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Help! Trying to repair an RCF312a woofer is off

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  • Help! Trying to repair an RCF312a woofer is off

    Hi all i bought this rcf 312a mk1 think! And would love to try to repair it, it did have lim protect light on and the fuse marked for +high voltage connected to mos3 had blown.

    I opened it up removed the pair of sap16n p's and two irf940 9140s all tested ok with meter diode check,
    So I refitted put a new fuse in and also the speaker and tweeter wires to amp board i cut back and resoldered in as they where very weak flimsy about to snap feeling ! possibly stressed with vibration there good and solid now, so i,turned amp on with light bulb limiter and limit protect has gone out.

    The main woofer 12" speaker itself is ok, but has no output!
    The tweeter horn works has output but sounds a bit quiet, i was using a light bulb limiter but still didn't want to push volume up more than just over half way waiting on a dummy load i ve ordered,

    Also i noticed the idle current 8mv test point checked it and it's off the scale on meter reading around 220mv. I didn't alter the idle current pot in case of a fault elsewhere and there was very little dc on the speaker outputs. 3.5mv woofer terminals
    1.2mv on tweeter

    I checked the supply rails but I'm not sure what they should be and checked bridge rectifiers out of circuit tested ok.

    -HV AND +HV 164V / to gnd -82hv +82hv
    -LV AND +LV 75.2 / to gnd -38LV +38LV

    12v rail measures +c68 +11.47v
    -c69 -11.56v
    Also measured 11.8v on pin 5 of the first op amp 4560 from pin2 xls input
    P.s one half the underneath of the pcb has no component markings I'll post schematic I have but again not sure which version I have did a lot of googling and looks like the mk1 the power supply section looks same mine component wise

  • #2
    Hi,
    Looks as if you have other things happening.
    Don't forget to change out C52 I just use a hole thru component 10uF 16V 105 degrees C.This component fails with age.....and is well documented on this forum.
    John

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    • #3
      Click image for larger version

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ID:	856261Hi,
      Diagram showing the position of C52

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      • #4
        Thanks for reply john I check that cap,it seems fine but I'll replace it while I'm on would you know if my rail voltages are correct I posted my readings they seem stable and similar values - and +v's not sure where to go next I'm thinking a signal traces from input but more worried about the main amp being down it's way out and magic smoke is it called ! If is was a guitar amp I'd put a signal into fx return bypass preamp straight to power amp to check if fault is in preamp only but like you say that idle is 220mv should about 8mv any help would be great I can take readings etc thanks again

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        • #5
          Hi,
          Sorry I haven't got any voltages, and the job has gone back to customer. The only fault I have had with this series is C52, and the faulty transistors on the filter board.
          PS your second drawing on your other post is blank.
          If the rail voltages are close to each other i.e the + &- voltages are similar then maybe the PSUs are ok.
          Just one thing, is the Protect light on all the time.....if it is you will get nothing out....................
          John

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          • #6
            Hi the protect light is not on the tweeter or horn works but not the main 12 inch speaker I can't post schematics off my tablet doesn't seem to be an option I'll put them up,tomorrow though from my MacBook

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            • #7
              If you are still on the bulb limiter you can't expect the numbers that are called out on the schematic. Was the protect bulb ever lit while on the bulb limiter?
              Do you have the speakers disconnected?
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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              • #8
                Hi I'll double check and repost voltages as i ran amp with limiter both on and off and may have tested voltages with it on, sorry!

                When The protect light was connected it lit a few seconds quiet bright protect light, then very dim if on at all till I started to push the amp a little I put a 1khz sine wave in 1/4 jack input back of amp from my phone sounded clear on tweeter but nothing on 12inch woofer though it didn't seem very loud i didn't want to max out amp volume was about 2/3rds volume then bulb was really bright, similar to when I test bulb on a good working behringer monitor a while back,

                when I measure for voltage on speaker terminals do I disconnect terminals from the speaker first! If so is volume turned all the way down? Thanks for reply.

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                • #9
                  here's schematics I retested my voltages were correct still same phew!.
                  Attached Files
                  Last edited by basher106; 12-07-2019, 06:13 PM.

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                  • #10
                    Hi John I'm missing the surface mount layout as in this pic so have no component numbers on smc side of board, is this complete for the rcf312a ? If you have any more info could you possibly post or send me it, would be a great help thanks.

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                    • #11
                      I've built an audio signal tracer and can follow a healthy signal from pin2 hot at xlr input jack all the way to pin 7 u15b 4560 and lose signal at one side of r135 also can anyone tell me expected voltage at the base of q13 and q14? I'm getting 0.2v thought it would be nearer 12v

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