Tweet
A GK Fusion 550 found its' way back into my shop, this time No Output Signal. I verified I had no output, though I did have signal at the Rear Panel Send jack. Plugging into the Return jack, I did find signal at U13 Pin 7, but it had no headroom, and, still no LF output. I did find low level HF output from the Tweeter Amp, but then it clipped at a low output level, less than 3V RMS.
I don't yet have service documents on this amp, which I need to get from GK. Meanwhile, I also didn't have any tiny adapters that would allow me to inject signal into the harness cables that feed the Woofer and Tweeter inputs of the Power Amp. What do I have to cobble something that's reliable? What about making a 2-pin adapter from my in-line machine pin sockets? Pin dia measured 0.018" dia, close enough to mate with the socket. Those headers/sockets GK uses are 0.098" spacing, while the in-line machine socket strips I have are 0.100" spacing. Close enough for a 2-pin adapter.
I selected a 1N4148 diode, thinking those component leads are stiffer than 1/4 W resistor leads. Fetched one of those, cut the leads short, then using my small round nose pliers, fashioned hoop test point, cut the lead short, and plugged it into the socket. Made the GND lead shorter than the Signal lead, mounted the two-pin socket into my small machinist vise and soldered them into place.
That seemed reasonable. Now to try it out. That allowed me to verify the two power amps worked fine. So, had to track down the fault.
I had signal at U13 pin 7 on the Preamp PCB, but not on any of the other IC's...there only being three TL072's on this board. Looked at the I/O PCB on the rear panel, finding a TL072 and a pair of J113 JFET's. I removed that PCB, unsoldered the JFET's and the TL072, replaced those, put the board back into place, finished putting the preamp board back into place and powered it up.
Now have full output signal on both Woofer and Tweeter outputs. And a cute little adapter to allow that sort of interface on the rest of the GK amps in our rental depot.
I don't yet have service documents on this amp, which I need to get from GK. Meanwhile, I also didn't have any tiny adapters that would allow me to inject signal into the harness cables that feed the Woofer and Tweeter inputs of the Power Amp. What do I have to cobble something that's reliable? What about making a 2-pin adapter from my in-line machine pin sockets? Pin dia measured 0.018" dia, close enough to mate with the socket. Those headers/sockets GK uses are 0.098" spacing, while the in-line machine socket strips I have are 0.100" spacing. Close enough for a 2-pin adapter.
I selected a 1N4148 diode, thinking those component leads are stiffer than 1/4 W resistor leads. Fetched one of those, cut the leads short, then using my small round nose pliers, fashioned hoop test point, cut the lead short, and plugged it into the socket. Made the GND lead shorter than the Signal lead, mounted the two-pin socket into my small machinist vise and soldered them into place.
That seemed reasonable. Now to try it out. That allowed me to verify the two power amps worked fine. So, had to track down the fault.
I had signal at U13 pin 7 on the Preamp PCB, but not on any of the other IC's...there only being three TL072's on this board. Looked at the I/O PCB on the rear panel, finding a TL072 and a pair of J113 JFET's. I removed that PCB, unsoldered the JFET's and the TL072, replaced those, put the board back into place, finished putting the preamp board back into place and powered it up.
Now have full output signal on both Woofer and Tweeter outputs. And a cute little adapter to allow that sort of interface on the rest of the GK amps in our rental depot.
Attached Files