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Overhead Console Computer

Thanks for good answer!
The 3 pin crystal has an integrated capacitor that fails. I find it easier to solder in a standard 2 pin (4MHz) crystal and then add a 47pF capacitor between one of the leads on the backside (I use the one closest to the middle of the board) and the center hole (now unused - which ties to ground).

HTH
Todd
What does this 47pF capacitor do? If I don't use this and use a standard 2pin crystal. Would the display stay dead then?

I did put a 4MHz 2 pin crystal there. So maybe if I add that 47pF it would light up?
 
Thanks for good answer!

What does this 47pF capacitor do? If I don't use this and use a standard 2pin crystal. Would the display stay dead then?

I did put a 4MHz 2 pin crystal there. So maybe if I add that 47pF it would light up?

Yes, you need the 47pF capacitor off one lead of the 2 pin crystal or the board won't light up. I ran into this with a board that came with the 3 pin design from the factory. I couldn't get the board to work after changing the crystal to a 2 pin version (the 3 pin versions are no longer available) so I used an oscilloscope to check the output. When I compared it to an older board that came from the factory with a 2 pin crystal I noted that they had a capacitor installed and when I checked both boards with the oscilloscope I found that the amplitude was lower on the version with the capacitor so I installed the the 47pF capacitor and the board worked perfectly. When I re-checked with the oscilloscope, it was the same as the original board.

I don't know if it will make a difference but I always connect the capacitor to the pin on the crystal that is closest to the center of the board because that is how it was configured on the board that came this way.

HTH
Todd
 
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Hi from Australia,
Dragging up this very old thread again. Great work by everybody on working out a fix for the overhead console temp/compass PCB. Mine is really fried. I have replaced the tactile switches, 4 large caps, and 3 smaller ones.. run a heap of traces as a lot are burnt up. I just have one question if anybody still monitors this thread and can help.. Input power seems to run along a circuit to pin 1 input of the L4812CV 3 pin voltage regulator. It continues on to a component that i can't identify. From there, the circuit continues to the positive side of the large 100 uf cap and goes to pin 1 of the big 5v voltage regulator. I have attached a pic with a red circle around it of the component i am inquiring about. (pic is from Ebay and also a member on here). Looks like a zener diode or something. Mine is fried and unintelligible, and from the pic, i cannot ascertain exactly what it is. Any help would be appreciated in trying to get the old board running.. thanks and stay safe..


y4mgifW1pQ1twLmaFqoy15CzG9KMNSfLfD32EKETpmq0PLkDTR6L8ikNjOeeHJQBJ4EOWXcxvn9BJCFs-zpuPZFgaWDHisvj9dX4hiAdMoaI8bPrkajNeVyd90vylmU4d5AgyjGrG_5lHk_Jq1JzvDHR4yXBE-oPQETPiEJN56iSQqJUA4nqW8GrY5t37ectliIY1KWkf0CIDGHqcvN6qdwig
 
It looks like a regular old diode to me. A zener looks identical but there's no reason one would use a zener in that place in that circuit. That diode is likely there to prevent battery reverse connection or backwards jump starting from blowing up the ohc electronics. You can probably replace it with something like a 1n4001 rectifier diode and it'll work fine. Desolder the diode and see if there are any other markings on it though. Make sure you get the cathode band in the right place, you want it towards the 5 volt regulator.
 
Thanks for the reply kastein. That pic is from the repair service on Ebay, and who is also a member on here. He got back to me last night and said it is a MELF diode. So this would have some built in resistance value?? However, my board looks like Pearl Harbour and that diode was pretty beat up. I am stuck at home so just playing with it and hopefully breath some life back into her. I have no markings left on the diode, but i did test it with multimeter set on diode. On one multimeter i got open both ways. On the other, i got open one way, and 0.6v the other (but sometimes 2.5v).



Here is a pic of my board and what i am dealing with.. almost nuclear war..


y4mGfRyPAWN7n4yQ0RNAFzS46DG8bSgl01m3Y4evgACvDdjxDraznk_XxwYgjdikH_bzfVxitnnnPM7Xfxr-tI_r4BPUBXMictnzvO5K3Tv_l3xCUviMHOao-xcKA5dydyV9YFo315GEUoUi4zFveuE8Cq-MciySd7i8Xr25BrQNi2CR-hUE1iZBlxe845ZSRAbdD-Z8n1kyxSzmHF5-7iBnQ
 
Jesus, what the hell happened to that thing? That's a lot of damage. I'll be surprised if you bring that one back, that much charring means open flame at one point, which probably means other parts are smoked too. Good luck.

MELF is just a package type - means Metal Electrode Leadless Face, a kind of surface mount package.
 
Thanks Kastein. I took your advice and replaced the MELF with a 1N4001.

Your right.. she is smoked. But i looked at all the pics on this forum, and followed the repair that a guy made on another forum (that used this forum). Kind of had to draw up my own schematic based on the jigsaw puzzle i had.


Well... she fired right up!! You can't keep a good Jeep down!!



y4m6IZhCtELQc6LJPm_UbH_GUdALz5JiLjez_ML77WoarKkkdr_7o9ioTQ3YMbBSJzidZDPLeVIQ9IpUxsuW-HP4QIjuiE4eU2L5-klSIfKO3lKPbYdbaSjqL02xgz1-25hbezRhSQ9fzVqdwRNbuw9L3RYubOk3Mt6nuv5Z3iKiynKG_v9j_AWzl1hYD28YrQ6t5lqOYMD1EvWQezemZG4aA



y4mdCIg6JJp9niNN7R2LjwO6oKN6po5JJFmUayfPyaLdahLEzmtRXOFrpmrGDFb4tG8VtQyJbr_hVMKDmi78CxU6g6QlXxLuO85SEJSrkbfoY5HS4nW3nCWySWzdo4cxa_Qfq_ehIHy-2F-CQNHRy2QvQe9qKgALArjTK08kkbIe1uAaMDRc7e7bT6Sbw7mW7HQu1ms32EVuyV9_r3U4u7tPA



y4mQ49au3UV5grK9-Incg9vooG2MldNhgtuFRNuxRLCrJqJRtllRkeHz3pUX9--LGeM_w0jNeF8PYYoxZEUwmDvzxjJEPOYx12KfOPZOW1644m_PeLSXh6DL1jrZewDMFgT9QFwbeJjnvQp_9tXlBFSXe5qKi8OoT2y2vvb75-zW9IzvEKEbgLpuU_ULfAJC-ThP-0MLZqabqCnmm1omtNhNA



I still have to heat the heat shrink up, use liquid electrical tape, seal the board and check everything over, but its a step in the right direction.


Cheers,
 
Holy shit, I never would have expected that board to live again. Well done.
 
This thread has been just what the doctor ordered, thanks.

My board has a burn spot next to the right most capacitor which I believe has damaged the traces to the display. I have looked at every picture I can find of the board, but can't see exactly where each trace goes so that I can repair them. If you are looking at the board in the correct orientation for the display, with the voltage regulator at the top left, I need the traces for the top right 5 pins on the display. It looks like 3 traces come from the top right , snake next to the right most cap, then connect. Anyone have a good picture that clearly shows where the traces connect?

capacitors_6b504280a9f319a734ca1333f2609eeabbff4704.png
 
I was going to edit my post to add more pics, but forum won't let me yet (maybe low post count). I am using these pics from Cherokee forum.
instructions10_3002b4da5082a121d26a2b797011e1aeddd4071e.png

You can see in this image(upside down, but I will talk to them as we see them in the pic), the bottom most traces on the left side coming to the display. The top most of the three looks like it connects to the 2 pin from the left, the middle connects to the 3rd pin, and the bottom connect to the 4th pin. Can anyone confirm?

Looks like maybe the trace between the 4th and 5th pin doesn't connect to either, but it is quite hard to see. Those pins then go on to connect to the other side of the display.

Thanks for any help.
 
I don't have time to post a picture but will try to describe these connections:


Starting at the top two circled in green Let's call these 1 and 2 (left to right) The next row would be 3 and 4 and the bottom row would be 5 and 6


1 2 green
3 4 red
5 6 red



The one that everyone misses is that #4 and #5 should be connected together. This trace ALWAYS burns up.


#1 goes to pin 4 (which also goes to pin 5 which also connects to pin 3 of the regulator i.e the ground)
#2 goes to the + side of the electrolytic capacitor next to it
#3 goes to pin 2 of the regulator
#4 goes to the ground pin of the electrolytic capacitor (as well as pins 1 and 5 and pin 3 of the regulator)
#5 - same as #4 and #1
#6 goes to the pin 3 of the regulator to the left (marked as L4812CV)


HTH
Todd
 
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Todd, thank you for the thorough write-up, and I will use the info, but unfortunately I shouldn't have used someone else's picture that had circles items. I don't only need to know how to connect the burned up capacitors, I also need to know where the traces for some of the display unit pins go. Specifically, the top right 5 pins. I cannot really tell if my traces are still connected in the burned up section under the far right capacitor.

My second picture and post shows much more of what info I am looking for.,

Thanks again, Aaron
 
That can be a challenge. Almost all of the pins on the display route down to the square chip in the middle (TMS017) however, the main CPU (the other square chip marked as SC93704) routes a number of signals between the pins of the display. I have seen these break from time to time but in most cases, the easiest way to fix them is to remove the display first.


I have fixed dozens of these and the number one cause of failure on these board is those 3 electrolytic capacitors but the second most common cause of failure is the crystal. Specifically, the version of the board that uses the 3 pin crystal (aka oscillator) has a built in capacitor that breaks down. If this isn't working, the Main CPU can't boot. In the pictures below, the crystal is the blue "blob". They don't make these any longer but it is relatively easy to adapt the design back to a 2 pin crystal.


Documenting the display pins will take some time but if you want to send me a PM with your email address, I will try to carve out some time to pull it together. I hesitate to fill this thread with all of that info since most people will never use it.


HTH
Todd
 
Todd, thank you! I have ordered the three capacitors and a new crystal. My board used the 2 pin crystal, not the 3, but I ordered a new one anyway.

It is very difficult to see if the traces below the right most capacitors have actually failed, so I may replace the crystal and capacitors first, then if there are still problems, work on the traces.

Thanks again, Aaron
 
Two direct questions since my board still isn't working after replacing all 4 caps, the momentary buttons, and the crystal:

1. Post 60 used to include a pic of where to put the 47 pF cap in the crystal circuit. I originally had a 2 prong crystal, but I did install a new 4.0 crystal and assume I need the cap based on post 60. Does it go in parallel?

2. Since I have the older 2 prong crystal, I have an additional tan cap on the board to go along with the other three near the 100uF caps. It was broken and I removed it. Do I need to bridge anything to replace it?

Hopefully the 47 pF cap fixes my problem after I find out where it goes. Otherwise, I may have more traces to check (and questions about where they are). Thank you!
 
Thank to Todd, my board now works! I had to repair two traces that run in-between display pins at the top right corner near the right most capacitor. I also had to add back in a 47 pF capacitor since mine was destroyed for the crystal. I have a two post crystal board originally, which has a cap right below the crystal, so with mine broken, I had to replace it with the 47 pF going from the crystal to ground (used PIN 3 on the regulator)

I went ahead and replaced all 4 large capacitors, the crystal, and both the momentary buttons.

I have some great pics to add to this board, but I still don't have enough posts to allow attachments.

Thanks again to Todd: he went out of his way to help me, and is the finest example of a person with knowledge just wanting to assist others in need. Hats off!
 
Really glad that this worked out for you!!


I read the other post and it appears that the author was referencing pictures that I had posted in this thread. Apologies to anyone that this affected but they were stored on Photobucket and were deleted when Photobucket decided that I would need to pay them to keep them on their site. If anyone needs them, feel free to PM me.


HTH
Todd
 
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