Marantz 7 phono help needed - Page 2 - diyAudio
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Old 26th December 2012, 03:42 PM   #11
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I bypassed the gyrators (now CLCRCRC) an no change at all.
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Old 26th December 2012, 03:43 PM   #12
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the copper foil is heat sink
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Old 26th December 2012, 05:06 PM   #13
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I changed the feedback network on one channel (single polystyrenes, values like marantz). no change.
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Old 26th December 2012, 05:08 PM   #14
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I had changed the first coupling cap to 0,02u. but a look at b2pice says, the equalization curve changes quite a bit. maybe a new network?
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Old 26th December 2012, 05:22 PM   #15
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I changed the first coupling cap to 0.043 uF. Changed the riaa network too. the voltage at the cathode is now at about 40 volts, but running around wildly. its crazy. I removed the copper foil too. no change.
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Old 26th December 2012, 05:38 PM   #16
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I wanted to know if it is still listenable. No, know I could hear the oscillation farting around. I give up for today.
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Old 26th December 2012, 09:01 PM   #17
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I checked the power supply without the feedback network. There is "no" hum at the output of the gyrators, at least the little pocket scope doesnt show it. There is hum of 0,59 mV at the plate of the first stage. Then at the output tubes cathode there is "something" going on. The scope doesnt give a definitive result. Something around 100 Hz in the region of 100 mV. Picture maybe tomorrow.
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Old 27th December 2012, 05:26 PM   #18
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It works! I put a 1k resistor into the RIAA network like in the original circuit and raised the feedback cap from 2nd stage to first from 10 pF to 20 pF (also like in M7). Chris from preservationsound suggested this. It sounds great, nice flow.
I have to make everything ordentlich now. And put back the cover.
Thank you all again for your suggestions.
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Old 27th December 2012, 05:58 PM   #19
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Thanks for updating the thread. Pleased its all OK now
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Old 29th December 2012, 10:10 PM   #20
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Thanks for the kind words! I am happy too. I have been listening to this now for a while (I have a Lenco L75 on heavy wood plint with Rega RB251 with modified counterweight and a Denon DL103 through beyerdynamic SUTs). I am still surprised by how good it sounds. I have built and listened to JE Labs 6SL7 phono, EAR 834p (with adjustable air caps), some from Sound Practices, the jfet Pacific, opamp-based units (VSPS and phonoclone (Peter Daniels version with op27 and lesser parts)), Pearl 1 + 2 and ONO/XONO and others. With more or less care, so my judgement is very - mmh - so so. Anyway I like this Marantz clone very much, as is said before: timing, tone -very good, a very nice flow. I want to finish my ONO one day though, I thinks its better overall. but I am in no hurry. Maybe I am surprised, because I knew this circuit "all the time", since I learned about electronics. But it had NFB and 12AX7 and cathode followers - it seemed very ugly!

OK. So, the schematics i attached works for me. (The power supply is different.) It started working after I changed the values for the RIAA network to something closer to the ones shown in the schem. There was hum though, exactly what Kevin suspected: coupling through the input tube next to the rectifier tube. Tube moved, copper screens for the input tubes - thicker wire in some places, twisting, shorting some wires, (don't mind the loose socket, please)... its quiet now, tubes hiss only. I had the nerve to try if I could go back to tho old RIAA values. It oscillated again. But not a pity, the graph looks good and it sounds correct. - I love vinyl!
Attached Images
File Type: jpg m7.JPG (425.5 KB, 58 views)
File Type: jpg tt.jpg (327.8 KB, 49 views)
File Type: png m7 phono def.png (15.8 KB, 49 views)
File Type: png m7 graph.png (24.0 KB, 45 views)
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