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GK Fusion 550 Turn-on/off tranients, MV Pot defects

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  • GK Fusion 550 Turn-on/off tranients, MV Pot defects

    We have a number of Gallien-Kruger's Fusion 550 Bass Amp heads in our rental inventory. Interesting amp, though I'm still somewhat puzzled by the motorized pots in the front panel preamp assembly. I had one earlier in the year with substantial Turn-On/Turn-off Switching transients, which were again from the Preamp board. I got replacement preamp board to cure that. Now I have another, which also has the 'nice' benefit of a Master Volume pot that can generate Rail to Rail transients by merely rotating the Master Volume pot in the first 120 deg arc from full CCW. I pulled the PCB assembly out to see if it was something I could pull apart, but....these aren't built for disassembly like other simple single-gang pots are.

    Click image for larger version  Name:	GK 550 Woofer Turn-on & off Transients-1.JPG Views:	0 Size:	1.58 MB ID:	941967 Click image for larger version  Name:	GK 550 Tweeter Turn-on & off Transients-1.JPG Views:	0 Size:	1.62 MB ID:	941969 Click image for larger version  Name:	Master Volume pot 'noise'.JPG Views:	0 Size:	1.45 MB ID:	941971

    Unplug the tiny Woofer and Tweeter connectors on the top side of the Preamp PCB assembly's interface harness to the power amp, and the turn-on/off transients are gone. After seeing the pot 'noise', I think I'll take the easy way out, rather than mess with trouble-shooting that preamp board and removing that motorized dual-gang pot. Another adventure in Lead-Free solder, double-sided PCB with tiny-tiny solder pads. Life it too short to mess with those.

    Click image for larger version

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    Attached Files
    Last edited by nevetslab; 09-24-2021, 12:32 AM.
    Logic is an organized way of going wrong with confidence

  • #2
    Nice way to overcomplicate a good product for nothing.

    I agree that watching all those knobs move on their own can be VERY interesting .... for all of 5 or 10 minutes.
    And then?

    Once had to repair a Soldano rack preamp, which did the same.
    Juan Manuel Fahey

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    • #3
      After I moved the second GK Fusion 550 to the bench to see why it didn't power up, I found the internal T10A AC Mains fuse open. Same power amp PCB as used in the 1001RB. So, I unplugged the power xfmr secondary harness plug from the board, and checked the three Bridge Rectifiers. On BR3, I found one of the diodes in the block shorted. Checked to see if C38 was at fault, but definitely appeared to be the bridge. I removed the PCB assy, then removed C45...a 0.1uF/100V bypass cap that was across that particular diode in the bridge, removed it, and the short remained, so it wasn't that.

      Getting parts in that area off the PCB is a royal PITA. The solder pads on the Bridge are either small annular rings, or the solder mask is reduced to the same size, so no proper surface area to conduct soldering iron tip heat to the joints as needed, and the leads have been trimmed way too close to the PCB surface, AND....Lead Free Solder is used. Plate thru holes, of course. I unsoldered the buss cap C38, so I'd at least have access to the top side of the bridge to have a fighting chance. Started by sucking up solder with solder wick, then applied 60/40 lead/tin solder on those terminals of the cap, and again used solder wick to suck it up, which did finally free the leads enough to tackle the RTV underneath the caps. I did get that big cap removed, but along with it came most of the plate-thru sleeve. Great....half-expected that!

      Then went thru similar process with trying to get the bridge unsoldered. Working from both sides of the board, I finally got it removed and didn't loose plate-thrus. The Bridge definitely was shorted. I didn't have a right angle replacement bridge, so that's where this amp stopped. But, it had an assumed working Preamp PCB assy. I removed that and moved it to the other amp and installed it. That restored the GK Fusion 550 having the bad Preamp PCB assy. So, finished putting it back together, finding one of the top cover holes not having alignment with the chassis threaded hole. Figures.

      So, cobbling two amps together at least yielded one working amp, and one with both the Preamp bad, and the power amp waiting for a replacement Bridge. I did check with DMM in Semiconductor mode for any other surprises, but none found. So it goes with maintaining working amps in the rental fleet here.
      Logic is an organized way of going wrong with confidence

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      • #4
        A 220nF capacitor across each motor should stop motor interference. Otherwise there may be a timing issue with the driver stage to the motor.
        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
        If you can't fix it, I probably can.

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        • #5
          Originally posted by Jon Snell View Post
          A 220nF capacitor across each motor should stop motor interference. Otherwise there may be a timing issue with the driver stage to the motor.
          While that may be true, manually rotating the offending Master Volume pot, which in the first 120 deg arc of rotation, racking the pot in that region from CW to CCW yielded lower rail noise excursions, so that doesn't seem like a motor activation function. Plus, the turn-on and turn-off transients are something that I've seen on these amps before. I left well enough alone this time, requesting replacement PCB assy for GK's equipment that we have on some sort of loan for our rental inventory (don't know the details, but it's attractive enough to them to provide their gear for our renowned clients.
          Logic is an organized way of going wrong with confidence

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          • #6
            The Replacement Preamp PCB arrived today, along with some replacement bridge rectifiers. I re-connected the top side C38 Buss Cap connections to the bottom side, where I had lost the plate-thru sleeves during unsoldering/removal of that cap, using 28AWG buss wire and scraping away more solder mask so I could tack-solder them down to serve as a known path instead of a chance that solder would flow to the top side PCB.

            Got it all back together, having to rob hardware from the bad preamp board, as well as having to solder all of the rear motorized pot support tabs, which weren't soldered. Perhaps those are soldered in AFTER the PCB is mounted? Dunno.

            Anyway, got everything installed. I had gone thru the Power Amp PCB looking for other failures besides that BR3 Bridge rectifier, but didn't find anything.

            As I had power connected via Variac, sitting at 0VAC, and power analyzer in the path, I slowly nudged the Variac upwards. It drew high current immediately, as I shut down after it got to 2.5A. Obviously more calamities not yet found residing on the board. Sigh........... I can think of more enjoyable tasks than trouble-shooting GK 1001RB Power Amp PCB's! Maybe as this IS the property of GK's, sending that power amp PCB is the easiest way to restore the amp. It did at least get me a replacement Preamp. Power amp board from one 550, the Preamp board from another 550. Cobbled working parts to restore one amp, as stated earlier.
            Logic is an organized way of going wrong with confidence

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            • #7
              GK included two bags, each containing a pair of 2SA1380's and a pair of 2SC3502's. These are the Voltage Gain stage parts Q14 (2SA1380) and Q11 (2SC3502). I seem to recall these parts being mentioned previously on 1001RB amp's being a common failure part(s). I hadn't requested them, but now pulling that power amp PCB back out (has to come out anyway), to see if I missed finding faults on those. PITA to extract/replace, but what isn't on these dense-packed boards?
              Logic is an organized way of going wrong with confidence

              Comment


              • #8
                I found the problem! When I installed C45 0.1uF/100V bypass cap around one side of the Bridge input terminals, I managed to leave a solder bridge from the large adjacent solder pad. Their layout has alternating 'large' and tiny annular rings for solder pads, with the other two 'large' pads on the opposite side of the PCB. So, that buss cap C38 must be removed to deal with a bad bridge or those tiny mylar bypass caps. So, after correcting that problem, and having again measured all of the other power xstrs and such on the amp PCB, I put it back into the chassis, restored all the hardware and finally got it re-connected to tempt fate with AC mains.

                It powered up without any issues, while the control panel motorized pots did their dance. After everything stopped turning, and I had nominal AC mains current/power draw, I set the controls for Ch 1. Out of curiosity, I switched to Ch 2, and the controls all turned CCW. I randomly set them similar to how I set Ch 1, then switched back to Ch 1. Nothing like I had set them, so turned the amp off, and pulled up the Owners manual, expecting to find a clue to how to save the control settings.

                All I found was....on the page describing the control panel functions: 3) Channel Select Switch: Allows you to store two different settings via the motorized knobs. Switch to Channel 1 and adjust your settings then switch to Channel 2 and adjust the settings for your second channel.

                That's it. Powered back up, the controls danced to a new setting, so on Ch 1, I made arbitrary zero'd Tone Control settings, 11 o'clock settings on the Gain and Master Volume, left the HF Volume CCW, and Contour to 11 o'clock. I then switched to Ch 2, and did the same thing. Switched back to Ch 1, and the Treble control turned fully CCW, the other tone controls went where they wanted, but not at 12 o'clock as I had set them. Switched to Ch 2, Treble control wandered around but never came back to where I set it. Switched back to Ch 1, and it turned the Treble control CCW again.

                Turned the amp on and off, with no repeatibility of settings. Nice to have motorized pots. Good bragging rights.

                I went back on line to see if there's been any updates with this issue. I found many similar complaints on different forums, such as Talk Bass about the randomness of these high tech features on this amp. Now, this Preamp Board I just received from GK was supposed to have been a working / tested PCB to retrofit into their amp for us to put into our rental fleet along with many other 550 Fusions, 1001RB,s, 2001RB's and such.

                I guess all I can say for now is....the amp is working again. Don't bother with channel switching. And, after it wakes up from being powered down, reset your controls like any other amp, and forget it. Though, turning off, then back on again later, don't expect the same setting to come up. This one comes with a little brother who can't keep his hands off the neat little knobs.
                Logic is an organized way of going wrong with confidence

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