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Amp Repair

Amp Tubes





      It can be a real find to unearth a warehouse full of vintage jukeboxes.  Imagine the thrill of finding 10 or 20 jukes untouched for maybe 30 years!  Of course, that's the problem:  they've been untouched for 30 years.  During the 40s and 50s, the operators that placed jukes on location were a thrifty lot.  They never replaced a jukebox that still worked.  When a jukebox became too troublesome to keep on the route, it was pulled back into the shop to become a parts donor.  If the amp was good, it was swapped out with one that wasn't.  The same held true for credit units, selection receivers, power supplies, and in some cases entire mechanisms.  After all, the parts box had no value other than it's individual pieces.  After a few years of serving as a parts box, it was stored in a back room and forgotten.  This procedure was repeated over and over until the storage room was full.


      Enter the person looking to buy "an old jukebox to fix up".   The kind hearted operator then shows them the back room full of jukes and offers to sell one or all for a reasonable price.  What he doesn't tell you is that it's unlikely you'll have any luck getting any of them to work!  So you pick out the one with the best chrome, glass and cabinet and take it home.


      The first thing any unsuspecting novice will do is plug it in. This is mistake #2.  You get to watch smoke drift lazily over the top or out the back as you recoil in horror.  After weeks and weeks of trying to get the juke to work you realize that almost every component in it is shot.  You stand back, scratch your head, and wonder how so much could be wrong with just one jukebox.  Of course, this doesn't happen with every single juke, but it happens often enough.


      The other scenario is:  you take your new prize home, plug it in and it actually springs to life and plays a record.  Hey!  A juke that works.  You ponder a trans-Atlantic call to the Vatican to report this miracle.  Instead you congratulate yourself on selecting the one good jukebox in the lot, and proceed to load it with records.


      Somewhere around the tenth record, you notice that it isn't as loud as it was when you played the first record.  When the 20th or so record comes up to be played, you can barely hear it.  This is when you should start looking for smoke.  Time is not kind to electronic components.  They are designed to have a certain life span.  In the cas of electrolytic capacitors, it is usually around 10 years.  Of course this can vary, but you can count on most of them failing somewhere in this time period.  Instead of merely opening up, which gives you a loud hum, they may choose instead to short.  This is what causes major damage to amps and selection receivers.


      Coupling capacitors can be trickier.  They usually last longer and will continue to work long after filters fail.  What they do best is leak under load, or become temperature sensitive.  This is why an old amp will sound OK when cold, but the sound will deteriorate the longer it's on.


      Of course, there are many other things than can cause poor or no sound.  A short list would include resistors that have drifted out of value, bad controls, poor solder joints, weak or gassy tubes, the the biggie:  bad transformers.


      Transformers are necessary.  They must be in perfect condition or the performance of the amp is affected greatly.  A bad power transformer can blow fuses, smoke, fail to produce the correct voltages necessary for the amp to operate efficiently, or shock you if you touch the bare chassis.


      Output transformers also can cause many problems, but generally they will cause low volume, lack of bass or extreme distortion.  Certain other problems can pop up, but in most cases they are not temperature sensitive.


      One of the worst things a person can do is plug in a jukebox that has been sitting for an extended period of time.  If you are lucky, all that will happen is a blown fuse.  In the worst case, you will fry the entire sound system and possibly the selection system.  If you MUST plug the juke in, at least unplug the rectifier tube, usually a 5U4 or 5Z3.  This way you will prevent expensive damage to the amp while you work on the mech.


      Get the mech and selection system working properly and then turn your attention to the amp.  Remove it and look for broken or missing parts.  Next, look at the bottom of the filter capacitors, sometimes referred to as can capacitors.  If you see any sign of leakage or swelling, replace them first before anything else.  This will save you a lot of money and time.  If you have any doubts, be safe and change any part that looks suspect.


      If you plan to repair the amp yourself, be sure to change every single capacitor in the amp.  Do not make the "amateur" mistake of  thinking you can change a few of them and it will be OK.  Anything less than a full cap change is a waste of time.  Then, check every resistor for value.  Look for any that might have gotten hot over the years and replace any that seem questionable.


      Check your audio output transformer.  A digital meter will not work, use only an analog meter.  Check for shorts to ground.  Connect the negative lead to the chassis, and measure resistance to the primary.  It will drop low and slowly raise as the filter caps charge. Any reading less than around 10,000 ohms is cause for concern.


      Next, find the primary center tap lead.  It will usually be a red wire that runs to the power supply.  Connect the negative meter lead to this wire and measure to each plate lead.  The reading should be the same from side to side.  Any open winding will easily be detected by this procedure.  Some transformers also have a screen grid lead, and they can be checked in the same way.  If the transformer checks bad, there is no reason to go any further since nothing you can do to the amp will fix it short of changing the transformer.  Capacitors will not help this condition.  Also, you stand a chance of damaging the output tubes if they have to drive a bad transformer.


      One other condition can occur:  rarely, windings will short against each other for a couple of turns.  This cannot usually be detected by a simple resistance measurement.  The result is a slight loss of voume, with a noticeable loss of frequency response.  Sometimes there will be a loss of bass, sometimes the high end will be distorted. A condition such as this is best found with a scope, or by substituting a known good transformer.  "Old timers" can listen to an amp and tell if the output is bad, but this comes only from experience!