CD-4 demodulator adjustment

[RockaMarka88]

Hi, everyone.

I am a new arrival on QuadraphonicQuad. I apologise if this has been covered before, but I was wondering if anyone could help me.

I caught the quadraphonic bug some time ago, and after using the SQ system, I soon became very intrigued about CD-4. I recently invested in a new old stock JVC 4DD-5 demodulator (with low capacitance cables included), and an Audio-Technica AT20SLa cartridge, capable of tracking frequencies up to 50kHz.

However, there seems to be no separation between the front and rear channels, i.e. what should only be heard in one speaker can be heard in both front and rear. So essentially, it’s more like 2-channel stereo with four speakers than discrete 4-channel.

I’ve tried calibrating the demodulator with the adjustment record included in the box. As instructed in the manual, I lowered the front and raised the rear speaker volume, then turned the CD-4 Adjust screws at the back of the unit so that the sound from the rear was at its lowest. I also adjusted the 30kHz screw at the bottom until there was no distortion. The CD-4 radar light illuminates when playing Quadradisc records like it should, but the front and rear still mirror each other, sounding no different to 2-channel stereo.

Could anyone please advise me on what I might be doing wrong? Could I have connected something incorrectly, or bought the wrong equipment?

These are the connections I have made:


I am using the following equipment:

Turntable: Audio-Technica AT-LP120XUSB

Cartridge: Audio-Technica AT20SLa

CD-4 Demodulator: JVC 4DD-5

Receiver: Realistic QTA-753

Speakers (x4, of course!): Wharfedale Glendale XP3

Any assistance would be greatly appreciated.

Mark.

 

[J. PUPSTER]

Hi, everyone.

I am a new arrival on QuadraphonicQuad. I apologise if this has been covered before, but I was wondering if anyone could help me.

I caught the quadraphonic bug some time ago, and after using the SQ system, I soon became very intrigued about CD-4. I recently invested in a new old stock JVC 4DD-5 demodulator (with low capacitance cables included), and an Audio-Technica AT20SLa cartridge, capable of tracking frequencies up to 50kHz.

However, there seems to be no separation between the front and rear channels, i.e. what should only be heard in one speaker can be heard in both front and rear. So essentially, it’s more like 2-channel stereo with four speakers than discrete 4-channel.

I’ve tried calibrating the demodulator with the adjustment record included in the box. As instructed in the manual, I lowered the front and raised the rear speaker volume, then turned the CD-4 Adjust screws at the back of the unit so that the sound from the rear was at its lowest. I also adjusted the 30kHz screw at the bottom until there was no distortion. The CD-4 radar light illuminates when playing Quadradisc records like it should, but the front and rear still mirror each other, sounding no different to 2-channel stereo.

Could anyone please advise me on what I might be doing wrong? Could I have connected something incorrectly, or bought the wrong equipment?

These are the connections I have made:
View attachment 62037
View attachment 62035View attachment 62036
I am using the following equipment:

Turntable: Audio-Technica AT-LP120XUSB

Cartridge: Audio-Technica AT20SLa

CD-4 Demodulator: JVC 4DD-5

Receiver: Realistic QTA-753

Speakers (x4, of course!): Wharfedale Glendale XP3

Any assistance would be greatly appreciated.

Mark.

Welcome to the forum Mark; someone that's more informed than myself will be along shortly I'm sure, to help you out on this (my CD-4 experience is fairly new also.)
Just out of curiosity though, what specific LPs are you playing through this setup?

 

[Doug G.]

Does that adjustment record have a channel identification track on nit? I have a JVC record but can't remember, off hand, if it does. If it does, what happens when you play that track? Your connections look correct and it sounds like your procedure is correct.

Doug

 

[RockaMarka88]

Does that adjustment record have a channel identification track on nit? I have a JVC record but can't remember, off hand, if it does. If it does, what happens when you play that track? Your connections look correct and it sounds like your procedure is correct.

Doug

Hi, Doug.

Yes, Band 3 on the record is the channel identification track. When the announcement says “Front Left, Channel 1”, it sounds in both front and rear left; the same with rear left, front right, and rear right. You hear the announcement in both front and rear at the same time, rather than individually, so it’s more like stereo than discrete quad.

This is the record I’m using...



Many thanks,

Mark

 

[RockaMarka88]

Welcome to the forum Mark; someone that's more informed than myself will be along shortly I'm sure, to help you out on this (my CD-4 experience is fairly new also.)
Just out of curiosity though, what specific LPs are you playing through this setup?

Hi, J. Pupster!

Thank you for the welcome!

I currently have three Quadradisc records -


Mark.

 

[Doug G.]

I guess if I had that problem, I would just try setting the separation controls to different positions and see what happens. If you can't get separation at any position, it points to a problem with the demodulator. Even though it's NOS, components can deteriorate with age.

When you adjust the separation using the record, can you hear a fairly good difference in volume of the back channels as you turn the controls? They will never go silent, of course, or perhaps not close to it but you should be able to hear quite a difference in the volume level of the tone as you turn them.

Another possibility is the receiver not actually being in discrete mode when switched to it. You could try feeding a signal into each of the four inputs and see if the receiver sends output to only the associated speaker. You can even just use a small screwdriver and stick it into the four input jacks, one at a time and see if the buzz just comes from one speaker at a time.

Actually, I guess I would check the receiver out first to ensure it's working in discrete mode.

Doug

 

[quadsearcher]

Expanding on Doug's idea above, I looked for info on the Realistic QTA-753 receiver, and it has an option for "bridging" like some Marantzs did. If bridged, it becomes two channels of more power, instead of the four channel amp it is natively. Might want to check if it is in bridged mode, if the test of each channel shows you can't get sound to each speaker separately.

 

[jaybird100]

I guess if I had that problem, I would just try setting the separation controls to different positions and see what happens. If you can't get separation at any position, it points to a problem with the demodulator. Even though it's NOS, components can deteriorate with age.

When you adjust the separation using the record, can you hear a fairly good difference in volume of the back channels as you turn the controls? They will never go silent, of course, or perhaps not close to it but you should be able to hear quite a difference in the volume level of the tone as you turn them.

Another possibility is the receiver not actually being in discrete mode when switched to it. You could try feeding a signal into each of the four inputs and see if the receiver sends output to only the associated speaker. You can even just use a small screwdriver and stick it into the four input jacks, one at a time and see if the buzz just comes from one speaker at a time.

Actually, I guess I would check the receiver out first to ensure it's working in discrete mode.

Doug

It couldn't hurt, also, to try a different adjustment record.

 

[Q-Eight]

Make sure the switch on the DD-5 is set to 4Ch Auto.
Failing that, you might have a bad demodulator. I had a Pioneer QD-240 that acted similar. Light was on but no demodulation.

 

[Doug G.]

It couldn't hurt, also, to try a different adjustment record.

Yes, in spite of original intentions, they aren't all the same. My favorite one is the Harman-Kardon LP, pressed by RCA, which agrees with real world conditions. When separation is set with it, it's right on with other CD-4 records.

 

[Doug G.]

Make sure the switch on the DD-5 is set to 4Ch Auto.
Failing that, you might have a bad demodulator. I had a Pioneer QD-240 that acted similar. Light was on but no demodulation.

He shows his set to "Discrete 4" The more I think about this, the more I say he needs to be sure the receiver is working correctly, first. Otherwise, he may be wasting time trying to get a demodulator to work that's already working. Make sure the receiver isn't in bridged mode as quadsearcher suggested.

EDIT: Bah! I don't know why thought I was looking at the front of the demodulator when it's the receiver. Sorry. But, see his post below.

Doug

 

[RockaMarka88]

Thank you for the suggestions, everyone.

I’ve taken all your advice on board, but unfortunately the problem still persists.

Expanding on Doug's idea above, I looked for info on the Realistic QTA-753 receiver, and it has an option for "bridging" like some Marantzs did. If bridged, it becomes two channels of more power, instead of the four channel amp it is natively. Might want to check if it is in bridged mode, if the test of each channel shows you can't get sound to each speaker separately.

My receiver does indeed have a bridging system, known as “Stereomax”. But I’ve checked, and it’s definitely in “Discrete 4”.

Make sure the switch on the DD-5 is set to 4Ch Auto.
Failing that, you might have a bad demodulator. I had a Pioneer QD-240 that acted similar. Light was on but no demodulation.

Yes, it’s set to 4CH Auto. But you’re right, it may be faulty, even if it’s NOS.

Yes, in spite of original intentions, they aren't all the same. My favorite one is the Harman-Kardon LP, pressed by RCA, which agrees with real world conditions. When separation is set with it, it's right on with other CD-4 records.

I’ll give that a go.

He shows his set to "Discrete 4" The more I think about this, the more I say he needs to be sure the receiver is working correctly, first. Otherwise, he may be wasting time trying to get a demodulator to work that's already working. Make sure the receiver isn't in bridged mode as quadsearcher suggested.

Doug

You have a point, Doug, there’s a chance it could in fact be the receiver. I might have a look inside.

Thanks everyone; I’ll continue to persevere!

 

[Doug G.]

The way to be sure is to take a small screwdriver and insert it into the input jacks, one at a time, with your finger on the blade. This is called hum injection. Sound (hum) should only come out of the associated speaker for each jack. Of course, if you have a signal generator, you can use that instead of the screwdriver/finger. Same objective, however. It will confirm each of the four channels is independent.

Doug

 

[gvl_guy]

Yes, it’s set to 4CH Auto. But you’re right, it may be faulty, even if it’s NOS.

I bought a NOS JVC 4DD-5. I was so excited! It even came with the JVC CD-4 cartridge! (Also brand new!) But when I plugged it in, it didn't turn on. I took it to a local repair guy. He recapped it and "put in some failed relays" but it's never been quite right. I get much better CD-4 from the internal demodulator on my recently purchased QRX-9001.

I guess the NOS wasn't a good deal after all.

 

[Doug G.]

Yes. Unfortunately, electronic equipment that's been stored/sitting around, for many years, deteriorates even if not used. Most pieces need to be restored the same as if they HAD been used.

My two demodulators, an SE-405 and SH-400 both work flawlessly and they had been used over the years.

Doug

 

[quadsearcher]

The way to be sure is to take a small screwdriver and insert it into the input jacks, one at a time, with your finger on the blade. This is called hum injection. Sound (hum) should only come out of the associated speaker for each jack. Of course, if you have a signal generator, you can use that instead of the screwdriver/finger. Same objective, however. It will confirm each of the four channels is independent.

Doug

Agreed. This should be the first step. Confirm the basics. Troubleshooting can lead to spinning one's wheels and not going anywhere, unless one starts with a known, proven functioning point in the system and work from there (and share the results of that test with anyone trying to help). If uncomfortable with a small screwdriver, simply have it playing through all speakers, unplug all four LF, RF, RL, RR and take one of the front RCA cables, put it in each of the four inputs separately, and see if each speaker plays independently. I don't think I've seen the results of that test reported in this thread.

 

[RockaMarka88]

Agreed. This should be the first step. Confirm the basics. Troubleshooting can lead to spinning one's wheels and not going anywhere, unless one starts with a known, proven functioning point in the system and work from there (and share the results of that test with anyone trying to help). If uncomfortable with a small screwdriver, simply have it playing through all speakers, unplug all four LF, RF, RL, RR and take one of the front RCA cables, put it in each of the four inputs separately, and see if each speaker plays independently. I don't think I've seen the results of that test reported in this thread.

The way to be sure is to take a small screwdriver and insert it into the input jacks, one at a time, with your finger on the blade. This is called hum injection. Sound (hum) should only come out of the associated speaker for each jack. Of course, if you have a signal generator, you can use that instead of the screwdriver/finger. Same objective, however. It will confirm each of the four channels is independent.

Doug

Thank you both, Doug and quadsearcher.

To be honest, I didn’t like the idea of placing a screwdriver in the back of a live unit, so I did what quadsearcher suggested...

I put a Quadradisc record on the turntable, disconnected the CD-4 RCA cables, then placed one of them in each of the four inputs. They all hummed, and played independently. I connected them all back, and I’m pleased to say that miraculously, it’s now all working, and I’m experiencing the glory of discrete quadraphonic sound! The Discrete 4 mode on my receiver is working fine.

Interestingly though, the CD-4 adjustment record still doesn’t separate front and rear; could it be that it was just a poor record?

Many thanks for all your help, everyone!

 

[furui_suterioo]

It might just be my imagination but my JVC demodulator seems to sound/perform slightly better after about 20 minutes of warming up, maybe a demodulator that has been sitting unused for decades is more prone to failure than one that was used regularly. Keep excersizing/warming up the demodulator and it might get better.

 

[RockaMarka88]

It might just be my imagination but my JVC demodulator seems to sound/perform slightly better after about 20 minutes of warming up, maybe a demodulator that has been sitting unused for decades is more prone to failure than one that was used regularly. Keep excersizing/warming up the demodulator and it might get better.

Yes, it probably just needed “breaking in” after sitting on the shelf unused for 45 years!

 

[Doug G.]

Or, as we said when we were kids, it just had to get used to working again.

I guess I should have explained there are no hazardous voltages at the input jacks. Heathkit had the hum injection procedure in all their amplifier manuals. It would be the same as if you touch the positive (inner) conductor at the other end of an RCA cable plugged into a jack.

Doug