Here’s one for the old timers, I’m working on a model 771 amplifier from a Wurlitzer 616 and the complaint is,
“ There is significant hum primarily at 60Hz as seen in the attached spectrograms. The hum is 30dB SPL over the noise floor, is constant regardless of volume and is still there when the input is disconnected.”
I set up the amplifier on my bench and played an iPod through it with field coil and volume box from the jukebox and I get perfect sound and dead quiet with or without input.
I rebuilt the amp a few years ago but was surprised when I saw the volume box modification, it just had straight wires to volume pot and back out again. I’m only familiar with the mods for the 1940’s volume boxes. Is it different for these early amplifiers? I’m at a loss as to what might be causing this hum in his jukebox but was curious about the lack of resistor or capacitors in the volume box. Anybody familiar with these? Thanks, Mauro
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Hi Mauro,
If it all works fine on the bench then the setup in the machine must be the problem.
Check that the volume control box and the mechanism chassis aren't 'Hot' - connected in some way to the 'mains!
Set your meter on AC volts suitable for your line power and touch one lead to the volume control box and the other to earth ground. Should not be over a few volts leakage no matter which way the power plug (if reversible) is inserted. If good then check the metal chassis, as well as the tone arm and amplifier.
Here is a link to make a simple leakage detector which is a step up from the above crude test procedure.
🔗 https://www.flippers.com/pdfs/ElectricalSafety/Electrical_Safety_Testing.pdf https://www.flippers.com/pdfs/ElectricalSafety/Electrical_Safety_Testing.pdf
John :-#)#
On 2026-07-29 4:21 p.m., M De Simone via Jukebox-list wrote:
Here’s one for the old timers, I’m working on a model 771 amplifier from a Wurlitzer 616 and the complaint is,
“ There is significant hum primarily at 60Hz as seen in the attached spectrograms. The hum is 30dB SPL over the noise floor, is constant regardless of volume and is still there when the input is disconnected.”
I set up the amplifier on my bench and played an iPod through it with field coil and volume box from the jukebox and I get perfect sound and dead quiet with or without input.
I rebuilt the amp a few years ago but was surprised when I saw the volume box modification, it just had straight wires to volume pot and back out again. I’m only familiar with the mods for the 1940’s volume boxes. Is it different for these early amplifiers? I’m at a loss as to what might be causing this hum in his jukebox but was curious about the lack of resistor or capacitors in the volume box. Anybody familiar with these? Thanks, Mauro
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On 7/29/2026 7:35 PM, John Robertson via Jukebox-list wrote:
Hi Mauro,
If it all works fine on the bench then the setup in the machine must be the problem.
Check that the volume control box and the mechanism chassis aren't 'Hot' - connected in some way to the 'mains!
Set your meter on AC volts suitable for your line power and touch one lead to the volume control box and the other to earth ground. Should not be over a few volts leakage no matter which way the power plug (if reversible) is inserted. If good then check the metal chassis, as well as the tone arm and amplifier.
Here is a link to make a simple leakage detector which is a step up from the above crude test procedure.
🔗 https://www.flippers.com/pdfs/ElectricalSafety/Electrical_Safety_Testing.pdf https://www.flippers.com/pdfs/ElectricalSafety/Electrical_Safety_Testing.pdf
John :-#)#
I don't like the idea of the loading resistor in that tester. I've had chassis that give that little *buzz* shock when touching the juke and the resistor can ground that out. When I've ran into the hardly noticeable buzz, I'll measure from ground to the chassis. The buzz some of us feel sometimes can be "shorted out" by simply grabbing onto the machine instead of just your fingertips or whatever body part is barely touching the machine using a cheap DMM without enough ohms/volts. I've been there and done it. I now always use one of my higher end meters to do that test. Often the chassis to a real ground is as high as the line voltage, yet not give a hell of a shock. It's so mild that your body's conductance to ground can *short* it down close to zero, yet unnerving to others. The cheap meter can measure close to zero volts from chassis to ground, but a good meter can measure as high as line voltage.
Check with Bill Bickers. He will probably tell you resistor(s)
On Wed, Jul 29, 2026 at 7:22 PM M De Simone via Jukebox-list < jukebox-list@lists.netlojix.com> wrote:
Here’s one for the old timers, I’m working on a model 771 amplifier from a Wurlitzer 616 and the complaint is,
“ There is significant hum primarily at 60Hz as seen in the attached spectrograms. The hum is 30dB SPL over the noise floor, is constant regardless of volume and is still there when the input is disconnected.”
I set up the amplifier on my bench and played an iPod through it with field coil and volume box from the jukebox and I get perfect sound and dead quiet with or without input.
I rebuilt the amp a few years ago but was surprised when I saw the volume box modification, it just had straight wires to volume pot and back out again. I’m only familiar with the mods for the 1940’s volume boxes. Is it different for these early amplifiers? I’m at a loss as to what might be causing this hum in his jukebox but was curious about the lack of resistor or capacitors in the volume box. Anybody familiar with these? Thanks, Mauro
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I seem to remember there's two versions of that amp. One has the volume control built into the amp itself and is a pain to adjust because it's down near the floor. I've seen where someone modified the machine by adding the separate volume control box in order to make adjusting the volume easier. If your back door has a crude cutout for the volume control, this is probably the case. On Wednesday, July 29, 2026 at 07:22:06 PM EDT, M De Simone via Jukebox-list jukebox-list@lists.netlojix.com wrote:
Here’s one for the old timers, I’m working on a model 771 amplifier from a Wurlitzer 616 and the complaint is,
“ There is significant hum primarily at 60Hz as seen in the attached spectrograms. The hum is 30dB SPL over the noise floor, is constant regardless of volume and is still there when the input is disconnected.”
I set up the amplifier on my bench and played an iPod through it with field coil and volume box from the jukebox and I get perfect sound and dead quiet with or without input.
I rebuilt the amp a few years ago but was surprised when I saw the volume box modification, it just had straight wires to volume pot and back out again. I’m only familiar with the mods for the 1940’s volume boxes. Is it different for these early amplifiers? I’m at a loss as to what might be causing this hum in his jukebox but was curious about the lack of resistor or capacitors in the volume box. Anybody familiar with these? Thanks, Mauro
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I've had a hum that was fixed by turning the tone arm 180° in it's socket. I don't recall if the plug and socket are originally polarized. Also the mains plug can make a difference by turning it, (then putting a polarized plug on it) Maybe even a ground, although some believe it is more of a danger than a non-polarized plug if you don't run a ground wire to each and every metal piece in the jukebox.
As for the gutted volume control, it's the way to go when installing a light weight ceramic cartridge. Some add a resistor and a cap in the pico farad range but I haven't heard a difference with or without them. I've seen them with a pot of anywhere between 10K and 100K.
Tony
On 7/29/2026 7:21 PM, M De Simone via Jukebox-list wrote:
Here’s one for the old timers, I’m working on a model 771 amplifier from a Wurlitzer 616 and the complaint is,
“ There is significant hum primarily at 60Hz as seen in the attached spectrograms. The hum is 30dB SPL over the noise floor, is constant regardless of volume and is still there when the input is disconnected.”
I set up the amplifier on my bench and played an iPod through it with field coil and volume box from the jukebox and I get perfect sound and dead quiet with or without input.
I rebuilt the amp a few years ago but was surprised when I saw the volume box modification, it just had straight wires to volume pot and back out again. I’m only familiar with the mods for the 1940’s volume boxes. Is it different for these early amplifiers? I’m at a loss as to what might be causing this hum in his jukebox but was curious about the lack of resistor or capacitors in the volume box. Anybody familiar with these? Thanks, Mauro
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Appreciate this response, if the hum persists we will try your recommendation. It will be a few weeks before I can get the amp back to the jukebox, out of town. Although, I’m still curious why I would read 23 vac on the chassis with my Fluke multimeter and less than 1 vac with my 1000 ohms / volt Simpson analogue meter? I’ve done this comparison before when calibrating my tube tester and the difference was minimal?
Sent from my iPhone
On Aug 2, 2026, at 7:40 AM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
I've had a hum that was fixed by turning the tone arm 180° in it's socket. I don't recall if the plug and socket are originally polarized. Also the mains plug can make a difference by turning it, (then putting a polarized plug on it) Maybe even a ground, although some believe it is more of a danger than a non-polarized plug if you don't run a ground wire to each and every metal piece in the jukebox.
As for the gutted volume control, it's the way to go when installing a light weight ceramic cartridge. Some add a resistor and a cap in the pico farad range but I haven't heard a difference with or without them. I've seen them with a pot of anywhere between 10K and 100K.
Tony
On 7/29/2026 7:21 PM, M De Simone via Jukebox-list wrote: Here’s one for the old timers, I’m working on a model 771 amplifier from a Wurlitzer 616 and the complaint is,
“ There is significant hum primarily at 60Hz as seen in the attached spectrograms. The hum is 30dB SPL over the noise floor, is constant regardless of volume and is still there when the input is disconnected.”
I set up the amplifier on my bench and played an iPod through it with field coil and volume box from the jukebox and I get perfect sound and dead quiet with or without input.
I rebuilt the amp a few years ago but was surprised when I saw the volume box modification, it just had straight wires to volume pot and back out again. I’m only familiar with the mods for the 1940’s volume boxes. Is it different for these early amplifiers? I’m at a loss as to what might be causing this hum in his jukebox but was curious about the lack of resistor or capacitors in the volume box. Anybody familiar with these? Thanks, Mauro
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