Wurlitzer Cobra Tonearm to Magnetic Conversion
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These are instructions for converting a Cobra equipped Wurlitzer jukebox to use a magnetic cartrtidge in the existing tonearm. An alternate method is to find the tonearm from a stereo model (2300S-????) and use that since it will accept a half inch mount magnetic cartridge without modification.
Fitting the Pickering EP-DJ/Stanton SM.V3 to the Cobra Tonearm
- A replacement magnetic cartridge. In this case, a Pickering EP-DJ or a Stanton SM.V3 since that is the type of cartridge used for these instructions.
- A phono preamp.
- Replacement tonearm wire. You can use the wire from a scrapped set of headphones/earbuds. Headphone/earbud wire is different between the manufacturers and can be difficult to work with. You might try a different make of headphones if you are having difficulty.
- Clips for making the electrical connection to the magnetic cartridge. DO NOT solder to the pins on the cartridge. They sell clips specific for this purpose, or you can try the pins for a female d-sub connector.
- A small 250k ohm potentiometer. Use a trim potentiometer but not the multi-turn variety.
- A new RCA plug or reuse the old one from the cobra tonearm wire.
- A new RCA jack. Use a panel jack mount that is insulated from the panel to replace the existing non-insulated jack or use an inline jack that is not mounted to the panel. For this circuit you want one ground point and that should be where the preamp connects to the amplifier.
Check and Adjust the Tonearm
- Strap the two channels of the new cartridge so that they are connected in parallel. Strap R to L. Strap RG to LG. I stripped a length of 30 AWG wire wrap wire and used that to strap the pins together. Be sure not to leave a wire tail that could possibly short to the other pair of pins and make sure you leave enough room to attach the clips from the tonearm wire.
- Remove the tonearm brush. Removing the brush will help you from getting in trouble by having it damage the stylus cantilever before you have had the chance to adjust it and will give you more room when setting the tonearm pressure.
- Unplug the tonearm wire.
- Rotate black metal plate that is partially obscuring the hole where the tonearm wire feeds through the arch until it locks to allow for extra clearance when removing and installing the tonearm.
- Remove the tonearm assembly from the jukebox. Take care to secure the spring for the tonearm pressure when removing the tonearm. It can fall out once the tonearm is no longer limited by the tonearm release bracket.
- Remove the rear snap in cover.
- Remove the rivets for the cobra socket by drilling out the rivets from the back of the tonearm.
- Remove the Cobra Socket and tonearm wire if you have not already done so while removing the rivets.
- Remove the mount for the cobra that is closest to the tonearm pivot by either filing down the casting or using a Dremel tool with a suitable bit. The casting is rather soft and using a file to remove the mount is not a lot of work. File it at an angle so that cartridge has room to fit without making a large opening. After removing the mount you may need to remove an extra 1mm of material to make the opening a little larger. Doing this will keep the casting from getting too thin as you file it down. If it gets too thin it will crack and leave a rough edge. Below is an picture of the tonearm after using a file to remove the mount and make room for the cartridge.
Modified Cobra Tonearm
- If using a EP-DJ / SM.V3, trim the mount on the cartridge so that it can fit inside the tonearm body.
- Mount the cartridge to the rear cover. I used three layers of 3M foam mounting tape. This raises the cart 3mm off the surface of the mounting plate and when mounted in the tonearm allows the cartridge to be at the appropriate height relative to the tonearm. I aligned the mount on the cartridge using the center line of the bumps as a guide. The tape is a bit too pliant for my liking, but the forces exerted on the cartridge during normal playback do not seem to be enough to cause the cartridge to shift or move. I may make a more rigid mount at a later time.
Cartridge Mounted to Rear Cover
- Fit the tonearm wire to the tonearm and connect to the cartridge. It is probably best to replace the tonearm wire since the wire from the cobra setup is probably stiff and the exposed shield could possibly make a ground connection when you don't want it to and lead to a problem with hum.
- With the tonearm completely assembled, do a rough check of the tonearm balance before mounting it in the jukebox. Adjust the counter weight and possibly remove drill out some of the lead from the tonearm to get the tonearm balanced. Make sure you have all of the parts reinstalled on the tonearm when checking the balance, including the spring.
- Replace the panel mounted RCA jack on the record changer so that the tonearm wire is no longer grounded to the record changer chassis.
Follow the instructions from the service manual
for these items.
Installing the Preamp
- s. Tonearm feed in adjustment.
- t. Tonearm latch bracket clearance.
- u. Needle pressure adjustment. You will want to use the pressure recommended for the replacement cartridge. For the SM.V3 this is 2-5 grams.
- v. Trip switch adjustment
- w. Tonearm balance
- Tonearm brush setting - Now is the time to install the tonearm brush. Set it so that it barely touches the needle if at all. Do not worry about following the procedure in the manual.
I'm not going to tell you exactly where to place the preamp and how to mount it, but I will tell you where not to put it. Do not
mount it so that it comes in contact with any of the grounded metal items in the jukebox if the preamp has a metal case. Do not
mount it near anything that contains relays or any other coils. That means do not mount it on or near the playrak or the chassis. Once you have found a suitable location for the preamp, run the wire from the tonearm to an input jack on the preamp and then use a good quality RCA cord to connect the output jack for the same channel that you used to connect the tonearm to the input of the amplifier.
Modifying the 532 amplifier.
These instructions may work for the 530 amp too.
Schematic available here
Modified Circuit Diagram
The Tonearm with the Magnetic Cartridge Installed.
Reducing the Pop Noise When Making a Selection
- Remove C6
- Remove C7 Just cutting the leads is sufficient to remove the components. If you leave enough of the leads from C6 behind, that will provide you with a place to attach the potentiometer later. Check the PCB layout for the location of C6 and C7.
- Add the 250k ohm potentiometer to the ground side of C7. Use one of the non-wiper connections of the pot to make the connection. You will want to mount the potentiometer to the top of the PCB. Mounting it to the top will allow you to adjust the potentiometer while the amp is installed in the jukebox and will reduce the risk of you lifting or damaging a trace on the solder side of the PCB.
- Connect the wiper of the 250k ohm potentiometer to the junction of C6, C7, R4, R5 (the other side of C7 should work).
- Move the wire for the center conductor of the input RCA jack from its current location to the unconnected non-wiper connection on the potentiometer.
- Set the potentiometer to pad the output of the preamp so that it falls within a reasonable range for the ALC circuit. Measure the ALC control voltage and adjust the potentiometer so that when you play your loudest record the voltage is no more than the voltage when the amplifier is on but muted. On the 532 amp, you can measure the voltage at pin 4 of the muting plug. This is a high impedance circuit and the meter will have an effect on the voltage. Take a measurement for only a moment at a time. How long depends on how quickly you meter can give a reading. After I adjusted the pot, it was set for about 200k ohms padding. Your results may vary.
Here are some strategies that can help if you notice that your getting pop noise from the sound system when you make a selection. There are a number of relays and coils that are energized and then switched off when making a selection. The noise generated when switching these coils on and off can easily be picked up by the preamp.
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- Tonearm Wire Routing
- In my case, I found that the routing of the tonearm wire to the preamp was the most effective in reducing the noise. I recommend experimenting with the routing of the tonearm wire first.
- Preamp Grounding
- At one point I attached a ground wire from the preamp ground terminal to the record changer. This helped reduce the noise slightly, but was not as effective rerouting the wire from the tone arm to the preamp.
- Preamp Location
- The location of the preamp can be a factor as well. I mentioned this in the conversion instructions, but you should not locate the preamp near any coils or relays. This means that you should not mount the preamp on the playrak or the chassis. I have mine located on the bottom of the jukebox and approximately 1" (2.5cm) away from the chassis.