[time-nuts] TENMA 72-5000 Counter Mods

Chuck Forsberg WA7KGX N2469R caf at omen.com
Sun Oct 2 08:27:16 UTC 2011

I have a manual for the counter which shows the block diagram but no 
An ICM7226A does the logic.  The A channel has a 10 mHz clock using a 
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
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Chuck Forsberg WA7KGX N2469R     caf at omen.com   www.omen.com
Developer of Industrial ZMODEM(Tm) for Embedded Applications
   Omen Technology Inc      "The High Reliability Software"
10255 NW Old Cornelius Pass Portland OR 97231   503-614-0430

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