Post by thespianator on Nov 9, 2016 16:10:41 GMT -5
Greetings! I picked up a free Hafler DH-200 that was built from a kit. The giver told me it had fried one of his speakers, so before testing I checked the DC offset and found that it was 20mV on one channel and -55 Volts on the other!
I thought this meant there was a fried MOSFET output transistor (Unobtainium), but I pulled each of the 4 outputs and tested for a short between the pins. No short, so I figure they must be OK. I'm in the process of getting some reasonably closely matched 2N5550 and 2N5401, but need to be sure that these don't blow out if the -55V is caused by something else. I've tested the electrolytic caps in circuit with my ESR meter, and nothing comes out as out of spec. I put the correct rating slo-blo fuses in both channel mains and when turning it on, they don't blow out. I'm presuming this means that the outputs are not shorted. There's a small daughter board that hangs next to the PC-6 amp board, btw, and the caps read OK. Is there a particular pre-driver that is blown?
I plan to re-cap this once I get the DC-offset figured out. There's no pot for DC-offset on this one as there is on the DH-220. One would just set the bias and the DC offset "should" be in range.
I'm at a little loss on where to proceed from here, and these are good little amps. Could anyone provide a little guidance? My pile of gear that I am stuck somewhere on fixing is large enough! and I would like to get this one working.
Thanks for any help!
Hafler PC-6 Schematic.pdf (384.37 KB)
I thought this meant there was a fried MOSFET output transistor (Unobtainium), but I pulled each of the 4 outputs and tested for a short between the pins. No short, so I figure they must be OK. I'm in the process of getting some reasonably closely matched 2N5550 and 2N5401, but need to be sure that these don't blow out if the -55V is caused by something else. I've tested the electrolytic caps in circuit with my ESR meter, and nothing comes out as out of spec. I put the correct rating slo-blo fuses in both channel mains and when turning it on, they don't blow out. I'm presuming this means that the outputs are not shorted. There's a small daughter board that hangs next to the PC-6 amp board, btw, and the caps read OK. Is there a particular pre-driver that is blown?
I plan to re-cap this once I get the DC-offset figured out. There's no pot for DC-offset on this one as there is on the DH-220. One would just set the bias and the DC offset "should" be in range.
I'm at a little loss on where to proceed from here, and these are good little amps. Could anyone provide a little guidance? My pile of gear that I am stuck somewhere on fixing is large enough! and I would like to get this one working.
Thanks for any help!
Hafler PC-6 Schematic.pdf (384.37 KB)
