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HPSCHD-L  July 2017

HPSCHD-L July 2017

Subject:

Plastic Jack Provenance

From:

Rob Brooke <[log in to unmask]>

Reply-To:

Harpsichords and Related Topics <[log in to unmask]>

Date:

Wed, 19 Jul 2017 11:04:00 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (94 lines)

I have been called in to do some maintenance work on what is said to be 
a Burton kit harpsichord , built in 1976. It is slab-sided, compass of 
five octaves and has a piano-like keyboard - white plastic natural 
covers and black plastic sharp tops. The instrument has apparently had a 
significant amount of rework as it is fitted with white Delrin jacks 
which are not Burton jacks - the one with the dread 
plectra-cast-in-tongue feature. My problem is not jack repair; it is 
missing jacks. I have not seen these specific jacks before during my 
maintenance career (admittedly a fairly short one). The instrument has 
aluminum registers and what appears to be some sort of composite strip 
as a lower guide.

I have pawed through the List archives back as far as 2009 looking for a 
matching description without success. So now, I will describe the jack 
as best I can in hopes that some of you may recognize it. If so, please 
let me know where other such jacks can be purchased, if possible.

The jack is white Delrin with a wide upper portion reducing in a quick 
taper to a thin square section to fit a round hole in the lower guide. 
The length of the upper portion from the jack top to the narrow end of 
the taper is 2 3/8". My example is a 4' jack and the length of the 
square lower section is 3 1/2". The lower section of an 8' jack would be 
about 1/4" longer but both could come from the same mold. The thickness 
of the jack is 11/64", which is also the dimension of the side of the 
square lower section of the jack.

The jack has an end screw for adjusting jack height. The head of the 
screw (slotted, not Philips) is about the same diameter as the side of 
the square lower section in order to fit through the lower guide hole. 
There is also a tongue-adjusting screw in the top of the jack, also 
slotted. It is threaded through a cast horizontal "bridge" of 1/8" 
thickness which connects the two sides of the window in which the tongue 
lies. The lower edge of this bridge is 3/32" above the top of the tongue 
and its upper edge is 3/16" below the top of the jack. The 
tongue-adjusting screw is threaded through the middle of this bridge and 
extends down to contact the beveled top edge of the tongue. The head of 
this screw is cylindrical and lies entirely within the gap formed by the 
top of the bridge and the top of the jack.

There is an additional widening at the top of the jack to allow for a 
damper groove. The top of the jack is 1/2" wide, including the damper 
slot, and extends down 11/32" from the jack's top.

The tongue is a quite intricate casting. The tongue body is 1" long, 
3/16" wide and has no axles or axle hole. The top of the tongue is 
beveled to contact the tongue-adjusting screw and the bottom of the 
tongue body is beveled to meet a similar bevel in the tongue window of 
the jack body to prevent forward movement of the tongue. The tongue body 
casting is split into a narrow window from a point 1/2" below the top of 
the tongue. The plastic strip which is to act as a bristle is cast as 
part of the tongue body, starts at the top of this small window in the 
tongue and extends downward 11/16" from the top of the window. It is a 
flat strip which terminates in a small equilateral triangle whose base 
at the bottom of the strip is 3/16", wider than the 3/64" "bristle".

The jack body has no axle hole. There is a slot cast downward 3/32" from 
the bottom of the jack body's tongue window and wide enough to admit the 
flat "bristle" emerging from the tongue without contact. below this slot 
is cast an indentation in the jack body about 1/32" deep, 1/4" long and 
with sloping shoulders where the indentation meets the bottom of the 
slot. These shoulders match the slope of the equilateral triangle cast 
at the lower end of the "bristle". When the jack is assembled, the 
tongue is poised in its window, constrained from forward movement by the 
screw at the top and the bevel in the bottom and supported by its 
"bristle", the triangle having been snapped or otherwise fitted into the 
indentation in the back of the jack body. There is no bend in the 
"bristle", which lies along the same plane as the tongue itself.

This is entirely more information than is necessary for positive 
identification but I have seen other jacks with tongues similarly 
propped up by their plastic "bristles" which are not the same, so I 
provided this nest of measurements in case any possible helper wants to 
check. The salient points are: the overall dimensions of the jack body; 
the quite elegant casting of the tongue; the bridge in which the 
tongue-adjusting screw is threaded and the lack of any sort of tongue 
axle. I shall be grateful for any assistance in identifying this jack.

Rob

-- 
ROBERT BROOKE HARPSICHORDS
4214 18th Ave W
Bradenton, FL 34205

www.robertbrookeharpsichords.com

941-746-6725

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
Note:  opinions  expressed on HPSCHD-L are those of the  individual con-
tributors and not necessarily  those of the list owners  nor of the Uni-
versity of Iowa.  For a brief  summary of list  commands, send mail to
[log in to unmask]  saying  HELP .
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