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Thread: IC-745 No RF output

  1. #1

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    Default IC-745 No RF output

    My ICOM IC-745 stopped transmitting. I found one driver and PA transistor bad. I replaced ALL transistors, except Q7, on the PA unit plus a burned up R14, still no RF output. All transistors are getting the required input voltage, 14.5VDC. Also I have only 2.5mA across W35 and adjusting R23/R27 make no difference to this reading. I'm suspecting it may be in the RF unit. I don't have a signal generator/tracer so am using the "N5ESE" RF probe and my digital multimeter. I have an old photo copied IC745 Maintenance Manual, which is of poor quality. Any ideas or tips would be greatly appreciated. Thank You. Dale P. Appleton K2DPA.

  2. #2
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    It's always a good thing to have a general-coverage receiver in the shack -- one that you can listen to and see if the transmitter is producing any RF at all.
    73
    Pat K7KBN
    Semper ubi sub ubi.

  3. #3

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    what you really need is an oscilloscope to test the output of every stage to see where the magic stops happening.

    A good quality service manual is available at http://www.radiomanual.info/schemi/I...C-745_serv.pdf

    I must say.... 14.5v on the final seems a bit high to me. Every radio I've seen has this regulated down a bit under 13.8 somewhere to stabilize it against changing with load conditions... and the fact that it is this high (without studying the schematic to know for sure) would make me suspicious of the voltage regulator transistor. Typically, when the finals fry in a manner capable of visible damage, so does the regulator that supplies it the power. Its hard to make any accurate assumptions not knowing the cause of the burn out or not knowing the mode in which the voltages were checked.

    I would recommend, if you have no receiver capable of picking up HF, to attempt a listen on a near-by police scanner set to the nearest programmable harmonic to see if the final mixing stage is active at least. If you have RF on frequency (and can hear the scanner picking up a harmonic of it), it would likely be in the amplifier stage or power regulator. Perhaps tomorrow I can look at the schematic and see if I can come up with possibilities, but for now, that's my best guess. Can you provide any other information as to how it happened?

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    Brandon. I have a Yaesu FT450D and the N5ESE RF probe of which neither indicate any RF out put from the 745. I have that service manual referenced in your reply. The radio was serviced by an ICOM Tech in Tampa FL, when I got it back it was working fine. Then one day while CWing on 40M it just quit sending. A couple of months ago I retuned my vertical and my worst match is on 10M at 1.5:1. I'm using the ICOM PS-125 power supply and 14.5v is at idle, can .7v make that much difference? I'll keeping reading over the service manual, may be I'm missing something.

  5. #5

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    Having the supply at 14.5v is within spec, and I do not see any regulator for the final transistors, so that's fine.

    Adjusting those resistors is for bias control, and from what I see, there should be an idle current regardless of what the RF stages are producing as long as the radio is in ssb transmit mode

    setting the bias:
    1) remove W35 and W10 jumper
    2) attach dummy load (or antenna if you have no high power dummy load) to radio output (shouldn't matter without both stages on and RF coming through... this is a DC adjustment assuming no mic input. Just being safe)
    3) set radio to SSB, mic gain minimum
    4) with ammeter attached across W35 location, key the radio and check bias current while transmitting. Adjust R27 for 100mA.
    5) Now place the ammeter across where W10 was and shoot for 600mA via adjustment R23 while in transmit mode.
    6) replace jumpers

    EDIT: If you are still getting no current to the bias networks, it might be a bad resistor on the 8v regulator board. You should see 8v on the W5 wire (in the corner by L12) of the PA board (the one you switched final transistors on) while keyed up. Look on the regulator unit (see picture) for R1, a 4.7ohm half watt resistor. Ohm it out. If found to be bad, repair, then attempt bias adjustment again. And visually inspect the regulator IC when you have the 8v regulator unit removed.

    s-l300.jpg
    Last edited by brandon lind; Mon 31st Dec 2018 at 04:32. Reason: added new info

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    Brandon. I've tried checking for the 100mA and 600mA at W5 and W10 two times once using the instructions from the Svc manual I down loaded, then again using the instructions you provided and both times I got only 2.4mA and adjusting R23 and R27 made no change in that output. While I did this I had my Yaesu on and tuned to 14Mhz, and there was no response on it. I've checked both D1 and D2, they are both good. Not sure if I mentioned I did replace R14 as it was burned up. You mentioned P3, all wires/cables coming from and going to the PA unit are IDed as W1/W10/W4 etc, would "W1" be the same as "P1", etc.?

  7. #7

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    W5 (according to the pic in the manual) appears to be where the radio sends in power to feed the bias networks. Its an 8v source that is active during TX.

    If you are getting nothing through the bias networks, I assume the 8 volt regulator unit has a problem. Go to page 66 of the manual and look at the picture of the PA circuit board. Notice in the top right corner a marking labeled W5, this is where the 8v comes in during tx to supply the bias networks. If you key the radio and there is not 8v there, take a look at the regulator unit, the image on page 39 shows its location. According to research online, this commonly has a 4.7ohm half-watt resistor fry out. The resistor supplies power for many of the control functions of the radio and often makes everything but the display seem dead. Start there.

    EDIT: W1 and P1 are not the same thing. W is wire. P is for Plug (holding multiple wires), J for jumper etc. Unfortunately Plug 4 dont always go in Jumper 4
    Last edited by brandon lind; Wed 2nd Jan 2019 at 23:47.

  8. #8

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    Brandon. I found R1 (4.7ohm) on the regulator board, removed one side and got 6 ohms so I know it's good. There is 14.8vDC going into and in transmit 8.3vDC coming out of that regulator and 8.3vDC at W5 in the PA unit. I'm frustrated not knowing where to turn to next. I use to repair xray and xray processing equipment and always had the best service manuals on hand. FYI, I do hear RL1 on the PA unit clicking when transmitting so I know it's good. I sincerely appreciate your assistance. Thank You.

  9. #9

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    During TX with both the W35 and W10 jumper removed, do the following and see if anything stands out. If you get through this without issue, we will look further. If you hit a problem, stop and tell me.

    Assuming W5 has 8v on tx:
    1) is there 8v at + side of C21? If no:
    possible shorted C21
    possible damaged L12 (from shorted C21)
    2) is there 1.4v at the anode* of D2? (*--|>|---) If no:
    possible shorted C22
    possible burnt R23 (0-470ohm)
    possible burnt R24 (150ohm)
    possible bad D2
    3) is there .6v at the anode of D1? If no:
    possible shorted C27
    possible burnt R27 (0-100ohm)
    possible burnt R28 (68ohm)
    possible burnt R7 (22ohm)
    possible burnt R8 (22ohm)

    Is there 13.8v (or 14.5v in your case) at R20?
    Lets start with that and see what happens...

  10. #10

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    Brandon, here are the results:

    There is 8.04vDC at the + side of C21

    There is 1.31vDC at the anode of D2

    There is .53vDC at the anode of D1

    There is no voltage at R20, I removed the bare wire at W10. However there is 14.5vDC on that bare wire.
    I discount any small voltage differences are a result of my DVM error rate.

    I appreciate all the help your giving me, so don't put this on your priority list, I still have the Yaesu FT-450D to work from. Thank You.

  11. #11

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    A closer look at the board confirms there would not be voltage on R20 with jumper W10 removed. I didn't realize R20 was AFTER the jumper as the schematic did not depict its location, no worries there. But ohm out R20 just for fun...

    Otherwise, those voltages look close but a bit low. Again, I am assuming the transistors you installed are identical replacements or suitable equivalents and are in working order AND installed correctly. The R23 and R27 pots, when adjusted, should give a current through the removed jumpers.

    Forgive me for asking, but are you making sure the ammeter is in DC mode and not AC mode and BOTH the probe tips each connect to where the W jumper ends were soldered in (using the ammeter as a jumper unlike a voltage measurement with the black probe to ground somewhere)? If done right, based on those measurements only 4 possibilities should remain: 1) the transistors have a much lower beta than the originals and the bias resistors need changing to different values, 2) the transistors are not functioning correctly or are installed wrong, 3) the variable resistors R23 and R27 are half smoked and allowing just enough current to let a diode junction just barely begin conducting, or 4) the diodes somehow changed their knee voltage to a lower value preventing the transistor from turning on (highly unlikely, never heard of it. Did you change them?).

    From what you are describing, you have supply current to the transistors' collectors (when jumper or ammeter completes the circuit) and there is voltage to the bias network which should be making it to the transistor bases.

    If your DVM has a diode setting, I would remove Q2 or Q3 and see what the diode drop is between the base and emitter, and then with Q2 or Q3 in circuit see if that same voltage of higher is attainable at the base connection via adjustment of R27 (after ohming out R27 to make sure its not burnt up, ohm across outer terminals looking for 100ohm). If it cannot deliver the turn-on voltage, we know where to start next.... a transistor beta measurement.

    EDIT: do that very last voltage measurement at the base pin step with W jumpers installed
    Last edited by brandon lind; Sat 12th Jan 2019 at 03:12.

  12. #12

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    Brandon. I’ve ohmed R20, I don’t have a precision ohm meter so the reading I got was 0.1 ohm. The svc manual IDs it at 0.012 ohm and 5 watts.

    All transistors were replaced with OEM Mitsubishi parts, and matched pairs for the drivers and PAs. I checked Beta for each transistor before installing them and they were all good. Using my transistor tester I find the Hfe/Beta Q1 is 144, Q2 is 72, Q3 is 78, Q4 is 83, Q5 is 82.

    After I replaced Q1 (for the 3rd time) I ohm checked all transistor leads for good solder connection. I removed R23/R27 and ohmed them also, R23 was 435 ohms and R27 was 90.5 ohms.

    As I said earlier Q1 has been replaced twice before in trying to fix this problem. I have 7.61Vdc between R29 and R3, 180mV at the Q1 emitter when the mike is keyed.

    With an amp meter in series with W35 and W10 and the mike keyed, I get 3mA and adjusting R23/R27 show no increase or decrease.

    This is where I now stand.

  13. #13

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    I am very confused...

    Let’s start with the known facts. You measured 8.04v at the + side of C21 (which also happens to be directly connected to the higher side of R29). You measured 7.61v between R29 and R3. This gives a voltage drop across R29 of 0.43v. Using ohms law, we can determine the current through R29 as being (.43/10ohm) = 43mA.

    The manual calls for 0.4v at the emitter of Q1 (you measured 0.180v). Since the emitter resistor is 2.2ohm, we should expect an emitter current of 0.4/2.2 = 182mA. Given a beta of 144 (you measured), there should be a current of 182mA/144 = 1.264mA at the base. Generally, a voltage divider type bias for a class A amplifier is designed to have at least 10X the current through it to stabilize the voltage at the base. Being the voltage divider is comprised of R3 and R2, and knowing R3 has the same current flow as R29, we know we are well within spec at 43mA for the divider circuit. Knowing we have 3.4X more than the suggested current through the divider (R3 and R2), it is safe to assume the effects of Q1 should have little effect on the voltage at the base of Q1. Therefore, the voltage at the base of Q1 should be approx 7.61v*(R2/(R2+R3)) = 1.33v.

    Knowing the voltage at the base of Q1 is roughly 1.33v and the emitter voltage (per your measurement) is 0.180v, the base-emitter voltage drop is thus assumed to be 1.33v - .180v = 1.150v. I’m sorry to say… but for a properly functioning transistor, that is very unlikely. Being there are no electrolytic capacitors to dry out in this particular path, and assuming the solder job is proper, id say either a resistor somewhere is not good, the transistor beta is not 144 as you suggested, or something is being measured wrong. I would at least grab a magnifying glass and check thoroughly for rouge solder blobs shorting things out.

    Perhaps I lack the experience needed to solve this one through a keyboard alone, but I do know when I am beat… That said, I highly recommend sending the radio in to the same Icom tech that repaired it the first time. Perhaps having it in front of the tech doing the diagnosis will clear things up. I greatly enjoyed the mental exercise and apologize I was not more helpful.

    73 and good luck!

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    I would like to extended my sincere appreciation for your assistance. I think I have the problem narrowed down to Q1, as it keeps burning out. When I ordered that transistor it was cheaper to order 10 of them, due to the shipping cost. 73s and you have a safe year.

  15. #15

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    That is interesting, I was unaware of the fact Q1 was burning out. It is possible that the replacement has the mounting tab connected internally to the emitter pin and may need insulating mounting hardware like mica or a sheet type silicone heat sync material and ceramic screw support. If the mounting tab is internally connected to the emitter pin, directly shorting it with the mounting screw bypasses the emitter resistor completely! The emitter resistor is CRITICAL. The base-emitter junction wants to stay at about .7v, without the emitter resistor to take up the rest (1.2v - .7v = .5v), it will saturate and dump as much current as it can through the device.
    Last edited by brandon lind; Thu 17th Jan 2019 at 07:55.

  16. #16

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    Oooopppppppps
    Brandon. I was sure that when I replaced the transistors the insulating mica and plastic retaining screw came from Q6. Could this be my problem?

  17. #17

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    one easy way to find out is to ohm between the tab and the pins. if q1 shows conductivity between the tab and emitter (or any pin for that matter) then YES.

    It is possible the mica came from q6 and that the new replacements for q1 are different internally with respect to tab isolation and also require the mica.... ohm the pins on q6 as well. if any pin on q6 shows continuity to the tab, taking the mica away from q6 could toast that one as well. you might need it on both!

    EDIT: Another thought worth adding... Make sure the W35 and W10 jumpers are removed until you resolve the Q1 issue. It is possible that Q1 being instantly cooked (thus damaging the base emitter junction), it was the cause of the lack of bias current for the other stages. If you repair Q1 and suddenly there is enough current to supply the other stages with those bias pots set wrong, you could burn those transistors up too. Keep those jumpers out until you are ready to make the bias current measurement!
    Last edited by brandon lind; Wed 23rd Jan 2019 at 17:51.

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