Good morning Members,

I'm at it again with a list of components for the FD tachometer circuit board. First, here's an old link for the https://www.rx7club.com/3rd-generati...c-info-405772/.

This is a partial part number list because there are some components on the backside of the circuit board that I cannot access without destroying the tach. If someone has a circuit board with full access to both sides, then please post your findings.

Here are the part number breakdowns:
IC1: AN8352UBK
C1: 1uF, 50V electrolytic capacitor, DigiKey P/N: 493-14556-1-ND
C5: 153J50 (.015uF, 50V, ceramic type) DigiKey P/N: 399-4161-ND*
VR1: 204:37
D2(J3): Green, Blue, Blue (1N566)**
D3: C2***
Motor: 24024-16

* - Closest match on DigiKey to the part number on the capacitor.
** - This link Diode identification shows a figure with color codes for diodes. Based upon this site, a partial part number of 1N566 was derived.
*** - Partial part number.

If anyone has more accurate info regarding the diode part numbers and capacitor C5 then post your findings below.

Photos of the board are posted below.

Cheers,
George

 

Diodes D2(J3) and D3 need additional info on part numbers.



IC1 shown here.



VR1 and C5



C1 is a 1uF, 50V capacitor.



Tachometer motor

 

This is an updated part numbers list that adds C5.

CAUTION: Refer to FD Speedometer-Odometer Post #25 for more info on C5 before replacement.

IC1: AN8352UBK
C1: 1uF, 50V electrolytic capacitor, DigiKey P/N: 493-14556-1-ND
C5: 0.015uF, 50V, film type, Mouser P/N: 80-MMK5153J50J01TA18*
VR1: 204:37
D2(J3): Green, Blue, Blue (1N566)**
D3: C2***
Motor: 24024-16

* - Closest match on Mouser to the part number on the capacitor (153J50).
** - This link, Diode Identification shows a figure with color codes for diodes. Based upon this site, a partial part number of 1N566 was derived.
*** - Partial part number.

 

This post provides a cross-referenced link to "Troubleshooting the FD Speedometer-Odometer-Tachometer Circuit Board" thread, Post #34.
Here it is: https://www.rx7club.com/3rd-generati.../#post12217648

That post answered my question regarding the availability of IC1's chip, AN8352UBK, from the original manufacturer, Panasonic. In short, the chip is obsolete and no replacement exists. Therefore, the only way to replace IC1 is through cannibalization from other tachometer boards or from a different car that uses the same chip. The parts list will be updated to reflect IC1's unavailability.

This 2nd link separated from the parts list that provides a caution remark when replacing C5.
https://www.rx7club.com/3rd-generati.../#post12201933

This 3rd link provides diode identification that shows a figure with color codes for diodes. Based upon this site, a partial part number of 1N566 was derived for D2(J3).
Diode identification

Cheers,
George

 

This is an updated part numbers list that provides amplifying remarks for IC1, C5, and D2(J3).

IC1: AN8352UBK, Obsolete Part Number, no known replacement, (1)
C1: 1uF, 50V electrolytic capacitor, DigiKey P/N: 493-14556-1-ND
C5: 0.015uF, 50V, film type, Mouser P/N: 80-MMK5153J50J01TA18 (2)
VR1: 204:37
D2(J3): Green, Blue, Blue (P/N: 1N566) (3)
D3: C2 (4)
Motor: 24024-16

(1) - Refer to Post #4 for information regarding IC1 part availability.
(2) - CAUTION. Refer to Post #4, "2nd Link". This provides a caution remark when replacing C5. Original marked P/N on capacitor: 153J50.
(3) - Refer to Post #4, "3rd Link". This provides information to decode diode color codes.
(4) - Partial part number for D3.

 

Thanks to George advice I finally fixed my Tachometer.

For guys like me that removed the needle and then lost the correct position.
I found, after many tries, that the end scale of the needle should be approximately that the back of the needle should point about 3300rpm

 

So does this mean that the original c5 part number does not work as a replacement? (399-4161-ND)

 

@Aramir,

The part number for the capacitor may be in stock. However, if it is not in stock then use an alternate part number. Both Mouser and DigiKey will usually show alternate part numbers. Otherwise, you can search for the proper component based on the specs that are listed in this thread.

Does that help you?

 

I had an intermittently failing Tacho that worked most of the time but then randomly go to zero or 9,000 rpm or just drift somewhere else for seconds or many minutes at a time. I performed the usually recommended capacitor replacement and resolder and the problem went away for quite some time before coming back much worse than before. This offered the opportunity to check out the tacho in a failure mode.

I set the tacho up on a bench in a test configuration; see here: https://www.rx7club.com/3rd-generati.../#post12619951.

Before resorting to the freeze spray, I prodded the board and components with a plastic pointer looking for dry joints and quickly located the culprit and realised that I can be an Idiot at times. What component has been banned from mission critical military circuits? Variable resistors.

RECOMMENDATIONS:

If you are replacing tacho components just on spec, by all means change the electrolytic but change VR1 as well. If you are not recalibrating, just remember to measure the in-circuit resistance before removal.

VR1 is 200kOhm full scale. I happened to have two 1MOhm 25 turn “precision” trimmers. Second lesson: don’t use unknown brand trimmers. Suntan brand = Piece of ****. Trying to adjust at 6000 rpm the needle would jump several hundred RPM the wrong way before overshooting way back the other way then drifting off somewhere else. I bought a Bourns unit:

VR1 recommend replacement: Bourns 3296W-204-ND, Trimmer 200K Ohm 0.5W PC Pin Top. Bend the pins to match the triangular holes in the board.

Other substitutions that are working in my tach.:

C1: Kemet 399-15737-ND, CAP ALUM 1UF 20% 100V RADIAL (high reliability Lifetime 8,000 hrs @ 105 dec C)

C5: WIMA FKP2C021501I00HSSD 0.015uf film capacitor – pulse duty.

C5 does not warrant replacement as a matter of course – the wonky colouring is normal and this type is not particularly prone to failure.

I suspect lead free solder was used on my series 8 board as it didn’t respond to wicking, and my solder sucker as expected. I finished lifting a pad which required further rework. If I were to do this again, I would just snip the leads on the components and solder the leads of the new components to the old leads.

Here are some board parameters taken from my working tach:

No power:Powered up, Supply Voltage 13.75V, Tach Signal: 150 Hz square wave = 4500 rpm (135 degree swing) – cos and sine motor currents in the air core motor initially expected to be equal:
Voltage drops:

• D2: 0.5V
• D3: 4.9 V
• VR1: 2.882 V
• C1: 13.75 V
• Cosine motor terminals (1-3): 2.2V
• Sine motor terminals (2-4): 2.25V

IC1 voltages with respect to 0V

Pin 13 waveform:


Pin 3 waveform – ringing on what I presume to be a reset on each pulse (Pin 20 similar):



Expansion of Pin 3 ringing:



Peculiarities, oddities, open questions and speculation:

 

Error above. Bourns 3296X-204-ND should read 3296X-1-204-ND