EVX-4 Decoder Repair

[anthonywc]

I recently got hold of a EVX-4 decoder. The left channel was bad and it appears to be a simple fix of replacing a capacitor. I noticed a chip on the inside and was curious if they even made this chip anymore. I can still find SQ MC1312 chips but cannot seem to find this one anywhere. Was this something special, or just a simple chip that's called something else now?

 

[Soundfield]

Has anyone got the circuit schematic of the EV-44X? I've never actually been able to find it.

 

[quadsearcher]

I recently got hold of a EVX-4 decoder. The left channel was bad and it appears to be a simple fix of replacing a capacitor. I noticed a chip on the inside and was curious if they even made this chip anymore. I can still find SQ MC1312 chips but cannot seem to find this one anywhere. Was this something special, or just a simple chip that's called something else now?
View attachment 65432

Do you have a schematic for that EVX-4? That might explain the function of that chip. The numbers don't come up as anything that makes sense. There is a 14 pin FM detector IC, but that can't be it .
It sure looks to be house-numbered with the EV logo on it, but I doubt they made their own chips or even had them made for a project like that. Maybe rebadged or selected.

 

[Paul Pettegrove]

I’ve got an EVX-44... and eBay NOS, open box, still wrapped, comes with semi-useless hookup instructions, no schematics... I haven’t been able to locate those, as others have pointed out. IC’s are un-obtainium... I have a dead EVX-4, but I have enough projects... I will say that the EVX-44 is one of the most pleasant surprises of the year... for those looking to locate chips, I’ll take a look inside the EVX-4. Otherwise, a quick eBay check yielding a Radio Shack equivalent... Realistic ARS-4 Quad Decoder With Record Stereo-4 Quadraphonic Electro-Voice | eBay

 

[par4ken]

The EV-4 decoder chip didn't do very much, it would of contained some op-amps and perhaps some blend resistors. With a dead EV-4 you could easily build a replacement circuit or better yet use the enclosure to house an Involve evaluation module. I know there is a magazine article or two about the EV-4 decoder and a construction article or two that illustrates the use of their special chip. Likewise there could be one about the EV-44 but if so I don't recall seeing it. I'll try to locate the EV-4 article.

 

[par4ken]

I've found a construction article for a homebuilt version of the EV-44 "Build a Four-Channel Universal Decoder". The special chip is similar to the SQ chip MC1312, but would have slightly different blend coefficients compared to the SQ decoder.

 

[Paul Pettegrove]

Just opened the dead EVX-4.

 

[par4ken]

I'm sure you problem are those old electrolytic coupling capacitors. Look at how the ends are swollen. A very cheap fix, use film types for better sound!

 

[par4ken]

Here is a service manual of a decoder/amplifier.

 

[Soundfield]

So the EV1244x and the EVX-4 used the 14pin 87403 and the POP-1972-12 / EVX-44 version used the 16 pin 87743. All rather confusing, especially as there seems to be no trace of either of those chips!

 

[par4ken]

Special made for ElectoVoice, I would guess.

 

[par4ken]

Here is link to the Heathkit Stereo-4 Decoder manual, the manual itself was too large for me to post. https://www.vintage-radio.info/download.php?id=818

 

[Paul Pettegrove]

Thank you. Hadn’t planned on opening it, but I’ve got the AA-2010 wide open and the EVX thing is on my mind... can say enough about the “liveability” of the EVX-44 unit itself... while Heathkit used the same chip, its circuit has no adjustment, other than full volume & EQ for Front & Rear Pre/Main, the decoder tucked inside, position are “on or off”.

Some searching earlier I did find some historical context:

In 1973, Electro Voice signed an agreement with Columbia, and produced the EVX-44 Universal Decoder. The idea here was to make a decoder that can play the EV, QS, and SQ records mixed on a record changer without having to change decoders. They later suggested the same coefficients as an encoder, but no records were ever produced.

The decoder has a front-oriented automatic variable blend that reduces back separation when center front material is present. It can also be set to be blended all the time (lower room diagram), or never blended (upper room diagram). The lb and rb dots farthest apart on the sphere are not blended, the ones closest together are blended. The Metrotec decoder is the EV universal decoder without the blend in the "synthesized quad" position.

https://midimagic.sgc-hosting.com/quadrafon.htm

 

[bob.stek]

From my archives . . .

 

[Doug G.]

The Heathkit universal decoder uses the SN16852 IC from Texas Instruments.

Doug

 

[kfbkfb]

I built myself a matrix decoder in the Spring of 1983 (using off the shelf opamps and wire wrap for the circuitry), it has 2 decode modes (modeled after the Pioneer Quadralizer):
LF=L, RF=R, LB=L-R, RB=R-L and LF=L+0.5R, RF=R+0.5L, LB=L-05R, RB=R-0.5L

I used it sporadically until 2000 when I got a Pioneer VSX-D209

Stereo-4 decoders are simple compared to QS, SQ and EV Universal decoders, just some addition and subtraction circuitry, one could be built the same way (opamps + wire wrap).


Kirk Bayne