|
|
Post by stevend on Jul 8, 2018 4:42:06 GMT
Hi All. I ust put the Seeburg 100B I had restored in my son's house, and low and behold, a pronounced 60 cycle hum. After all sorts of usual interventions (ruling out ground loops, checking the tonearm source, tubes, having rebuilt the amp and knowing there was no hum on the bench either through audio or on 'scope), I by happenstance noticed that the hum was only present when the mute plug was in place...I went back and forth, and indeed the hum is only present when the mute is plugged into the amp. It plays fine without the mute circuit engaged, but one doesn't want to leave it that way for the "pops" which could hurt the speaker.
Tomorrow, I will take the mute plug apart and make sure it is grounded correctly, and I will likely see if my new DVM with Hz can "see" the origin of the noise. I probably won't bring my scope over quite yet, as it is old and heavy!
This is a new one for me...
Anyone have any ideas?
Thanks!!
|
|
|
|
Post by Ron Rich on Jul 8, 2018 6:52:49 GMT
Steve,
Which amp is installed in your phono-?? AVC kit,or not?
Ron Rich
|
|
|
|
Post by robnyc on Jul 8, 2018 6:59:08 GMT
Steve, I don't know about the oldest series such as yours, but the mute ckt floats on the V-VL-etc. So, in these models the ground shield on the mute cable will not show any continuity with the mech body. Check that the shield is connected to Pin 1 in the mute plug.
RobNYC
|
|
|
|
Post by Ron Rich on Jul 8, 2018 7:15:58 GMT
Same on yours--Ron Rich
|
|
|
|
Post by stevend on Jul 8, 2018 13:50:35 GMT
Sorry, folks..should have told you..it is a MRA1-L6; has the 6v6 push/pull, no AVC..
Part of the testing I will do today is to take cover off the plug, and check to make sure it is grounded appropriately.
Ron, you had mentioned taking out the 0.1 cap from the mech...do you know why it was recommended to do that? The old one is still in place...is it possible is leaking signal to the mute contacts on the cam stack?
Thanks all.
Steve
|
|
|
|
Post by stevend on Jul 8, 2018 14:21:38 GMT
I should also note that I left the old 0.1 cap in place until I had a chance to ask about the reasons for its removal...i intended to either remove or replace...my question about signal relates to my experience with tube radios...they can be sensitive to the placement of caps, as the signal might get distorted. I know the 0.1 cap on the mech is not in the mute circuit, but I was wondering if anyone thought a leaky cap nearby might inject a signal into the mute circuit...
|
|
|
|
Post by Ron Rich on Jul 8, 2018 14:31:38 GMT
Ni Steve
Going by my great, but not so long memory, I believe this is common one the mech end-see the schematic. Ron Rich
|
|
|
|
Post by stevend on Jul 8, 2018 16:58:34 GMT
Hi...I am not sure what you mean by common one...if you are meaning the 0.1 cap is common to the mute circuit, I don't think so...I have attached a picture with arrows...C2 is the 0.1 cap... Screen Shot 2018-07-08 at 9.52.41 AM.pdf (228.39 KB) sorry for the blurry..did it as a screen shot... I am hoping it is just a bad connection to ground...will check later today when I can get over to my son's.. Any other thoughts are welcome! |
|
|
|
Post by Ron Rich on Jul 8, 2018 21:27:01 GMT
Steve,
Screen shot won't open--is there a "note" beside it ?
|
|
|
|
Post by stevend on Jul 9, 2018 5:23:55 GMT
Hi..I don't know what happened...I saved it this time as a JPEG... Attachments:
|
|
|
|
Post by robnyc on Jul 9, 2018 12:16:40 GMT
Steve, eliminate that capacitor on the motor terminals. It has nothing to do with your hum problem, but should be gone as it can pop violently if it gets leaky or shorted.
I neglected to mention to check that you have ground-shield continuity up to the shields depicted in the mech schematic. The constant flexing of the mech tail can break it.
RobNYC
|
|
|
|
Post by Ron Rich on Jul 9, 2018 19:44:57 GMT
Steve, Sorry--I was in the hospital when I wrote that--I have no idea what I was saying either-- The C-1 I was trying to describe is on the input line of the amp--only needed with one type of cartridge, produced when the model A was in production. C 2 on the mechanism was eliminated as one of them blew up in an engineer's face--causing damage, so I was told--I have seen a few mechanism covers that had holes in them, I assume, from that cap blowing up ? Ron Rich |
|
|
|
Post by stevend on Jul 15, 2018 4:51:23 GMT
Thanks all! I haven't had much time, as my son is going on vacation, but I bypassed the existing mute plug/shielded cable with a spare three prog plug and a new shielded cable (just ran the cable from the amp up to jumpers on the M switch on the clutch switches, and no hum...I will get to unwrapping the mech wire bundle and replacing the mute circuit shielded cable next week. BTW, there was continuity from prong 1 to ground...but that doesn't mean there isn't some holes in the shield somewhere...Now I just have to work on the warble!
|
|
|
|
Post by robnyc on Jul 15, 2018 9:34:38 GMT
" BTW, there was continuity from prong 1 to ground...."
Steve, if you mean that pin 1 on the male mute plug shows connection to the mech chassis -that is wrong and probably the cause of hum. That cable must be floating.
RobNYC
|
|
