How to Fix Dim Digital Dash Instruments (MA61)

[VA61DETT]

Hi All.

My digi dash in the old MA61 half packed it in the other week, both Fuel / Temp gauges went extremely dim, I put it off for about 2 weeks until they became totally un-readable.

As I have had plenty of experience in electronics I thought I would put together a quick guide on how to troubleshoot & repair these instruments, as good 2nd hand parts are getting difficult to come by.

1st thing was to completely disassemble the instrument cluster down to individual modules.

In order to power up the VFD’s you must have the Speedo connected to the Fuel/Temp boards via the small 8 pin connector.

I soldered a couple of wires to the board to make life easier for moving it around and making measurements.
Pin 1 (Red) : + 12v
Pin 5 (Black) : GND (almost anywhere along edge of board)


This was how dim the displays were


Compared to the Speedo


VFD’s work in a similar way to CRT’s
They have a heater (filament) / grid / cathode and anode…
Various voltages to make electrons flow and light things up blah blah blah…..
For a good explanation of how they work see:
http://hem.passagen.se/communication/vfd.html

Anyway, first thing I did was inspect the board for obvious component failures / dry joints etc…


Note the pads in the centre have little cracks around them, classic dry joints.
Still, not bad for a 28+ year old board
Its worth while going through and re-soldering all the sus looking ones.

Ok, well that didn’t fix anything for me. ..
Time to look closer at the fault
I located the filament pins on the fuel/temp cluster as this sort of dimming is commonly due to low filament voltage.

Its quite easy to spot where the filaments connect to the pins on the VFD tube.
They should measure about 2-4 ohms. (make sure to turn it off first )

When the display is running it “should” have about 3volts across the filaments.
Now this is what confused me for a bit. There is no DC voltage present at all.
I first thought that may have been the problem, so I compared the voltages on the working speedo, with the multimeter on DC & AC modes none at all!!

Ok this doesn’t make sense, time to break out the scopemeter…
Righty, now we have something interesting, looking at the waveform on the scope there is a 8v PkPk sine wave present on the speedo, however the waveform is not entirely stable and is fluctuating enough to fool the multimeter.

I probably should have taken pix of the waveform, can’t remember the exact frequency, was fairly low (about 50-100 Hz)

Looking at the Fuel/Temp filaments, the AC waveform is only present in small bursts of voltage, about ~20% duty (still about 8v pkpk)

VFD Power-supply


Note the small ferrite transformer on the upper RHS and Black 104K poly capacitor.
These are critical parts of the filament driver circuitry.

I made a closer inspection of the power-supply components & found that the 104k (100nf) capacitor was slightly bludged on the sides!

Conveniently I found a suitable replacement floating around the workshop, I also decided to change both of the old orange electrolytic caps while I was at it.


New (yellow) 100nf / 100v poly cap & 2 large electros (black). 100uf 25v (100v version was all I had around at the time) / 47uf 25v
*make sure to use good quality 105c caps, as they will have a hard life in summer!

Fixed
Works like a champ now!


Re-assembled and re-installed, so far so good.
Hope this may be helpful!

-A

[RA35GT]

Good Stuff Aaron !

[ZZT231]

Great write up!

Thanks for sharing.

[JustCallMeOrlando]

Nicely done dude H1TMAN should get you to look at his dash for him

[Mos]

So did you take a look at the filament waveform after the repair?

[Talasas]

Excellent writeup, I like electronics ones, there's always something to be learned. The norm really seems to be that electrolytic capacitors just die over time.

[VA61DETT]

-Mos,

Yep, came up with a nice reasonably consistent sine wave, much like the speedo VFD.
Me thinks the reason for the more complex AC drive system is to allow for variable display dimming.

-A

[TERRA Operative]

One thing I always do is replace the caps. Even if they're ok, it's good to replace them anyway to be sure.


Good writeup, I'll know where to look when I come across one of these displays.