Old TOYOTA speedometer board - Zener diode value

[Aller]

Hi All. My name is Jerry and I am a new member here, so greetings to everybody. I am an electronics repairs technician and my business is called All Electronics Repairs. I am based in Buderim, Sunshine Coast, Queensland.
I have an old TOYOTA speedometer board (#403), which was extensively corroded, with one leg of electrolytic capacitor missing and a Zener diode damaged beyond recognition. Replacing the capacitor and rebuilding the tracks and pads was not a problem, however, I don't know what was the value of the Zener diode (see pictures).

Can anybody help me with the value of the Zener diode?

The board is very similar to the one in Stephan's post. I was hoping that somebody may have the information. Thank you in advance, Jerry

[fzabkar]

The TB9213P IC requires a 5V supply, so the Zener is probably a 5.6V, 1/4W type, assuming it is connected to the base of an NPN regulator transistor.

https://www.land-cruiser.ru/index.php?/topic/127327-elektronnyy-spidometr-tlc-80/

[Aller]

Thank you so very much! It saved me reverse-engineering the circuit!

[floobydust]

What is the part number of the power transistor/IC the zener connects to? If an NPN transistor, then it's a 5.6V zener and a straightforward circuit.
I thought a 1SMA5913BT3 SMA package would fit, but did not look so great because it is rated 5.6V at high current 67mA. Here it looks like the speedo is pushing ~8mA through the diode. Maybe a SOD-123 packaged part would fit as well like BZT52C5V6 or MMSZ5232C.
another similar speedo: https://www.land-cruiser.ru/index.php?/topic/264876-dohlyy-spidometr/

Edit: the zener is 9.1V on all other Toyota speedometers, giving an 8.5V output from the 2SD2033 power transistor to power the sine/cosine meter driver IC TB9213P.
Replacement 9.1V zener could use 1SMA5924BT3G or BZT52-C9V1 or MMSZ5239.
Power transistor is 2SD2033 (insulated TO-126 ECB pinout).

[fzabkar]

This simple supply would have a negligible tempco. Vbe has a tempco of -2mV/C whereas a 5.6V Zener has a tempco of +2mV/C (according to Google).

[floobydust]

This simple supply would have a negligible tempco. Vbe has a tempco of -2mV/C whereas a 5.6V Zener has a tempco of +2mV/C (according to Google).

Who's bringing up tempCo?
It's finding a zener that fits with the right wattage and voltage. The original some large martian package w/1k to B+ minus a diode drop, ballpark 42mW. So a "voltage regulator" type zener which are spec'd at a lower test currents. As long as the IC gets around 5V.

[fzabkar]

Sorry, tempco has no relevance to the OP's problem. I was merely wondering why Toyota didn't just use a 7805 regulator. Then I started wondering how the transistor-zener regulator would perform in an automotive environment, and that got me wondering about tempco. Just my idle curiosity, basically.

[floobydust]

I think the transistor regulator is better for transient overvoltages. There is a 27V MOV on the speedo board with a max clamp of 53V.
7805 I knew one company using them on 24VDC trucks and they roasted pretty often, max. Vin is 35V and there is a SMBJ33A TVS (53V max.) but too high for the IC.
What's funny here is the custom stubby transistor that fits under the odometer dials, Japan makes custom parts for the oddest reason. OP's zener even looks weird, sorta 1W but I've never seen that SMT package before.

[james_s]

It was probably cheaper too, which matters when you're building millions of something. That circuit didn't need all the features of a 7805, it just needed a few mA of reasonably regulated 5V.

[fzabkar]

Earlier this year I worked on a Nissan speedometer, probably 1980s era. It used a 7809 regulator and a series diode for protection from a reversed battery. I can't remember if there were any extra protection devices on the supply rail. The regulator had a rubber standoff for vibration resistance. It had been filled with some kind of glue(?) which became brittle, corrosive and conductive, so much so that it took out a fuse.

[Aller]

Unfortunately, the transistor is totally BLANK (so was the original Zener). I have installed the only 5.6V Zener I had in my inventory, which is 4W  More than enough. It was a through-hole package and a bit too big for this side of the circuit, so I modified the board by drilling 2 small holes and installing it on the other side of the board  . I had also a missing electrolytic capacitor that I didn't know any specs of, but I managed to find the answer on one of the videos on YouTube. I simply watched the video, saw what I needed, took note of the values, and voila! Thank you all for your input.

[fzabkar]

I'm a little confused. The Russian forum showed a diagram with a supply voltage of 5V, but that Volvo thread shows a Vcc of 8.5V and Zener voltage of 9.1V.

The markings on the Zener in this photo (from the LandCruiser thread) appear to be "9.1":

https://www.land-cruiser.ru/uploads/monthly_2019_05/IMG_20190511_114131.jpg.8a1efd27f52e818532f1a9978b8263d2.jpg