Rock-Ola 1478 Amplifier Connections

[juke46]

I just completed the cleaned pinbank and installed it.  Actually I cleaned the pinbank a couple of years ago but got on too many other projects.  Cleaning the pinbank is a really tough job of getting all those pieces lined up for the top plate to go back on the pinbank.  If I have to do that again I'll let my smart wife do that nasty chore!  The re-installation went well without any difficulty in particular.  It selects every selection made now.

Now on to the amplifier.  I recapped it and it seems to work well.  The current speaker connections are obviously incorrect with the Amp-Lok plug labeled "Machine Speakers"  not connected to the machine speakers.  Instead someone ran wires from the four screw terminal connections on the top side of the amp labeled Left/Main/Right to the 12" speakers.  The small horn speaker is not connected to any terminal.  While the sound is adequate I think it could be much better if the speakers were wired correctly.

The schematic shows a crossover network in the wiring that would (should) go into the speakers from the Amp-Lok plug.  The signal is divided between the two large speakers and the high frequency horn by the crossover network.  I do not have the crossover network but it looks simple enough that I can (might) (maybe)  probably can build that one I acquire the necessary parts.  I am one who carefully replaces capacitors (shotgun) and hope everything works in an amp.  If more complex than that I have someone a lot smarter than I fix it or tell me what I messed up.  If the crossover was in a box I'll check the "usual people and places" for a spare.  One way or another there won't be electrical components hanging around on the speaker wires.

I just wondered if I am correct in all or any of my assumptions?  Thank You All!!

[robnyc]

"Juke" according to this schematic: www.verntisdale.com/schem/1478-85.jpg This is the typical R-O speaker wiring of that era. The internal 16 ohm 12" speakers are loomed across both output transformers in opposing phase to give monaural output. This is a better choice as it helps prevent acoustic feedback from the internal speakers to the pickup. The Jensen or Atlas "tweeter" used didn't have much output and was padded down further as shown in the diagram. There is no real crossover -just a high-pass capacitor to protect the tweeter.

Depending on how far you wish to go in improving the sound of this phono, I suggest at-minimum adding a choke in series with the woofer to remove all the highs and most of the upper midrange. Simple, but adequate is a 6db/octave crossover. here you would just add a choke coil in series with the pair of 12" speakers. I suggest either a 1.82 Mh or 2.55 MH as shown here: www.parts-express.com/resources-crossover-component-selection-guide

I had several R-O's over the years and added newer magnetic carts & preamps along with tonearm counterweight, modern efficient horn tweeter and 12db packaged crossover. Rock-olas were always economy machines, but with a little extra work they could rival any of the era.

RobNYC

[juke46]

Rob,

Thank You! I'll study that info so I know a little better about this subject. I really appreciate your assistance!!