Ampico B Valves and Drawer Motor
By A. B. Bonds, forwarded by Robbie Rhodes
[ Editor's Note: Below is a message from Robbie Rhodes which
[ contains, as an attachment, a message he received from our new
[ subscriber A.B. Bonds. Within that attachment are comments
[ which are delineated with {} marks. Thoese comments are
[ Robbie's, not mine.
[
[ There's a possibility here to get an ongoing thread regarding
[ the engineering subtleties of reproducing pianos. I'd like
[ to see this happen for two reasons. One, it will help some
[ of our readers troubleshoot certain kinds of malfunctions.
[ Two, it will help those here who are interested in writing
[ software emulation of these systems (for the MIDI interpretation
[ of roll data).
In response to my note in digest 951214 I received this interesting reply from new member A. B. Bonds. Can any piano techs out there give further advice about repairing B valves? {I have snipped out portions of my original letter to save space. -- Robbie Rhodes }
========== attachment follows =====
To: rhodes@foxtail.com
From: "A. B. Bonds" <ab@vuse.vanderbilt.edu>
Date: 14 Dec 1995 11:38:15 -0600
> Welcome, A. B. Bonds, and thanks to "BamBam" Bill Jelen for telling
> the world about the Automatic-Music Digest.
Thanks. I have access to fairly substantial (disk) storage here, and have been contemplating setting up an Ampico web page.
{ Neat-O! Tell us more! }
> Have you compared the Ampico A versus B control systems? Quite
> a difference.
Agreed. I have put a sensitive electronic vacuum gauge on my B and find that the level (which is pretty wobbly over time) can shift substantially in about 50-80 msec. Cool. The "servo" nature of the control flaps that Hickman designed is astonishingly clever.
My A is rather slower, but it is in some ways more reliable, and certainly works better when things are slightly wrong.
{ snip - description of interference filters for drawer motor }
All I have done so far is to hang a 0.1 uf ceramic across the points. I'll try your suggestions. I don't get that much interference, but have had trouble keeping the points clean. Dirty points are usually first apparent in the form of a very slow rewind.
{ You need a small resistor in series with that capacitor,
in order to limit the spark current when the points _close_. }
When I first got my B'eeste, the motor ran very poorly and there was a lot of arcing in the brushes. Turns out that it had not been run since the early 50's (the rewind mechanism had given up and so they used it as a coffee table). The motor sat in the same position so long that the goo that holds the armature together had sagged. The armature was very slightly out of round and the black stuff had oozed out between the commutator segments. I chucked it in a lathe and very carefully turned the commutator true, then undercut the insulation very slightly with an X-acto knife. Motor very happy now. This may contribute to the interference you were experiencing.
{ snip -- regarding the poor valve performance }
Yeah. Try the Gershwin "Rhapsody in Blue" (recut from D/A) or "Islamey." Low vacuum + long hose = trouble, especially in the notes from the middle of the tracker bar. I have tried to minimize the length of the hose on the back of the stack, which is about the only place one has any flexibility in that regard, but I am still not satisfied.
{ snip -- cleaning the ball check valve }
I have cleaned the ball valves in acetone in an ultrasound bath. (Note: the acetone container was sealed, and sitting in water in the bath, else you can get a Big Bang.) They generally seem okay. Every once and a while a unit valve will stop working, which can be traced to some junk (usually a splinter of wood) in the ball valve. My greatest concern is that I did not repouch the unit valves (save one or two that especially needed it), although I did reface the buttons. Most of the pouches seemed pretty good when I tested them "by mouth", and I dropped neatsfoot oil onto the pouch surfaces via a hypodermic needle. The exercise they now get seems to have taken some toll, and I am now getting a fair amount of leakage in some valves.
Even when the unit was freshly set up, though, I got lousy action in those middle notes, which did not seem to depend on whether the unit valve was "good" or "bad". I swapped several different ones in and got about the same no matter, which leads me to believe that there is some problem there.
{ snip -- adding primary valves to B system }
I haven't had a chance to argue the merits of the A vs the B type valves. I like the easy adjustment of the B button travel as well as the bakelite seat, but they do seem awfully twitchy in terms of reliability. We'll have to chat more on this topic. I am impressed that someone is making a whole set of valves, which would seem to be best done these days with a numerically-controlled milling machine. True? And where do you hide the primary valves? It's awfully crowded down there....
{ Primaries are above the action stack, same as in the A. -- Rob }
Regards ab |
(Message sent Fri 15 Dec 1995, 04:45:02 GMT, from time zone GMT-0800.) |
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