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Buying a broken amp as learning bench

Discussion in 'Amps and Cabs [BG]' started by eukatheude, Jun 27, 2014.


  1. I have had, for some time now, an ad asking for gooched basses, pedals, amps and whatnot for recycling and repairing purposes.
    I just got contacted about an amp for the first time: this guy is offering me a Fender Eightyfive combo for 35 euros - I don't know yet whether it's dead or has any other kind of issue.

    I'd like to know from the infinite TB wisdom pool... Is it worth it to get it and experiment with it? Do I ***need*** testing equipment outside of a multimeter, if not to repair it at least to learn some basic troubleshooting?
    If it's not repairable, will scavenging the driver and electronic parts be worth the price?
    Or am I just better off not wasting time and money on this?
     
  2. What's a Fender 85 combo when it's at home?
     
  3. Not sure what you mean.
     
  4. Never heard of that model and not inclined to google.
     
  5. timonvh

    timonvh

    Oct 16, 2010
    Googling takes less time than writing two posts (I know, I just did).
     
  6. fender-eighty-five-71720.
    fender-eighty-five-125799.

    At least that's what google gives me. Solid state, unfortunately, and of course for guitar.
     
    Last edited: Jun 27, 2014
  7. That looks like an old enough thing without surface mount components. Perfect for your purposes. No good for bass.
     
  8. Hi.

    Unfortunately?

    Solid state is much more forgiving, and more repair-noob friendly than any tube amp.

    If the amp is in semi-decent condition, the cabinet, speaker and the amp chassis are well worth the €35.
    Heck, those potentiometer knobs are probably 2€/piece.

    If for nothing else, I'd buy that to house a DIY amp.

    You "can" diagnose and repair any amp with just a multimeter (and a lot of experience), but you can also use a spoon to dig a ditch.
    Neither is going to be feasible, or guarantee a satisfactory result.

    Fortunately, old oscilloscopes for light AF duty are dirt cheap compared with what they used to cost.
    As You probably noticed on a recent thread which got derailed a bit onto the world of oscilloscopes, almost any will do.
    An old PC functions well as a signal generator, and that's about all Youll need to start.

    What's the most demanding on the material side these days is the lack of schematics though.
    There's only a limited amount of circuit topologies used in AF amps, but still diagnosing and repairing anything but the simplest amp is a major PITA if a schematic can't be found.
    Some nmanufaturers also have the habit of grinding the markings off the components, in that case even the simplest of OP amps can become unobtanium.

    By far the deepest wolf-pit on any DIYers first steps in anything really nowadays is the abundance of mis-information.
    Everyone is an expert on the 'net, and weeding out the mis-information, and picking only the accurate information requires quite a bit of knowledge.
    Knowledge that the newbie obviously does not posess.

    For that reason, I'd suggest that if it's in any way possible, try to take a community college course or eqvivalent in el appliance repair safety procedures and basic electronics.
    If that's not possible, Your local library most likely has some old vocotional school textbooks on the subject.
    I'd choose those over anything on the 'net.

    Also keep in mind that in most EU countries AFAIK, once You tamper (mod/fix/whatever) with a wall voltage device without the proper license, you will be personally held accountable for any injury or damage the said appliance causes.

    That said, be safe.

    Regards
    Sam
     
  9. Thanks for your post. Did not see that thread, can you give me the title?
    Purchasing lab equipment would be an issue now, as I don't even have the space to house it.
    I only have basic electronics knowledge but I'm slowly starting to study a bit deeper. The problem is that I can't focus on a single things and attempt 10 projects at once. That's why you saw my thread about building a cab myself, then diying a 100+W tube head with zero experience, and so on. And I fear this might end up on the "to do" list and just sit there taking up space.
    Scavenging it for parts however is something I can do. Do you think it's worth it?
     
  10. charlie monroe

    charlie monroe Gold Supporting Member

    Feb 14, 2011
    Buffalo, NY
    T-Bird already said the parts are worth it, but...only if you do something with them.

    Sounds like you won't. Back away...SQUIRREL!
     
  11. Well I could use them in the next pedal build. I don't suppose I can sell them easily.
     
  12. will33

    will33

    May 22, 2006
    austin,tx
    What's 35 over there, like $50? That's pretty cheap for a complete amp, including speaker, to learn on.

    It's the setting aside enough time and applying ourselves to do the learning that is the stumbling block for most of us, including me.

    I'd start by making sure you can find a schematic for the amp at least before you buy it. Service manual/instructions even better, but at least the schematic and parts list. If you can't find that, I'd wait for an amp to come along you can find those documents for, or it'll be a whole lot of shooting in the dark, failure and frustration.
     
  13. Hi.

    Here's AFAIK the most recent thread that wandered onto the o'scope discussion, but there has been many others in the past:

    http://www.talkbass.com/threads/beringer-powered-pa-goes-into-over-load.1079995/

    The skinny of it IMO: Any will do for basic troubleshooting.

    Only 10 projects at once?
    Consider Yourself lucky ;).

    You're probably a bit younger than I am, so You'll get there, just give it some time.
    I could advice You to focus on one project at a time, but I try to preach what I practise :).

    Scavenging for parts will work these days IME only if YOU will use them, people are constantly hunting for that "sweetest deal" and will not pay enough to justify the time spent on trying to flip easy to find parts.
    Do not ask me how I know that :(.

    Regards
    Sam
     
    eukatheude likes this.
  14. The 85, Studio 85, and Deluxe 85 evidently are very close in design. There are schematics for the Studio 85 floating round and it looks pretty straightforward.

    So we troubleshoot by starting with the basics. Is the amp powering up at all (any lights come on? any noise that indicates power is on?) If not, look at the basic: is there a blown fuse? Test the fuse for continuity with a meter, don't just eyeball it. If the fuse is blown, build a Dim Bulb Tester (Google it) before replacing the fuse, then power the amp up with the DBT to look for excessive current draw. If fuse is OK, though, then follow the incoming AC power through the power switch, fuseholder, and on to the primary windings of the transformer. If power is getting to the primaries, then we look for voltage on the secondaries. If we can't get power to the primaries though we find out where the power stops: bad switch? bad fuseholder? etc.

    If the fuse was blown and the amp is brightly lighting up the light bulb on the DBT, then either there's a major short in the incoming wiring or transformer, or there's blown output devices.
     
  15. So, the guy says it worked before he took it apart, and just needs the input jacks changed...
    If it's truly that, I can easily fix it up and sell it. I'm feel like I ain't getting the whole story though...
     
  16. Riiiiiiight, the old "just needs the input jacks changed" story...

    If it worked, why did he take it apart? Just to break it? How did it go from "working" to "needing the input jacks changed" ? Input jacks don't normally need changing either, unless he's talking about the EFX jacks (pre amp out/power amp in jacks) which sometimes need cleaning with Deoxit.

    Offer him less than half what he's asking. Wave a little cash at him. If he doesn't accept, shrug your shoulders and walk away.
     
    eukatheude likes this.

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