Turntable help!

madmax

I have a Michell Gyro Dec with a heavy (read that Very Heavy) platter mod. The idea is that when the LP is clamped to it the LP itself becomes very high mass. Also the high mass keeps the speed very constant. Along with this mod is an improved speed controller.

Here is my problem. When the LP turns and the stylus is engaged I get a very substantial speaker pump onced every RPM. It may have been this way since I got it. Above about 1/2 volume you can see it pump the speakers out significantly every rev, whether the subsonic filter is on or not. I have watched the cartridge very closely and I can see a little jerk on it on each RPM. It looks more horizontal than vertical but of course it is quite hard to tell for sure. I've unhooked the motor drive and turned it by hand and the result is the same. The platter seems very tight and in no way can I detect the problem by feel.

So, what do I do? The bearing and recepticle is replaceable but will certainly be severall hundred dollars. One question might be whether this is a problem or just a characteristic of turntables. On a quiet passage does anyone else have this problem? You cannot hear it directly, it just robs the amp of power. (which you can hear). Has anyone else (or could they) looked at their drivers closely while playing a soft passage loudly? I just hate to spend that kind of cash to find out the problem is characteristic and does not get fixed. Yes, it is on every lp I try.

Thanks!
madmax

dorokusai

Every LP....spun by hand, no motor....I think you've isolated the potential problem. It shouldn't be inherent with design, especially on a rotating mass. Cheap TT's rotate at varying rates and rise....this isn't a high end quality, or at least shouldn't be. I would say it's a bearing, flat spot or dimensional(Size-Tolerance)issue. I would presume it's a breakdown, so pop it, and spin the bearing 1/4, and see if the issue moves to a different location upon rotation.

If the tracking is off at that spot, platter wise, depending on the condition, the gain could possibly change if I recall correctly.

I presume this is a retrofit?

madmax

Great idea! I'll mark the exact location, tighten the bearing about 180 degrees off and see where it happens then. Hey, this may sound simple but I've already spent two days on it. Just goes to show you how important another persons input is!
Thanks,
madmax

sucks2beme

If the bearing is bad, try going directly to a bearing suppler for the replacement. Get some measurements with a micrometer or vernier. (know any gun nuts? they'll have these as well as a good mechanic). That will be the difference between big bucks, and a modest cost. It will be a hassle ,though.

madmax

Unfortunatly it is a two inch tall affair with oil ports and a built in mechanical oil pump and sump. Not your standard bearing. I haven't tried Doro's idea yet to verify the bearing. Hopefully I can do that tonight.
madmax

scottnbnj

i don't know anything about gyrodecs, just what i've surfed across. one thing i remember reading somewhere is that there where (/are?) hard and soft belts. the hard belt could develop kinks or humps from sitting idle for very short periods.

anyway, i couldn't find that link, but, if you haven't seen the links below, maybe there's something in one of these that could help.

http://www.angelfire.com/music5/gyrodec/

http://pedersensgyro.hjem.wanadoo.dk/test_english_tekst.htm

)

madmax

Great links!
Thanks,
madmax

BrentMcGhee

All right I am defiantly waking up a dead horse but I am now too having this same exact problem. I remember running across this thread last month and did not think anything of it because I did not know I was experiencing the same thing.

Just about three months ago I brought myself to try out some vinyl’s (me being 19 I grew up with cd) after learning about all the downfalls of digital recordings. I went out and bought me a cheap record player from cc just as a starter unit until I learned a lot more about them and could actually know what to look for in a record player(and from reading on this forum there is a whole hell of a lot more to learn about them).

Well anyway just last night I was playing a record at rather high volumes as I was cleaning up and when I took my speaker grill off to vacuum it I was extremely shocked to see what was going on behind there. Just like madmax was saying with every rotation of the record the cone would flop back and fourth a great distance. (It looked as if the cone would fully extend and then retract several times really fast). At first I thought it was part of the music but it did not seem to be in beat and then the song came to a softer part and still the same thing.... tremendous cone movement with every rotation. The amount of movement was in direct relation with how loud I had it turned up because as I turned the volume knob down the distance that the cone would extend and retract would greatly decrease but it would still flop back and fourth with every rotation.

At first I though it was with just that record, so I put on another record and it yielded the exact same results. I then realized that both of those records were newer ones that I had bought so I though maybe they make the records differently now then they did 20 years ago so I grabbed one of my older records and still the exact same thing. (however with the older records it was not as prevalent)

Now after my long explanation that was probably not needed.... I read through everything that you guys had recommended madmax try but the thing is I still know very little about turntables and would not know how to do those things or even if I could on such a cheap turntable ($150.00 for the whole thing).

Some help would be greatly appreciated (with either my cone flopping problem or just some education on turntables haha:))

Jim Shearer

What you describe sounds to me like the old problem of the center hole not being punched exactly in the center of the vinyl disc. This was always an issue, and for poorly made discs, the only solution I recall was a low frequency filter. My 1970's vinage receivers have such filters.

If the turntable platter does not show in-out wobble, then it must be the discs. To check the platter, just spin it while watching the edge (and maybe putting a reference object near the edge to make it easier to see any movement.) Now put a disc on & repeat the procedure. If the platter does not wobble & the disc does, then blame falls on the disc. Different discs should then show different amounts of wobble, with the good ones having no detectable wobble.

Yes, I'm old as dirt. As a kid I remember playing 78's as well as those new 45 & 33.3 rpm formats.

Cheers, Jim

BrentMcGhee

thanks Jim, I am going to try that as soon as i get home from work.

madmax

I removed my platter, filled the oil sump and re-assembled. The problem was still there. A few days later I'm seeing the problem only slightly. It seems to come and go. I'll have to keep track of which lps do it and which ones don't. Pretty annoying little problem. I'm hoping it has something to do with the holes not being centered but I doubt it because I see the needle jiggle once per revolution. Could still be off centered I guess.
madmax

Polkersince85

Find a machinist friend who will loan you a good dial indicator and set it up on the platter edge. Check the runout with the record playing. This will let you know to a few tenths (.0001 inch) where you stand. I tend to believe the disk punchout theory myself as stated above.

madmax

I actually got one of those, took it home, didn't see a good easy way to mount it (although certainly I could figure it out) and then got to thinking maybe the platter itself may not be perfect anyway. I mean, it doesn't have to be. I brought it back because people use it but I'm still tempted to try it again.

A little more thought on the punchout theory: when the disk hits maximum offness (new word I just made up) and starts back in the other direction it would cause the little jerk I see on the stylus so yes, it is totally reasonable that could be the cause. Now, since that is such a low freq why doesn't my rumble filter remove it? Just thinking out loud.
madmax