Translate this page:

Hi - I have a pair of KRK VXT4 studio monitor speakers. Very expensive! When I stored them, they were working fine. When I tried to use a few days ago, one of them simply hums loudly at 50Hz. I have tried replacing the two 12000uF PSU capacitors, as I found on Google that sometimes they can become depolarised but this has not solved the problem. I am trying to find a schematic for both the main board (which has the PSU and the PA stage) and the pre-amp board. - Thanks, Kevin

This one had been sitting in a barn for years. Powered it up and the large power transistors on the heat sink would get hot
after a time....and the heat sink would also get quite hot. Speakers sounded like they had some DC on them....with a loud thump

Found the manual on Hi-Fi engine,
[ http://www.mediafire.com/file/b8mlsu7q3pz0bvf/hfe_nad_3140_service.pdf/file ]
and saw a video on youtube saying lots of capacitors dry out
due to their locations near resistors that get very hot, I guess sorta a poor
design. Went in and recapped almost the entire
product save for the (4) large 50V PS caps which appear to check ok.None of
the high power or driver transistors
mounted to the main heat sink were shorted..

Powered on but no audio, and noticed the fuses to Bridge D805 were melted open.
I believe they were ok before.
Replaced them and they started to light up like fiilaments, so shut down,
removed the bridge D805, then checked it, its ok.

There is about a 16 ohm short on the + / - lands where the bridge was. It
seems like something I did maybe is causing this,
like a capacitor in backwards, which I am doing alot of lately for some
reason.

Turns out, yep I put C805 (470/ 63) In backwards ! Capacitor still checks
ok, so will re-use it.

Short is still there though Im guessing R803 is adding in 5.6 ohms or so,
that means theres a 10 ohm short someplace. Q801, 802 arent reading short,
and looks like this becomes a +43V ps feed.

Any ideas out there on what to check next? I am guessing this cap in
backwards caused the voltage applied to a component to be
reverse and its shorted. Or would this be normal with a reversed cap installed,
and putting it in right would be how to fix it?

Steve Hearns
Technotronic Dimensions [USA]
WWW.TECHNOTRONIC-DIMENSIONS.COM
 

Sziasztok! Egy TEAC X-20R tipusú szalagos magnónak az a hibája, hogy végtelenített üzemmódban nem vált előre játszásra, ha a fémcsík eléri az érintkezőt. A normál lejátszáskor mindkét irányban játszik, és kezelhető. Egy rajz is segíthetne talán, mert amit a fórumon találtam (X-2000R) az nem ugyanaz sajnos. Köszönettel Attila

Sziasztok! Van nekem egy CD lejátszós csoda. Hiba leirás: ACC táp rendben megy, B+ csont zárlatos, (6 A vesz fel ennyit bir leadni a táp) (pozíció számokat pótlom később)de volt egy 10 ohmos állat a stand by áramkörben, ami megnyúlt, cseréltem. TDA Végfok IC zárlatra gyanakodtam kiszedtem,a zárlat nem szűnt meg továbbra is fent áll, 78L15 stab IC meg mértem működik (B+ áramkör), 78L05 stab cserélve (Stand by áramkör), hibás volt, kapcsoló tranzisztorok mérve jónak tünnek, Nem tudom most merre tovább... Javított már valaki ilyen magnót? Állítólag hallgatták és egyszer csak el hallgatott, majd egy másik autóba valaki beakarta kötni és testre zárta a magnó bekötésekor az áramkört. Ha van valakinek erről kapcsolási rajza nagyon hálás lennék érte! Ha valaki javított már ilyesmit kérem segítsen nekem! Thx! Üdv Stewe!