External Speakers to USC2

Ron, one of the SHP's that I had to take care-of while my friends Bill was in the hospital in early 1992 actually had the jumpers taped over and an earlier un-modded driver board in there.
to be honest, I saw so many hacked, burned up boards in those amps that I hated when one was brought to me. It didn't help that I had zero service info and had to figure out what the design was. the pre-AGC board was similar to the silicon TSA pre's and the outputs were a typical quasi-comp, but I had no info on the mod's needed to prevent meltdowns. Many driver boards had holes burned in them and I had to do point-to-point to get them going again. By then all the shops on Tenth were gone and all I had to work with were piles of old doc's that I'd saved from the ending of the jukebox era in hell's Kitchen -as the area was called. I figured things out and kept notes on what I'd heard and seen done on amps that I dealt with. My interest in jukes was mainly the older machines and I didn't pay too much attention to the "console era". The internet would have been a help there.

Torin: Yes you can fairly easily connect a turntable to that amp, but I suggest a test run beforehand because the presence of the AGC circuit might complicate things a bit.

The AGC works to keep all records at the same approximate level but it is a simple, ungated circuit. This means that if no signal is present as in between tracks on an LP, the gain will increase as the AGC is not getting compressed by voltage from the record. This -can- lead to a bit of "overshoot' -that is a fraction of a second of high level while the circuit responds to the level of the record. There is no simple way to deal with this but it can be done. First, thing is to see if the level from your turntable is so low that this won't ever really be a problem. The output from Seeburg's carts and styli is mush higher than a typical hi-fi magnetic cart.

To try it out:

1) Remove the mech plug to the amp. this allows the mute relay to fall out and eliminates the squelch voltage to the AGC.

2) With power off connect you table and don't forget the green turntable ground wire to amp chassis.

Plug it in and see how it works.

If it works out we can discuss ways to make it work more conveniently.

Rob

Thanks Rob!

Quick question but I want to be sure. Which inputs do I use on the amp to plug my turntable in?

Remove the red and blue pick up plugs from the mech and plug your table in there. They are really the only RCA plugs on the amp.

Switching this low-level pickup signal from mech to your turntable will be one of the complications in making a more permanent installation, but for now just plug it in.

Rob

Alright, with the mech plug disconnected, I plugged in the turntable and played a record. I couldn't hear anything coming out of the speakers at all.

I know with this turntable, I have to use a pre amp for it to work with my receiver. I did not want to try to use it in this situation and so I took it out of the equation.

So, yes I believe this turntable is low level.

Torin

You have to remove the mech plug in the amp. That is the orange-ish one right next to the speaker connections. This un-mutes the amp.

Make sure that the amp is powered.

It seems you have a magnetic pickup in your table, that should work fine but as is mentioned, the output from a regular cart is slightly lower than what seeburg pickups produced. Same tech and compatible.

Rob

Ok, Whoops. Got it now.

I unplugged the orange connector and now I can hear the turntable just fine. It basically sounds just the same volume as when the jukebox is playing. Sounds great.

What are my next steps? I really appreciate your help.

Torin

There are two mods required to integrate an external table into that amp.

First the mute relay must be made switchable. This is done by placing either a manual switch or a relay controlled by the mechanism motor supply. Looking at this schematic:
http://arcarc.xmission.com/PDF_Jukebox/ ... hp1-3b.pdf -you can see that the relay switching pin is #3 of J5103. When modifying equipment the decision of how and where to place the mod is based on convenience and what component is least likely to ever be replaced. Since the mech is least likely to be replaced, in this case I would choose to place the opening of the circuit in the mech harness at-near the J5103 male.

The switching of the pickup carts input is a more complicated and critical operation due to the -very- low level signals involved (around 1-3 millivolts). This means that any switch or relay used must have very low resistance contacts.

Manual switches such as these would be OK: https://www.e-switch.com/system/asset/p ... 29/100.pdf
Since is is desirable to keep the cable lengths as short as practical using a relay located near the amp and in a grounded metal box would be my choice. Relays with mercury-wetted contacts have intrinsically low contact resistance and are ideal but generally more expensive.
A standard relay can be used but it must be DC powered to keep AC hum out of the signal and it must be placed in a shielded-grounded enclosure for the same reason. I mention 5 volts as this is easily powered by a discarded cellphone charger or similar device and the coil voltage is low so as to keep noise induction into the signal minimized.


Whether relay or switch, you will be switching the center leads of the two shielded cables from the mech and external table. The contact configuration we would use is called 4 pole -double throw or 4PDT. More about relay configurations here: http://en.wikipedia.org/wiki/Relay

There are inexpensive switching boxes normally intended for line-level sources (CD player etc) and these -might- be OK here but they are generally not shielded and their contacts do tend to build resistance over time.

Rob

Great information and thanks for the long write-up.

I am going to try this. I will post back my experience and any questions that may arise. Thanks again Rob!


Torin

Rob, a couple of quick questions. When creating a switch for the mute relay, do I use a 4PDT switch as well?

Also, when looking at the schematic, would I just insert the switch before pin 3 hits the molex connector and then also run the ground from the harness to the switch as well?

Torin, the mute interrupt switch would be a single-pole single-throw. SPST of any convenient type. You are just opening one wire. No ground is needed here.

Rob

Easy enough. Thanks, I think I can do this.

Ok, got the mute switch done and it works fine.

I'm thinking of going this route for the cart swap. It specifically states for low line use and it has pretty good recs in some of the turntable forums. Any thoughts?:

about halfway down the page $37.50

http://www.esotericsound.com/elect.htm

Torin, yes, that seems OK for the purpose.

The fact that it is just a simple straight-thru switch is important in this situation. Seeburg used the resistance of the cart as the voltage divider for the forward bias of the fist stage. So the cart must actually be electrically connected and not buffered with any intermediate electronics. Why Seeburg did this....I don't know...it did save exactly ONE resistor and ONE capacitor......

Where are going to mount it?

Rob

Rob,
I have had "wars" with the cost cutters, at Seeburg over as little as 1/8 cent per phono (and about three seconds of "production time") !! Some of these guys could not see the forest for the trees ! Meanwhile, back at the ranch, $$ were flowing out the "back door", like water----Ron Rich

Looks like I'm going to try to mount both switches outside the jukebox in between that and my external turntable. It will mount next to my PRVC3 control.

That will require some longer extension rca cables and I'm hoping that doesn't mess with anything as far as sound quality or interference.