JVC M-7050: restoration and revisions

[redk9258]

Super work!

 

[GD70]

Is there any chance you can post close-up photos of your Super-A assembly?

I’ll post some pics tomorrow. Sorry for the delay.

 

[Jcandy]

I’ll post some pics tomorrow. Sorry for the delay.

Oh, I'm sorry, I don't mean to pressure you! I really appreciate your help and feedback.

 

[GD70]

No worries!
Here's a few pics of the two variations of the Super A Bias Board. Top pic is the Japan Victor version, the other U.S. JVC version.
Cheers, Glenn

 

[Jcandy]

No worries!
Here's a few pics of the two variations of the Super A Bias Board. Top pic is the Japan Victor version, the other U.S. JVC version.

Thanks Glenn. It looks like:

- Japan version: from 07/1981 (on mini relay), thermistor attached to VC5022 chip
- US version: can't make out the date, thermistor not attached to anything (?!?)
- My version: from 03/1979, has thermistor attached to prehistoric Super-A block

I will post the schematic soon.

 

[Jcandy]

Work on this will be delayed since day job and loudpeaker project are taking priority now. However, I may be able to post schematic this weekend.

 

[Jcandy]

Here is the reverse-engineered schematic of the circuit. Note that Q7-Q9 is really a single dual-transistor 2SA995. Similarly, Q8-Q10 is a single 2SC2291.

 

[Jcandy]

Here is my first prototype of the new Super-A board. It uses BC547 and BC557 transistors to replace 2SC458 and 2SA1029 (as well as the original dual transistors 995/2291). The 5*4=20 possible pin-pin diode voltages match my JVC working version. New transistors have higher gain than the originals but function of the board should have low sensitivity to that.

 

[zenith2134]

Hi,
Fantastic work! These amps are special and deserve a proper servicing.

 

[Jcandy]

Hi,
Fantastic work! These amps are special and deserve a proper servicing.

Thanks! My intention is to create open-source versions of the TAD-198 (the original board with resistors and transistors circa 1979) and hopefully the TAD-200 (updated board for 1980+)

 

[ElectTech]

Thanks! My intention is to create open-source versions of the TAD-198 (the original board with resistors and transistors circa 1979) and hopefully the TAD-200 (updated board for 1980+)

Congratulations! This is nice and hard work! I compared the reverse engineering you did of the Super-A block (TAD-198), and it looks more like Version #1 of the VC5022 integrated circuit. (I have a JVC A-X1 made in 1980, and I really like how Super-A sounds.) Please inform if your first prototype of the new Super-A board met the specifications (voltage, calibration, etc.) in the JVC service manual of your equipment. Thanks.

 

[Jcandy]

Here's a revised schematic (with updated transistors), PCB layout, and pin-to-pin voltage drop (diode) numbers comparing original to prototype (V3). The numbers are voltage drops between test points with multimeter in diode mode (forward/reverse). Agreement is nearly perfect but the multimeter gives OL for 3V drops because this exceeds the capability of the multimeter. I had put this project hold but will resume soon.