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.

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.

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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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