Here's an interesting one I worked on this weekend. Bought a parts/repair Marantz 1070 online a few weeks ago as it appeared to be in great shape and had a high-quality custom case. Got it out and hooked it up to the light bulb and...dead as a doornail. 3.5A fuse on the back good. Start looking for AC and I get it at the switch but not on the secondary of the transformer. No resistance on the power cord with power switch engaged, so I'm thinking "did I get a doorstopper with an open primary?". No, as it turns out in the Canadian model there is a second undocumented axial fuse between the two courtesy outlets hidden in heatshrink tubing. It was blown spectacularly. Oops. Get that replaced and bright bulb.
Ok, so it has fried outputs on the right channel. Not a big deal. Get those swapped and then bulb goes dim, but no relay click. Hmm, look more closely and there's no relay. Well there is, but about 20% of a relay. Everything above the base and the NO/NC contacts is gone. WTF is going on with this unit? Get that replaced and now the unit comes out of protection.
Ok, put some signal through it and the right channel that I replaced the outputs on has good signal, but nothing on left channel. Good signal on both amplifier inputs, so somewhere on the left channel amplifier. I should mention that someone had clearly been monkeying around on the amp board (thermal compensation diodes had been cut and then resoldered back together). Take a close look at what parts had been soldered and find diode H717 on the left amp had been installed backwards. Boom, now have good output on left channel.
Set bias without issue. Check DC offset and left channel sets to 0 without issue, but right channel won't go above -100 mV of offset. Ok I'm thinking input differential pairs. Pull them (2SA763) and check gain. Wildly mismatched gains at 55 and 110 on left channel, 111 and 245 on right channel. Replace all with KSA992s and thinking all will be good, but no change. Offset pot measures ok at 4.5k and adjusts so what am I missing? Turns out R714 (4.7k) which connects to one end of the DC offset pot is open. Was covered in glue from C706 right next to it. Check the same resistor on the left channel and it reads 6.6k, so I swap that one as well. Now everything adjusts fine and the amp is running well. All bad parts highlighted yellow on the images. Now on to the rebuild.
So that was a bit of an adventure I thought folks might enjoy. Cheers.
Ok, so it has fried outputs on the right channel. Not a big deal. Get those swapped and then bulb goes dim, but no relay click. Hmm, look more closely and there's no relay. Well there is, but about 20% of a relay. Everything above the base and the NO/NC contacts is gone. WTF is going on with this unit? Get that replaced and now the unit comes out of protection.
Ok, put some signal through it and the right channel that I replaced the outputs on has good signal, but nothing on left channel. Good signal on both amplifier inputs, so somewhere on the left channel amplifier. I should mention that someone had clearly been monkeying around on the amp board (thermal compensation diodes had been cut and then resoldered back together). Take a close look at what parts had been soldered and find diode H717 on the left amp had been installed backwards. Boom, now have good output on left channel.
Set bias without issue. Check DC offset and left channel sets to 0 without issue, but right channel won't go above -100 mV of offset. Ok I'm thinking input differential pairs. Pull them (2SA763) and check gain. Wildly mismatched gains at 55 and 110 on left channel, 111 and 245 on right channel. Replace all with KSA992s and thinking all will be good, but no change. Offset pot measures ok at 4.5k and adjusts so what am I missing? Turns out R714 (4.7k) which connects to one end of the DC offset pot is open. Was covered in glue from C706 right next to it. Check the same resistor on the left channel and it reads 6.6k, so I swap that one as well. Now everything adjusts fine and the amp is running well. All bad parts highlighted yellow on the images. Now on to the rebuild.
So that was a bit of an adventure I thought folks might enjoy. Cheers.
