jumping in and replacing it and I'd especially make sure the voltage on pin 6 of Q1 varies up and down as expected as you adjust the trim pot VR2. Compare voltages and behaviors on your good working regulator board to the bad board. Sure Q1 might be blown but it could just as easily be a bad resistor or a cracked circuit trace near the adjustment pot.
     
  4. KE4CQ

    KE4CQ Premium Subscriber QRZ Page

    The trim pot, VR2 was gone from the circuit when I purchased the radio. It is a skeleton type pot but on this one the guts have been removed. When I have attempted to install a 3 leg multi turn pot, I loose ALL audio to the radio, it shows about 3 volts at the pot and adjustment on the pot has no effect at all. I tried 2 different pots and verified with my MM that they were working. So right now there is no pot there but I will try again after I install the new IC. The 2 pots I have tried are 2k multi turn which although reaches higher than 1K should cover what i need just fine.

    I studied the data sheet for this IC and have compared it to the circuit on the schematic. Here is a link to the data sheet. https://www.datasheets360.com/part/detail/upc141c/-6272254657789446884/ If you look at the "applications diagrams on the data sheet you will see that the 10V 200ma regulator diagram is what appears to be what Yaesu has followed. So, I would be curious to put in my good board and measure the voltage directly out of pin 8 to see if it shows 10v or not (what the datasheet diagram shows). If that is the case, 10v is not far off from the raw 11.5v that I am showing on my bad board. Makes me think my problem may also be where the pot is.

    Thanks again for your thoughts and expertise! I will be back at it again this evening.....

    Kevin
    KE4CQ
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, it sound like IC1 is blown.
     
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Sorry I haven't been back on here to comment . . . but the PSU in the FT101 is VERY simple . . .

    The 13V line is a simple unregulated supply with 2 diodes and smoothing capacitors.

    The 6V regulator provides the ONLY regulated line, to feed the VFO and other oscillators.

    So as I suggested before, the only fault can be with the 6V regulator IC.

    I've never known one of these fail on their own . . . but I'm guessing it damaged the IC when you shorted it out. (it sounds like it's not only not regulating properly, but going into some sort of oscillation)

    Roger G3YRO
     
  7. KE4CQ

    KE4CQ Premium Subscriber QRZ Page

    Thanks Roger.
    My IC regulator chip will be here on Friday. I am also encouraged that it will fix the problem. Until then, I have other things on the radio to work on such as a paint job :)
    The previous owner had installed 6JS6C tubes of different brands! One is a Mullard and the other I believe is a GE brand. It's stuff like this that needs some attention! :)
     
  8. KE4CQ

    KE4CQ Premium Subscriber QRZ Page

    Dave,
    As you suggested, I injected a stable 6V into R12 and happy to say that I have a good sounding radio. Looking forward to getting the regulator IC on Friday.........
     
    K7TRF likes this.
  9. KE4CQ

    KE4CQ Premium Subscriber QRZ Page

    I found and fixed the problem with my regulator board.
    After all it was not Q1 or Q2. It was R10, which is a 1.2K to ground for the 6v VR2 variable pot. Whew! Finally.
    I did something a few minutes ago that I should have done much earlier in the fact finding mission... I simply took a resistance measurement at pin 13 and compared it with my "good" board. Big, Big difference as it was reading very high! Well, a good lesson learned in problem solving and it was an easy find because there are not too many components in that circuit. R10 was actually busted at the base where it solders to ground but you could not see it. It was only found when I desoldered it and saw that it had been disconnected from ground.

    Thanks for all the good suggestions!

    Kevin
    KE4CQ
     
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Glad you found the culprit.

    FWIW, that's the advice I gave you several days ago:
    "...I'd check all the passive components around Q1 before jumping in and replacing it and I'd especially make sure the voltage on pin 6 of Q1 varies up and down as expected as you adjust the trim pot VR2. Compare voltages and behaviors on your good working regulator board to the bad board. Sure Q1 might be blown but it could just as easily be a bad resistor ..."

    Looks like it was a bad resistor.

    Future troubleshooting tip. When you find an obvious problem, like the voltage adjustment pot problem you described and trying to replace that pot with a suitable substitute doesn't fix the problem then don't go off looking for other problems or worrying about audio quality or other high level functions. Troubleshoot and fix the problem in front of you to avoid chasing red herrings and forgetting the obvious problem that isn't resolved yet.
     
    KE4CQ likes this.

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