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Kenwood TS-940S display problem with a twist

Discussion in 'Radio Circuits, Repair & Performance' started by KA2JNF, Oct 30, 2011.

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  1. KA2JNF

    KA2JNF Ham Member

    Hi Guys!

    Hate to do this to you. I have a 7million serial Kenwood ts-940s that I have owned used for over 10 years.
    For a long time, ran great. Now if I leave it unplugged for several months, will get the dreaded dot dot dot display issue. If I re-set the processor a few times, and run it for awhile, it is ok again.

    I Played with it yesterday, and until I reset the processor, it was no good. Now it is running fine, breaking into all sorts of pileups on 10, 12 15 and 20 metersng. I thought this was an all or nothing problem, not an intermittent one. Is this the typical "loose connector" issue or does it sound like something else?

    I've been inside this rig, and am squeamish about moving boards around, and I know the 940 suffers from cold solder joints, but have not seen this spin on this problem before. Should I not worry about it, or is it something I need to delve into? and, any way to restore the print on the A=B and A/B buttons that is worn off?
    Thanks, Dave Ka2Jnf
     
  2. G4COE

    G4COE Ham Member

    Dots usually indicates pll trouble, usually out of lock. TS940/930 all same breed, I'd carefully do a resolder on all boards paying attention to the power regulator section, also check those mini rf connectors for tarnishing thus causing problems.

    Here's a old thread from QRZ.COM on the very issue.

    Dave
     
  3. W0FRU

    W0FRU Subscriber

    Dave,
    You might check the condition of the lithium batteries. Low or dead batteries cause strange things to happen.
    Jack, K4FRU.
     
  4. KA2JNF

    KA2JNF Ham Member

    thanks for the response, I appreciate the link. I suspected a cold solder condition of sorts, will study the link and see if I can handle it.

    73's

    Dave
     
  5. G4COE

    G4COE Ham Member

    Further to K4FRU here's a jpeg of where they are along with the display.... I forgot about the LCD I assumed the main dial!

    I ought to have asked first.

    Dave
     
  6. KA2JNF

    KA2JNF Ham Member

    Yes, it is the main display. the LCD sub display works fine, although I need to replace the grain of wheat bulbs with lcd's. I've run this radio on and off all day, and it has not even made a hiccup.

    and yes, It definitely could use replacement of the batteries, which was what I was hoping it would be, but apparently is not. these batteries are who knows how old, and could use replacement.

    any idea about re-silkscreening the a/b and a=b buttons, which the writing has worn off? this is the only cosmetic problem with my radio.
     
  7. G4COE

    G4COE Ham Member

    I've never tried it in my 930... but have done it in my Yaesu FT290, I used a hi-bright water clear 3mm LED with a 330 Ohm resistor (12V supply), the current was only 28mA, the lamp in these is a pain to change, hopefully changing the lamp for the very last time and it appeared a wee tad brighter and much cleaner looking - worth a thought! I'll certainly be trying it again should I ever have to change a lamp... stick to hi-bright water clear!

    One TS940 I repaired I could literally pull three bridge rectifier diodes out, they were that loose!

    I should have asked, my bad! But you confirmed the main dial then I would suspect a PLL unlock or problem. As far as I know all Kenwood's with a PLL fault will display a row of dots on the tuning dial.

    I don't know if there is anything here, but certainly worth eying over! KB2LJJ excellent article on the 940.

    For spare 'cosmetics' try this place, they are in the UK though.

    Dave
     
    Last edited: Oct 30, 2011
  8. KA2JNF

    KA2JNF Ham Member

    I am torn between really getting my hands dirty on this. Like I said, I have looked at a bunch of the web pages on this one. I know that there is a lot of controversy on the 940 with the "reversed" transistors. for a 7 million serial rig, all the mods have not been done, I am wondering if it really is worth putting 500 dollars in repairs and shipping into it. I want to think I can do it myself, but am realistic about the fact that I am not just soldering a bunch of wires here. I am worried about separating tracings on the boards. If it came down to that, it would be a heck of a learning (and $$) experience.

    I have been inside the rig 2 times prior, and it is a marvel of how they got all that stuff to fit inside the box. Now to have to undo all that stuff, and fix it? that scares me.

    If I would decide to bail, who would you suggest to do the repairs?
     
  9. G4COE

    G4COE Ham Member

    Sorry, being the other side of the pond I can't say who's the best place to send it to... maybe someone on your side will advise.

    I guess these are the last of the 'serviceable' rigs, all seem to be going surface mounted components these days, they sure do look like dead bugs... all hot air and soldering paste.

    The service manual for it ought be available, the very last link, KB2LJJ I gave seem pretty informative on the PLL problem, just 'may' want a tweak. Try blowing some warm or cold air around the PLL and see if it drifts back to life.... don't want it too warm though, knowing my luck it'll drift the wrong way, hi!

    Hope you get it resolved, 73's

    Dave
     
  10. KA2JNF

    KA2JNF Ham Member

    thanks for the responses. It is drifting to life, it is running well, just wondering if the other shoe will fall.

    I am putting my feelers out locally to see if I can find a qualified person to look into it. thanks for your help.

    Dave Ka2Jnf
     
  11. W9GB

    W9GB Ham Member

    The most common problem with these radios (TS-930 and TS-940) are corrosion of the plug connectors
    between the wiring harnesses and PC boards. Cleaning those connection with proper solvent
    (One that does NOT melt the nylon or make the situation worse)

    The number of technicans that I would trust with a TS-940 is a VERY SMALL LIST.

    Why? Too many HF radios are being "parted out" and sold on ebay for quick cash -- that require minimal service to fix.
    The occasional ex-CB, radio/TV applaince technican, while qualified for general repairs, often becomes quickly overwhelmed
    with the number of parts and technology (complexity) that Kenwood shoe-horned into this chassis.

    Kenwood did have Service Technican classes in USA on this radio (TS-930 and TS-940) -- that would be my first question to anyone claiming service abilities !

    As Jeff King, ZL4AI / DU7 has pointed out -- they are repairable and have been used in earlier DXpeditions - BUT you need a degree of patience !
    http://homepages.ihug.co.nz/~jaking/TS-940_02.htm

    G8WRB has placed Jeff's TS-940 Notes into a PDF file - that you can download (151 pages)
    http://www.g8wrb.org/data/Kenwood/TS-940S/Kenwood_TS-940S_Fixes_Reviews.pdf

    A phone call to Clif Holland at Avvid -- can be an initial query for assistance.
    http://avvid.com/

    I do know of other good Kenwood techs -- but none that live in Buffalo, NY.

    w9gb
     
    Last edited: Oct 31, 2011
  12. KA2JNF

    KA2JNF Ham Member

    qualified radio techs seem like a dying breed. If this were a television it would have been in the garbage by now....
     
  13. W9GB

    W9GB Ham Member

    CRT based -- yes.
    There is one gentleman up the street from me, that handles O'Hare Airport's monitors (CRT/flat screens (LCD, LED)) he stays busy.
    Ironically some fo the latest LED/LCD displays fail and can usually be easily fixed (if screen ok) by reapir or changeout of the
    switch mode power supply (failure).

    The move to surface mount technologies in 1990s .. marked a change for board level service / repair operations.
    It is amazing the quality and amount of equipment that hit the surplus market from 1995 to 2005 due this
    change, change in Corporate Support/Service policies and manufacutring changes (RoHS compliance).

    The community and techncial school sin the Chicago area switched their teaching toward comptuer software during this
    same period. Component level repair is slowly becoming a "lost art" for the USA --- time and patience.

    SOME are working to keep the skill sets alive and training a new generation --
    BUT the promises made of "dumbing down" tests and requirements (Leave No Child Behind) leaves a country vunerable (no defenses).

    w9gb
     
  14. KA2JNF

    KA2JNF Ham Member

    the more and more I think about it, I should move this post to "for sale" but I won't do that just yet.
     
  15. W9GB

    W9GB Ham Member

    As Jeff King points out, this is really the LAST of the complex analog radios that IS repairable.
    Uses thru-hole components, the cosmetic/plastic parts are the only ones that are "hard to find",
    electrical parts are still available OR easily substituted.

    For the "non-technical" amateurs -- stick to current production radios -- still under factory service.
    Treat the hobby like your mobile/cellular spending -- change your radio every 2 to 5 years --
    that IS what the mfg. desired and were VERY SUPPORTIVE of lowering the licensing bar.


    YES, that approach can be more expensive -- especially for those economcially challenged.

    w9gb
     
  16. KA2JNF

    KA2JNF Ham Member

    Greg, you still have to consider, this rig is 25 years old (plus). It's exceeded the life of it's design, and for all intent and purpose, "ought" to be retired. (trying not to do that!!)

    this radio had so many bugs in it and yet we love them to death. The more you know about them, the more it is hard to believe any of them have survived this long!!

    I don't think it is necessary to "replace" a rig every 5 years, like a cell phone. Even my Kenwood Ts-2000 is older than that, but still runs well. The 940 is a very complex piece of equipment, and it is surprising with all it's issues (and the 930 as well) that Kenwood did not go under due to all of these "design flaws."
    It sounds like it would probably be cheaper (and more reliable down the road) to refurbish my ts-830s. At least there is room for air inside that box to circulate. the 940s is a hornets nest inside. How did they do that?
     
  17. N8YX

    N8YX Ham Member

    The problem lies on the PLL board. It is under the Digital A unit and requires you to dig into the radio a ways. Once you have the PLL board in your hands, look at the back (solder) side and identify a square piece of black foam. Remove it and resolder every joint underneath, then install the board in its spot and reassemble the balance of the radio.

    If you still get the 'dot' problem, realignment of the VCO whose circuitry was "cushioned" by the foam is in order.

    Kenwood put the stuff there to damp out vibration-induced microphonics. Unfortunately, the material they used is hygroscopic - it attracts water and will eventually cause the solder joints underneath it to corrode. This results in VCO failure and subsequent PLL unlock.
     
  18. YB7MRK

    YB7MRK Guest

    Hi Dave Ka2Jnf
    Hi... all

    I like Vintage Radio
    Display shown dot.
    I think this problems on PLL, let see on Service manual for re setting VCO and adjust up again all voltage of VCO, if not fixed the problem, let check again on IC MN6147 or replace this part, and check again VCO on main unit board on bottom, vibration on audio is not only from microphone, however the PLL signal not linear let check sinus signal on Osciloscop equipment.

    Hope so you have nice day with your radio.

    Regards

    Martin de YB7MRK
     
  19. KA2JNF

    KA2JNF Ham Member

    Kenwood 940S

    Thank you Martin,

    Many people have emailed me indicating the pll is the fault. thanks for suggesting the part #, will consider pulling this apart.

    Right now, it is working fine though.. I appreciate

    73's

    Dave Ka2Jnf
     
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