PoDuck
Active Member
I recapped this 2250, and all was well except for the phono section. I didn't have a signal generator with low enough output to not overdrive it when I first went through it. I left it as it was, using everything else just fine for a couple months, then I got a proper signal generator that would do a low enough output. I went to adjust the phono stage, and I had nothing but trouble. Nothing I did seemed to show what I thought it should show on the oscilloscope, and as I was working with it, I noticed the left channel getting weaker and weaker. Then I went to the right channel and it did the same thing. I tested the aux and tape inputs and was getting nothing there either except when the input is turned up pretty loud and the volume is turned up all the way on the Marantz. When I put a signal on the main input, I get great sound, so it isn't the amplifier, and I get sound on AM and FM, so it would seem to be the preamp.
I assumed it must be an active component on the preamp board, so I started there. I tested the transistors, and one of them tested bad, so I matched up a pair of KSC1845s, and a pair of KSA992s and replaced the transistors, and I started hearing sound from the phono board, but no change in the audio output from other inputs. I then decided that since the only other active components were the BA312s, and I happen to have some of those sitting around, that I would replace them. Still no bueno. Nothing changed. Now I went back to test the transistors again, and another one is testing bad, so I replaced them all again, just for good measure. After all, I have about a thousand of each.
Anyway, I am quite perplexed here. What would allow me to hear audio from the phono input at 2 mV, but not from any other source, no matter the input level?
It may help knowing that I am not getting anything like what I expect to see when I test the tape output ports, but I do see a distorted 1khz signal.
What would be common between both channels, not be in the amplifier stage, not affect AM and FM, and cause distortion on Phono?
Thanks
I assumed it must be an active component on the preamp board, so I started there. I tested the transistors, and one of them tested bad, so I matched up a pair of KSC1845s, and a pair of KSA992s and replaced the transistors, and I started hearing sound from the phono board, but no change in the audio output from other inputs. I then decided that since the only other active components were the BA312s, and I happen to have some of those sitting around, that I would replace them. Still no bueno. Nothing changed. Now I went back to test the transistors again, and another one is testing bad, so I replaced them all again, just for good measure. After all, I have about a thousand of each.
Anyway, I am quite perplexed here. What would allow me to hear audio from the phono input at 2 mV, but not from any other source, no matter the input level?
It may help knowing that I am not getting anything like what I expect to see when I test the tape output ports, but I do see a distorted 1khz signal.
What would be common between both channels, not be in the amplifier stage, not affect AM and FM, and cause distortion on Phono?
Thanks