SIPs ? Plenty of scope for getting one of those in back to front as well

 

They don't have a common pin, it's an 8 pin case with 4 x resistors. So I don't think they can be wrong.

 

Hope it's a 10pin SIP with 5 resistors! yes you cannot get it backwards

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Remove Q4 and test VPP again, if all is well with Q4 out of circuit, your problem lies around Q6 or SW6 and other reset components, R16, 17, D3 & SW4. Perhaps U2 also.
The fact that you are generating 12.35V suggests that the SMPS is likely fully functional, I suspect that something is pulling excess current, perhaps a leaky diode or transistor junction, or reversed polarity or orientation.



rgds

 

VPP on the ICD header reads the same as +C1 (as far as my meter is accurate).

Check for cold solder joints on the 4.7K and 10K SIPs.[/QUOTE]

I'll have a look along them in a while and touch up any 'bad' ones. If that doesn't work, I've got some time to sit and look over the circuit again. If you have any suggestions based on the above reading, that would be great.

 

I've touched up a few joints but no difference. My soldering isn't brilliant but everything else seems to work

VPP on the plastic header above the 18F2550 also reads correctly.

Any other ideas?

 

Well if VPP is fine (and it appears it is) then test the VPP run (+5) & reset (Gnd) modes (also in troubleshooting)

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Aren't they for PGC and PGD? Both appear fine, GND is around 0.09v and high is around 4.75v (the USB port on the machine I tested with is a little low).

I'll scour the circuit again later.. any other suggestions?

 

What should the resistance be between pin 20 on the 18F2550 and pin 23? Mine shows they are connected.. this shouldn't be possible because I don't think the 2N3904 (Q3) would conduct from collector to base.

Even if an NPN transistor did conduct from C to B.. there should be at least 120K resistance due to R1B, R1D and R7 (assuming the test voltage from the meter gets through L1 and D1). Right?

Please correct me if any of the above seems wrong. I could be completely out on the theory.

I ask because it appears there might be a little defect between the traces on the top layer from pin 20 and pin 23. This is barely visible between the 28 pin socket and the connector at the top of the board.

If there shouldn't be 0 resistance between the two pins, could this possibly be the cause of my problems? I can get out a macro lens and try to take a picture later if the resistance shouldn't be 0.

 

Around 28 megohms on mine.

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0.6 on the 200hm: scale, 0 on all other scales. So our readings are different, any thoughts Bill?

 

Mine agrees with Futz.

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Can you explain how you work out from the schematic? I've obviously got that bit wrong and it would be good to know why

However.. the fact that I get virtually no resistance suggests that there might be a short somewhere. It could be where I think there is a fault on the PCB, I'll try and get a picture.

 

Remove U1 and measure again without power being applied, if the measured resistance is still a few ohms, I would agree with you in that you have a short on the board. If the resistance is no longer only a few ohms, measure the pins on U1 for a possible short

rgds

 

He He. You read that prior to me nuking the bit about the schematic. There are so many poeple here who know more about this then me that it makes no sense for me speculate. I am a CS type by training.

But here is what I was thinking
It should be infinite on an unpopulated board. No path between the two pins.

On a populated board with the power off I do not see any reasonable path between the two pins. I can see a possible path from pin 20 to the inductor D1 R7 Q3 collector to base, R18 to pin 23. But the resistance between the collector and the base of the 2n3904 (Q3) is for practical purposes infinite. It is a good spot to look for a short but even if it were you would have more the the .6 Ohm you are seeing because the other resistors in the path.

I do not put much faith in the logic. Any EE should know better. I will take my lumps

Now

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