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Dynacord PowerMate 600 Powered Mixer - Protect Light On

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    Dynacord PowerMate 600 Powered Mixer - Protect Light On

    Hi guys
    I'm trying to repair this powered mixer. Sorry this is a bit long but I want to give as much info as possible to help get an answer,


    As soon as it powers on the protect light comes on. There are relays that connect the speakers to each channel of the amplifier but they never click. If I connect a 60W light bulb in series with the mains supply it comes in bright then goes to maybe 30-50% brightness - it seems to dim a little further over time

    I have power to the amplifier, I think it was as about plus/minus 50-60V for the main amp, and plus/minus 15V to the mixer. There are some other voltages marked on the silk screen near the ribbon cable that connects the main amplifier board to the mixer (5V, 15V etc) and they also check OK.

    Just to mention the +/-60V supply reads about +/- 30V with the light bulb limiter in circuit so it looks like this amp is drawing about 30W in quiescent state, either with or without the preamp board connected.

    All the output devices check OK in circuit, the emitter and base resistors check OK. I have the amplifier PCB on the bench, mixer ribbon disconnected to make it easier to work on.

    I've traced the circuit back to the driver or pre-driver stage and the DC voltages read the same on both channels (this is the four transistors bolted to the PCB either side of the large electrolytic capacitors).

    Measuring the NPN output transistors on both channels (base-emitter) I have about 0.65V and the PNP output transistors (base-emitter) ones I have about 0.57V. This is the same for both channels - also if I measure from ground to the junction of the emitter resistors on both channels I get about 0.06V so it looks like the output transistors are biased correctly and I don't have one or both channels presenting DC voltage to the output. By the way I don't have speakers connected at the moment.

    So to me all that looks OK but when I was putting the amp PCB back in the cabinet I noticed something is getting warm on the rear edge of the PCB behind the heat sinks.

    So I used my thermal camera and I can clearly see that one transistor (Q140) is getting warm/hot. This transistor is type BF393 and checked OK in circuit so I removed it and confirmed it is OK on my DCA55.

    I then looked for a schematic and found one (attached) but it does not match my PCB as there are no transistors Q140, Q141, Q142 etc on the schematic.

    So I'm a bit stuck now. There is a whole row of these transistors behind the heat sink, 16 of them in fact. Two sets are in blocks of six and have 2x BF393 plus two pairs of MPSA42 and MPSA92 (complimentary NPN/PNP) then there is a block of four more in the middle of the row.

    As none of these transistors behind the heat sink seem to exist on the schematic, and I have to confess I have no idea what they are supposed to be doing, I don't know how to proceed now. Quite clearly something is wrong if one is getting hot and I do suspect this is causing the Protect error but that is just a guess

    Can someone help me understand and repair this?

    Cheers
    Rich
    Attached Files
    Last edited by dicky96; 05-22-2022, 05:07 AM. Reason: more info added
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    #2
    Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

    I found a faulty MPSA92 transistor nearby that reads a low resistance C --> B
    probably that was the cause of the problem but I need to order some
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      #3
      Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

      hi check all the small diodes around mpsa trans. and resistors.disconect the amp 60v+- cables from the transformer and left connected the +-15v and see if start the relays.

      Comment


        #4
        Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

        Hi ... I've repaired a few of the larger version of these. They share the same amp design as Electrovoice Q44 & 66.

        Virtually all the one's I've looked at are stuck in protect ...

        Q140 etc. ARE on the schematic you provided but it's a busy diagram ... If you follow the centre line on Left channel to the L100 output inductor, then look down you will see Q140 just before the -VCC rail. If you look above L100 you will see Q142, 143 and friends ...

        I really like these amps but they are easily triggered into protect.

        Main problems I've found have been the MPSA92 (Q141,142 etc.) on either channel triggering protect on the time in line with intermittent performance and sometimes intermittent base resistors (one tested fine in and out of circuit) but when soldered back in and wiggled with chop stick would change resistance ... so now I just change them and the transistors in the SOA.

        Also I've found the I104,I304 (TL074 quad op amps top and bottom left of drawing) to give limiter and intermittent protect problems so now to save time if these amps are in protect I just change these immediately to save time.

        And the last common point of weakness is the Quiescent current adjusting trim pots, these are awful in most cases and I change them for better quality 27turn pots.

        That's all I can think of at the moment (it's been a while since I've seen one).

        You can check the protect and fan circuit (bottom right of drawing) there is a comparator type circuit ... test voltages behind the signal diodes to see what circuit is triggering protect.

        Then if it's "time in" ... you can lift D116 or D316 etc. to confirm which side is causing it and trouble shoot from there ...
        Last edited by check12; 05-29-2022, 09:09 AM.

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          #5
          Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

          Here's a block diagram for the circuit ...
          Attached Files

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            #6
            Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

            Thanks for the info check12

            I did eventually fix it (without schematics) - the problem was not quite what you suggest, but it was rather interesting

            I made a video of the repair, it's rather long but it covers a lot of ground

            https://www.youtube.com/watch?v=1NTNfTgSzgw

            cheers
            Rich
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              #7
              Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

              Yeah that's just some faults I've noticed consistently with the larger one's ... and parts I just replace as a matter of course now to save time & increase reliability with these models.

              I'll go and watch the vid, I see a few of these so it's always interesting to see other faults ...

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                #8
                Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

                I did watch the looong video. Interesting repair as I figured the problem being either the final amp stage or in the pre amplifier circuit. The final fix was kind of interesting, but it would have been nice to have figured out where the other problem was, instead of putting a wire jumper in. Obviously there was something open somewhere. Never the less... it was still a successful repair!

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                  #9
                  Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

                  Hi Capleaker, if it was pre-amp then there would have been imbalance of voltage on the collectors of the output devices.

                  If there was another problem it would have been obvious after fitting the jumper.

                  Most of the time these issues are caused by "pilot error" when the owners do outlandish stuff with other items they connect to them, that repair was fine ... might be back soon though if it was caused by e.g. a set of speakers with a partial voice coil short.

                  The "other problem" was most likely not associated with the amp or maybe even the venue I've seen named DJ's destroy piles of equipment in a single night gig with stupid things they've connected to house systems ... :o

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                    #10
                    Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

                    if i was seeing it right the repair could have been better as one track is now doing the work of 2 tracks .

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                      #11
                      Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

                      Originally posted by petehall347 View Post
                      if i was seeing it right the repair could have been better as one track is now doing the work of 2 tracks .
                      Did you see the schematic? ... The repair is just connecting Q139 (left CH) & Q339 (right CH) emitter's to -VCC.

                      Maybe I'm wrong it's been a few days now since I saw the vid.

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                        #12
                        Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

                        Yes that was the fix - blown track to -HT (65V)

                        @Petehall347 @CapLeaker - I did locate the actual blown track (showed it clearly on the video from 2:01:45) but it was running underneath a whole line of resistors and similar PTH components so it was very hard to repair the actual track, hence my 'piece of wire' compromise. One track is not doing to job of two now, the original -65V track goes to the emitter on one channel, then across under a lot of components to the other channel. I just bridged the section of track going from one emitter to the other.

                        What I am not sure is what caused the track to blow in the first place. The nearby transistor I had already replaced that had a low resistance C-B was connected via resistors so I can't imagine a lot of current flowed that way when the transistor failed.

                        Anyway the mixer has been working fine in the live music venue for a few weeks now so the repair seems good.
                        Last edited by dicky96; 06-13-2022, 04:03 AM.
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                          #13
                          Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

                          Originally posted by dicky96 View Post
                          What I am not sure is what caused the track to blow in the first place. The nearby transistor I had already replaced that had a low resistance C-B was connected via resistors so I can't imagine a lot of current flowed that way when the transistor failed.
                          Yeah that is a good question ... I see a high percentage of various failures in this area of the board.

                          I've had the chance to check a few of the speakers these damaged amps were attached to and quite often they have partially shorted voice coils resulting in very low impedance .. but how that translates to the damage I have no idea ...

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                            #14
                            Re: Dynacord PowerMate 600 Powered Mixer - Protect Light On

                            Dynacord mixer amps have a time in circuit , i think -11Volt, each amplifier , the psu, thermal trip on mains traffo, dc at o/p, will pull this down , through a diode D320, D120, D121, in Pmate 1000, The time in diode that is pulling down the _10Volts will quickly indicate which unit is causing the problem

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