Calibrating the JVC 4DD-5 Demodulator

[berninahusq]

Okay, I have two of these puppies and don't know how to calibrate them to get true discrete CD-4 playback. I don't have a user's manual, only a service manual I got from the Quadfather. I am using an Grado F1+ and AT 15sa cartridges. The following problems happen identically with both of my 4DD-5s. I wonder if anyone can please help me.

I just bought this CD-4 Quadradisc 4-Channel/2-Channel Test Record SB4X-002 by Sutton-Miller Ltd. It has all the CD-4 tests you'd expect, (CD-4 carrier present test, 4 channel balance test, etc.). What I was surprised to see was that both the front and rear channels had a signal almost of equal strength whether or not just the rear or front channel was supposed to play alone. The same happens for the left channels. This is like playing a SQ matrix record through a non-Tate passive matrix decoder. Lots of front-to-back, back-to-front channel bleeding. I've adjusted the CD-4 Left and Right Level controls in the back and also the 30kHz Carrier Level adjustment underneath the unit.

The CD-4 signal light goes on and off when the test record says it should, but I'm not getting discrete sound like I should. I don't have a CD-4 record that I can compare with its Q8 tape equivalent. I just assumed that my CD-4 records' program material was not as discrete as some on my Q8s. Are the CD-4s supposed to be AS DISCRETE as their Q8 counterparts?

Not having ever heard a pristine CD-4 demo in my life, I don't know what I should expect of CD-4. Could both my 4DD-5s be broken? Any help would be appreciated.

Thanks--Ed

 

[The Quadfather]

Dear Ed:
It sounds like your subcarrier demodulators aren't functioning. If you turn down the separation controls all the way, you should get nothing but the subcarrier audio. It will have no front to back separation. If it isn't there, you have a problem probably with the subcarrier mute circuit. (I think it has one) If you have this, adjust carrier control for the least distortion. It should sound hollow, but again, with no front to back separation. Adjust slowly, when you get it right, there will be almost no distortion. Then bring up the separation adjustments about half way, using your test record to judge separation. You are adjusting for front back separation, left to right separation is maintained by the vector modulation of the record (just as it is on a stereo record). You are adjusting for null, not loudness. The best way to do this is to turn off all channels except for left front. Now play the test track for left rear. Now, adjust the left separation control to cancel out the signal. This will occur in the middle of the adjustment, not towards the end. This happens when the main audio level (left front+left rear) matches the subcarrier audio (left front-left rear). In this case you are cancelling left rear from the left front channel. Now repeat this procedure for the right channel. Hope this helps.

The Quadfather

 

[berninahusq]

Dear Quadfather,

Thank you very much for my first instructions on how to calibrate the 4DD-5 and for helping me out this last year and a half as I make this journey through my old quad days.

I'm a systems analyst by trade and I should have understood your instructions, but my brainless mind these last 2 days is missing a few things here. I hope you can please bear with me and kindly give me your advice because I'm not having any luck. In fact, I got so frustrated that I'm listening to an SQ record with my TATE.

Before I play the band with the 30kHz subcarrier, I turned down both the left and right CD-4 adjust controls in the back of the unit fully counterclockwise. While hearing the subcarrier and turning the 30kHz level control at the bottom of the unit (with tick marks from "0" to "10"), I think it sounds most hollow (or what I think is undistorted) at "0". At higher settings it sounds "nasal". I don't know what undistorted should sound like and it's kind of hard for me to distinguish distortion from the crackle and pop of the record. Is the "nasal" sound what you mean by "distorted"?

So with the 30kHz level control at "0" (or any other position for that matter), I turned only the left front speaker on and adjusted the CD-4 adjust left control in the back of the unit while playing the left rear. The only way I cancelled the left front was to turn the left control fully counterclockwise (where I started). Anything higher than that (or in the clockwise direction), the left front speaker progressively got louder and louder. The "cancelling" out did not occur in the middle of the adjustment like you suggest. Of course as you know, with the left or right adjust controls at their fully counterclockwise position, no sound comes out of the front speakers. The same thing happens when I follow these steps for the right channel.

Does it matter whether the 2Channel Direct Out is switched "on" or "off"?

I repeated the same steps with my other 4DD-5 and got identical results.

Can you please tell me what I'm doing wrong or if I have two duds of a demodulator and I have yet to hear what discrete CD-4 sounds like? I do think I'm getting some separation, not nearly as much as I do with an SQ and the Tate, but maybe it just my imagination and desperation wanting to make it work. CD-4 is a hard animal to tame!

Help!!!

Thanks Quadfather.

Ed

 

[The Quadfather]

Try that carrier level adjustment with a music record, rather than a CD-4 adjustment record. With the separation pots all the way down, if the "radar" light is lit, you should get the "difference" signal. It will sound like the music, but will sound hollow. There will be no separation between front and back, but you will hear it in all speakers. Make sure that your SQ decoder isn't on (your amp is in discrete mode) If you don't hear this signal, there is no point in trying to do any further adjustments. I looked at the schematic for the 4DD5, and it appears that the direct out switch rerouts the turntable to a stereo preamp through the direct out jacks so that you can use a separate preamp for stereo. It has no effect in four channel mode and it shorts out the direct out jacks so that the stereo amp will not amplify hum. I use the 4DD5 as a preamp when not playing quadradiscs. (it puts out double stereo in two channel mode). So check if you have the subcarrier signal (the hollow signal I discussed earlier), and get back with me. Start with the carrier control about halfway, and the separation pots all the way down. If you get no sound at all, it's broke. There is a subcarrier muting circuit, and if it isn't functioning properly, it can kill the subcarrier signal and still light the "radar" light. (they actually call it that) Anyway, let me know what you find out from this test, and I can let you know what to do from there. If you want to Email me, it's [email protected] . We really don't need to be troubleshooting this on the forum. I can give you my phone number there, with out broadcasting it. Good Luck,

The Quadfather

 

[AllAroundSurround]

<blockquote><strong><em>Quote:</em></strong><hr>We really don't need to be troubleshooting this on the forum.[/quote]
Hey! It was interesting reading, don't stop know!
No , really it might help others who have the same problem.
If you guys get the problem fixed, post a note on what you came up with!
8o
Rob

 

[berninahusq]

Okay, here's my offline reply directly to the Honorable Quadfather sans my superfluous comments (it's long enough!). Nevertheless I wish CD-4 wasn't so tempermental. The Tate is so effortless--true plug and play. And when you can get near-discrete SQ with the Tate, I ask myself "why bother" but us quaddies want it all!
<span style="color:blue;font-family:Times New roman;font-size:small;">
Dear Quadfather,

You are a lifesaver! Thank you very much. With your patience and guidance I was able to come as close to discrete CD-4 with one of my 4DDs and determine that the other one is broke. While doing these tests I had a 4DD connected directly to two stereo Yamaha main amps so the signal could be as undistorted by any other processing. I did the tests on both 4DDs with both my Technics SL-1650 direct drive with the Grado F1+ and Garrard Zero 100C with the AT 15sa (I know you aren't particularly keen on the Zero...but I still love it and it tracks my badly warped SQ Bernstein's Mass at 1.5 grams effortlessly as compared to my Technics which had trouble tracking it even at a dangerous 3.0 grams....ouch!).

On the first 4DD I bought in May 2001, the subcarrier muting circuit is malfunctioning (I know I should have stopped at that point but I went ahead and confirmed it with the other cartridge just in case!).

Fortunately the second 4DD works! and I was able to hear discrete (or semi-discrete) CD-4 finally! I say "semi-discrete" because even after calibrating the 4DD the best I could, there is still some bleeding between the front and rear channels when my test record says that the sound should only be coming the rear speaker. Nonetheless, it is still better that SQ.

Not knowing what the fine line between "distortion" and just a "different" timbre of the hollow sound, I settled on a setting of the 30hKz carrier of about 4.75. I was not able to cancel out the left rear from the left front with the left CD-4 adjust as much as I was able to with the right rear/front channels. Is that typical? But the best "cancelling" setting for both channels was at about the 10 o'clock setting. Perhaps because I couldn't "cancel" out the rear channels to near silent, is why there is some bleeding of the rear into the front channels??? These settings seemed to be optimal for both cartridges. Is that usual? I thought the settings would be slightly different for each cartridge.

In awe (I know you'll understand that us "quaddies" experience moments like this with reverence), I sat back and listened to three of my 20 odd CD-4 records (I have 4-times the amount of SQ records). Wow! I'll have to have a CD-4 weekend!

Anyway, is true "discrete" (like from a Q8 or 4-channel reel tape) possible from CD-4 record, or does the physical limitations of a record and stylus allow us to only "approximate" discrete? If so, I'll not only need to get my broken 4DD fixed, but have my working one "tuned". I read on the ezboard that the PLL chips can still be obtained. If this chip will fix the 4DD, is this something I can do myself? If I replace them on the working 4DD, will it improve the front/rear separation? I'm pretty good with a soldering iron but I haven't had any formal audio repair training.

Thanks again so much--Ed
</span>

 

[The Quadfather]

I am glad to see that you got at least one working. The quality of the separation is maintained by maintaining phase integrity between the main audio, (heard when you turn up the separation pots) and the subcarrier audio (heard when the pots are down). The best separation is achieved when these two signals are equal in amplitude. Normally you wouldn't have to worry about phase integrity, but these units are old. An electrolytic capacitor that has lost some of it's value can cause a phase shift as well as a reduction of bass response. The phase shift will reduce separation, because the opposing waveforms will not match in time and cancel or add properly in the summing circuits. (these circuits are often called matrix circuits, but don't confuse them with matrix quad, they serve a different purpose and theory). It may improve performance to recap the demod if you are technically savvy enough to conduct this operation. (don't put in an electrolytic capacitor backwards, they tend to explode when you do that) It is doubtful that any other components will cause this phase shift problem. A new set of chips might enhance performance, but I don't know if modern made chips are necessarily better than the old ones. I have heard that the chips were improved during the manufacture of the 4DD5, so you might have the older kind. I don't think the numbers are different. Back then the phase locked loop circuit was a new idea, and it seems logical that the first chips might have not been as good. I run my original chips in mine. Incidently, the separation won't be perfect, just better than SQ, and you will definitely notice it in a listening test. Happy listening!

The Quadfather

 

[sspsandy]

Hey Quadfather, thanks for you expertise with this unit, very informative. Looks like I'm gonna want to get mine serviced eventually, even though I'm getting great separation now. I had all the old component replced in my Tate, and what a difference. Just wondering, if the quadradisc was not designed to be stereo compatable, but had only the front channels in the low frequency tracks, rear channels in the high frequency tracks, would it be totally discrete?

 

[The Quadfather]

Yes, it would. However, there would be a quality difference between the fronts and the rears. It works out better this way. FM stereo radio works the same way, and a high degree of separation can be achieved. Once you get the system working properly, there will be no need to fiddle with the carrier and separation pots. It will seem as effortless as the Tate. Set it and forget it. You can even change a stylus and still not have to readjust. Be sure to align the cartridge for proper tracking angle. This should be done for playing all records, not just quadradiscs. Alignment guages are hard to find these days, but not impossible. I got one from Consolidated Electronics. Happy Listening!

The Quadfather

 

[Circular Vibes]

Hey Quadfather!

Would you be willing to copy your service manual for the 4DD-5? I have one I got from EBay, but it was a little wonky. It works better now that I tightened up the screws on the bottom, but I'd like to try recapping it and possibly replace the trim pots (they are quite corroded like tarnished silver). I can't see any breaks in the board, so I am suspecting a loose connection. Before I try anything heroic with it, I'd like to have the manual in front of me. Let me know if you can copy it or possibly make a pdf file. I am willing to send you a reasonable amount for copying. Thanks.

Matthew

 

[gulbranson]

several years ago i bought a grado cartridge because they were relatively cheap and supposed to be able to play cd-4 records,,,after hooking it up for cd-4 use i noticed that my rear sounds were coming from the front and the front from the rear!,,,after i reversed the wire connectors going to the cartridge everything came out fine,,,,,when i brought this up to the dealer that sold me the cartridge,he told me something to the effect that i had discovered the reason for the certidge's "unique sound",,,

 

[The Quadfather]

Hey Circular Vibes:
I've been on vacation. Do you still need a copy of the 4DD5 quadradisc demodulator? If so, Email me at: [email protected]

The Quadfather