Back to the amp for a moment.
So I read an article by a guy named Carl Haines
http://www.carlhaines.com/jukebox/cobra.htm
About modding the wurlitzer 532 amplifier to accept the phono input.
Here is that circuit
Now it seems he is jumping the volume control tube valve and putting the signal after the caps he pulled.
What I do not understand is the why. Are the caps acting as filters? The cobra needle apparatus seems to be refered to a frequency generator.
I do not understand the circuit, and I have not seen anything explaining it yet.
Here is the 518 amplifier partial schematic:
Similar but different....
This is where I am at...
For postarity here is the rest of the instructions for modifying the 532 amp. Again Carl Haines.
"Modifying the 532 amplifier.
These instructions may work for the 530 amp too.
Schematic available here.
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."
Any help is appreciated, I am working and have not been able to really attack this problem.
Edit-
Apparently in haines got the idea from another on the jukebox listserv. That information is as follows:
"Re: [Jukebox-list] Wurlitzer 2304 Car...
From Sent On Attachments
Columbus Coin-Op Shop Apr 3, 2012 8:30 am
James Alexander Apr 3, 2012 9:09 am
Columbus Coin-Op Shop Apr 3, 2012 11:10 am
James Alexander Apr 3, 2012 2:14 pm
Jim & Laurie Dirksen Apr 3, 2012 2:15 pm
CCOS Apr 3, 2012 3:29 pm
Chan Gade Apr 3, 2012 4:06 pm
Ron Rich Apr 3, 2012 4:06 pm
Jay Hennigan Apr 3, 2012 5:17 pm
CCOS Apr 3, 2012 5:32 pm
Chan Gade Apr 3, 2012 6:59 pm
James Alexander Apr 3, 2012 9:02 pm
CCOS Apr 3, 2012 10:24 pm
CCOS Apr 3, 2012 10:26 pm
Chan Gade Apr 4, 2012 12:52 am
Ron Rich Apr 4, 2012 9:04 am
Jay Apr 4, 2012 9:23 am
Ron Rich Apr 4, 2012 9:43 am
Jay Apr 4, 2012 12:45 pm
James Alexander Apr 4, 2012 2:11 pm
Columbus Coin-Op Shop Apr 5, 2012 7:06 am
Carl Haines Apr 5, 2012 9:26 am
Jay Hennigan Apr 5, 2012 11:39 am
Columbus Coin-Op Shop Apr 5, 2012 12:49 pm
Carl Haines Apr 5, 2012 4:53 pm
James Alexander Apr 5, 2012 9:23 pm
Mauro Apr 6, 2012 3:29 am
Chan Gade Apr 6, 2012 5:47 pm
Jim & Laurie Dirksen Apr 7, 2012 7:42 am
Chan Gade Apr 7, 2012 1:00 pm
Subject: Re: [Jukebox-list] Wurlitzer 2304 Cartridge permalink
From: Mauro (
[email protected])
Date: Apr 6, 2012 3:29:02 am
List: com.netlojix.lists.jukebox-list
Jim,
Thanks for putting this info altogether in one document. I'm printing it out and
including it into my 2304 manual.
(as you know) I am planning on doing 2 1950's Wurlitzer conversions in the
immediate future (2150 and 2304) this information will save me much trial and
error and frustration.
This has been great thread!
Thanks Jim and all,
Mauro
On Apr 6, 2012, at 12:23 AM, James Alexander <
[email protected]> wrote:
Gentlemen:
RE: stereo magnetic cartridge conversion for a mono Wurlitzer 45-carousel
changer jukebox.
1. Original phono cartridge is the mono Zenith cobra cartridge. It's stiff
compliance, poor sound quality (judging by modern standards) and heavy tracking
are killers for stereo records. Most 45's pressed after 1965 are stereo
recordings. Some of the after market stylus makers have issued a
retro-designed .7 mil tip size cobra needles which helps the tracking issue, but
it's an engineering compromise. It's becoming more difficult to buy cobra
replacements (this translates into expensive) anymore.
2. The cobra cartridge is not a conventional voltage generating pickup,feeding
the grid input circuit of a tube amplifier stage. The cobra element modulates
a tuned RF circuit (oscillator and detector) where the resulting audio signal
is picked off and fed into the input stage of the first audio amp. A new
conversion cartridge with roughly .5 volt signal level needs to be connected
here,bypassing the RF tank circuit. Many ceramic element cartridges with their
stereo outputs "Y"-ed together can be plugged in at this stage. These (stereo)
jukes originally used Sonotone 8T, 9T and variations of Astatic 133 as original
equipment. If installing fresh ceramic cartridge, the Sonotone models are both
freshly replicated. The Pfanstiehl P-132D is being used as a universal
replacement for the others. There will be some minor fitment/arm
balancing issues to address when changing cartridge types. Mag element
cartridges can be used if a self powered phono preamp is inserted at this stage.
3. Pickering NP-AC ,EP-DJ cartridges and older style Stanton 500AL are
discontinued but NOS can still be found in the sales pipeline. Stanton 400V3
is the closest current physical/electrical match to the Pickering models. The
400V3 differs electrically in that it has a higher output level than the
Pickering originals did. The 400V3 has a contoured freq response (the ad lit.
promises a bigger bass response) that boosts lows and highs compared to other
Stanton cartridges. One of the Stanton 500 series cartridges has a flat-response
output. The newest Stanton cartridges use different-design stylii and don't
interchange with the needles for the older cartridges. There is some stylus
compatibility among NP-AC and older 500AL models.
All of these cartridges have the same physical footrprint that with a little
modification can be fit and glued into a cobra tone arm, which does not have the
standard 2-screw cartridge mounting setup. It is necessary to ream out the
rectangular hole in the tone arm to accept the larger-body magnetic
cartridge. On mono jukebox models 1900 thru 2400 salvaging a complete stereo
tone arm assy from a junked jukebox 2300S up through 3300 is a trouble-free way
to mount a stereo cartridge. As an aside there is an Audio Technica brand
cartridge (AT3482 possibly?) and a Sanyo-Fisher MG-29 that can fit this
application. The stylus needs to be a .7 mil conical type, tracking at about 4
to 5 grams to work properly. Don't select a cartridge using an elliptical shaped
stylus for this app. Jukebox models 1700 and 1800 have different style tone
arms,but are the same operationally, so these need to be modified to take the
new cartridge.
4. It will be necessary to add weight to the tailpiece of the tone arm as any
of the magnetic cartridges are heavier in actual weight than the stereo
ceramics used in this app. The ceramic carts are apparently lighter than the
cobra, requiring addt'l weight to be added to the tone arm at the cartridge end.
5. stereo outputs of either the ceramic or magnetic cartridge should be
parallelled together for best mono sound quality. Some of these cartridges have
ground straps at the terminals, bonding the LG terminal to the metal cartridge
body. This strap needs to be lifted. The cartridge's audio grounds should
connect to the amp input using the shells of the RCA input plugsand not touch
the steel changer ground potential.. A hum loop can be set up if this happens.
The improvement in audio resolution, lighter tracking,less record wear and
assurance that you can find replacement stylii at reasonable prices are the
reward that makes this conversion worth the effort. If this is the first time
you're doing this, some trial-and-error testing will be needed to get all the
settings at optimum. With practice the conversion process becomes easier to
accomplish.
Jim Alexander "
Additional reading:
https://www.thevoiceofmusic.com/cartridges_help_substitution.html