Using a m/c phonostage. It's a kit-built Phonoclone, which is known on this forum. I have an old Heathkit preamp that I built years ago. It has a m/c circuit; the noise problem is the same with both phonostages. Moreover, after building the Phonoclone, I discussed the problem with the kit designer, after some testing, we both agreed that the fault was not in the Phonoclone, but somewhere in the turntable.
I think Kevin's prognosis is likely correct: that the PL-570 was designed with a m/m cart in mind; and to perform properly with a high gain m/c, the table has to be at its absolute best. Also, I think the rebuilt cuing motor is highly suspect; but there's virtually nothing I can do about that. I'd settle for reducing the other problems.
I bought and reconditioned the Pioneer two years ago. Basically, I just replaced the vinyl veneer with walnut, reconditioned the dust cover, repaired the tonearm motor, and adjusted the mechanics. I thought about replacing the tonearm wiring and/or replacing electrolytics, but, since it was working fine, decided to avoid those jobs. I know stripping and soldering tone arm wires is a headache.
The turntable is about 2 feet away from the closest electronics (including the phonostage). I could probably move it another eighteen inches, but I'd have to install longer leads. If anybody thinks that worth doing, I could try it with some rca extension cords to test.
As to grounding, I think everything is connected properly. I suspect that I'm going to end up replacing the tonearm wires, but I'd like to try everything else before tackling that. Eliminating the muting switch from the loop is easy enough. I could do that now if it seems worthwhile.
I would start by getting rid of the switch and doing anything you can to improve the shielding of the existing wiring. The internal arm wiring may be OK, but I'd probably replace everything else with good shielded cable. Double check that the arm tube is really grounded, and even if so it would not be a bad idea to bring that and any metalic components of the table back on its own ground lead.
It will be a few days before I can get to these suggestions, but I should be able to do all of that fairly easily. I'll post the results.
Some progress
The tone arm wire bundle contains six wires -- four colored, two black -- which are routed to a small terminal block. The two black wires were joined to one of the colored ground wires at the block. I assumed that the black wires function as a tonearm ground, so I routed them directly to the turntable output ground post.
As mentioned, there had been two types of interference: background hum and loud hum during tonearm operations. Since rerouting, the background hum has, essentially, disappeared. That's good. The cuing motor noise, however, was not affected.
I'm going to experiment next by removing the muting switch circuit from the circuit and see what happens. The wiring from the terminal block to the output pcb does not appear to be shielded, so I'll replace that in due course. Should I be thinking about trying to create an additional ground from the tonearm motor? I'll post as i learn more.
Thanks.
The tone arm wire bundle contains six wires -- four colored, two black -- which are routed to a small terminal block. The two black wires were joined to one of the colored ground wires at the block. I assumed that the black wires function as a tonearm ground, so I routed them directly to the turntable output ground post.
As mentioned, there had been two types of interference: background hum and loud hum during tonearm operations. Since rerouting, the background hum has, essentially, disappeared. That's good. The cuing motor noise, however, was not affected.
I'm going to experiment next by removing the muting switch circuit from the circuit and see what happens. The wiring from the terminal block to the output pcb does not appear to be shielded, so I'll replace that in due course. Should I be thinking about trying to create an additional ground from the tonearm motor? I'll post as i learn more.
Thanks.
DON'T "OHM" MC CARTS!!!
When building a Phonoclone for my med output Benz, I decided to measure the Benz's resistance with my Fluke 88? DMM and promptly destroyed the cartridge.
The meter accurately measures very low resistances-and I guess that requires low impedance/high current. You'll be fine measuring MM carts, but take the above advice: insert a 1K resistor in series, check the reading across the resistor, then measure the cart, deducting the 1K reading for a fairly accurate/safe measurement!
When building a Phonoclone for my med output Benz, I decided to measure the Benz's resistance with my Fluke 88? DMM and promptly destroyed the cartridge.
The meter accurately measures very low resistances-and I guess that requires low impedance/high current. You'll be fine measuring MM carts, but take the above advice: insert a 1K resistor in series, check the reading across the resistor, then measure the cart, deducting the 1K reading for a fairly accurate/safe measurement!
Final Report
Kevin was pretty much dead on:
I mentioned earlier that my two tonearm ground wires had been routed through one of the tonearm common wires directly to the phono preamp ground; so I rerouted both directly to chassis ground and got immediate relief from much of the hum problem.
I also mentioned that I was getting loud feedback when the cuing motor was running. Likely as a result of the mute switch circuitry. I was able to solve that over the weekend. I did three things at once, so I cannot directly tie the solution to the muting switch, but that's pretty likely. First, I removed the terminal pcb eliminating the mute switch and associated circuitry as well as the soldering posts for the patch cords. Then I installed panel mounted rca jacks. Finally, I routed the tonearm signal output directly to the rca's using shielded, twisted pair wires. These actions eliminated all the signal hum as well as the cuing motor feedback noise.
Thanks to everyone for the help.
Kevin was pretty much dead on:
I mentioned earlier that my two tonearm ground wires had been routed through one of the tonearm common wires directly to the phono preamp ground; so I rerouted both directly to chassis ground and got immediate relief from much of the hum problem.
I also mentioned that I was getting loud feedback when the cuing motor was running. Likely as a result of the mute switch circuitry. I was able to solve that over the weekend. I did three things at once, so I cannot directly tie the solution to the muting switch, but that's pretty likely. First, I removed the terminal pcb eliminating the mute switch and associated circuitry as well as the soldering posts for the patch cords. Then I installed panel mounted rca jacks. Finally, I routed the tonearm signal output directly to the rca's using shielded, twisted pair wires. These actions eliminated all the signal hum as well as the cuing motor feedback noise.
Thanks to everyone for the help.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.