1979 Dodge Magnum Digital Chronometer clock repair

[Diesel01]

I have two digital chronometer clocks that I need to repair. I was wondering before I reverse engineer the clock if anybody has successfully repaired them. One works but 5s and 6s come out 9s and 8s. The other burned up a resistor to the filaments of the display and a coil that gives constant power to the chip. My guess is the zener diode got very leaky and burned out the coil like a fuse. They are a pretty basic but I cannot source the main chip which is a National Semiconductor 7839. I also cannot source the zener diode of a 7841. I am guessing it is a 12v or 13v.

One part that I think I have sourced successfully is the VFD display. It seems that an IVL2-7/5 appears to be a direct replacement and are made in Russia and sold by a guy in Ukraine on ebay. They are inexpensive at 2 for $15 with shipping. There is another guy in the US that sells 6 for $23 on ebay. The only thing about these displays is they say they are 24v and docs are in Ukrainian or Russian. I have seen other docs specify that this display will work from 12v-36v. The original display is made in Japan part #3697639 but I cannot find that exact part either.

 

[WileE]

Have you attempted to source replacement used or NOS clocks ?

 

[Diesel01]

I got both on ebay. I have done this before reverse engineering devices and successfully fixing them as long as I can find a replacement part.

 

[Billccm]

The National Semiconductor DS7839 is a 5V TTL device. Are you sure you are dropping 13.8 to 5V via a choke coil? That is burning lots of current. Here is a link to the datasheets for the DS7839 device by National Semiconductor:

http://www.bitsavers.org/components...75_National_Interface_Integrated_Circuits.pdf

 

[Diesel01]

I am guessing that the zener diode has fatigued and is leaking to ground causing the choke coil to fry. The coil is open via my ohm meter. So the Zener must be a 5v then. Zeners do this over time. I have repaired quit a few things where the zener started drawing way too much current and fried components and traces. Do you know what type of coil it is?

 

[Billccm]

I probably need to give this some thought, but I would suspect a big coil. You are dropping 9V basically. Maybe 150 ohms DC resistance and 40mH? Maybe some pictures of the PCB and coil? Are you certain there is nothing else dropping voltage from the battery to the Zener?

For troubleshooting purposes you could sub the coil with a 150 ohm 1W resistor and see if the Zener regulates at 5V (and power the rest of the circuit).

 

[Billccm]

Here is a coil that might work:
https://www.digikey.com/short/bmjvppfj

Here is a Zener:
https://www.digikey.com/short/f4qjjzh3

BUT, try the resistor substitute first and see if the circuit powers up (and Zener knee is hit).

 

[Diesel01]

I desoldered the zener and coil. I checked resistance to ground on north of the coil. It is virtually open on the 2k scale.
I guess the only reason for the coil is to filter high frequencies from leaving the clock? I could replace this with a ferrite beed and appropriate sized resistor.

Checking the resistance in circuit of the working one and it is 14.5 ohms. It is physically bigger to by about twice.

The wire wound filament resister R1 is pretty burnt too but measures 77ohms. In my working one, it measures 82 ohms.

There is a big difference between the two clocks. The working one is two boards sandwiched and the clock chip is a National Semiconductor 814.

 

[Diesel01]

Burnt open coil

 

[Diesel01]

This is the working clock with the messed up 5s and 6s digits. This is totally different consisting of 2 sandwiched boards. One board appears to be the interface to the car and the other is the driver to the VFD and clock.

 

[Diesel01]

The National Semiconductor DS7839 is a 5V TTL device. Are you sure you are dropping 13.8 to 5V via a choke coil? That is burning lots of current. Here is a link to the datasheets for the DS7839 device by National Semiconductor:

http://www.bitsavers.org/components...75_National_Interface_Integrated_Circuits.pdf

I looked at the DS7839 chip in this pdf and it is not mine. Mine is a 22 pin. The chip in the pdf is an 18 pin.

 

[Billccm]

Here is a coil that might work:
https://www.digikey.com/short/bmjvppfj

Here is a Zener:
https://www.digikey.com/short/f4qjjzh3

BUT, try the resistor substitute first and see if the circuit powers up (and Zener knee is hit).

 

[Billccm]

For the clock with segment issues did you try reflowing the solder connections to the display?

I think if I were to tackle this I would just place my bench DC supply at 5V, current limit it to 100mA, and jump the voltage into the circuit at the zener through holes. See what happens.

I'd also look at finding junk clock radios at Goodwill and see if you can retrofit the timekeeping circuit.

 

[Diesel01]

Using a zener site to double check, for a 12v to 5v zener, it recommends a 13.66ohm resistor will result in <5mA.

I was going to try and resolder the segment one or try one of these new IVL2-7/5 VFD. However, I doubt it is the VFD unless somehow the segments got shorted together internally in the VFD.

There are clock projects using the IVL2-7/5 VFD as a wristwatch using a small smd display driver running on a 1.5v AA. I think if I cannot get these going, I would gut them and interface that little smd board to this.

 

[Diesel01]

For the clock with segment issues did you try reflowing the solder connections to the display?

I think if I were to tackle this I would just place my bench DC supply at 5V, current limit it to 100mA, and jump the voltage into the circuit at the zener through holes. See what happens.

I'd also look at finding junk clock radios at Goodwill and see if you can retrofit the timekeeping circuit.

I don't have that fancy of a supply to limit the current. Why do you think my filament resister got so hot? Display issue? I read where the filaments are only suppose to get 3-4v max.

 

[Billccm]

Hard to say what is failing. You can take three D cells in series and have a current limited 4.5V supply. I've done this before when I need a quick and dirty voltage source to check gauges at flea markets, etc.

I know GE clock radios from the 1980s used very similar display and timekeeping architecture as what was used in the automobile applications. I once fixed a Buick digital clock using a GE clock radio PCB.

 

[Diesel01]

90s Polaris Jetskis have a display that has a 1/4 amp fuse. When the zener ages and gets leaky this fuse starts failing a lot. Then the user will increase the fuse or replace with a slow-blow. I have seen where the zener has cracked open and burned traces. I have a strong feeling that this was the case with the coil but it is puzzling me on the filament resistor. Maybe it got weak after 20-40 years of use.

I am going to try and wire a 1/4 amp fuse in line.

 

[Diesel01]

Well partial success. I put in a 10 ohm resistor for the coil and original zener. The clock powered up and worked. I was able to set it and the ":" was blinking. However, my resistor was getting hot. I measured at the zener and it was 12v to gnd. I measured the filament resistor and it was 2.4v which sounds right. It was not getting warm.

I did some test measures on the one working with the messed up display and the zener was also 12v to gnd and filaments were 2.6v.

So is the Zener really a 12v? I know most TTL 5-15v Vcc. I dont have a 12v zener but a 13v. I could swap that to see if I an still getting the hot resistor. With the engine off I am running about 12.5. To both red and blue wires.

 

[Diesel01]

I ran a test on the Zener diode. I put a 1k ohm resistor to positive of my power supply and the other end to the cathode of the diode then the anode to gnd of my supply. I watched the voltage across the diode up to 15v. It never regulated to 5, 6, 9, 12 or whatever it is. I ran the same test with my new 13v zener. Measuring across the diode, as soon as I hit 13v, it stayed there all the way to the max of my supply which is 15v. I have came to the conclusion that the original diode is bad, however, I don't know what voltage zener I should replace it with the 13v for test but a 12v?

What is weird is that the original Zener acts like a diode when tested with an ohm meter. I would think it should be shorted both ways. It went silently.

 

[Billccm]

So a Zener has a 'knee' that once the current gets there it clamps the voltage. You can make a Zener by connecting the Collector to the Base of an NPN transistor, and depending on the transistor Beta a 'knee' will occur, etc.

More info than you want to know.

Try to power the circuit off of 5V and see what you get.