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SWR SM900 acting up, need some diagnostic advice.

Discussion in 'Amps and Cabs [BG]' started by Thor, Jun 11, 2014.

  1. Thor

    Thor Gold Supporting Member In Memoriam

    My SWR SM-900, circa 1996 is acting up and I could use some advice in diagnosing it. (I have both 1992 and 1997 schematics available) US made unit.

    First, the amp started making ‘egg frying” crackle noises through the cab while I was playing at medium volume, same volume as always. I swapped out basses, then input cords. No change. I unplugged the bass and cords and loud crackling continued, so it wasn’t the bass jack or the cord.
    When it crackles, both the preamp clip light and power amp clip light flicker on.

    I opened it up the next day and checked the passive input jack socket. Nothing unusual there. The amp has a 12AX7 tube in the gain circuit and I was thinking it might be going microphonic. I put in a new GT 12AX7 soviet made tube from GC. This didn’t cure it, but the symptoms changed slightly.

    Now, when turned on hooked to a 8 ohm cab, it made no crackling noise when no bass is plugged in. I plugged in the bass, and played. After about 1 minute the crackling returned, but only when certain notes were played. The E on the 7th fret of the A string and the A note on the 7th fret of the D string, approx. 80hz and 110 hz respectively drove the amp into crackling and both clip lights started flickering when those notes were played. Also, the sound seemed to oscillate a bit. I surmised the tube may have been part but not all of the problem. I had the limiter off and the amp top cover open. The fan did not fire up as far as I could see.

    I checked bridge mode and right and left stereo output modes, both channels. Same crackling thing happened either of the three ways

    I noted that cutting the amps parametric eq at around 100 hz reduced, but did not eliminate the crackling. I also found that if I turned the limiter up half way, the sound oscillated oddly as well.

    I typically use the amp in a rack bag, and the fan operates fairly consistently on and off, especially when pushed a bit in bridge mode. I use an 8 ohm SWR goliath 3 jr. 4x10. The flaps are open of course, it’s a 4u rack and the other 2u have short bodies, power conditioner and tuner, so there is air available in the back of the unit, it’s not ‘sandwiched” in.
    Is it possible the fan or the temp switch controlling the fan failed? In the schematic I have, it does not show the voltage the plastic fan runs at, just a block diagram notated ‘fan’. That was on the later schematic. The 1992 schematic drawn by Steve Rabe doesn’t even show the fan.

    I’d like to put current though the fan and see if it operates. Any one know what those typically run at? 3 volts? 5? 9?

    Is there a way to check the thermo switch without frying the output section? And finally, is there a quick way to check the tube using a voltmeter across any of the pins? I guess the new tube could be bad, but I kind of doubt that. Still, I’d like to eliminate that as a problem.

    I am starting to suspect fan failure and overheating damage resulting somewhere. I can’t see any fried caps or transistors, but that may not mean anything.

    Any thoughts on this?

    (Also fried an Eden WT-400 last week, Separate thread for that… :( ) Probably off to the tech shop unless I can figure something simpler.
  2. Hi Thor.

    Did You try the good old patch cord in the FX loop "trick".

    My ST220 shows similar symptoms when the FX jack switch is oxidised.
    Which is almost always when I dig it up, that fine amp recieves way too little playtime since I bought that Fame of mine :(.

  3. Thor

    Thor Gold Supporting Member In Memoriam

    I did not. That's a new one one me. So patch a cord in both send and return, work it in and out a few times to de-crudify it? Or spray some De-Oxit on the plug and work it in? Definitely worth doing asap before consigning to the black hole of the amp tech's shop.
  4. Just put the patch cord from the send to the return. If the noise stops, then you would need to clean the jacks with deoxit.
  5. Hi Thor.

    You don't need to work it in and out, just plug it in.

    The patch cord bypasses the normal signal chain and the switch that's on that signal chain.

    IIRC the switch in my ST220 at least can't be cleaned properly without taking the lid off.
    The one in Yours may be different though.

    If You decide to clean the switch, DO NOT use abrasives, just some soft cloth/paper soaked in the liquid of your choice.
    Most people who run into this problem who don't use the FX loop, have a patch cord there permanently :).

    I suspect that someone in the past has cleaned the switch on mine with emery paper/cloth and that's the reason it always oxidises when not in use.
    The plating's been damaged.

    Bass_Pounder likes this.
  6. Thor

    Thor Gold Supporting Member In Memoriam

    Ok, will give that a test tonight and report back.
  7. Nighttrain1127

    Nighttrain1127 Supporting Member

    Nov 27, 2004
    Near Worcester MA
    Also Just because a tube is new does not mean it is working properly. Find someone with a tube tester and Check them both out. Also a quick microphonics test is put the tube in and let the amp warm up for a minute or two. Then lightly tap on the tube with a pencil with the eraser end(soft rubber). If it rings then the tube is Microphonic. If not then at least that part is working right.
  8. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The switch inside the return jack can not be reached with any cloth or such. The only way to clean it is to spray a smalll amount of de-oxit into the jack's switch.

    I suspect that this is not the problem however. Make sure all of the connectors are fully seated. If that doesn't take care of it, I recommend that you find a good service tech tyo look at it rather than muck around and create a bigger, possibly unrepairable problem.
  9. If I remember correctly, the fan is 18V. I replaced mine recently. If I remember, I'll look at the old one I pulled out and confirm. You should be able to google the model number printed on the fan and come up with quite a few replacement options.

    I have January 1996 and November 1997 SM-900's.
  10. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    18V fan does not sound correct.
  11. OK, found it. The fan is a TA300DC made by Nidec. Also says model #M33407-16. 24V D.C., 0.18amp
  12. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Consult with SWR (Fender Consumer Relations: 480-596-7195 or swrcustserve@fender.com) regarding the correct fan for your amp.
  13. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The reason for my suggestion is that it looks like there were several fans used over a period of time.

    In general, when replacing fans it's important to select one with not only the same voltage and current, but also something that is not shown on the data sheets that will vary from manufacturer to manufacturer and from model to model is the starting curve. This is important on any amp that operates the fan at reduced voltage, either a switched low speed or continuously variable operation. Under some conditions, an inappropriate fan that otherwise looks like it's ok will not start because it operates on a different commutating curve.
    Last edited: Jun 12, 2014
  14. If you get a definitive and actionable answer from Fender on appropriate fans or fan specs, please post it here.
  15. Agedhorse, don't get me wrong. I appreciate your coming at this from a technical viewpoint. The commutating curve info is new and useful. I replaced one of my fans with a similar, but not identical one. It's working fine, but perhaps just a lucky choice. Now the question is if Fender will provide complete enough specs on the fan to make an educated choice.
  16. I'd be surprised if the fan died. You can simply short across the thermal switch to test it.
    Most likely one of the front panel pots has developed a loose connection on the preamp board, that's what happened to mine and it behaved in the same way.
  17. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Without either the complete can specs or the OEM model number, guessing on a fan will always be a crapshoot in a variable speed or reduced voltage/current start fan circuit. I have seen the bad results of random fan substitution enough to bring this up.
  18. Thor

    Thor Gold Supporting Member In Memoriam

    Thank all of you for your sage advice. After a weekend of gigging (using an Acoustic 320 and my first amp, Acoustic 140 as a backup) I am ready to come back to this. Also @djkool thanks for that link, there was a ton of good info there.

    The OEM printed on the back is 126 1056. I will shoot that off to customer service and see what they say, just to document that. The patch cord thing is next, the connector fully seated check and rechecking the front panel pots.

    Just for the record, the unit is always used in a 4u rack padded bag. I am careful about transporting it and any physical pot damage.

    @agedhorse Good tips all and I did spend the time to find the top tech in the area. This is what I was looking for, what basic things to check first, and if those all check out, seek professional assistance as the problem is beyond my current (expanding) knowledge scope.

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