[Jukebox-list] ami continental sound problem

[Jay Hennigan]

Daniel Gray wrote:
> Hello again!
> 
> So I have sorted the crossover problem. I have another problem I need to 
> sort out. When the jukebox is at a high volume during lead out section 
> of the record, the jukebox hums really loud. Sometimes it sounds like 
> its going to take off! The woofer shakes really violently, I turn it 
> down because it can't be good for it. It doesn't happen on the lead in 
> section of the record. Do you know what could be causing this?

Hello, Daniel and welcome to the list!  I had somewhat of the same thing 
with my Wurlitzer 1900.  The underlying cause is probably mechanical 
rumble coupled to the stylus.  It could also be magnetic hum pickup from 
the motor to the cartridge, or even acoustic feedback from the woofer. 
Try carefully lifting the arm just off of the surface of the record 
right at the end of the record and hold it there for a few seconds to 
see if the hum appears.  If so it is electrical or magnetic from the 
motor.  If not, it's mechanical.

There's a reason for waiting a few seconds, and it is why you don't hear 
the hum at the beginning of the record or while it's playing.  The 
amplifier has AVC, automatic volume control.  To compensate for records 
of different loudness and have the jukebox volume relatively constant, a 
circuit in the amplifier boosts the volume for quiet records and lowers 
it for loud records.  There is a time constant of about one to four 
seconds for this to adjust so it doesn't obviously "breathe".

The muting circuitry forces the AVC to be at its lowest volume during 
record transfer and immediately after it unmutes.  This plus the time 
constant brings up the volume up slowly at the beginning of the record, 
hiding any crackle in the lead-in groove.

At the end of the record, however, the lead-out groove will have a few 
seconds of silence.  The AVC treats this silence as very low volume 
music and turns up the amplifier to compensate.  Thus, any problems with 
rumble or noise that exist are amplified.

You'll want to find and fix the source of the noise, of course, but it 
may also be worthwhile to make the AVC a little less aggressive.   The 
aggressiveness is adjustable on Seeburg amplifiers, but I don't think it 
is on the AMI.  This might require some component changes in the 
amplifier.  Depending on your expertise you might want to look into 
this.  Fix the cause of the underlying noise, as well.

--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
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