[time-nuts] TENMA 72-5000 Counter Mods

Dave M dgminala at mediacombb.net
Sun Oct 2 02:16:50 UTC 2011

> 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.

dgminala at mediacombb dot net

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