In 1963 JVC VN900 amp, circuit designation STV-3, probably a triple diode for bias seting, any info on it?. Parts listing absent from the manual, just
68C on black body 4x7x10mm that looks like a slotted opto device but instead of square bottomed notch , semicircle curved, as though for locating on a rod, but mounted on pcb here.
just
instead
aJust found a side ref to it in the manual, a varistor, part number E03094-002 circuit symbol 3 triangles in a row, like 3 diodes without 2 of the cathode bars, not skew Z over a resistor, or back to back triangles Probably varistor diode but no useful data found yet , whether varistor or 3 diodes or varistor and 1 diode or whatever, or that symbol found elsewhere
I tried to email you some data on that diode but it bounced. You can email me at
mark snipped-for-privacy@labolgcbs.net
and reverse the domain to read "sbcglobal"
and I'll resend.
BTW I think the VN-900 was closer to 1970...
I own one, also a VN-700...
Mark Z.
diode
on
All received, thanks. Next task - to learn Japaneso
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I was taking 6303 on the Elno main caps as datecode , dark brown phenolic pcb board, wire-wrap type wiring and some strange transistor packagings as being consistent with 1963 but will burrow again
Not that 150A package but Imax 120mA, Vr max 50V, Ir max 10uA, and at 7mA, 1.65V forward V, -7mV per degree C
As one 1n4148 is about -1.6mV per deg C then 3 in series would be about right forward V but not enough thermal response at -4.8mV , is Ge more thermally responsive ? or some sort of NTC thermistor in there as well
No other datecodes found, so presumably ELNA QEY 6303 is a type number for
6800uF 63V electros. Light grey cement/epoxy? cased resistors are more 70s than 60s Some strange-to-me 2SC960 bolt-downable (but not here, like the varistors) packages , different to my 1977 databook listing, but all types in here are Si not 60s Ge
Some diodes intend for temperature compensation used actual transistor B-E junctions, to get a better match to the "real" transistors over temperature.
Isaac
diode
on
Presumably about 1973 made, from www cites, although I can find no indicator inside the amp. Can't find 6303 as an ELNA type number , so maybe old stock caps used. Those 2SC960 with great bolt down mounting flanges, that are not bolted, floating in mid air look mighty strange also , no package pic/type number found for them, somewhere between TO3 and TO37 but flanges twice as thick but TO5 size cap . Amp serial number 046017**
As a teenager in the '70's I used to like these models, (the VN-900 and VN-700) but now as a technician I find they are really kind of junky as regards the amp section and overall quality, compared to the Pioneer, Kenwood, Marantz etc of the time. Seems like they put a little too much of the manufacturing cost into the SEA tone section. Also, I find one can clean the controls until you are blue in the face, yet within a couple months they are acting up again. I seem to have much better luck on this with other brands. My VN-900 has low level noise problems I have so far been unable to resolve. Coupling caps, power supply, signal transistors, no luck. Had to set it aside for some future date...
Mark Z.
Mark,
We used to have problems with noisy resistors in Japanese amps of that vintage. In a pinch, for testing purposes, we'd stack 3 silicon diodes to sub for a STV-3. If the STV3s aren't available these days, perhaps mounting the diodes to a piece of aluminum and attaching it to the heatsink might suffice. Chuck
manual,
locating
number
thick
SEA
yet
better
resolve.
This VN900 originally had sun damage rack and pinion pivot switch (pink noise sw), metal actuating pin heated up relative to plastic and cracked the plastic. Also worn pot. They went to a High St repairer and a 2SD188 went C-E short on one channel on initial powerup and they gave up. And for good measure had coffee poured into it, in its very distant past, when the owner was a student.
Have ground off the pivot rivet and got the switch dissassembled, metal ground down a bit to give some slack and hopefully capillary superglued back as one. Will probably replace o/p pair with 2955/3055 before wasting anything more contemporaneous.
SEA
yet
better
resolve.
Placed switch pivot in a plastic bag and glue passed 1lb load on end of pin. Next the vol control, quad ganged pot with bass comp CTs, not many of them around no doubt. To disassemble requires grinding off the swaging on the end of the shaft, presumably drill + maybe tap for screw and pad/washer to refit. A grey small electro on the main board has leaked at some time
SEA (punningly in blue print on front) stands for Sound Effects Amplifier , from the user manual, me thinking Selective Equalisation Adjustment
SEA
yet
better
resolve.
All the nuts for the VN 900 ,TO3 were loose, not to point of dropping off , but not as tight as I would expect, so worth checking your one, compressed nylon insulator/spacers?. Sw rebuilt , quad pot graphited paste pushed in for the moment, won't know whether requires rebuilding until I can get some power on it.
so far not broken an age-hardened hookup wire. Easier to work on than a lot of 70s stuff, can remove sw and pots etc with room to manoeuvre, wired in place. And the edge-connectored multiple cards makes it a lot easier, presumably can swap between channels for diagnostic purposes if required.
resolve.
As far as intrusive noise. Anyone know why +74V DC rail to a preamp ? producing low current shot noise? Particularly the third transistor in,
250V rating 2SC1103 and associated Rs would be my guess at noise source. Assuming its not bleed over from the pink noise generator
Definitely not bleed over, the noise varies too much in nature and intensity. I suspect the internal wiring to the back panel could be an issue - they used individual (cheap) shielded phono cable and if you look you'll see the grounds don't tie together at the rear panel (not all of them anyway). I am also suspicious of an oily residue on the tone board. Perhaps someone tried to clean the controls with WD40 or the like and made a mess of it. It's definitely a preamp issue, as the main amps work fine when divorced from the preamp section. I noticed the higher than usual voltages. Not being a old radio guy, the idea of "noisy resistors" still bothers me a bit (even though I acknowledge it is a known phenomenon), although if it's due to their resistance shifting, I can definitely see them causing noise in a given circuit.
Mark Z.
them
acknowledge
I put some manky oily residue (not all over) down to a leaky nearby cap , replaced , but tested ok. Perhaps a breakdown product of that dark phenolic 60s type pcb, like "dolls disease" in soft plastics. I'm wondering if the SEA board , populated with
70s Si Tr and Rs &cs ,along with the main caps, is a hang over from earlier 60s model.
The noisy resistor problem in the preamps of Kenwood receivers of the early 1970s was fairly common (If I remember correctly, they were emitter resistors). The company I worked for didn't carry JVC receivers so I don't know if they had this problem. Chuck
With bipolar transistors, the way to get low noise is to run low emitter current. Low emitter current means high Rbe (intrinsic base resistance). High Rbe means low gain (for gain determination, Rbe in in series with the external emitter resistance). In that environment, one way to recover some stage gain is to run a high collector resistor, and that calls for a high rail voltage.
Isaac
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