Time Machine (-) CV input & bypass modification – Rev 02/27/02

Notice: I have revised this document to remove one the modification for negative
voltage protection of the External Mod CV. The modification designed to protect
the Delay CV input actually protects both quite well. Thanks to Scott Juskiw for
pointing this out to me.

This document contains all of the tested modifications I am currently aware of for
the Blacet Time Machine module when constructed in the MOTM format. Certainly,
I would love to hear from you if you have other useful additions or changes to this
superb module.

The first modification involves adding a bypass switch to the Time Machine. What this
switch actually does is mute the "wet" part of the signal. Any dry signal in the mix is
allowed to pass. This modification information is included in my assembly instructions
if you care to use them. Or you can find the original documentation at Dave Bradley’s
Hot Rod MOTM site:

Bypass switch theory: You can see from the schematic that the external "cancel" (or
bypass as identified on the stooge panel) causes current in U13c by applying positive
voltage through a 100K resistor to pin 9. The bypass switch modification does also.
The positive voltage is pulled directly from the power supply +15v rail (on my PCB, I
used the untrimmed lead of PS1). The voltage is switched and then applied through
a resistor to pin 9 of U13c (on my PCB, I used the untrimmed lead of R42). The output
of U13c in turn applies negative voltage to pin 3 of the SSM2164 causing its output to
go to zero.

Here are some links to photos of my bypass switch modification too. This one shows
the resistor mounted to the switch.
This one shows how I left component leads long to
provide an easy wire attachment point.
Here are the wires attached to the PCB.

The other modification is the result of a problem that can occur if you apply negative
control voltage to either the Delay CV input or the External Mod CV. The negative
voltage can cause the clock to lock-up and your Time Machine will not function until
you turn it off and back on allowing the clock to reset. We are protecting two inputs
with only one modification.

Theory of the Delay CV modification: This modification protects the Delay CV input.
My first attempt at this solution did just that, shunted negative voltages to ground at the
input with a diode and resistor. However, I found that had a negative side effect. Since
the (-) input of U1d is normally held negative to set the initial range of the HF VCO, the
negative shunt on the input was a path to ground for current that otherwise would have
been summed into U1d. The effect was limiting the upper frequency of the HF VCO to
about 250 KHz. John specifies calibration to 400KHz. Therefore, that idea was not as
good as I originally thought.

What I found by measurement is that the voltage on the output of U1d at pin 14 is about
1 volt positive at 400KHz and goes toward zero and significantly negative as the delay
time is increased. I found that negative input voltages push this voltage over 2 volts
positive (it is an inverting summing amp) causing the HF VCO to approach a frequency
of 1MHz and then the clock locks up. So, the solution is to limit the output of U1d at 2
volts positive and not effect its range below 1 volt positive. The solution does just that.
In fact, the voltage is limited to only 1.4 volts with a full 5 volt negative voltage. I could
not apply enough negative voltage to get the clock to lock up. And, protection at this location fortunately protects both "Delay CV" and "External Mod CV" inputs.

Construction of the CV input protection modification: This assumes you have already
built your Time Machine and you are retrofitting this modification after the fact. If you are
just building your Time Machine, just follow the direction of my assembly instructions.
You need one 1K 1% resistor, and two 1N4148 or equivalent signal diodes. I also
recommend a little heat shrink.

De-solder and lift one end of R11 (45.3K ohm) so that it is in a standing position. You
are lifting the end closest to the edge of the PCB. R11 is now standing in the hole that
is closest to Q1. At the opposite hole (toward the edge of the PCB), insert your extra
1K ohm 1% resistor also standing on end. Now, tie together by twisting the two tops of
these two resistors. They now have a "tee pee" style appearance on the PCB. Do not
solder the tops together yet. Do solder to PCB.

Take your two remaining 1N4148 signal diodes and connect them end to end about ˝
inch apart (removing a significant portion of their leads), Connect so that one cathode
connects to one anode, or so the cathode stripes are both facing the same direction.
I stuck about 1 ˝ inches of heat shrink over this so that I end up with a component
inside (composed of two diodes in series) and I have a nice long lead sticking out of
each end. The heat shrink is not required, but makes the modification bullet proof in my
opinion. If you do add the heat shrink, be sure to mark which end is the cathode. Now,
you are ready to install your diode combo. The anode end connects to the twisted
junction point of R11 and the 1K resistor that stands on end tee pee style at the R11
location. The cathode lead connects through the via hole just below R14 and just above
the "S" in RESEARCH. This location is electrically ground. Now you can solder the top
connection of R11, the new 1K resistor and the anode end of your series diodes.

Here are photos of preparing the diodes to be installed and covering with heat shrink.
Here is a
photo of what it should look like when this modification complete.

Disclaimer: I am not an engineer. I am not recommending that you modify your
Time Machine. However, I am sharing the results of my own modification for your
consideration should you find you have a similar need. I am not responsible for
damage to your Time Machine.

Larry Hendry 02/27/2002 – Revision 2

Download the MS Word document here

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