Icom IC-736 Power Supply Help

Discussion in 'Radio Circuits, Repair & Performance' started by K7JOE, Dec 31, 2017.

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  1. K7JOE

    K7JOE Ham Member QRZ Page

    Hi, I have had on my desk an Icom IC-736 that wont turn on. For those unfamiliar, this is an HF+6M HF radio with built in ATU and internal AC-DC switching power supply. Troubleshooting it reveals an issue with the switching PS. I have removed it to try to bench trouble shoot but the PS issue is beyond my technical and ham shack limited test equipment capability. No obvious burn marks or leaking caps and I do not want to just start replacing parts here and there in hopes of reviving it. Anyway....

    The design of the PS is a little bit unique - As far as reading up on it, it is 100-240VAC input switching ps with dual voltage outputs - running 13.8V to the receiver circuits and computer circuits, and 28V to the PA transistors I presume this 14V/28V design is to minimize IMD. I am not sure, nor could i find in the service manual, what current output the internal PS delivers, but doing a little math, I speculate it's 3 amps for the 14V side and approx 11 amps for the 28V output. The wire size supports this approximation.

    A google search of the power supply board/model number is a Rubycon (a Japanese PS maker) model is: RPS-3604, but digging reveals no schematic or technical info. I started out wanting to repair this board or just find a replacement board. It is not labeled Icom, so Icom purchases these from someone else to install inside the 736.

    Got to thinking about this a bit more and am now wondering if shouldn't I just make an internal/external power supply using the relatively inexpensive switching power supplies as seen on ebay/amazon.

    They sell 24V power supplies capable of 15 amps although too big to fit into the chassis of the radio (bummer). I'll hunt for a smaller PS and then use an inexpensive voltage converter to provide the 3A at 14V too. Total cost would be around $45. Seems like a reasonable approach.

    Another approach is to try to find another dual voltage power supply of similar current characteristics that fits inside the chassis -- tall order but possible??? .

    Any other thoughts here? Last option is to make a "special" power supply for the rig that provides 28V to the radio, and just mount the voltage converter/regulator inside the chassis to provide the 14V for the RX.

    Looking for ideas on ways to do this cleanly. I considered using an Icom speaker, say, a SP-21 and hide the external power supply inside that chassis. Would be easy to hid the external power supply...

    Any other ideas or thoughts I may have missed?

    thanks
    Joe
     

    Attached Files:

    Last edited: Dec 31, 2017
  2. G4COE

    G4COE Ham Member QRZ Page

    If the power supply is dead it ought to be easily repaired, even without a circuit. Make sure the filter caps are discharged!

    Following the +ve terminal and check all resistors you come across, here I am thinking there will be a 'start up voltage' to get the circuit going, sometimes you will see a zener diode going to ground from this resistors, this is more than likely the component you'll want to check, remembering high value resistors normally go high, if this supply is faulty then there will bo no supply to 'boot' it into operation.

    I would be tempted top replace that small electrolytic right in the middle using ONLY 105 deg C high temperature types LOW ESR - avoid bog standard capacitors we are handling ripple currents of about 22-30Khz or so not 50Hz, replacing them all will do no harm at all with good quality low ESR units.

    Dave
     
  3. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    Hi Joe, I havent had a lot of luck with switching supply repair but ...
    heres a link to a free book. I downloaded it just to read and I dont know if it will have anything usefull
    https://readerspage.files.wordpress.com/2016/07/smpsrepairebook.pdf
    also I read that a fellow found an intermittant break on the main cap solder pad
    thats here: http://www.eevblog.com/forum/rf-microwave/icom-ic-736-power-supply-problem/
    and I also found the service manual for the ic736 here: http://www.radiomanual.info/schemi/IC736-IC738_serv.pdf
    it does show voltages as you say. I fear that if IC21 or IC81 is shot that it might be hard to come by.
    If it were me I'd see what is happening on IC21 but it gets signals from the secondary side thru 2 optos and there looks like some fusing on the input with a 10ohm R1 and relay contacts controlled from the 28v output.
    Lots of things controlling IC21 so I looked up the chip and found this:
    http://pdf1.alldatasheet.com/datasheet-pdf/view/890/MITSUBISHI/M51977FP.html
    then, I looked up IC81 and got this
    http://datasheet.elcodis.com/pdf2/87/29/872920/mb3759-p.pdf
    lots to digest
    I have an IC751A and am going to look & see what its internal supply uses, just in case.
    I built a linear supply for my 9100. I f you are thinking of making a 28V supply theres one in an old handbook using a lm723 and pass transistors. You could put the 13.8V regulators inside the rig. there are some hi current regulators like the lm338ksteel that have have fair current output and can be (properly) paralled for more current. There is some over voltage trip circuits around that are simple to do as well.

    anyway good luck, hope some of this helps

    Bob VE3CGA
     
  4. K7JOE

    K7JOE Ham Member QRZ Page

    Hi Bob, thanks much. I spent the day researching and came up with fairly similar information. troubleshooting switching ps is difficult - they are complex compared to linear PS.

    There is one item that I am investigating as possible option: https://www.ebay.com/itm/311906663367/

    while it is 12V and 24V, i bet it's an adjustable supply and may be able to increase to 28V and thus work for my application. Size is just slightly smaller than original PS board, so maybe it will even fit internally.

    Thanks for info. I'll study the links you provided.
    Joe
     
  5. VE3CGA

    VE3CGA XML Subscriber QRZ Page

    I think the chip datasheets combined with the service manual schematic would give the best clues. I wonder if the secondary can be isolated briefly so you can see if ic21 is working. Stuff gets messy unsoldering etc with switchers. the ebay ps doesnt look too bad, I'm suspicious of switchers anyway, had one put RF noise on my 2M rig
    73's Bob
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    Did you check the internal circuit breaker and fuse resistor ?

    C59 might be worth replacing.

    Good Luck
     
    Last edited: Jan 1, 2018
  7. K7JOE

    K7JOE Ham Member QRZ Page

    yes, RY1 and R1 both removed from board, tested OK. I'll look at C59..thanks.
     
  8. NU4Y

    NU4Y Ham Member QRZ Page

    I fixed one a couple of years ago. Especially look around c53-c54 for any leakage on the pcb.
    As I remember I replaced those and c11-c12 plus another couple of electrolytics. Really not trouble shooting, more like shooting.
    Was on a dxpedtition no time to spend on it. All worked. But c53-c54 seemed to be the one's that did it.
    GL Jim
     
  9. K7JOE

    K7JOE Ham Member QRZ Page

    ok. May just see what it will cost to re-cap the board.
    looks like about 10 caps... No leakage on the board that I can see...
    I'll grab the parts list to get the values and see if I can come up with some inexpensive replacements.
     
  10. N8VIL

    N8VIL Premium Subscriber QRZ Page

    Change the smaller electrolytics. Should be cheap enough. If you have an ESR meter I bet you will find a few that are bad. I repaired one for a friend about 5 years ago and this did the trick.
     

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