‘Jukeboxes’ Category

rare Rockola jukebox amp hum problem

Rockola 1426 jukebox "O" amp

I wanted to write about a unique problem in a Rockola model “O” jukebox amplifier (from a Rockola 1426 jukebox, vintage 1946).  The amp is a very primitive design: class A, 4 tube operation consisting of 5U4 rectifier, push-pull 6L6’s, and 6J5 preamp tube.  An interstage transformer drives the P-P 6L6’s instead of a more modern phase inverter tube circuit.

The amp was recapped but had a hum problem.  The hum was not a single frequency, such as the common 60-hz or 120-hz hum that you would expect in a typical amp.  The hum was a combination of 60hz with a strong 3rd harmonic of 180-hz.

The problem turned out to be that the interstage transformer was inductively coupling hum from the power transformer.  This problem would have existed in this amp from the day that it left the factory.  Often it is necessary to carefully position and orient interstage transformers so that they will not inductively couple hum into the amp.  In this case, remounting the interstage transformer at a 45-degree angle from original mount, as shown in photo, “magically” eliminated the hum.  Trial-and-error (rotating the interstage transformer) is the only way to find what mounting orientation will cancel the hum, and each case would be unique.  In this instance, the hum cancelled best at the orientation that you see in the photo.

I wonder whether other Rockola model “O” amps were shipped with this problem?

The amp also had somewhat slightly reduced power output, which was tracked down to a resistor/capacitor combo from grid to ground on each 6L6 tube.  Scope analysis showed that these components added no benefit but did reduce the power output a little more than preferred, therefore the parts were removed.  Finally, the amp had poor reproduction of treble notes, and this was tracked down to a plate-to-plate capacitor on the 6L6’s.  This cap was probably intended to prevent oscillation in some circumstances.  Removing this cap dramatically increased the frequency response of the amp and without any oscillation.

Seeburg jukebox stepper repair

This articles explains stepper repair of a 1950’s Seeburg jukebox.  High voltages are present.  Repairs should only be attempted by a qualified technician. ©2010, Bob Putnak.

This article uses a stepper from a Seeburg C as the repair example, but the 160/200 play steppers found in late 50s and 60s Seeburgs are fundamentally the same concept.

The purpose of “the Stepper” (Step Switch & Relay Assembly) is to energize a selector coil and group solenoid in the jukebox pinbank according to the selection made on a 3W-1 Wall-O-Matic wallbox.  A 3W-1 is a remote selector that was installed at each diner table to promote jukebox play.

Stepper repair is straightforward, although many problems can arise.  The system is entirely electromechanical, and is based upon contacts opening and closing at proper times.  This article will explain how I approach stepper repair.

1.  You need a reliable working wallbox.  Do not try to fix a stepper by using a wallbox of unknown condition.  Don’t rely that someone told you that wallbox works.  If you have not seen the wallbox working with a stepper and pinbank, then the condition is still unknown.


1950s Seeburg free play

Converting a Seeburg jukebox for free play operation is a popular modification for home use.  It eliminates the need to find coins and eliminates coin gear jams.  The “old” way to free play a Seeburg was to bend the little ramp inside the CCU so that selections were not cancelled as the CCU wheel rotates.  The pitfall to that approach was that the latch bar solenoid inside the selector would remain energized for long periods of time; this causes the latch solenoid to burnout, and it can even be a (slight) fire risk.

Another method to free play a Seeburg is to wire a push button switch into the coin gear switches, and ask the user to reach around to wherever that you mounted this switch.  This method is rather crude in my opinion, and often requires drilling a hole that could be avoided if a different freeplay method was chosen.

Here is a popular freeplay conversion for 1950s Seeburg jukeboxes M100C, 100W, HF100G, HF100R, 100J, 100JL.  This conversion is not my own.  It has been floating around the internet for quite a while.

Obviously, common sense disclaimers — do not try this at home — experienced technicians only.  Remove all power to jukebox and practice safety precautions.

  • Locate the CCU (Credit and Cancel Unit, which is mounted on the selection receiver).  Unplug the coin gear and leave it unplugged, as it will no longer be used.  You may want to lightly coat the 4-pin coin gear plug socket with some hotmelt glue so that no one accidentally plugs the coin gear back in.
  • Make sure that no credits are on the CCU.
  • Underneath the octal socket in the CCU, jump together pins 1 and 6.  Remove the connections from socket pin 7, and insert a 1N4007 diode between those connection and socket pin 7.  Anode side of diode goes to socket pin 7, cathode side of diode goes towards the connections that you removed.
  • I suggest that you put an instruction sticker on the CCU metal case that explains this mod, so that future owners of the jukebox will understand why the selector is working in this manner.

That’s all there is to it.  The latch bar solenoid will now only engage when someone pushes a button (either letter or number).  It will stay engaged until both a letter and number button are pushed.  A selection will be made, and the latch bar solenoid will release.

Obviously, if someone were to engage only one button (either letter or number), the latch solenoid would stay engaged.  This behavior is the same as previous coin operation — if someone had inserted a coin, pushed one button, and never finished making a selection.  In this situation, you would still risk burning out the latch solenoid, but obviously such a situation would be extremely rare.  I am not aware of anyone who presses one button and never finishes making a selection.

Free playing a Rockola 1493 Princess Jukebox

This article explains a more professional method to “free-play” a Rockola Princess Jukebox (model 1493). This theory can be adapted to many other jukeboxes, including other brands such as 1950s Seeburgs.

This method will: (1) not require any freeplay button, (2) prevent the Lock Bar Solenoid from energizing until the person begins selecting a song, and (3) keep the Select Light illuminated. Benefits: eliminates Solenoid noise during machine operation and standby, eliminates unnecessary solenoid wear, and increases usability and enjoyment of the jukebox.


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