Indicator lite circuit board repair

[wheeledhorseman 574]

With the exception of the low oil pressure lite, the others weren’t working properly on my tractor and the previous owner had been told that the circuit board needed to be replaced. As the tractor ran fine without he hadn’t bothered, cost probably being the main consideration. There’s an excellent thread by Chuck (Save Old Iron) which ran way back in 2009 in which he proceeded to reveal the secrets of the board and resurrected a very badly corroded (pretty much worst case) board from a 416-5.

If you’re new to this topic and looking to fix the status lites on your tractor then reading the original thread is a pre-requisite. I would normally have added this content to the original thread but as a key image, the circuit schematic, appears to have been lost in the move across from the old RS site I thought it better to start a fresh thread on the topic and reinstate the missing diagram.

I examined the board on my 227-5 during the summer and found that the sealant had cracked open in one or two places so, assuming that corrosion would be the likely problem, removed the rest of the rubbery compound only to reveal a pristine board. Clearly, the components on the board would need examination and testing, a job which could wait till winter when the tractor would be out of use. The situation was that the oil warning lite worked fine, the parking brake lite was on all the time, the PTO and clutch lamps glowed at various brightnesses when they should be off, and the seat switch never lit up at all except when the test switch was used.

The parking brake lite issue was simple to fix though it took a while to track down and was unrelated to the board.

A standard micro-switch was used here which sounded really good as it operated with a clean click but the issue was that someone in the past had put the cables in the wrong place. These are three terminal two-way switches with normally open and normally closed contacts. The common terminal is on the far side of the switch and out of view so I could see how the mistake had been made. Moving the yellow lead to the side terminal fixed the problem, well partly.

I now had three lites which lit a full brightness when they should do but glowed at various other levels when they should be off and the brightness of the glow depended on which of the others were on, and there was still a lamp that never lit when it should. Oh well, the board has to come out to be fixed. It may not be the case with all tractors fitted with the indicator system but removal and reinstallation of the board on my tractor was made much easier by first removing the bottom two lite lenses otherwise there is a danger of damaging the little lamps.

The board is pretty much identical to the one Chuck dealt with in the original thread though in this case it included the additional components round a 555 timer chip (bottom left) to make the oil lite flash.

Chuck produced the following schematic for the board, though obviously the 555 circuit was not included.

In tackling the non-functional seat lite first, I removed the relevant transistor to establish if it was the transistor or chip that was faulty. It turned out that the logic gate in the 54LS86 chip was non-functional in that the output remained low even when the input conditions should make it go high and turn the transistor and hence the lamp on. Now the 54LS86 was a military spec chip, it is still available from specialist suppliers, but read on before attempting to order one. Chuck pointed out that a 74LS86 has the same logic function and is much more readily obtainable but more about making this substitution in a moment.

The puzzling thing was that the three other logic gates in the chip were still working though struggling to turn the transistors fully off. The chip had obviously been fried in some way but not a total wipe out. Chips cannot be repaired but in this case it was worth experimenting and trying to give it a helping hand so I added a 1k resistor between the base of each transistor and ground to help pull the outputs from the chip low. The residual glowing in the off state was much improved but did not entirely go away.

Determined now to force it into submission I took a few voltage measurements and discovered that by reducing the supply down to about 10 volts the three logic gates worked perfectly. Further investigation of the voltage at the supply pin (14) on the chip revealed two things. Firstly, unlike most 54 and 74 series chips which are designed to operate with a a nominal supply of 5 volts, this particular 54 series chip appears to have an internal feature allowing it to operate from the 14 volt tractor supply provided the supply is via a 470 ohm resistor. I think the clue to this might be in the suffix. The chip used on the board is actually and very specifically a 54LS86A/BCAJC.

Much searching on the net hasn’t turned up much by way of an answer other than the BCAJC version costs considerably more than the plain old 54LS86A though as far as I can make out all 54 LS chips have a supply voltage range of 4.5 – 5.5 volts. All that I can suppose is that the version used here includes some kind of internal shunt circuit designed to deal with spikes on the supply line as part of its MIL SPEC and that this has been exploited in the circuit to act as a regulator. My voltmeter indicated that it isn’t a steady regulation and that by the time the supply to the board reaches 14 volts the supply to the chip at pin 14 had reached pretty much the 5.5 volt limit. Easy to see then how the chip could get fried.

My solution was to add a zener diode to clamp the supply voltage to the chip. I actually used a 4.7 volt zener though a 5.1 volt zener would have done just as well and…….. the lites on the board now worked as they should. I tried removing the pull down resistors but the glow returned. I wasn’t surprised as the chip has been internally damaged and actually needs replacing. What I’ve really done is worked round it and learned from doing so that if you are going to replace the chip with anything other than the original 54LS86A/BCAJC then I’d add a zener to be sure. It’s an absolute must, I believe, if you use a 74LS86. Note the polarity from the photo – the end marked with a band (black in this case) goes to the track leading to pin 14 of the chip.

When I’m next in town I’ll pick up a standard 74LS86. They’re about 70 pence here in the UK so I’ll guess about 70 cents in the US. When time permits I’ll use it to replace the damaged chip on my board but for now it works again so I’ve put it back.

Ok, so I still don’t have a seat switch lite that works but I haven’t yet reached the age or state just yet when I need a visual reminder that I’m not actually sitting on the seat. I’ve not been that impressed by the lite system up to now but now the others are working properly, I think they’re actually quite useful.

For reference I made the cables out to be as follows (left to right):

BLACK – Earth

ORANGE – 12 volt when ignition is on

BROWN – 12 volt during start only

WHITEish (perhaps once a pale pink) – Clutch switch

PINKish RED – PTO switch

DARK GREEN – earth feed to one of the switches

LITE GREEN – seat switch

WHITE – parking brake switch

YELLOW – test switch

ORANGE – 12 volt when ignition on

RED – 12 volt ever present

BLUE – Oil pressure switch

The colours used for the cables are I think different on other models of tractor but the function should be the same.

Comments welcome and feel free to chip in if you have more to add or you've spotted a mistake.

Andy

[meadowfield 2,570]

Good explanation Andy !

milspec 20-30 years ago = standard spec now (ok only 0-105 dec C usually)

...so almost any 74x86 variant should work (as long as it's not LVC...)

Are you saying it relies on the input clamping the voltage to +5V ? I can't see a 5V rail on the diagram?

I agree it needs a 5V1 zener, but 6K8 shunt sounds a bit high...

mark

[Save Old Iron 1,566]

Very timely post guys - I'm currently investigating a test / repair fixture at SOI University. In the future, we may be able to offer reasonably priced repair services of the older 5486 based indicator boards.

I have also sourced a supply of useable edge connectors, Its a far east source and necessary to buy at least 10 pieces per order. Extras are on hand in the event someone needs a replacement connector.

Great work guys, thanks for adding to the knowledge base on the electrics.

p.s.. I believe there is also a 3rd version of this board which is total diode based logic instead of the 5486. The 555 flashes the oil pressure low light.

Parts are on order to covert the grain o wheat bulbs to longer lasting and brighter LED lamps.

and please don't let the lads in R&D know I let this one slip out

[can whlvr 993]

i have 2 boards that need work,good to hear from ya again chuck

[73-18 automatic 8]

I love the look of the new board that you can read how much ????????

[wheeledhorseman 574]

p.s.. I believe there is also a 3rd version of this board which is total diode based logic instead of the 5486. The 555 flashes the oil pressure low light.

You included a comparison photo of this in the original thread - so here it is again for completeness in this thread.

Andy