Hello,
I got a SP6C-1 Audio Research preamp to repair. The owner told me it began to blow fuses, and he had to change it from times to times. But now, the fuse always blows.
When I opened the preamp, I noticed that a wire from the power transformer was stuck under the PT cover, and that the PVC sheath was slightly cut. It was the center tap wire of the high voltage (red-yel, TP36 on the schematic), and I guess it gradually made a short cut with the ground on the PT cover.
I restored it as it should be, and I measured 2x205VAC with no load on the high voltage. But when I connect the wires to the circuit, the fuse blows again. I checked the diodes, transistors, chips and caps of the board and they're all fine (the preamp had been restored). At last, I tried with another power supply of high voltage, and the preamp works great.
So I guess I have to admit that the power trans has an issue. And I'll have to make another PT manufactured. I measured the voltages and intensities of the secondaries, here is what I got for those who are interested :
High voltage : 2x205VAC CT – 50mA
6DJ8 heaters : 22VAC – 600mA
12AX7 heaters : 16VAC – 1A
But I don't understand how the short cut blew the secondary. Could someone enlighten me on this ? And could the PT be possibly salvaged ?
Thank you very much.
I got a SP6C-1 Audio Research preamp to repair. The owner told me it began to blow fuses, and he had to change it from times to times. But now, the fuse always blows.
When I opened the preamp, I noticed that a wire from the power transformer was stuck under the PT cover, and that the PVC sheath was slightly cut. It was the center tap wire of the high voltage (red-yel, TP36 on the schematic), and I guess it gradually made a short cut with the ground on the PT cover.
I restored it as it should be, and I measured 2x205VAC with no load on the high voltage. But when I connect the wires to the circuit, the fuse blows again. I checked the diodes, transistors, chips and caps of the board and they're all fine (the preamp had been restored). At last, I tried with another power supply of high voltage, and the preamp works great.
So I guess I have to admit that the power trans has an issue. And I'll have to make another PT manufactured. I measured the voltages and intensities of the secondaries, here is what I got for those who are interested :
High voltage : 2x205VAC CT – 50mA
6DJ8 heaters : 22VAC – 600mA
12AX7 heaters : 16VAC – 1A
But I don't understand how the short cut blew the secondary. Could someone enlighten me on this ? And could the PT be possibly salvaged ?
Thank you very much.
