Car RPM meter gauge repair - Page 1
Forum section: electronics / repair

author: lukas.mican
author_rank: Newbie
timestamp: April 02, 2019, 10:03:51 pm
content: Hello,I am renovating an oldtimer with a friend and we needed car engine RPM meter gauge, so some time ago I bought one. I hooked it up to the wiring in the car and it didn't work, so I took it apart and discovered inside was clearly a hand-made stripboard with IC socket, but the IC was missing. After some digging on the internet I found out that the missing IC was frequency to voltage converter LM2917 (https://www.ti.com/lit/ds/symlink/lm2907-n.pdf). I ordered the IC, fitted it into the socket and after fixing a short circuit caused by stripped wire the circuit was finally working well. It worked with Arduino for testing purposes and also later in the car.In the car it was working well for a few rides and then I noticed the shown RPM were decreasing with every start. In the end it was showing always value around 500RPM and engine rotation speed had no effect on the displayed value, so I took it apart and tried to repair it again. The circuit is exactly the same as the schematics of typical applications in the LM2917 documentation, figure 21, Voltage Driven Meter Indicating Engine RPM. The input voltage is 12V. When there is very low frequency on the input it shows 0 RPM, but if I increase the input frequency the gauge moves and shows around 500 RPM and the needle coil voltage is 130mV. Optically I didn't found any problem. I probed all resistors and diodes in the circuit and they are working, so I suspect some capacitor or the IC could be bad?Now I don't really know how to proceed to solve the issue, so before I start ordering some new parts I am asking here. How can this be repaired, so it won't break again? Should I try to fix this circuit or rather make new one with different parts?Thank youLukas Mican

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 02, 2019, 10:30:00 pm
content: check that capacitor C1, wether it's ok (if being electrolytic capacitor, could be defect.)Is there an additional power supply on the PCB?And yes, do you have a picture of that instrument, best of the rear side, and of the PCB, showing manufacturer name and part numbers?Frank

author: lukas.mican
author_rank: Newbie
timestamp: April 02, 2019, 11:14:32 pm
content: Unfortunately I don't have the equipment needed to check capacitors.There are no additional power supplies on the PCB, it is all powered by 12V from car electric system.On the instrument there is only name Mera Lumel on the front. There are no other information on the back side. This instrument was frequently used in czech cars Skoda 100, 110 and 120 manufactured in 1970s.The PCB is some hand-made replacement I guess. There is no manufacturer name or part number on the board and it is made from stripboard.I attached photos of the board, the red/pink wire is 12V power, white is ground, green is signal from engine, red and gray wires are for the needle coil.The big blue capacitor is some old capacitor from czech electronics manufacturer called Tesla, but I couldn't find any further info about the capacitor.

author: floobydust
author_rank: Super Contributor
timestamp: April 02, 2019, 11:30:43 pm
content: The blue electrolytic capacitor has popped. It needs to be replaced. I think it goes to pin 3, typically 1uF.This schematic is mostly like the board you have.

author: lukas.mican
author_rank: Newbie
timestamp: April 02, 2019, 11:42:37 pm
content: I will take a look on the blue capacitor.The schematic is exactly as the one in datasheet, as I have mentioned in the first post. To make it clear I attached the schematic to the first post.

author: floobydust
author_rank: Super Contributor
timestamp: April 03, 2019, 12:03:30 am
content: Hey does the red wire go to a 470R resistor? I don't see that. If it goes directly to pin 9, that is a big mistake and would cook the IC.It's wired OK.If you can remove and test the capacitor, it looks strange but Tesla capacitors are always a bit odd looking. I would use 1uF 25V tantalum to replace it.I did not see a diode anywhere but there are secret parts inside the heatshrink tubing. I'll mark that schematic I posted as not it.

author: tooki
author_rank: Super Contributor
timestamp: April 03, 2019, 12:48:28 am
content: Quote from: lukas.mican on April 02, 2019, 10:03:51 pmJust a little terminology tip: In English, "oldtimer" doesn't mean a vintage car, but rather an elderly person!(And the RPM meter is called the "tachometer".)Oldtimer for classic cars is classic Denglisch, which is why I am familiar with it.

author: floobydust
author_rank: Super Contributor
timestamp: April 03, 2019, 02:11:09 am
content: Yes I would say it's a datasheet (fragile) circuit and needs a few more parts. LM2917 max. is 28V and protection against vehicle transients is an additional zener and resistor. The NS schematic I posted earlier shows them.For 16-pin versions "...input protection is removed"Internal +ve clamp diode D1 is only for the 8-pin versions. That is something not to miss. SeeLM2907/LM2917 datasheetsFig. 9.2, maybe a substrate diode does the work or I am mistaken.An ignition coil can generate over +400V primary pulses, so I would be adding a zener clamp diode at pin 1.OP, the point is the datasheet circuit could use a few extra parts for protection against surges and transients damaging the IC.