Greetings all- I am repairing an old JVC L-A55 turntable, and I am stuck with a bad problem with the PSU. I have replaced the entire motor assembly & PCB, and I still have the same problem I had with the original motor assembly. So, my problem is that the motor turns intermittently, like it surges, then turns off, surges, then off, etc. The surge happens in a pattern, like every 2/3 of a second, at all times.
I have replaced all of the components in the PSU, however, some of the old transistors are, of course, unavailable (and I haven't spent the extra money to buy NOS components on ebay).
There is a good service manual on vinylengine (you need an account to view) - JVC L-A55 Direct-Drive Auto Return Turntable Manual | Vinyl Engine
I've uploaded the same service pdf to my website: http://aveastga.com/JVC_LA55_Service.pdf
the PSU is on a small daughterboard on pg. 15, the power transistor X601 is on it's own pcb, just above the PSU on the pdf.
X601 (2SD325), X602 & X603 (both 2SC945) are the transistors which are difficult to find, as is a zener diode called RD5.6EB3.
I am not an repair tech or anything, just a hobbyist, so I can't diagnose the problem any further. I replaced X601 (2SD325) with a BD239C, and X602 & X603 (both 2SC945) with BC546B B1G.
I couldn't really find a perfect match for the bridge rectifier, so I put a basic one in place, but I have all of the original components for re-installation.
Anyways, it still doesn't work, and I think I keep frying the zener diode.
I'd like to simply replace the PSU with a 24vdc wall wart, then regulate it down to 23v, which is the rail. I'm just curious if the motor is sending feedback to the transistor pair on the PSU daughterboard for speed regulation.
Thanks for reading, I'd really love to repair this in any way possible, and I'd really appreciate anyone's help!
I have replaced all of the components in the PSU, however, some of the old transistors are, of course, unavailable (and I haven't spent the extra money to buy NOS components on ebay).
There is a good service manual on vinylengine (you need an account to view) - JVC L-A55 Direct-Drive Auto Return Turntable Manual | Vinyl Engine
I've uploaded the same service pdf to my website: http://aveastga.com/JVC_LA55_Service.pdf
the PSU is on a small daughterboard on pg. 15, the power transistor X601 is on it's own pcb, just above the PSU on the pdf.
X601 (2SD325), X602 & X603 (both 2SC945) are the transistors which are difficult to find, as is a zener diode called RD5.6EB3.
I am not an repair tech or anything, just a hobbyist, so I can't diagnose the problem any further. I replaced X601 (2SD325) with a BD239C, and X602 & X603 (both 2SC945) with BC546B B1G.
I couldn't really find a perfect match for the bridge rectifier, so I put a basic one in place, but I have all of the original components for re-installation.
Anyways, it still doesn't work, and I think I keep frying the zener diode.
I'd like to simply replace the PSU with a 24vdc wall wart, then regulate it down to 23v, which is the rail. I'm just curious if the motor is sending feedback to the transistor pair on the PSU daughterboard for speed regulation.
Thanks for reading, I'd really love to repair this in any way possible, and I'd really appreciate anyone's help!
why ? did you measure the voltage across it ? if it is roughly 5.8 as indicated in the schem it is O.K.
how do you operate it now, with the original PT or with your wall wart ?
if the problem is only when you use the ww then it could be not powerful enough to provide the surge current when the motor starts up
no, it doesn't, motor speed is controlled by frequency, not voltage.
X601, 602, 603 form just a standard stabilized voltage regulator
Also, measure & check DC voltages given in the schem, start with the 23V on emitter of X601 which is the output from the regulator, it should not twitch as it should be stabilised.
Same with 15V output IC2
Did you replace C601 1000uF/50V ? and if so, is it properly soldered ? If it has lost capacitance or being replaced by lower microfarad value it cannot provide surge currents nor proper filtering.
There are a couple of DC voltages indicated on the schem, check them, too ...
