[HAM] Percussion Trigger

[Dave Bishop]

Here is a response  By Dave Dillon to a similar question for an M3 that 
pretty much covers the possibilities.  My guess would be dendrites as the 
cause of the short.

Regards,

Dave Bishop
-----------------------------------


> There is no percussion.  However, if I hold down a note on the
> upper manual and flip the perc off/on switch on you can hear
> one perc tone and then it disappears and will not retrigger.

It is likely that your problem is one of the following:

(1)  Short in the percussion ON/OFF switch assembly,

(2)  Short in the wire (probably color-coded white) that connects
     the percussion ON/OFF switch to the 1-foot busbar for the
     upper manual,

(3)  Short in the upper manual 1-foot busbar,

(4)  A key contact, for 1-foot tones/frquencies, on the upper
     manual that is stuck shut or making a partial "teaser"
     contact.

By "short" in the above, I mean a low-resistance path to chassis
common/ground.  Such shorts are often caused by the dreaded
cadmium dendrite "whiskers"; less often by a pinched wire or
a bent contact.

To confirm this:

With percussion switch OFF, repeat the "K" terminal voltage
measurement.

You should read 25 volts or so.

--- This verifies that the circuit from terminal "K" to the
    percussion ON/OFF switch isn't shorted.

Now, with the voltmeter still on terminal "K", and all upper
manual drawbars pushed in, hold down any key on the upper manual,
and turn the percussion switch ON.

You should hear one percussion tone, and the voltage at terminal
"K" should drop to zero.

--- This verifies that the percussion circuit in the Hammond
    amplifier is working correctly.

Release the upper-manual key.

Terminal "K" should again read 25 volts -- but probably won't
in this case.

--- If terminal "K" remains at (or near) zero volts, this verifies
    that the problem is a short in one of the four areas listed
    above.  The short prevents terminal "K" from returning to
    25 volts, and also prevents more than one percussion cycle
    when you press keys on the upper manual.

    As a final double-check, turn the percussion switch OFF.
    Terminal "K" should now read 25 volts.  If it doesn't,
    something has gone awry in your test setup.

================

> I have tried this on both 2nd and 3rd harmonic settings.

The probable short circuit is in the percussion trigger circuit,
which is independent of the harmonic setting.

> I did the "K" terminal test as per FAQ's and came up with the
> " little or no voltage ".  It reads .7 to be exact.

Fits the symptoms.

> So I totally disassembled
> the perc switching unit, cleaned the contacts with a non res de-ox
> spray, reassembled (which went much better than expected) and I have
> absolutely no change from what was written above.

Cleaning -- good move; this sometimes gets rid of nasty cadmium
"whiskers" that can short out the trigger circuit full-time.

No change -- Another clue that the problem is either in, or is
"downstream" from, the percussion ON/OFF switch.

> I have also changed the 6c4 tubes and the 12au7.

Those are the percussion control tubes ... but they won't help in
this case.  The single percussion tone has already verified that
the percussion electronics in the amplifier are healthy, and the
percussion cutoff control is adjusted properly.

> It has been suggested and explained by a local repair shop how
> to use "flashing" to fix this, but would rather not do this
> unless its a last resort.

"Flashing" is one method of clearing shorts caused by cadmium
"whiskers" or other conductive particles.  Tech opinions are
mixed about the advisability of the procedure.  Pro: It's quick
and easy.  Con: It may not work, and may damage organ components
(in this case, manual key contacts, tone generator components,
percussion switch elements), depending on the flashing method
(source of current), the circuit location of the short, and the
exact cause of the short.  Also, the cure may be temporary,
because the cadmium whiskers can grow back again.

> What's next?

First -- eliminate the possibility of a stuck or "teaser" contact
on the upper-manual 1-foot tones by these steps:

  Turn percussion OFF.
  Pull the upper-manual 1-foot drawbar all the way out.
  With the organ on and no keys pressed, do you hear any high-
   pitched tone(s)?  You shouldn't -- but you might if a 1-foot
   contact is stuck shut on any upper-manual playing key.

  You can double-check this in another way, using your
  voltmeter on terminal "K" again:

  Turn percussion ON, and all drawbars in.
  "K" should read 25 volts, but will probably read zero because
    of your percussion-killing short.
  While watching the voltmeter closely, press and release the
    lowest key on the upper manual several times.  Does the
    votage at "K" ever rise above zero volts?  If so, the problem
    lies in that key contact.
  Repeat this process for every key on the upper manual, while
    watching for "K" to rise above zero volts.

Next easiest -- shift the upper-manual busbars.  This may clear a
short or whisker in the 1-foot manual busbar.  Procedure should
be in the FAQ.

Next -- try to isolate the short.  You'll have to be creative with
your voltmeter in this process.  If you succeed in locating it,
hopefully it'll be apparent what you need to do to fix it.

Next -- perform a busbar cleaning and lube.  Procedure in FAQ.

Next -- you may have to open up the upper-manual key contacts
to locate the problem.  At this point, the alternative is
flashing.

By the way, I'd suggest verifying that the short is indeed in the
upper-manual key contacts before doing the clean&lube, or opening
the key contacts, or flashing -- because of the effort and risk.

NOTE:  This article also applies, in principle, to console-model
Hammonds.  But consoles have the added switching complication
that percussion operates only when the upper-manual "B" preset
key is depressed.  This complication is actually a subtle aid
in troubleshooting:  It helps isolate whether a percussion short
is in the percussion switch or the upper-manual key contacts.Here is the 
tidbit of extra info we learned.

The day before yesterday, I had adjusted the bus bars of both manuals of
this organ just to be safe.  The contacts sounded fine but since this
thing had been sitting idle for the last 20 years, I thought I would
shift them over.  I noticed that the upper manual shifter was screwed out
counter-clockwise considerably more than any of the other organs I have
adjusted.  Assuming that this must have been done over time, I decided to
screw the adjuster out another 2 turns counter-clockwise.  THIS is where
the short must have occured.  The bus bar for the percussion must have
got so far over that it was shorting itself against the manual chassis.
Before I realized that this was the problem, I decided that since the bus
bar adjustment was the only variable in my scenario, I cranked the
shifter screw all the way in the opposite direction (clockwise) I reached
up and hit a key and PRESTO!  there was the percussion albeit a little
scratchy at first.  I backed the adjuster counter-clockwise two turns and
it cleared that up and now the organ is back to normal!

So...  At least on this particular Hammond, there appears to be a
possibility that you can pull the buss bars to far in the
counter-clockwise direction that they will actually short agaist the
chassis.  This doesn't seem to be the case from the full clockwise
direction.