Subject: Re: [time-nuts] TENMA 72-5000 Counter Mods
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I have a manual for the counter which shows the block diagram but no
schematic.
An ICM7226A does the logic. The A channel has a 10 mHz clock using a
discrete
crystal. The power supply uses a transformer and appears to be
linear.
A BNC connector next to the crystals could be repurposed to a 10 mHz
input. A small cap connected to one of the crystal leads might do????
On 10/01/2011 07:16 PM, Dave M wrote:
Picked up one of these today for $40.00.
Timebase to 10 seconds.
Supposed to use a 10 mHz timebase for channel A up to 100 mHz.
Any experience with this counter?
How hard to make it use the 10 mHz from a Thunderbolt?
Have no information on this counter, and assume that it's target
audience is consumer electronics service shops. Need to ask a couple
questions before making suggestions.
1) Does the counter have a jack on the rear panel for an external
time base, and a corresponding Int/Ext switch?
2) Do you have a manual or, at least, a schematic for it?
If the answer to Q1 is yes, then the solution is obvious... Connect
your Tbolt to the External Time Base input, and switch to External.
You might need a TTL level shifter to make it work.
If you have the schematic for the counter, and can scan the area
showing the internal oscillator, you might post it to a free file
sharing service so that we can look at the circuitry and make
intelligent suggestions.
If no manual or schematic, then you'll have to do a bit of
experimentation. Find the internal oscillator and scope the signal
level and freq. If it is, indeed, 10 MHz, then you'll need to find
out if the TBolt 10 MHz output can drive the circuitry. Is the
oscillator output at TTL levels (0/5 volts)? If it is, then you'll
have to build a level shifter to get the TBolt output to TTL levels.
Temporarily break the connection from the internal oscillator and
hook the TBolt in it's place. Scope the TBolt signal and make sure
that the counter circuitry isn't killing it. If everything works,
you're in good shape. If not, you might need to level shift the
TBolt signal to TTL levels. Find out as much as you can about the
counter and come back to the list with more info. We should be able
to get you going.
David
dgminala at mediacombb dot net
What do you mean by "The A channel has a 10 mHz clock using a discrete
crystal."?
A signal channel in a counter doesn't have a "clock" as such. The "A
Channel" handles the signal being measured.
Also, for clarification, "mHz" signifies millihertz; "MHz signifies
megahertz. Which do you mean?
OK.. about the ICM7226A... This chip has provision for using the on-chip
oscillator or an external oscillator input. Without a schematic of the
counter, I suggest that you get a copy of the datasheet for the ICM7226A
from http://www.datasheetarchive.com/. Read and pay attention to the areas
that discuss using the on-chip oscillator vs. an external oscillator. It's
not as simple as connecting an external oscillator through a capacitor.
Compare the circuitry around the oscillator in your counter to the
schematics in the datasheet. You'll probably find that they are very
similar in most respects, but only you can determine how similar, and the
exact steps you need to take to use a TBolt with the counter. Use your
scope to look at oscillator signals around the chip and see what they look
like. (TTL or analog) That will help you determine what you need to do
with the TBolt signal so that the chip works properly.
David
dgminala at mediacombb dot net
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