[time-nuts] time-nuts Digest, Vol 87, Issue 5

Dave M dgminala at mediacombb.net
Sun Oct 2 10:32:53 EDT 2011

> Subject: Re: [time-nuts] TENMA 72-5000 Counter Mods
> Message-ID: <4E882064.3020805 at omen.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 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 
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.

dgminala at mediacombb dot net

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