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Learning how to repair an amp (SWR combo blowing fuses)

Discussion in 'Amps and Cabs [BG]' started by GeneralElectric, Oct 26, 2016.

  1. GeneralElectric


    Dec 26, 2007
    NY, NY
    Hey everyone,

    I have an SWR Workingman's combo that needs some repairs. I figure that this old SS amp is a good thing to learn on.

    I got this amp for free many years ago and it had a crackly input that would cut out. I left it in a friend's rehearsal studio where I promptly forgot about it. It stayed there for several years before he brought it back to me. Someone had since stolen the fuse cap!

    So the first part to get this amp working was getting a new fuse cap. I got an OEM replacement off of eBay as well as a set of fuses off of Amazon. I got the correctly spec'd 3A Slo-Blow fuses on Amazon.

    Well I blew 3 out of 5 fuses before I realized that something is wrong.

    The amp powers on, and then quickly fizzles out. I haven't gotten it to produce sound because the fuse blows out before the amp fully turns on. The red power indicator light only glows for 2-4 seconds before fizzling out.

    Did they send me the wrong kind of fuse? Or does this sound like a blown output transistor?
  2. MrLenny1


    Jan 17, 2009
    Time to troubleshoot. If the fuse blows right away there may be a
    short in the power amp. It would have to be disconnected, then see if it is the culprit.
    If you have no training beware of workinh with 115 VAC.
    GeneralElectric likes this.
  3. charlie monroe

    charlie monroe Gold Supporting Member

    Feb 14, 2011
    Buffalo, NY
    This is the kind of issue that I fix with my telephone and credit card.
  4. GeneralElectric


    Dec 26, 2007
    NY, NY
    This amp, monetarily, is probably not worth paying someone $100 to fix. If I totally botch the job and end up tossing the amp, no biggie. Or I end up learning a thing or two. hopefully we end up with the latter scenario.
  5. GeneralElectric


    Dec 26, 2007
    NY, NY
    Life isn't nearly as exciting if you've never managed to electrocute yourself.

    How would I troubleshoot the power amp? What would I look for ?
  6. Please stop replacing fuses, one would have been enough. It’s like pumping up your tyre and not fixing the hole in it. You are perhaps causing more damage.

    Learning repair procedure is not done in a day, a week or even a year. It’s also something that you have to have an aptitude for.

    I advise you to take the amp to a tech for repair.
  7. GeneralElectric


    Dec 26, 2007
    NY, NY
    I'd like to think I've got somewhat of an aptitude for electronics work. I've made and repaired many a guitar and pedal and am pretty handy with a soldering iron. I've also done low-grade electrical work (outlets, switches, fixtures, etc)

    If you can point me towards some resources, I'd be very grateful. I have nothing but time on my hands at work to read things.

    Currently going through this, which, while for car amps, should prove somewhat useful. Basic Amplifier Repair
  8. Al Kraft

    Al Kraft Supporting Member

    May 2, 2016
    Northern Virginia
    First thing I would do, right after unplugging the amp and leaving the fuse out, is go online and see if you can get an SWR schematic for the amp. As odd as it may seem there are still a lot of those floating around out there on all kinds of sites.

    Once you have that in hand you could pull the top cover off the amp and look around to see if there are any obvious burn marks (components. boards, wiring), oddly shaped capacitors, burned smells from transformers, etc. This will give you a first order idea of some of what might be wrong.

    At that point you will need some electronics smarts and test instruments (pretty much in that order). The key to fixing electronic components as you have discovered is understanding that there are primary, secondary, tertiary, etc. failure modes. The trick is to identify the primary failed component(s)/failure mode and then trace the path from there to see what else may have been affected. This is the point in time where I like to engage an expert and get an estimate for what it will take to fix the problem.

    The more brave and better educated among us will simply make the repair themselves and check all the related components. That's the most helpful I can be. Good luck!
    M.R. Ogle, RiZzBot and DevinWard369 like this.
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Good, safe advice IMO.

    First of all, don't take my comments the wrong way because I do believe that you don't recognize the difficulty in the questions you asked.

    Do you understand how a transistor works? Can you read and understand a schematic? Can you understand how a global feedback amplifier works and how to safely troubleshoot?

    Being "handy" with a soldering iron and replacing outlets and switches are skills pretty much unrelated to troubleshooting and repairing amps.

    It's kind of like a butcher, because he has the skills of cutting meat and bone, thinking he is qualified to do surgery because that's no different than cutting meat and bone. It's not the cutting that's important, it's diagnosing exactly what needs to be cut and where... then there's the little issue of putting it all back together again after the cutting.
  10. GeneralElectric


    Dec 26, 2007
    NY, NY
    No offense taken.

    I know what a transistor is and does. (In this case it uses a small input current to control or switch a larger output current, or to amplify the input current) and I can read a schematic for a PCB as I've made, repaired, and modified pedals before. I will be honest in saying that I don't know what a global feedback amplifier is, but I wouldn't mind learning.

    But I've never repaired an amplifier before and I would like to learn. I don't mind the time and expense doing so, but I do mind paying someone else to do it. Again, if I had to pay someone else to do it I'd probably just toss it since its probably worth maybe $150 in top working condition. I'd rather use this inexpensive amplifier as something to learn on than anything else. If I fix it, awesome. If I somehow cause this thing to explode in a ball of flames, oh well. :)

    P.S. One of my first jobs was as butcher. :p
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Then learning about how amplifier circuits work, the various topologies and especially what global feedback is and how it affects troubleshooting of any linear amp that uses global feedback (just about all).

    Folks who do this successfully generally either learned electronics technology in school, in the military or spent a great deal of time self-studying.

    Butcher is a difficult job that requires dexterity, strength and the ability to cope with cold repetitive motion work. Not for me!
  12. GeneralElectric


    Dec 26, 2007
    NY, NY
    I screwed up and studied journalism in college. Studied butchering in high school when I started working in restaurants and we'd get whole sides of beef. Eventually ended up at Costco until I was let go for giving away free meat to my friends. Who'd have thunk it?

    @agedhorse if you have, would you please send me links to things I can study and read up upon? My current job is almost all downtime. Currently reading the news for 40 hours a week. Wouldn't mind reading something less depressing than politics.
  13. I will add... do not overlook the power supply as being a possible cause of blown fuses. While you are learning, learn some power supply theory. If this is the main fuse that blows, it is a part of the power supply circuit. Another tip, do not try putting in larger fuses or different types of fuses to try to keep it from blowing. You could end up causing more problems.

    Also, if you don't have a multi-meter I'd suggest investing in one. There are a lot of different ones out there, but they all pretty much do the same job. Measure Voltage, Resistance and Current. Learning to use one properly is a must. There is a lot to know about what to look for. That's a whole chapter in itself in the book on basic electronics. Good ones are reasonably priced these days. Just avoid the little cheap $5 jobs. One mid priced brand that has a nice selection is Craftsman. There are others as well, with Fluke being one of the top of the line in meters.

    OK... serious discussions aside, here is lesson one. Electronics works on smoke. If you let the smoke out then it won't work. If you can't repack the smoke you will need to replace the part with one that has the appropriate smoke installed.:laugh:

    Best of luck.
    Clutchcargo, Ewo and GeneralElectric like this.
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Search on line for basic electronics courses, transistor theory, basic audio amplification, "how it works" stuff. For audio, Rod Elliot has some nice articles but most assume that you understand basic electronic theory.
    GeneralElectric likes this.
  15. Ewo

    Ewo a/k/a Steve Cooper Supporting Member

    Apr 2, 2008
    Huntington WV
    Yes, yes...

    Because it's all about the smoke, the smoke--no sizzle.

  16. I think you should go for it! I don't know how old that amp is - the newer, the harder it will be to troubleshoot. But you should at least be able to figure out if it's the power supply and go from there.
  17. Also, while you try to learn how it works,
    see if you can find out what kind of life it led while
    your friend had it.
    Did it get used occasionally, crackle and all?
    Did it smoke and stop working abruptly?
    Did it just sit, all lonely and abandonded?

    If it was the last one, I'd vote for dried up filter caps that shorted.
    They die faster sitting around than they do being used.
    In any event, if they are 20 years old, they are suspect.

    Even if this WAG is correct, try to learn something from this.
    You are correct in thinking that the learning process is worth more than the amp.
  18. it sounds like you are intent on a DIY fix. That said, here is how I would proceed as a start:
    If the amp blows fuses repeatedly, it is drawing too much current. As others have mentioned, try to obtain a schematic so you can study the circuit layout and signal flow. It would be necessary to have a DVOM (meter) or better yet an oscilloscope standing by. Some rubber, foam or other insulating sheets to keep sub assemblies from shorting out against others as you disassemble and trouble shoot. Good lighting and a magnifier help. I would start by disconnecting the power supply (PSU) from its load. The load being primarily the preamp and power amp. Replace the fuse with proper value, disconnect the PSU from its load. Usually connected by heavier cables. Test to see if the PSU alone blows fuses. Yes? then probable short in the PSU. Could be power transformer, bridge rectifier, filter caps or voltage regulation. PSU alone does not blow fuse? connect the preamp section test and see. Do same with power amp section if necessary. Always refer to schematic. Be sure you can identify schematic symbols to real world components. Work slow. Be safe use your head. Stop if you are in over your head or sense danger.
    Last edited: Oct 27, 2016
    GeneralElectric and Engle like this.
  19. 40Hz

    40Hz Supporting Member

    Sounds like one or more blown output transistors. Possibly somebody shorted the speaker output or hot patched a cab with head powered on? That might explain the missing fuse holder cap when they tried a quick fix like you just did.
  20. Flabass


    Aug 11, 2008
    St. Petersburg
    Take it to a tech. Ask him what he did to fix it so if it happens again you can then do it yourself. My time to do all that research, studying and trial and error testing is worth way more than $100.

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