LPT top plate

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MTECH1a
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LPT top plate

Post by MTECH1a » 14 Jan 2019 21:20

Hi,
I wonder if anyone can advise me? I have recently realized that my “piano black” (black Perspex) LPT which I have owned since new has developed a sag of around 2.5 mm in the center relative to the corners. The attitude of the arm is therefore presumably wrong relative to the plane of the platter. Certainly the platter does not seem to sit quite level with respect to the plinth.
I am considering having a go at making a new top plate. Does anyone have any experience of doing this? Reputedly (according to Funk) there is some kind of magic in the construction - can anyone shed any light on this? What is the significance of the narrow slot that runs around the top plate? This would be quite hard to replicate with a router, its very narrow and very deep.

I’d really like to sort this out if possible. A new TT of comparable or better quality is not on the cards financially, but I had decided to rewire the arm (RB250/techno weight) and upgrade to Denon 103r. I guess none of this is worthwhile with the top plate in its current condition.

Any thoughts anyone?

Richard

abril
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Re: LPT top plate

Post by abril » 15 Jan 2019 08:54

First of all welcome to the forum.
I too am the owner of a piano black LPT and am a little confused how this could have happened,the only way I can think of is the slot you speak of actually goes all the way through the top plate and the centre section is held in place by three narrow bridges - any possibility one or more of these bridges could have broken?
Certainly if at all possible pictures with the platter both on and off might help with guidance.
I think if at all possible repair would be the best option as there are numerous drillings and cutouts on the underside which have to be exactly positioned.
Have you ever had the top plate off?
40025

MTECH1a
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Re: LPT top plate

Post by MTECH1a » 15 Jan 2019 13:51

Thanks for the welcome and responding in my hour of need!

Yes, its a strange thing to have occurred to this deck. I fear historical temperature or humidity fluctuations may have caused this problem. I have had the top plate off a couple of times (to repair the power supply of course :? ). However, its not been off for a thorough inspection since I noticed the sagging issue. I did check to see that the bearing was properly mated in its locating hole in the top plate though. Removing the platter shows that the three bridges connecting the central part of the top plate to the outer part are all intact - at least on the Perspex side, but there seems to be some debris (router debris from manufacturing) bridging the slot in a couple of places. I don't think this is relevant to the current issue though. I first noticed the problem because the platter is closer to the top plate on the left hand side (as you look at the deck from the front) compared to the right. A spirit level shows this is not massively out of alignment, but the relative alignment of the platter with respect to the arm is difficult to ascertain. Of course even small errors here could play havoc with things like azimuth.

Once I have found the relevant FAQ on uploading photos I will have a go at photographing the issue.

Richard

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Re: LPT top plate

Post by abril » 15 Jan 2019 14:04

Sounds like there isn't that much difference side to side,how much difference have you measured between sides on the platter?
Rotating the platter does not change the height?

MTECH1a
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Re: LPT top plate

Post by MTECH1a » 15 Jan 2019 14:23

No wobble when the platter rotates, no, so not a wonky spindle or dodgy bearing. Across the diameter of the platter I'd guess there is 2-3 mm difference in the gap from the top-plate to the bottom of the platter. I'll try to make some measurements & take a couple of photos this evening. (not easy because its so shiny and reflective...)

Richard

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Re: LPT top plate

Post by abril » 15 Jan 2019 14:39

Have you got a straight edge you can put against the top face of top plate to check the straightness?

MTECH1a
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Re: LPT top plate

Post by MTECH1a » 15 Jan 2019 14:43

Yes, I did that last night but didn't measure it. Definite bow in the top plate.

abril
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Re: LPT top plate

Post by abril » 15 Jan 2019 15:38

Seems you've already done all the basic checks (wish I could say the same for all the questions people ask here).
If you put the bubble level on the platter across width and then on the top plate near the arm post how much difference do you get on the bubble?
I know the out of level platter doesn't look pretty but just wondering how much error there is between the two positions?
I really don't expect very much azimuth error,probably less than on your average unipivot.

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Re: LPT top plate

Post by backtotheblack » 15 Jan 2019 15:59

the "piano black" top some do delaminate (mine also done it) as they were a bit frugal with the very expensive glue. just like the Roksan they all sag eventually.

there is no secret inside the construction and it was just Arthur being Arthur with his sales mumbo jumbo. i have taken one apart looking for the "secret" and there is none.

the GTI piano gloss top plates are made up of a sandwich of MDF and Perspex. the MDF sections were routed and glued using wood glue and the perspex top was waterjet cut then glued on using what at the time was expensive glue due to perspex not adhering to much.

i guess you have two options, make a new one like the old one or make a new one of different material say full perspex.

wither way its not easy as you will need to make a precise jig or get it waterjet cut. from experience of making a new top plate i would get a proper machine shop to do it as cutting the holes for the bearing has to be very precise.

back when the LPT was made it was fairly cheep to do but if you were to replicate it now it would cost a fortune.

is it worth doing.. very much so as to better it will cost a lot of money. or hunt down a PT1, PTTOO, Export and sell the LPT as spares repairs.

one other thought i had recently is buy a new Rega P2 trim it down to fit inside the LPT plinth and drill the holes for the "suspension" but you would get hanged by the pink brigade for doing that... lol.

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Re: LPT top plate

Post by abril » 15 Jan 2019 17:38

Had wondered if it had sagged if you could brace underneath with suitably sized 'U' steel section one at the front and another at the back with cosmetically suitable bolts (maybe stainless button heads) through at each end.
Not much room underneath but perhaps possible.
Not overly pretty maybe but hopefully effective at straightening it and a lot easier (and cheaper) than making a completely new top plate.

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Re: LPT top plate

Post by MTECH1a » 15 Jan 2019 20:59

Thanks for all the suggestions. I’m not really any the wiser on how to proceed, but I have plenty of food for thought.
backtotheblack wrote:
15 Jan 2019 15:59
one other thought i had recently is buy a new Rega P2 trim it down to fit inside the LPT plinth and drill the holes for the "suspension" but you would get hanged by the pink brigade for doing that... lol.
Interesting idea, but that would surely be more Rega P2 than LPT. Better surely to just use the P2 unadulterated? I like the idea of a solid acrylic top plate better, but where to get this made I wouldn’t know.

I wonder how much of the character of the LPT is actually due to the top plate? If I mess around with that won’t I just muck up the sound? I had considered making a 1/8” mild steel plate (with appropriate cutouts and so on) to screw onto the bottom of the top plate to try to remove the warping. But I wonder if this would be stiff enough to do the job - and what the sonic effects would be. I suppose there’s only one way to find out.

Anyhow, having made some measurements, I have found that the top plate is bowed to the extent of 1.11 mm at the spindle relative to the corners. The front edge has a dip of 1.25 mm in the middle. The curve in the top is such that the platter is skewed to the extent is 2.42 mm difference between the bottom of the platter to the top plate across its diameter, measured on a line from the arm to the spindle. This asymmetry may be due to the spindle bearing not being in the center of the top plate, so not at the lowest point of the bow.

Oddly then, putting a spirit level (of uncertain precision) on the platter shows that it is nearly parallel to the top plate in the region of the arm (again along this same line). I am wondering if it looks worse than it really is in terms of the effects on its sound.

Anyhow, I have some thinking to do and some decisions to make.

Richard

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Re: LPT top plate

Post by backtotheblack » 16 Jan 2019 18:51

i've been meaning to nip along to the waterjet cutters along the road for a quote but never found time as its always been down the list of priority's. i would guess getting a single one cut would be expensive and would be better to get a whole sheet cut.

i also have access to a milling machine and although its a proper big one it dont have CNC but does have DRO, this would do to cut the bearing and arm holes.

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Re: LPT top plate

Post by MTECH1a » 17 Jan 2019 14:14

I have been looking into laser cutting. Companies offering this service seem to be geared up for one off art or prototyping jobs. I think this is the way to go (maybe one in acrylic, 2 in mdf to make the top), but I have to learn how to do 2D CAD drawings first...

I think routing or milling is going to be difficult in acrylic because of the narrowness (c.a. 2mm) of the cut thats needed.

Btw did you complete your dc motor control project satisfactorily? Im quite interested in dc conversion.
The thread stopped quite abruptly.

Richard

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Re: LPT top plate

Post by tim_bissell » 31 Jan 2019 22:52

Hi,

I have picked up a couple of non-functional LPTs over the years; one is earmarked for an Abril-inspired DC remotoring, but the other I wanted for the platter more than anything. I think both are glossy-back top-plates, but one is lacking the pink plastic insert. I will check them for flatness over the weekend, and if one is suitable I'd be happy to sort something out with you (I will check one/both are Rega mounts, but they usually are...

-- Tim

abril
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Re: LPT top plate

Post by abril » 01 Feb 2019 10:53

one is earmarked for an Abril-inspired DC remotoring
Wot - not done yet [-X
but one is lacking the pink plastic insert
I use suitably sized red heatshrink to replace :D

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