AMI Continental Jukebox Amp

[mikeydee77]

Hi there anyone got better schematics from an AMI continental service manual that they can share with me?

I found the below on the web but the quality is low and so cannot read some of the resistor values.

[vintagekiki]

http://www.verntisdale.com/schemami.html

http://www.carlhaines.com/jukebox/manuals/#ami

http://arcarc.xmission.com/PDF_Jukebox/Rowe_AMI/

http://arcarc.xmission.com/PDF_Jukebox/Rowe_AMI/Schematics/

https://soundleisure.com/classic-jukeboxes/support

https://audiokarma.org/forums/index.php?threads/need-ami-continental-ii-jukebox-schematics.327049/

[mikeydee77]

Great selection of links there for all makes of Jukeboxes there... unfortunately I can't find any better than I had. But thanks anyway VintageKiki.

[The Dude]

Are you looking at the pictures at 100% size and not shrunk to fit a window or screen? I found them pretty easy to read and plenty legible.

[Enzo]

I also had no issue reading them. But I think also if you have a specific part in question, ask us. I'd bet my lunch money we can determine what it ought to be. I have rebuilt more old jukebox amps than I can count, and they are not mysterious. Only iffy part on them might be the automatic level control circuit, and I usually disable that anyway.

[g1]

The resistor values I had a hard time with turned out to be the word 'MEG' if that helps you. I right click and open the images in a new tab and it gives the magnifier option to zoom in.

[mikeydee77]

Actually you are right they are generally fine but the amp i have is a 1961 conti I had had few parts swapped out and I am checking all against the schematic as the amp has an irritating distortion like it is driving too hard.

There is one part especially I need help with if I can get suggestions. It is the top resistor of the potential divider just before the 470k on pin V3 pin 7. In the amp I have it is 680k old cc resistor so looks original but I thought the one on the schematic was ?meg . Also the 470k v3p7 on my amp is actually 270k cc sowing the seeds of doubt.

Good call on the magnifier though. I forgot all about that.

[mikeydee77]

Tell me more Enzo. I am guessing you mean the squelch output. I did wonder what was going on there.

It is unfamiliar territory to me as I mainly look at guitar amps but I am a sucker for pretty old jukebox too.

[mikeydee77]

Yes MEG and sometimes a toleranc or power exception exceptoon indicator I guess

[vintagekiki]

Irritating distortion can produce old, weakened power tubes 6973. If you are able replace them first.
Before checking value passive components, first check tube dc voltage readings (pin1-3; pin6-8).

[Enzo]

10 meg over 680k on your voltage divider.

By squelch, I assume you mean that which silences the amp between plays? SO it won't amplify rumblings of the transfer mechanics etc? I call it a mute, but same thing. And no, that is not an auto level circuit. Hmm, looking at it that does seem to be what they are calling a squelch.

REcords are recorded at all levels, well, really they are cut/pressed at all levels. SOme records are pretty loud and others not. This is the record itself, not the music on it. A jukebox doesn't want to be adjusted all day. A restaurant or bar wants to set it and have about the same volume song after song. AVC/ALC/AGC - for automatic volume control (or level or gain). it is a basic sort of compressor. It detects the overall average signal level and feeds it back as a control signal. The louder that average is, the more it turns the signal down. It should have a long enough time period that it won't chase loud and soft passages up and down.

The mono amp and the stereo channel;s seem to be about the same so lets look at mono. The basic preamp out is pin 8 of V3, cathode follower. Signal goes to volume pot, wiper is preamp out signal AUDIO. Look at V4. Preamp signal from V3 is fed to first triode, amplified and fed to second triode. Note second triode is wired as a diode, producing a DC voltage on pin 3. DC voltage follows amplitude of signal. yes, that is a 15 meg resistor nearby. The cap left of the 2.2meg resistor slows it down. That signal leaves the board as squelch, but is also fed to the grids of V2. V2 both sides - is wired as a variable resistor. The more DC on the grid, the harder it conducts, shunting more signal from the signal path through. The hotter the preamp out signal, the more V2 turns it down.

Look at the stereo, the two channels rectifier tubes are wifed together, and the control signal is wired to the "V2" tube in both channels. That way left and right are controlled at the same time evenly.

In a bar, this allows a volume setting to usually remain as long as desired. But it is subtractive, and in home settings we often want it as loud as possible, so in this case, simply pulling the 12AU7 ought to disable it. Note also that the last stage of V4, the rectifier as the bias supply on its plate, not B+. I believe the squelch signal is a switch on the mechanism, a switch that opens when a record is placed on the platter.

[mikeydee77]

Thanks Enzo....

Yes I agree 680k seems to be the value. I will take another look at the circuit and start another thread if required.


Thanks all

[mikeydee77]

Thanks for your help guys. Actuallyit it was power tubes in the end @vintage_kiki. I checked them out out on my tester and one was about 70% Ia and the other about 5%. New pair and sounds good as gold and very bassy.

[mikeau]

[Jazz P Bass]

Nice!