JVC 4DD5 problem - HELP

[radioheadrox]

Hey guys,

I just picked up a 4DD5 today and I encountered a unique problem. While hooking up to test, I decided to see if my stereo cart (I have yet to buy a cart with an extended frequency response) passed enough of the carrier signal to light the 'Radar' light. Surprisingly enough, it did light. With the separation pots all the way down, I was able to hear some of the difference signal in the left channels, but not the right. Upon further inspection, I achieved these results:

1. With the left channel output from my turntable plugged into the left phono input only and the separation pots all the way down, the CD4 lamp flickers (no doubt due to a very weak carrier signal) and I hear the difference signal flickering in the left channels.

2. With the right channel output from my turntable plugged into the left phono input only and the separation pots all the way down, the CD4 lamp flickers and I hear the difference signal flickering in the left channels.

At this point I came to the conclusion that I was getting a weak carrier signal in both channels (equally weak in both channels)

3. With only one signal plugged into the RIGHT phono input with the separation pots all the way down (nothing plugged into the left), the CD4 lamp is lit solidly and a signal is heard in the LEFT channels (NOTHING heard in the right channels). @:

4. Playing a 2 channel record through the 4DD5 results in proper double stereo (as it should) with strong signal in both the right and left channels.

Any ideas? I am purchasing a proper cart in the next few days but it is apparent that something is not occurring properly in the carrier signal demodulation step. Is my 4DD5 toast or do you guys think I can repair it?


Finally, does somebody here have the service manual for this unit (preferably in PDF format)? If so, please contact me at [email protected]

James

 

[sspsandy]

I'd suggest wait until you get the quad cart before making any conclusions. Once you get the cart installed, the general idea is to turn on only the rear channels and adjust the carrier level until it sounds the least snoggy. Then adjust up the left and right pots one at a time by first turning the balance all the way left to adjust, then all the way right. You try to get a balance between the fronts and the rears where there is the most separation. It works on this crazy phase principle that doesn't work right unless it's all balanced out, so trying to listen to the discreteness of different channels one at a time separately doesn't work. Some things I've noticed is if the carrier is too high you get a lot of record noise and high frequency distortion. If the front pots are off the rears will sound too loud. It takes a bit of experimenting to get the most discrete separation that CD-4 offers, but when it's right it's awesome. I also tried putting all the pots a the 12:00 position as a starting point, but ultimately kept playing with it till I got it to sound the way I liked it. Also all these units are 30 years old, so are hardly on spec, so it takes some juggling. I'm also willing to cut the discreteness a little in favor of less record noise.

 

[radioheadrox]

Yeah, I definitely won't do anything drastic until I can test the unit out with a quad cart.

What's bugging me the most is that when I send a signal into the right phono input, the difference signal bleeds into the left and is not heard at all in the right channels... (How that happens is beyond me...) yet the audio in the normal 20-20 000 Hz can be passed through to the right channels no problem.

James

 

[radioheadrox]

Alright, I purchased a new cart and I'm getting GREAT separation and fidelity from the beginning of a CD4 LP to the end ...



...but still only in the left channels. THe 4DD5 is still not demodulating the carrier signal on its right input.

I was planning on a recap.... but once I discovered that there are 49 capacitors inside I realized I should have a little more info first. Any suggestions to what part of the circuit board to focus on (ie - where in the demod it is located).

If possible, is there anyone who can provide a copy of the service manual and/or the schematic? ( [email protected] )

So close to great CD4 yet so far away.....

James

 

[The Quadfather]

I have written many posts about CD-4 demodulators including the 4DD5, as I have two of them. Just about any problem with this demod can be fixed if the parts are availlable, and many are. The right and left in a demodulator are totally separate, in fact, they constitute two demodulators. Right to left separation is maintained by the vectoring on the record just like in stereo. If right channel info is getting into the left channel, it is probably a failed filter capacitor in the power distribution chain. Not the main caps, but more probably one in the preamp circuit. There may be other bad caps causing the right demod to not work. If it has been on the shelf for twenty years, it is entirely possible to have multiple problems. Recapping is probably a good idea and probably will fix the problems. When I got my first 4DD5, I had to replace a transistor or two, as well as some caps. However, I did not recap the entire unit. I have a service manual, it is not complete, but it does have a schematic. A CD-4 demodulator is not a really complicated device. It is similar to a stereo decoder in an FM radio, or a stereo TV. It should be no great mystery to fix it for an experienced technician.

The Quadfather
P.S. If you want a schematic, contact me at [email protected]

Alright, I purchased a new cart and I'm getting GREAT separation and fidelity from the beginning of a CD4 LP to the end ...



...but still only in the left channels. THe 4DD5 is still not demodulating the carrier signal on its right input.

I was planning on a recap.... but once I discovered that there are 49 capacitors inside I realized I should have a little more info first. Any suggestions to what part of the circuit board to focus on (ie - where in the demod it is located).

If possible, is there anyone who can provide a copy of the service manual and/or the schematic? ( [email protected] )

So close to great CD4 yet so far away.....

James

 

[sspsandy]

One thing to try first is to spray some tuner spray into the switch in the back marked 2ch / 4ch and work it in. This switch when it is scratchy will cause channels to cut out.

 

[ress4278]

Pls let us know if you do the recap, and what your impression of the sound is.

 

[radioheadrox]

The Quadfather, please check your PM inbox.

James

 

[radioheadrox]

Update:

After receiving the schematic, my dad and I were able to isolate the problem which happened to be two faulty transitors after the FM decoding circuit for the right channel. My 4DD-5 appears to be functioning properly now and I must say, CD-4 is amazing!

Because I don't really have any idea what the mixes on my quadradiscs are supposed to sound like, I recorded one song and encoded it as a multichannel WMA file. The song is Albert Flasher by the Guess Who (http://strack.dyndns.org/demod/). The rear channels are a bit noisy but it's nothing serious. If anybody here is familiar with this mix, could you please try that file and let me know how it sounds? (I didn't use any pop/click removal or NR.) I don't have a test record so I tried to adjust the unit as best I could by ear.

Many thanks to the Quadfather; without his help my quadradiscs would be stuck in stereo land!

James

 

[Q-Eight]

Just had a listen to my Quadradisc with Albert Flasher on it so for the most part it has in the rear:

Left Rear: Electronic Keyboard
Right Rear: Piano

There are two instances where we hear Burton Cummings sing "Baby and me were ripe for the pickin'...." That line should come out of both front and rear to create a bit of an echo. There is also a guitar break mid-song, and it should alternate rear-front-rear.

Can I just ask what your setup is to get such good CD-4? My setup gets me decent sound, but the last song on every record I try, my carrier light stays lit, but it is definitely losing signal.

 

[radioheadrox]

I am just using a basic Dual turntable (CS 505-2). It has a straight tonearm... I hear that is more desirable than a S-shaped arm. As you probably gathered from the topic title, I am using a JVC 4DD-5 to demodulate.

I am using a Grado Prestige Green cartridge (10Hz-50KHz frequency response). It has an elliptical stylus, NOT a line contact or shibata but it seems to do a great job.

My setup seems to sound great and my copy of Jethro Tull - War Child plays clearly throughout. My copy of the Guess Who vol.2 does seem to get a little distorted by the end of the sides. That said, I have not properly calibrated my setup as I don't have a test disc.

Well, your description of the mix is bang-on with what I hear

James

Just had a listen to my Quadradisc with Albert Flasher on it so for the most part it has in the rear:

Left Rear: Electronic Keyboard
Right Rear: Piano

There are two instances where we hear Burton Cummings sing "Baby and me were ripe for the pickin'...." That line should come out of both front and rear to create a bit of an echo. There is also a guitar break mid-song, and it should alternate rear-front-rear.

Can I just ask what your setup is to get such good CD-4? My setup gets me decent sound, but the last song on every record I try, my carrier light stays lit, but it is definitely losing signal.

 

[sspsandy]

I don't find the test discs to be particularly helpful in getting a good CD-4 set-up. The warble tones don't seem too helpful, the high frequency tone is iffy, and the sound that moves through each individual speaker is also deceiving. The tracks on the 33 1/3 lps that are kinda easy listening popular stuff are better for hearing what's coming out of the speakers, but the only way to get it the way you want it is to take your favorite CD-4 lp and play around with the adjustments until it seems the most balanced, and has the least distortion. Two CD-4 records that stand out for test record purposes, in my opinion, are Hot Tuna America's Choice and Nilson Schmillson. There is a lot of discrete placement of the various instruments, and the recording quality of these lps is phenominal. The Guess Who lps, except for the Japanese CD-4 of American Woman, don't seem all that discrete, at any rate Flavors sure isn't. Haven't heard the War Child mix yet, but Aqualung is pretty good for balancing your system too. Volunteers by the Jefferson Airplane made me believe that something was wrong with my system, it just had such a different mix from the stereo lp, Grace Slick's vocals are really low on certain parts, and Jorma's leads are in the front and back right channel on Good Shepherd, which made me think it was bleeding through. As time goes by and you get a number of CD-4 lps, you'll get a feel for where it works the best.

 

[Q-Eight]

The Guess Who CD-4's aren't bad.... I have several. For the most part they're like the Q8's.... one instrument each in the rear speakers with a lots of front to rear play on some songs..... like the guitar in Albert Flasher. It bounces rear-front-rear. Definitely not as discrete as the Q8's for sure, but pretty good for testing purposes. The Volunteers scares me. I don't like the Quadradisc mix. It seems somewhat hap-hazardly thrown together.... like they were rolling dice to choose what went where.

 

[The Quadfather]

Hey Radioheadrox:
Try this: turn down the separation pots and put on a quadradisc. What you are hearing is the separation signal without the main audio. It will sound like the music, but strangely hollow. You will probably hear some CD-4 splatter as well. That is often described as a "sand papery" sound on loudness peaks. Adjust the carrier level pot to minimize this. When it is close to clean, make small adjustments, stopping to listen between adjustments. This is because the splatters will be getting far enough apart that you will not hear them constantly. You are trying to reduce the incidence of them. When it is the best it can be, turn up the separation pots not all the way, but you will find the separation is best at a null point about half way round the pot, and if you find that point, you can improve fidelity by raising the level a little higher, but not too far, for you are sacrificing separation as you do so. However, the CD-4 system has enough separation that you can afford to sacrifice a little for the sake of fidelity. If you are getting more noise at the end of the record, you need to work on the turntable stylus combo to see what is not right there. A cartridge alignment tool is a good thing to use here. Also, if you have a new stylus, it will get better as you play the records. They have a break in period as they get seated into the groove. The AT331LP requires about 2 grams of tracking force, and set the antiskate about the same.
I use a Marantz 6300 turntable with an AT331LP cartridge and ATN3472LC stylus with good results. A good quality linear tracking turntable of the quad era is best if you can find one. But a pivot arm will work good if you have a good one. You do not need a CD-4 test record to get good CD-4 performance. Some poorly recorded records never will play perfectly, but most records will play very well. Good Luck and Happy Listening!

The Quadfather

Update:

After receiving the schematic, my dad and I were able to isolate the problem which happened to be two faulty transitors after the FM decoding circuit for the right channel. My 4DD-5 appears to be functioning properly now and I must say, CD-4 is amazing!

Because I don't really have any idea what the mixes on my quadradiscs are supposed to sound like, I recorded one song and encoded it as a multichannel WMA file. The song is Albert Flasher by the Guess Who (http://strack.dyndns.org/demod/). The rear channels are a bit noisy but it's nothing serious. If anybody here is familiar with this mix, could you please try that file and let me know how it sounds? (I didn't use any pop/click removal or NR.) I don't have a test record so I tried to adjust the unit as best I could by ear.

Many thanks to the Quadfather; without his help my quadradiscs would be stuck in stereo land!

James