IT SOUNDED OK UNTIL IT STARTED SMOKING!
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
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
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
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!