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Thread: GK 2001RB Buss Fuse F4 open. This ALWAYS bothers me when just one 15A fuse blows!

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    Senior Member nevetslab's Avatar
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    GK 2001RB Buss Fuse F4 open. This ALWAYS bothers me when just one 15A fuse blows!

    A GK 2001RB came in last week from our rental inventory with the note (No OUTPUT). After pulling the cover for initial scrutiny, I found all of the main Bridge Rectifiers fine, and three of the four 15A Pico Fuses intact. But, F4 fuse was open. That's connected between the +38V LV Buss cap and the + output of the LV Bridge Rectifier.

    So far, I've not found any of the output stage xstrs shorted, lower tier output stage emitter resistors all intact. Drivers all measure ok, flyback diodes as well as large high current diodes (MUR410's) ok. The 50W Tweeter Amp doesn't measure short, nor do the +/- 15V regulator IC's (LM317T, LM337T). Now just going further to see what else may have shorted.

    I checked for bad buss caps, using a 5khz 30V P-P square wave across each of the filter caps, which will show ESR character, and all of the buss caps measure the same.

    I've yet to cobble any fixture to power these up outside the chassis, using the main power xfmr. Though I do have a dual +/-120VDC/0-1.7A Lamda dual tracking supply that could be pulled down from the shelf to see what I get slowly running up the rails.

    2001RB_Power_Amp.pdf

    The LV bipolar supplies (+/- 15V) are derived from the left Channel, which is where F4 is. If either/both F3/F4 fuses open, no output from either channel is the result.

    I did just find solder joint voids on the outside pins of the 7-pin power xfmr header, repaired both.

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    Last edited by nevetslab; 06-12-2019 at 09:19 PM.
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    Senior Member nevetslab's Avatar
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    Arciing evidence found on the lower voltage middle pins/sockets of AC Secondary wiring

    OK, I think I've found the culprits. On the 7-pin PCB header, which receives the three power xfmr secondary coils, the middle LV winding having a Center Tap, those three middle terminals showed evidence of Arcing on the header pins, as well as on the mating surface inside the Molex connector shell's single-wipe terminals.

    Getting at the surfaces of the header pins is a PITA, as GK doesn't provide proper solder pads to unsolder/remove that header, so I had to resort to using first an X-Acto Knife blade, No. 15 blade, then an Erem tiny 45 deg flush cutters as a surface scraper, then an offset flush lead cutter (which leaves a straight lead 1/16" above PCB surface), also as a scraper, along with a tiny flat coffee stirring stick wrapped with #400 grit emery cloth, and finally a small dia brass rotary brush chucked into my Foredom hand grinder, now having gotten all of the burn marks off the pin surfaces.

    Next, extracted the middle three contacts from the Molex Xfmr Secondary leads shell, scraped away the burn marks on the Molex terminals, then sanded that surface smooth, re-tensioned the terminals. Scraped off the burn marks on the inside surface of the nylon housing, washed that away with alcohol. Applied some Caig DeOxit onto the retensioned terminals and re-installed them back into the housing.

    I then scraped away all the solder mask on the +/- terminals of the LV bridge rectifier pins (bottom side PCB), re-soldered those, then re-soldered the AC pins of the bridge on the component side. Also re-soldered the middle header pins, bottom side of PCB, so now there's solid connections from the LV winding thru the header to the bridge. I had also scraped away the solder mask on the other terminals of the bridge terminals, as well as those of F4 Pico Fuse, instead of leaving just the 0.080 dia solder pad on a 0.20" dia pad, masked off!

    Click image for larger version. 

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    I've had other GK amps completely melt down with these connectors fused together. This one was on its' way, and I'm guessing the arcing eventually caused one of the 15A fuses to open. I never did find any failed semiconductors or other parts. So, trusting to luck and putting this back together to see if that's dumb idea or not.

    On second thought, I'll first try powering up the LM317T/LM337T circuits. What happened to the LM317T when F4 fuse opened? No voltage thru the (+) side of the middle Bridge, while the upper xfmr secondary coils would still conduct. Would that have taken out the LM317T +15V Regulator, I wonder.

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    Last edited by nevetslab; 06-12-2019 at 09:28 PM.
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    OK, having connected my HP 6227B 0-25VDC/0-2A Dual Tracking supply to the Ground pin & +/- terminals of the LV bridge (passing current thru the two supply fuses F3 & F4, I ran the supplies up while monitoring the outputs of the two IC regulators. With +/- 25VDC applied, I only got +/- 13.8VDC output, though there weren't any high current surprises which the HP supply would have indicated. I got no DC offset on the output of the Left Ch Tweeter Amp, so that too was a good sign. Not having any of the rest of the supplies connected, nor utility circuits to make the full power amp run, I couldn't assess anything else, apart from minimal current draw from the external supply and valid results. NOW I feel more confident about putting the amp back together to try again.

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    Senior Member nevetslab's Avatar
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    I put everything back together. Easy to loose track of which screw holes thru the two heat sinks go to the chassis standoffs, and which ones are for the two fans that only screw into the heat sinks, but got all the hardware back into place. Everything plugged back together, so with DMM's on the two output channels, I ran up the AC mains, monitoring the outputs as well as the AC mains current. All nominal (670mA/50W @ 120VAC Mains), no Fault/Protect mode, and DC offset on the two channels were in the +/- 10mV range. Already a good sign, as that indicates I have power supplies up and running on both. Pilot LED's lit.

    Patched in signal, and connected speaker, one channel at a time, and got Woofer and Tweeter on both channels, then set for Bridge Mono, and got that off the NL-4 Output connector of the Left Ch.

    So, after all of that exercise, I only had to replace a 15A Pico Fuse (as far as parts usage goes). And, of course, restoring metal conducting surfaces on the Xfmr Secondary connector and the mating PCB header, with the associated refinements in solder connections.

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