blake

sammichSID LCD troubleshooting help needed

7 posts in this topic

Hi There,

I'm trying to get my sammichSID up and running and am having an issue with the LCD. Specifically it is particularly dim with no backlighting at the moment.

After I assembled the board I went through and tested the voltage at all of the points recommended in the build manual without any problems, and at that point the LCD was quite bright with the backlight pot having an effect on the backlighting.

After I installed the ICs, minus the banksticks and SIDs, I hooked the sammichSID up to my computer to start loading the software. Shortly after powering the sammichSID on the LCD went quite a bit dimmer, although I still had the top row of rectangles, and something smelled in the unit. I shut the power over and checked all of the components to see if I could find anything overheated, but all of the components seemed cool so I assumed the smell might be the heatsinks or compound warming up for the first time. I checked my ICs on the control pcb and found that I had mistakenly put the 2 74HC165 shift registers in backwards. Since the ICs didn't seem overheated I put them in the right way round and checked for the 5v on the pins, which was present, and started again. The LCD still had the same problem. Assuming that I had fried the74hc165s, I replaced both with a couple of spares I happened to have and I still have the same problem.

Oddly, when I was poking around with a meter with one of my probes, I accidently touched the top 2 pins on the left row of the LCD together and the screen lit right up.

I'm using the LCD that shipped with the kit. I believe the LCD is considered "high voltage" so I have the shut on the J4 jumper and the other set to 12v.

I believe the problem is confined to the LCD. MIOS 2 is reading the pic and all of that seems okay, although I haven't tried loading the SID software yet.

I was reading through the forums and saw mention of the fact that the LCD needs to be "set" for a voltage input and can get set incorrectly if it sees a wrong voltage and needs to be "reset" somehow. Does this ring true to anyone?

Any advice on possible causes would be very helpful.

Thanks,

Blake.

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Did you use a highpower unregulated PSU? because it sounds like your backlight haz gone up in smokes.

Please check the output of the PSU with a multimeter on the connector. If you use an unregulated one you will measure something like 14V on it.

Edited by Shuriken

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Did you use a highpower unregulated PSU? because it sounds like your backlight haz gone up in smokes.

Please check the output of the PSU with a multimeter on the connector. If you use an unregulated one you will measure something like 14V on it.

No, my psu is regulated 12vdc. I tested the output before I used it and was getting 12.1 volts which I assume is within range.

I definitely had some ICs driving the LCD in backwards, and now I'm thinking that caused transistor T1 to blow. My meter is telling me its dead.

I'll get a replacement tomorrow and give it a shot anyways, rule it out at least.

Thanks.

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The ICs being in backwards should not have caused T1 to blow. That usually happens if the current going through T1 is too high, due to the brightness pot being faulty, or if it's at max. and the backlight voltage (when it's set to JBL=12V) is too high (when using unregulated input power). I'd recommend testing the backlight pot as well as replacing T1. If you need more help (or replacement parts), just email me.

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The ICs being in backwards should not have caused T1 to blow. That usually happens if the current going through T1 is too high, due to the brightness pot being faulty, or if it's at max. and the backlight voltage (when it's set to JBL=12V) is too high (when using unregulated input power). I'd recommend testing the backlight pot as well as replacing T1. If you need more help (or replacement parts), just email me.

Ok so I tested a new T1, swapped it with the old one, and also tested the trim pots, which gave me some resistance readings that I thought were low. Since I wasn't sure I powered the sammichSID to try it out and the LED lit up nicely... for a few seconds until it went dim and smoke came from the area of the base pcb around TI. I switched the power off and checked the components for heat. Only the housing on T1 was warm and when I tested it, it had definately blown. So too much current is going through T1 and I need to find out why.

When I check the pots I'm getting some odd readings. When I check P2 on pins 1 and 3 I get 3.6k and I assume I should be seeing somewhere around 10k. When I check on pins 1 and 2 I get resistance varying upwards toward 3.6k when turned fully anti-clockwise. To me this seems more or less correct other than much less than the 10k it should be providing.

When I check P1 on pins 1 and 3 I get varying resistance from 0 to 49.3k depending on where there trim pot is set. Its my understanding that I should be seeing the max resistance at these points, somewhere around 50k. When I check pins 1 and 2, I get no resistance regardless of trimmer settings. It is my understanding that I should be seeing resistance varying from 0 to around 50k on these points. I'm assuming this trimmer really needs replacing, and may have been part of the current overload damaging T1.

I'm a bit nervous about plugging my PSU back in after swapping these parts though. Its labeled as a 12vdc 1a output and the guy at the electronics shop specified that it was regulated and should put out a constant 12v at all times. I checked it before I plugged it in to the pcb and saw 12.12v so I thought it was fine. Would an unregulated PSU show around 12v when you test it but output more under load?

Thanks,

Blake.

Edited by blake

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I removed the pots to replace them and decided to test them, and they seem fine when off the pcb, so the readings I got must have been because they were on the board.

It may be that an incorrect PSU is the problem after all. I've ordered the Jameco psu and will see if that corrects the problem.

Thanks,

Blake.

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Ok, so I ordered the recommended PSU from Jameco just to be sure that the problem wasn't due to an unregulated power supply, and I still had the same problem with TI burning up and smoking when I switched everything on.

I replaced the trimpot for the backlight thinking that might be the problem and the light was bright again, so I was hoping that was the problem and everything would work ok. I went ahead and started loading the software, which appeared to go smoothly, although I didn't see a MIOS boot screen, simply a row of rectangles. I thought maybe the trimpot setting had to be changed, and when I turned up the backlight trimpot past a certain point there was a pop and some white smoke again. Probably from T1.

I replaced T1 again and got some funny behavior, initially the rectangles were quite bright, but they flickered between bright and very dim a few times and then stayed dim.

When I pulled the sammichSID apart again, I noticeD that there was a fairly large blob of solder on the top left connector pin (B-)to the LCD which looked like it was in contact with the nut holding the nearby screw in place, creating a solder bridge of sorts. I cleaned the excess solder up and tried powering the unit up again but everything was dim again.

I switch T1 again thinking another one was shot, even though it hadn't started smoking or felt warm, but the new one didn't change things either. When I test the new T1 with the meter I get 1 way signals between the base-collector and base emitter only so I'm hoping its actually ok. I'm getting nervous de-soldering those as its getting hard on the board.

I guess I'm wondering if its possible that the LCD has been damaged through all of this as well. The transistor feeding the backlight, as well and the negative backlight connection appear to be the components involved in this.

At the moment my LCD has dim triangles, no backlight, and wont show characters... although It appears that the software loaded correctly. I get the cluster of LCD flashes when I boot up, and a couple of LDs when I press switch 6 right now.

Any advice insight would be helpful. I'm running out of ideas.

Thanks.

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