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A minor issue with my 700M has me a little stumped

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by Alobar, Sep 20, 2018.

  1. Alobar

    Alobar Pulling out of the Last Chance Texaco.. Subscriber

    Messages:
    5,338
    Location:
    SE Alaska
    I have lately been noticing a fairly low frequency hum, or more like what sounds like a 60hz buzz coming from the left channel when I first start the amp up. I have isolated it to the power amp by removing the input to it. Also while this is going on the left meter will hold a steady -10db read where the right side which is quiet is where it should be,-20db..

    After switched off for several hours this issue occurs on startup, but goes away after it has been turned on for a length of time. After about 30 seconds from cold start, if I turn the left power level down and then back up the buzz goes away and doesn't return until the next day on a cold start.

    Any ideas on what this could be? The 3 main boards were rebuilt by echowars a few years ago and I replaced all the outputs with new ones that Glenn had recommended and since then this amp has been dead quiet (Except for the music of course ) . Although I haven't checked the DC offset lately it was quite low on both channels.

    One thing that has not been replaced was that big cap. . The protection board is all new as is the two main boards. .

    Hoping this thing isn't starting to develop a serious issue as living remote, it is too dang heavy to send by mail, and there is no one within a thousand miles I would trust to work on itl
    And besides, I already own a good boat anchor! :eek: Thanks for any advice you can offer.
     

     

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  2. BilboBaggins

    BilboBaggins AK Subscriber Subscriber

    Messages:
    3,360
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    Ottawa, Canada
    It sounds like noise coming off of your left bridge rectifier. Remember that although you have one transformer on it, you have two power supply units, one for each channel,if I remember correctly. So you are looking for where noise is getting into the circuit. I suppose you don’t have a scope or DVM. You can use a dvm as a crude indicator of where you see dc. Compare with the other side and play with the dc settings until you see something. There should be a difference from the two sides and that is what you want to chase backwords.

    Edit: that is where I would start anyway, with my limited and rusty analog skills. I stopped using my scope in the mid 90s unless I needed to see an image. When looking for a potential culprit, look for a signal opposite to what you should have (if expecting dc, set the meter for ac, and vice versa). This is just a starting point.
     
    Last edited: Sep 24, 2018
    Alobar likes this.
  3. KingBubba

    KingBubba "Too Much Stuff" Subscriber

    Messages:
    9,791
    Location:
    Brooksville, Fl.
    I believe you already hit upon a potential source of the problem. Chances may be good that the filter cap coinciding with the left side may be on it's way out. Have a look and see if there is leakage or bulging on the filter caps. The filter caps are typically the last to go bad, which is why echowars did what he did. The 700 is one of echowars favorites and his leaving the filter caps may have been to help save you some money. If you have any skills, you most likely could replace them yourself. If not, search AK for a tech guru near you. Echowars has been off the radar here at AK; so I can't suggest that he would be able to help.
     
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  4. Alobar

    Alobar Pulling out of the Last Chance Texaco.. Subscriber

    Messages:
    5,338
    Location:
    SE Alaska
    Thanks for the help guys! Gives me a place to start.. I have a new inexpensive Yamaha integrated that should be here by the end of the week and when it does I will pull the 700M out and put it on my bench and clean it up and check as much as I can with my very very limited knowledge.

    In the mean time, I want to stress this is something that is quite minor and goes away on its own and is such low level that I can't hear it with music, even at low level.

    The odd thing I'm observing is still the relationship with the L power level meter and the left output pot. When I first turn it on after tsitting off overnight, as I indicated in the OP, the left meter rises up from the peg to about -10db or so (with the meter range range at -20db). I can hear the faint buzz from across the room, Turning the pot from full left to full right doesn't change the sound much at that point, but if I wait a minute or so and turn the pot all the way down slowly and slowly bring it back up (I keep both levels straight up at 5 for music) the buzz stops completely, and the meter falls back to <-20db and it's good to go for as long as the amp is energized.

    Even after I have shut it off for a few hours, the buzzing is gone when powered back. It takes being switched off for several more hours before it returns. Definitely seems to make sense that it has to do with caps possibly bleeding off but thats what caps do it seems. Shouldn't they fairly instantly charge back up once energized? And why does turning the output level pot down and back up fix the issue after a time? Also another observation, the meter sometimes is all the way over to -20db and the buzz is there, so the meter rise doesn't always correlate with the buzz sound.

    When Glenn and I first talked about my 700's I sent my 700C out to him as it was light enough to get in the USPS (the only real shipping option I have here) and he completely went through it. But the 700M couldn't be shipped complete so we came to the conclusion that he could get it most of the way there (both sonically and reliability wise) by rebuilding 4 boards.The driver boards, relay protection board, and the meter board which he considered optional as it was doubtful it would affect the sound. Since that board was going to be a bugger to get out I elected to just do the 3 other boards and skip the meters.. BTW both meters still work fine and I really don't ever look at them.

    While having the amp apart it was noticed that the output transistors were not all the same brand. Since my brother had this amp in his possession for much of the time it wasn't clear but he may have taken it to a shop locally at some point. Anyway Glenn had me change out all of them to ones he recommended.

    Anyway, just wanted to explain the history of the repairs to this amp, and what EchoWars was up against by not having the complete amp to do everything he normally would. He had also mentioned some other parts that he could send me but that trail got cold after a while (and he could see how green I was at electronics repair possibly).

    Normally with a lessor piece of gear I would simply ignore this problem, and I may do that eventually anyway, so long as it isn't getting any worse. I love this amp (I bought it new when I was 20 years old!) and I want to have it working till they take it away at the doorway to "the home".. I guess the main point is I just don't want any other problems to arise by not dealing with this issue.. As for tech's in my area, there simply are none. I live in a very remote region, with no way to ship this thing, and short of putting it in my pickup and driving 1700 miles to Seattle, I am it.. Of course I run the risk of blowing up something just by poking around due to my electronics ignorance but I am also a retired electrician so while my electronics background is very weak, I at least know the dangers..

    Again, thanks for your ideas..
     
  5. KingBubba

    KingBubba "Too Much Stuff" Subscriber

    Messages:
    9,791
    Location:
    Brooksville, Fl.
    Remember to let the amp sit for a while after it has been turned off. This will give the filter caps a period of time to discharge and keep you from blowing up your amp like I did with mine.
     
    Alobar likes this.
  6. theophile

    theophile Pheasant Plucker. Subscriber

    Alobar, I don't know whether the following is an option in your remote part of the world: Here in Australia one can send freight via the buslines(think Greyhound style). It might be worth investigating unless you know that it is not an option.
     
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  7. Alobar

    Alobar Pulling out of the Last Chance Texaco.. Subscriber

    Messages:
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    Location:
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    Closest thing we have to a bus service is the ferry boat which does run all the way to Washington St but doesn't take any freight. We do have access to UPS as the air taxi will fly it to the nearest ups center but for an 80 pound box is too much money.

    All we really have that's practical for sending stuff is the post office (60 pound limit) or the weekly barge which is only practical for larger heavier pallet sized loads and only to Seattle where other arrangements have to be made in advance. At some point the costs of moving it to a competent tech and back again rapidly make the Kenwood not worth it's value just in shipping (even to me ).. Then finding and paying someone to fix it, we could be talking the expense of a new Mac amp..

    I'm going to need to solve this myself sadly. .
     
  8. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

    Messages:
    3,055
    Location:
    Tulsa Ok (C-Ville)
    I would pop the covers off and start poking those Molex plugs with a wood or plastic stick. See if you can make it act funny. There are some ground wires on the back that like to break as well. They go from the back panel to a screw on the chassis back there. Check that too. Tapping, poking, and flexing is in order.
     
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  9. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    538
    Location:
    San Diego, CA
    Start with the basics...with the amp cold and turned off, press all buttons and turn all knobs about 10-20 times. If, when powering it up, the meter for the suspect channel is showing any progress lower, rinse and repeat. I'm basing my response on the following statement: "After about 30 seconds from cold start, if I turn the left power level down and then back up the buzz goes away and doesn't return until the next day on a cold start."
     
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  10. slimecity

    slimecity Super Member

    Messages:
    2,891
    Location:
    New Zealand
    I would second large powersupply caps as being a potential cause of this issue.
     
  11. Alobar

    Alobar Pulling out of the Last Chance Texaco.. Subscriber

    Messages:
    5,338
    Location:
    SE Alaska
    So the other day the second Yamaha AS-301 came and I pulled the 700M out and put it on my bench. I left it sit for several days and yesterday I pulled the cover off as well as some internal covers and had a good look see.I exercised all the IC plugs I could get to on the left channel and I didn't find anything wrong that I could see. Nothing looked like it had been hot, and the big caps looked like new with no swelling, no leakage or corrosion. Basically everything I could see visually without deeper disassembly looked good.

    Then I plugged the unit in and with a small speaker listened to the buzzing sound. It was cool in the room, maybe 55 degrees or so as I normally don't keep the heat on. It took much longer to get the buzz to stop, leaving me to believe that it is temp related, possibly from expansion/contraction of the left channel heat sink.. While it was buzzing I poked around the outputs with a plastic stick but couldn't make any changes to the sound. Eventually after maybe 15 minutes it stopped after slowly rotating the output pot down to near minimum and back again.

    I also checked DC offset with my multimeter and all is well there, the left reading 1.5mv and the right about 3.

    My thought is that there are some solder joints on the small PC boards that sit on the heat sink to which the output transistors attach to from the other side. Those are fairly well known to crack due to cold solder joints from the factory or maybe just being worked from expansion/contraction of that big aluminum sink. Also since I heat with a woodstove, the house temp fluctuates more than most places do (often 10 degrees swing is typical in winter) which may aggravate the contraction . Also when I replaced all the output transistors a few years ago and had those little PC boards out I did find a cracked joint which I resoldered..

    I have decided to put the 700M back in service and keep it powered up 24/7 until I have more time this winter to pull the outputs and recheck the solder joints. They are impossible to see all of them visually without pulling them and I am not sure if I will need new Bergquist gasket pads for the outputs or if I can simply use the old ones.

    Anyway, this AM the 700M has been powered up all night and no buzzing sounds.. I really prefer its sonic sig over the little Yamaha 60wpc for bass but thats not to take away anything from the Yamaha. It sounds quite good, especially for an amp in the $300 range.
     
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  12. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    538
    Location:
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    Output transistor thermal pads >>> probably not a good idea to reuse the Bergquist silpads as they tend to compress over time. Mica pads do not. If the Bergquist silpads were installed dry on the first run, you may be able to buy some time on a second run by using a thin film of thermal grease on them. Ideally though, they should be replaced. And depending upon the texture of the surface, you might find this post, enlightening.
     
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  13. Alobar

    Alobar Pulling out of the Last Chance Texaco.. Subscriber

    Messages:
    5,338
    Location:
    SE Alaska
    That was interesting.. Had no idea the texture of the sink would make such a difference. Here is a picture of the heat sink I took a few years ago when I was in there last. Notice the mismatch in outputs. EW had me change these out and he recommended the Bergquist as probably easier for the novice like me to get right. Yesterday I did give the screws a little twist to make sure they weren't loose. Didn't reef but just a gentle twist.. They seemed nice and snug.. The picture shows the heat sink is fairly smooth, almost reflective but not quite..



    Capture.JPG
     
  14. rjsalvi

    rjsalvi Well-Known Member

    Messages:
    538
    Location:
    San Diego, CA
    If you want to eliminate the output transistors on the L CH as the cause for your buzz, get a can of freeze spray and spray'em after they've warmed up and the buzz is gone. If the buzz comes back after spraying, well, you know the drill. On cold start with the buzzing present, moving the L CH gain control back and forth has no effect?
     
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  15. Alobar

    Alobar Pulling out of the Last Chance Texaco.. Subscriber

    Messages:
    5,338
    Location:
    SE Alaska
    Thanks for the freeze tip.. Next time I put it on the bench I will try that.

    As for the output level and effect on the buzz, when first powered up after sitting for several hours the LH meter will read steady around -10db (with the range at -20db). This is at all gain levels except for 1.5 and 8.5 (or 9 o'clock and 3 o'clock) where both the meter and the buzz dips slightly. After a minute or so if I slowly turn the gain down to 1.5 area it will suddenly stop buzzing, and the meter will also drop to the peg.

    From there if I turn it back up to its normal level (5) too fast the buzz will return at that point. Then if I again slowly lower it to 1.5 and it will go away. Now if I slowly bring the gain up to 5 it will hold with no buzz, and at that point I can turn the gain up and down as fast as I want and the buzz will not come back again until the amp is shut down, and not after a few minutes, but several hours..

    The amp has now been energized for going on 2 days and no buzz has returned. The sound is its usual goodness. I have never been one to leave amplifiers on all the time so this has taken me a little getting used to. Think I am going to get an order to digikey for new Bergquist sil pads to have on hand for the next time I pull this I want to inspect those output solder joints.
     
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