JVC 4DD-5 lives again.

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Tim7099

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I was experiencing front - rear separation problems with my JVC 4DD-5 CD-4 decoder in fact there was no separation at all, adjustment of the left/right separation controls simply acted like a volume control, rather than varying the levels of front and rear individually. I also have a Pioneer unit but I find the sound from that unit harsh and unforgiving, probably in need of some attention, so I decided to have a look for problems with the 4DD-5, not that I am any great shakes at diagnosis.

Referring to the schematic and feeding what I thought to be an appropriate signal into the phono input I started to check voltages and waveforms and it was clear that there was certainly something amiss but the only oddity that I could find were some incorrect voltages on one of the transistors (X101, 2SC458 LGC). The transistor checked out perfectly OK on my component tester as did several other components in the same general area and, in spite of the usual contention that electrolytic capacitors can be blamed for most things, all of the ones that I removed for checking actually tested as well or better than replacements that I have. All rather strange. After many hours of fiddling and much cursing, out of desperation, I decided to try changing the errant transistor. I had some 2SC458Cs from a previous repair and I think that the only difference between the 2 types is a slightly better noise figure for the LGC but I'm no expert, quite the opposite in fact. I usually cause more damage, that I then have to fix as well, with my fiddling! However, in spite of my usual lack of success I'm pleased to report the problem seems to have gone away, at least for the moment, so whatever the issue was, the LGC seems to have been at the root of the problem. This unit incidentally was one of the NOS decoders offered for sale on Ebay some time ago and meant for the US market. The internal voltage adjustment for 220/240volt had been disabled but it wasn't too difficult to sort that issue out. I don't know if there is a mains wiring difference between decoders meant for all markets and those for the US market or if the selector plug is different according to market. No doubt many have been caught out with non-working units and wondered why. There was no mention of the voltage issue on the original Ebay listing either.

For CD-4 records I use my Sansui SR222 Mk1 fitted with an AT14Sa and either it's part worn original stylus or a Tonar pattern Shibata that I took a chance on from a supplier in Denmark. The sound from the 4DD-5 seems superior in every respect to that of the Pioneer. Records which are virtually unplayable due to dreadful distortion on that unit decode correctly with few signs of signal break up on the JVC. Job done (I hope)!
 

furui_suterioo

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It seems like every NOS/unused JVC demodulator that I've heard about is in need of repair, probably better to find one that has been actually used. My 4DD-10 still works for cd-4 records but will not accept regular stereo records without going into 4ch mode on any sound higher than 6khz, so it definitely needs something and there is no known schematic. Also have a Technics SH-3480 which has quiet-ish left channel, no schematic for that one either. I'd like to modify then and add line inputs so I can play raw CD-4 rips from a computer, maybe one day.
 

Doug G.

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Usually, units for the US market don't have a switch for line voltage. They are just wired for 110-120 volts.

The way it's usually done to allow for multiple voltage selection is, the transformer has two primary windings and for 110 volts, these windings are switched so they are in parallel and switched so they are in series for 220 volts. The rest of the circuitry is the same regardless of mains supply.

Doug
 

Tim7099

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It seems like every NOS/unused JVC demodulator that I've heard about is in need of repair, probably better to find one that has been actually used. My 4DD-10 still works for cd-4 records but will not accept regular stereo records without going into 4ch mode on any sound higher than 6khz, so it definitely needs something and there is no known schematic. Also have a Technics SH-3480 which has quiet-ish left channel, no schematic for that one either. I'd like to modify then and add line inputs so I can play raw CD-4 rips from a computer, maybe one day.
I suppose it's lucky that the 4DD-5 actually has a service manual available although it doesn't contain any kind of set up instructions. I presume that another publication was issued which has never made it into general circulation. I imagine specialised equipment would be required and the knowledge to use it also which leaves me out :) Like most obsolete items most of the paperwork eventually ends up in the rubbish. In respect of your own equipment, pending any service manuals surfacing, I guess all you can do is replace dodgy components in the hope that you come across those causing the issues, although a daunting prospect I'm sure.
 

Tim7099

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Usually, units for the US market don't have a switch for line voltage. They are just wired for 110-120 volts.

The way it's usually done to allow for multiple voltage selection is, the transformer has two primary windings and for 110 volts, these windings are switched so they are in parallel and switched so they are in series for 220 volts. The rest of the circuitry is the same regardless of mains supply.

Doug
This unit does have the selector, 110/120/220/240 volts and the selector changes between the 110 & 120 volt windings without issue but plugging into either of the others just results in an open circuit. I don't imagine it would be too onerous a task to re-wire it if a diagram were available but a link of fuse wire across a couple of the plug pins did the same thing enabling me to ditch my step down transformer.
 

kfbkfb

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I used stereo headphones to set the CD-4 front/back separation (I never had a CD-4 adjustment disc).

I connected L to LF and R to LB and listened to several CD-4 discs while adjusting the separation control (same for RF and RB).

Although it may not result in the maximum possible front/back separation, my method did provide a good, discrete sounding result (actually, only 12dB of channel separation is needed [research by Involve Audio]).


Kirk Bayne
 

Doug G.

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It's strange the service manual for the JVC doesn't include setup (calibration) instructions. Not much of a service manual without them.
Is this an original manual or copied? If copied, the instructions could have been deleted or left out at sometime. I have some copies of other manuals with missing parts. My SH-400 service manual has complete calibration instructions in it but I don't believe my SE-405 manual does. It's been quite a while since I looked at it. Of course, at my age, ten minutes is "quite a while". :D

Doug
 

Tim7099

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4DD5 also came with a 7" JVC CD-4 set-up disc:

Yes, the unit came with the record and the manuals, I did read somewhere that there is a problem with some versions of the supplied set up disc, I don't know what though.
 

Tim7099

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I used stereo headphones to set the CD-4 front/back separation (I never had a CD-4 adjustment disc).

I connected L to LF and R to LB and listened to several CD-4 discs while adjusting the separation control (same for RF and RB).

Although it may not result in the maximum possible front/back separation, my method did provide a good, discrete sounding result (actually, only 12dB of channel separation is needed [research by Involve Audio]).


Kirk Bayne
I do have the 7" set up record and also a Sylvania LP that has similar level setting bands on side one. I must admit setting the separation by trying to adjust the volume in the appropriate rear speaker to a minimum seems problematic (heavy distortion on some records) and I have reverted to either setting by ear or watching the VU meters on my Sony SQD2020 for a low enough reading, as it happens the two adjustment pots are more or less in the centre of travel anyway and all seems well. I am pretty new to CD-4 and I understand that there is always some sort of compromise between separation and distortion so I suppose I should not expect perfection
 

Tim7099

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It's strange the service manual for the JVC doesn't include setup (calibration) instructions. Not much of a service manual without them.
Is this an original manual or copied? If copied, the instructions could have been deleted or left out at sometime. I have some copies of other manuals with missing parts. My SH-400 service manual has complete calibration instructions in it but I don't believe my SE-405 manual does. It's been quite a while since I looked at it. Of course, at my age, ten minutes is "quite a while". :D

Doug
Doug, it's the one that can be found on Vinyl Engine amongst other places, Service manual No. 2289, 15 pages and a schematic diagram. There are similar instructions to those contained in the user manual in respect of connections and setting the separation and a description of how the thing works, full parts breakdown etc but information on actually setting any of the four pairs of adjustment pots scattered around the board none at all. It is possible that whoever supplied the manual, and I think it available elsewhere on line, may not have bothered to copy calibration instructions. I assume you would require special equipment to do so in any case and sometimes even the most simple things tend to be way beyond my limited skill set. I just used my signal generator to inject a 30khz signal with 1.3khz added from channel 2 into the phono sockets and went from there, my logic was probably flawed but it all seemed to work out OK in the end. As it happens I also decided to change the other transistor of the pair (X102) just to keep things balanced. All jolly good fun!
 

Doug G.

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The -202 JVC record is the one with the supposed problem in that instead of separate separation tracks for left and right channels, you just use the same track for both. The -205 record has the separate tracks.

I actually like the 33 1/3 RPM setup records better. The 45 RPM discs all seem to have some warpage and it affects the tracking. My favorite is the Harman-Kardon record.

If everything is done by the CD-4 rules, there's no compromise with distortion vs. separation, although there will always be a diminished chance for distortion with less separation and yes, using a meter is better than trying to just listen. My SH-400 makes it easy.

It could be that JVC decided not to include calibration procedures in service manuals meant for the field as they may have not wanted just anybody to get in there and mess things up. The procedures aren't that complex and you basically just need a signal generator, O'scope, and voltmeter but some of the adjustments take a little finesse with getting the right waveform shapes on the scope.

Doug
 

kfbkfb

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^^^
An article from 1973

Setting the carrier level by ear is also possible - play several CD-4 discs (the songs closest to the label, the carrier will be a its lowest level there) and set the carrier level so that the radar light is on steady, then increase it slightly.


Kirk Bayne
 

Tim7099

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^^^
An article from 1973

Setting the carrier level by ear is also possible - play several CD-4 discs (the songs closest to the label, the carrier will be a its lowest level there) and set the carrier level so that the radar light is on steady, then increase it slightly.


Kirk Bayne
Kirk, yes an interesting article, sets out the basics pretty well. In respect of the Radar light this is of course the final band on the JVC test record which, as a 7" disc, is probably a good test of set up. Most of the few CD-4 records that I have seem to have a very long run out, probably for the reason that you mention.
 

Tim7099

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The -202 JVC record is the one with the supposed problem in that instead of separate separation tracks for left and right channels, you just use the same track for both. The -205 record has the separate tracks.

I actually like the 33 1/3 RPM setup records better. The 45 RPM discs all seem to have some warpage and it affects the tracking. My favorite is the Harman-Kardon record.

If everything is done by the CD-4 rules, there's no compromise with distortion vs. separation, although there will always be a diminished chance for distortion with less separation and yes, using a meter is better than trying to just listen. My SH-400 makes it easy.

It could be that JVC decided not to include calibration procedures in service manuals meant for the field as they may have not wanted just anybody to get in there and mess things up. The procedures aren't that complex and you basically just need a signal generator, O'scope, and voltmeter but some of the adjustments take a little finesse with getting the right waveform shapes on the scope.

Doug
Doug, mine is the 202 record but it does actually have two separate tracks, however separation setting seems easier for the left channels than the right, something I have put down to oddities within the 4DD-5. You would have thought a company like JVC would have been able to get things like that right in the first place and, Incidentally, it is a bit warped too.
 

Doug G.

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OK, I was going by memory (I don't have a -202) and it may be that the track is actually only good for setting the left channel separation and the one meant for the right channel is not correct.

Actually, almost any CD-4 track with output meant only for the left or right front can be used to tell if there's anything coming out of the respective back channel. I do that all the time if I change cartridges instead of going through the formal setup. You can also use records where you know where particular parts are supposed to be coming from a particular channel, like Robby Krieger's solo in The Doors "Moonlight Drive" out of the left back or the crunchy guitar parts out of the right back in Carly's "You're So Vain".

Doug
 

sjcorne

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...or the crunchy guitar parts out of the right back in Carly's "You're So Vain".

"You're So Vain" is particularly handy since the discrete tape versions have no vocal signal in the rear whatsoever, not even a bit of reverb. Just solo the rears and tweak your separation controls until the voice almost disappears. The Doobie Brothers' "China Grove" can work pretty well too, since the right rear channel contains only tambourine strikes for almost the entire duration of the song.
 

Tim7099

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"You're So Vain" is particularly handy since the discrete tape versions have no vocal signal in the rear whatsoever, not even a bit of reverb. Just solo the rears and tweak your separation controls until the voice almost disappears. The Doobie Brothers' "China Grove" can work pretty well too, since the right rear channel contains only tambourine strikes for almost the entire duration of the song.
Doug I may just try the China Grove trick, thanks for the information!
 
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