Pilot 210 preamp/260 amp loud buzz

Discussion in 'Tube Audio' started by ronton3, Jan 11, 2018.

  1. ronton3

    ronton3 ronton3

    Messages:
    103
    Location:
    Macon Mo
    I have a Pilot 260 amp recapped, sounds great. I have a Pilot 210 preamp that draws power from the 260, I have never used it before because of loud hum. One of the can caps had leaked, so I just replaced the 2 can caps, and the rectifier with 4007 diodes. I hooked the pair up and they sounded really good, I was so happy, there was a very slight hum, from the 210(the 260 has been silent). I ran the the Sheffield XLO test CD, and then turned them off. Two hours later I turned them on and there is very loud, buzzing, hum that the volume control turns down. I ran the 260 with a Fisher 100-T preamp tuner and it is fine. I hope someone can tell me what is the problem. Thanks ron
     

     

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  2. cademan

    cademan Addicted Member

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    Usually when hum is volume dependent, it is something relatively simple such as ground loops, bad patch cords or connections.
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    Do you have the hum on both channels, or just one? All inputs, phono only, specific inputs? Does it make the same noise with no source hooked up to the preamp?

    Have you checked to see if the voltages in the power supply on the SP-210 match up with the schematic? Its a slightly odd arrangement with the way it has resistors to ground.


    The way this one controls volume makes it hard to use that to determine things. It cuts signal into the preamp tubes, but also cuts output from the preamp right at the output jacks. Bit unusual, I'd hate to have to source a replacement volume pot. 4 section with loudness taps? Yikes.
     
  4. ronton3

    ronton3 ronton3

    Messages:
    103
    Location:
    Macon Mo
    Thank you for responding, there has been a big change. When I first posted I had the unit out of the case, it is sort of two parts: the main chassis and a separate input cluster at the end of about 6 or 7 wires around 10 inches long. The loud buzzy hum, that bumped the woofers, and went up and down with volume control, may have been because of bad grounding and dirty connections. I spent a lot of time experimenting, deoxitizing the controls, and checking every solder joint, cleaning inputs and tube pins. I noticed that when I pressed the input cluster down onto the chassis firmly the buzzing stopped. When I moved it a bit it would start, and sometimes if moved just right and left alone it would not buzz/hum. I notice that if I turned it from stereo to mono it calmed down, until I moved the output cluster, then it would protest again. This morning I put it back into the case: the loud, obnoxious, scary woofer pumping is gone, replaced by a steady hum on both channels, the same with or without a source, moving the controls, including volume makes no difference, except the source selection knob: I am using FM-AM for an OPPO-83 Blu-Ray-CD player, each of those different sources has a different hum level when chosen with no component hooked to them. There is phono, tape head, mic, tape rec, and mpx, I intend to use the phono ( I have recently got back into records, and am loving it), can I use the other outputs for anything beside what they say. I wish I had thought to check the voltages before I put it back into the case. I replaced the can caps with individual caps, with matching mfd, but higher voltage ratings. The hum is loud enough to be heard at 20 feet, and is independent of volume. I am wondering if I need to add another filter cap? Any help on this new situation will be appreciated, at least it is not as scary as it was. I tried the phono hook-up same thing, just a bit louder. I am going to take it out of the case again to check voltages, this time I will screw the input section to the chassis hopefully to avoid the loud nightmarish buzzyhum. Thanks ron
     
    Last edited: Jan 12, 2018
  5. ronton3

    ronton3 ronton3

    Messages:
    103
    Location:
    Macon Mo
    A friend of mine looked this over and said the hum was from AC, we checked the caps i replaced in the pre amp, and they are fine, he thinks the problem may be coming from the Pilot 260 which provides power to the 210, any other ideas to try.
     
  6. gadget73

    gadget73 junk junkie Subscriber

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    Everything except phono and tape head (if it has a tape head) are effectively the same. You can use them for any line-level input you line. Tuner, DAC, CD player, whatever. If it does have a tape head, you can usually modify that to be a second phono input without a lot of fuss. I don't have the schematic open in front of me to see what its got going on.

    One possible thought is the umbilical wire placement. The 210 cable should have a 120 vac line to act as a switch for the 260, a pair of 6.3vac heater lines that go to the rectifier, a ground, and one or two high voltage DC feeds. Its not impossible that you're getting some hum from either of the AC lines induced into the DC supply, or maybe its high resistance in the ground, or maybe even a ground loop forming because of the signal cables. Not sure. I own a 260, but I do not own a 210 so the particulars of that unit are lost on me.

    Maybe worth trying, unhook the 120 vac switch wires at the umbilical connector and jump them across to power the amp. If the hum stops, you know its bleed from the power cable.
     

     

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  7. ronton3

    ronton3 ronton3

    Messages:
    103
    Location:
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    I have been running the 260 by using bridges in the connector, at 4-5 and 7-8, I have it plugged into a Fisher tuner-preamp and there is no hum, I tried bridging across the two large resistors connected to the connector and that did not work. Would it be possible to tell me which pins to connect inside the 260. I did change those resistors, they were both out of speck and one was blistered. I hooked it back up hoping that was the problem but the hum is still there and I notice now that it does go up and down with the volume control, when it is all the way down it is practically silent. Thanks ron
     
  8. gadget73

    gadget73 junk junkie Subscriber

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    The 120v wiring is pin 4 to 5. Pull those two off the plug and stick a jumper in to see if anything changes. Not honestly sure if it will, but its worth a try.
     
  9. ronton3

    ronton3 ronton3

    Messages:
    103
    Location:
    Macon Mo
    It sure was worth a try, but it did not make any difference, with volume still not affecting, and no hum when I have it hooked to another preamp, I also tried, a water pipe ground on both items no change. I am wondering if I need to add another filter stage to the preamp, and how I would do it.
     
    Last edited: Jan 20, 2018
  10. gadget73

    gadget73 junk junkie Subscriber

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    do you have a scope? That would help nail down where the noise is and whether more filter stages would help. Even being able to ID whether its 60 or 120 hz would help. 120 is power supply related, 60 is ground loop or open ground usually.

    I think I asked, but did you verify if all the voltages in the SP210 match the schematic, both the high voltage and the heater supply stuff? No heater supply caps grounded when they should not be? I know the schematic indicates some of the caps don't actually go to circuit ground.

    Unfortunately since I've never had hands on an SP210 I couldn't tell you what normal ripple or behavior is. I would venture to guess that humming is not normal but thats all I could say.
     
  11. ronton3

    ronton3 ronton3

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    103
    Location:
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    I have a a couple of older scopes but am not confident in either of them, and am not skilled enough to use them. I do not know the protocol here, but if you would like to get your hands on an SP210, I would be happy to send you mine, pay postage both ways, and a reasonable fee. With your SP260 you could do the testing I do not know how to do. Would that be possible?
     

     

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  12. gadget73

    gadget73 junk junkie Subscriber

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    So I have this sitting on my bench currently. This one I believe may have been a stand-alone model. The input jacks are on their own sub-chassis, connected to the rest of the unit through a bundle of shielded cables. I don't have any enclosure with it, so basically this is all sitting loose. The first thing I noticed was that there was no electrical connection between the outer ring of any of the RCA jacks and the preamp chassis. All of the shielded cables are connected only at the input end, they do not connect at the preamp end. There is a single solid wire wrapped around the bundle of cables but it ended just above the chassis.

    Does anyone happen to know what the original grounding setup would have been? I tied the solid wire to a grounded terminal strip underneath and it seems to behave itself but I can't find any evidence that this wire was ever tied to anything specific. It doesn't look broken off.
     
  13. kirk57

    kirk57 Some guy on the Internet Subscriber

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    I have a SP210 with a separate PS thanks to Redboy's schematic, and it hums very loudly unless screwed into the metal cabinet. Mine is the standalone version not intended for console use (the lettering is horizontally oriented)

    Can I help here Gadget?
     
  14. gadget73

    gadget73 junk junkie Subscriber

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    Probably. Kinda sounds like the same situation as the one I have here. I don't have the cabinet, and I'm not entirely certain if the owner does. I have just the chassis here, no faceplate, cabinet, etc.

    If you could take a look at the bundle of cables going from the input jacks to the main chassis, would you let me know if there is a single black wire wound around all that which seems to just go nowhere? This one was hanging loose just above the point where the cables go into the chassis.
     
  15. kirk57

    kirk57 Some guy on the Internet Subscriber

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    No I don't think so. Looks like these all terminate on the input jacks.

    [​IMG][​IMG]
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    Talked with the owner, the wire was added. I was confused because its a cloth wrapped solid wire, which usually says "old" to me.

    They must be relying on the cabinet mounting to complete the circuit then. The shielded cables are only connected to ground at the RCA end. No electrical connection exists between the amp chassis and the input jacks otherwise. Kinda seems strange to me, but whatever.
     

     

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