Tim7099
Member
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)!
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)!