Reverse Engineering a Clock Pulse Giver
Over the years my professional watchmaker friend brought me several of his professional devices when they stopped working or when he needed some advice. And I am always curious about the electronic products he brings me to investigate. Last and recent one was about his Gent of Leicester XC408 Master Clock which was fixed and works again like a sunshine! And another very big repair was about the Exachron DCF time pulse receiver that completely was examined and even got simulated circuit parts that worked in the Tina designer/simulator. Of which also several Youtube videos were added to that article on Jestine’s great Blog. And this article gets a Tina circuit too.
This one is about reverse engineering a Clock pulse giver he in the past had bought that no longer was available because the manufacturer and/or designer of this product no longer existed or worse past away. And my friend (and also old colleague of a previous job we both worked) therefore wanted to make sure it was examined and saved in case he needed to repair it or duplicate it in future projects.
The designer and/or manufacturer of this Clock pulse giver however had made sure that his product couldn’t be duplicated by removing all markings on the IC’s.
Above photo shows the boxed Pulse giver circuit. The red and blue wires are the about 9V DC + and – battery inputs. And both black wires are output to the Clock.
It worked splendidly but as said no longer could be bought from the designer/manufacturer. And neither could the schematic of this circuit be found anywhere. Mainly because the seller simply no longer existed.
Finding the used but unmarked IC’s is no problem if they are the standard digital TTL/CMOS types.
Most of my IC testers are capable in recognizing the IC function to find what type it is. If they are normal or the special Schmitt triggered type is however not shown. Per example: the 7404 hex inverter is also available as a 7414 hex Schmitt trigger inverter but they behave almost identical in the tester if the different trigger level is not checked. Some testers then do give more IC types as result. And we have to select the right type from the given results. Schmitt triggers are used if we want to eliminated any erratic trigger pulses influencing its functioning.
And as was concluded in my previous Gent of Leicester XC408 Master Clock, 2 of my portable universal digital IC testers had problems with disapproving bad counter IC’s like the 4024 7 bit binary counter that was bad but tested good anyway! My other PC controlled IC testers from Elektor March 1998, and the ELV PC slotcard controlled IC tester had no problem with disapproving the bad chip. To my surprice the TL866A nor the Genius G540 could handle all counter IC’s. Because the TL866A simply lacked types CD4024, and types 7490, 7492 completely! The Genius G540 universal iC tester did better with recognizing the 4024, but lacked also the 7490 and 7492 counter/divider IC’s. (Counters are used as dividers when only a selected output or a combination of outputs are used).
Next photos show the solder side of this Pulse giver.
So please bare in mind that most (portable battery operated) IC testers only recognize the IC function but not fully test them. Especially when they are the complexer counters!
To complete this reverse engineering article also the in Tina designed circuit of this Pulse giver is added.
Above the Tina circuit with explanation. Below some additional info.
Sadly simply measuring the exact frequency on the counter outputs in the Tina simulation failed if the integrated Multimeter was used. The integrated virtual Multimeter only gave the frequency of the used frequency generator and refused to probe on the signal outputs I wanted to measure. I first thought it was a bug in my version 11 of the Classic Tina Designer with the extra HDL package. I therefore asked the Tina creators why it can’t be used as the probe button on the Multimeter suggests but clearly fails. Because when connections are probed the previous running simulation just Halts and the Multimeter is of no use.
But after a quick and confirming helpful answer from the 24 hours a day available email support from the technical Tina designer staff they told me that the Multimeter couldn’t work in the Transient simulation mode because it only works in the AC/DC simulation mode. And checking the frequency on the counter outputs was still possible but only by using the virtual oscilloscope and its Wave Diagram in Transient simulation mode. And by using its cursors to find the pulse width and also the frequency of the counter signal outputs is very helpful. And the Diagram can be exported like all simulations can be saved as jpg, gif or as other screen snapshots.
Why do we need to know and double check the frequency on output Qc? (See Tina schematic). Because the counter which is used as a divider is confusing, and it is wrong thinking that counter U1 with only output Qc used (output active when 0100 is reached) is dividing by 4! Because it isn’t! Counter U2 output Qd (output active when combinations 1000 up to 1111 are reached) which is 16 combinations counting starting from 0000. Which means that it devides by 16. But because transistor T1 disables combination 0000, counter U2 starts at 0001 after combination 1111 was reached and output RCO became ‘1’. Which means that Counter U2 divides the frequency of the input signal by 15! And for Counter U1 is difficult to see that it divides by 8 but apparently it does!
Because it is confusing to check the real dividing factor of a Counter I simply use the simulator and the given Tina circuit and then input a signal with a known frequency. And check the result. In this case a 200KHz signal and examine the output signals at QD from U2 and at Qc from U1. See the result in the Wave Diagram and the signals on the Virtual Oscilloscope further below. VM5 is the signal at output QD from Counter U2. And VM1 is the signal at output Qc from Counter U1.
And we notice that we get a 5.42 uSec pulse at VM5 with a pulse length of 75.81 uSec. And we get a symmetrical output pulse of 299.64 uSec with a pulse length of 2 x 299.64 uSec = 599.28 uSec.
And Frequency = 1/T <=> 1/75.81 uSec gives a frequency of about 13.19087192 KHz. And for 1/599.28 uSec <=> gives a frequency of about 1.66866907 KHz.
So at an input frequency of 200 KHz we get an output frequency of about 13.19087 KHz at Qd Counter U2. Which is dividing by 15! And the frequency of the signal at Qc Counter U1 is about 1.66 KHz which is compared to the frequency at output Qd of Counter U2 dividing by 8!
The total dividing factor of both counters therefore is 15 x 8 times = by 120 !
With this knowledge we finally are able to determine the correct output frequency and the shape of the signal at output Qc at Counter U1.
The original Pulse giver input frequency in our original Tina circuit was 32768 Hz divided by 16384 is 2 Hz (by U3 the used 4060 cmos IC).
And output signal Qd at Counter U2 is <=> 2 Hz divided by 15 because the frequency slows down a factor 15 <=> frequency at Qd becomes 1/7.5 Hz.
Following the given dividing factor we previously found in the Tina simulator for Counter U1 at output Qc which is another 8. We get a frequency of 1/7.5 Hz divided by 8 = 1/60 Hz.
The pulse length at Qc Counter U1 therefore is T = 1/F = 1/( 1/60 Hz) = 60 seconds. And according to the findings we gathered from the Tina simulation that signal is symmetrical, so it also must be a square wave with 30 seconds positive going signal and 30 seconds negative.
Below the Wave diagram from our Tina simulation (in Transient mode) when a 200KHz signal was used at the CLK input of Counter U2. And showing result VM1 at Counter U1, and the output signal VM5 at Counter U2.
Above the Tina Counter test circuit I used in Transient mode with Oscilloscope and Wave Diagram to check the output signals of both Counters.
Although checking circuits with Counters can be very confusing , and also a problem when most IC testers fail to test them correctly, above mentioned simulation method is maybe the best way to examine these circuits fast and easy.
* Testing of the unmarked IC’s was of course only possible after carefully desoldering the IC’s from the very thin copper tracks of the circuit board first. And soldering them back afterwards.
By publishing this circuit we respect the work that was done by the original author/designer and or seller. And the only purpose of the reverse engineering done was to save this product that no longer can be bought.
This way making sure this great battery operated Pulse giver circuit as project will not get lost in time.
I guess that this exact 32768 Hertz crystal operated Pulse giver circuit will also be of good use to other Professional Watchmakers. And hopefully this article is a very useful document if you need help when Counter circuits need to be examined.
Note: Both e-caps on the board are 1000uF 16V. The transistors are plain BC547/BC557 npn/pnp’s. (See circuit). The circuit needs at least a DC voltage higher than 1.5V. (else the crystal simply won’t oscillate, which also depends on the clock’s coil used). I used my digital IC-testers (several of them to be sure) to determine their function successfully. In the Tina simulation circuit some other component values were used to speed up the simulation.
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous repair article in the below link: https://jestineyong.com/gents-xc408-master-clock-repair/