Solid state amp blowing fuses

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ToneGrail, Jan 9, 2006.


  1. ToneGrail

    ToneGrail Member

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    To you tech gurus:

    I have this ancient Crate amp that I bought in the 70s that keeps blowing fuses repeatedly. I know that there was a loud hum that developed recently, probably due to a filter cap. Could this also be causing the fuses to blow?

    Thanks in Advance.
     
  2. bob-i

    bob-i Member

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    Yes. It could also be almost anything even a shorted output transistor or a bad ground connection.
     
  3. John Phillips

    John Phillips Member

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    If the fuse blows more or less immediately, you have about a 90% chance it's one or more shorted main output power transistors.

    Occasionally you get bad rectifier diodes, or a failed driver transistor - which usually blows the main power transistor(s) immediately afterwards, so if you have failed mains, check the drivers too.

    The bad news is that if it's that old, getting exact replacement transistors may be difficult, so you're then into the fun of working out the circuit parameters and seeing what modern types will do the job.

    Filter caps are a possibility, but less likely - at the voltages in solid state amps, they tend to fail open-circuit and hum but not blow fuses.

    Also check the speaker since although it may not have caused the trouble, the DC output from the shorted transistors could well have fried the voice coil.

    Good luck :).
     
  4. donnyjaguar

    donnyjaguar Member

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    John's on the right track. Its almost certainly a blown output transistor. If this is a lower-powered ampliifer it may use a hybrid style of output section on a single slab of silicon. Getting replacements for these is sometimes not possible any more. I searched for 2 years to repair one and gave up in the end electing to modify the circuit. Should've done that to begin with. I digress...If it uses discrete transistors, these can be replaced easily enough with units of equal or better power dissipation. Most every transistor output section is an emmitter-follower so its not being called upon for much gain. In my experience, most output transitors are matched pairs so even if only one is whipped its wise to replace both (or all). There are often power resistors that are used to limit current and its wise to replace these also even if they do measure okay. Sorry for the less than pefectly concise answers, but solid state ampliifiers are more complex and varied than tube ones. Many hacks don't repair transistor ampliifers properly thereby assuring themself of repeat business. This isn't usually the plan, its just lack of competence. Some power ampliifiers have adjustments for idle current and if these are set too high its not immediately obvious but it will overheat and burn out.

    Blah, blah, blah...
     
  5. toddyjoe

    toddyjoe Member

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    You guys are not kidding when you mention that replacement output and driver transistors can be hard to find for old solid-state amps. I have a 1970's Triumph Silicon 100 that I have been fixing up. I had to drop some serious coinage to get the original type of RCA driver transistors.
     
  6. John Phillips

    John Phillips Member

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    A lot of old solid-state amps just get tossed out because of the difficulty of finding transistors - none of them are worth enough to make it really sensible to spend a lot to get them fixed, and if you're paying by the hour for someone to work out what modern parts will do the job instead it can mount up quickly. They're also inherently more difficult to troubleshoot or modify than tube amps because the stages are often DC-coupled, which not only leads to one bad stage frying another, it makes it harder to measure things with the parts in place.

    It's a pity as some of them are actually quite decent amps - pretty good-sounding and reliable, provided you don't overdrive the power stage (at which point they mostly sound horrible and usually die quickly, they were not designed to be or capable of taking the sort of abuse tube amps can), mostly with good transformers, big filter caps and in the case of combos, often ply cabinets and good speakers.

    If you've got one you like the tone of, it can sometimes be simpler, quicker and more economical to strip out the power stage entirely (especially if it's on a separate PCB, many are) and substitute one of the kit power modules that electronics hobbyist companies sell - eg Maplins in the UK, not sure about the US. These are available in any power rating from a few watts up to about 150W, usually IC or Mosfet based and built on small self-contained PCBs, and will take a wide range of supply voltages - some of the lower-powered ones will run from single-rail supplies too, which are more common in older amps. I've saved quite a few old SS amps like this.
     
  7. ToneGrail

    ToneGrail Member

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    Thanks for all the info guys! I think I may just retire the amp to the graveyard. I know that I definitely got my money's worth out of it after 25 years of use. I guess it's just the sentimental attachment since it was my first ever amp. :(
     
  8. donnyjaguar

    donnyjaguar Member

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    I wouldn't give up that easily myself. :) Oftentimes you can repair a high-powered transistor amp for much less than a tube amp. One of the awkward things to repair is an amplifier I had once which used germanium output transistors with transformer coupling. These transistors have been out of production for so long now that you can't get replacements. The transformer coupling ensured that using silicon units would create a lot of crossover distortion. I don't think Crate has been around long enough to have used germaniums though.

    DJ
     

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