Wednesday, January 18, 2012

Fixed a dead 1284p

The Problem:
Through the 1284p saga I ended up with a 1284p that I couldn't burn the bootloader to. It was working (sprinter running, random serial errors and restarts though) then the next day it quit. Later, when I tried to burn the correct fuse bit and bootloader I received a Device Signature = 0x000000.

Unless something had corrupted it, it had the Ext Crystal Osc. setting. I figured the fuse problem out after this chip died. I came to the conclusion that either something was messed up with the fuses or the micro was completely dead. I wanted to build a Fusebit Doctor, but didn't have the time to start another project at this point, so I bought a 644p and shelved the 1284p.

The Solution, or "I Had a Lucky Break!":
I kept thinking about the dead micro, it was really bugging me. Finally I decided to try it with a crystal oscillator instead of the resonator to see if that was it. I found a 16mhz crystal in my spare parts box and replaced the resonator on my breadboard with it, couldn't find 22pf caps, and the caps I found didn't work with it.

I still managed to burned a bootloader on it with just a 16mhz crystal, no caps. Now it seems fine again with the resonator and the new fuse settings, go figure.

Tuesday, January 17, 2012

Parallax 7345 RV Power Converter (SMPS)

See Also:
Parallax 7345 Repair Tips

A friend had a Parrallax 7345 power converter for a RV that didn't work. We know what happened, It's designed for 120VAC only and somehow got hooked up to 240VAC. So it released some of it's magic smoke. (and probably made a big BANG noise I imagine)

Basic Info:
  • The big heatsinks may have 300VDC+- on them, Be careful! (obviously if it's on the heatsinks, it could be anywhere, like those 2 big caps) It doesn't just go away if it's not plugged it either.
  • 45A@13.8VDC (7300 series 45A)
  • 120VAC input
  • Despite the fancy "converter" name it's just a big Switch Mode Power Supply (SMPS)
  • Schematics are not available (Parrallax has on their page "We do not give schematics to anyone perod", or something to that effect), as with most things in the RV universe, they are expensive to replace and don't expect any help repairing them.
The Diagnosis:
It was pretty easy to locate the fault, as there was black stuff all over one spot. (Note: I'd already removed the heatsink from the bridge rectifier in the first picture and cleaned to bottom a bit when I took the second picture)
I looked it over good. It seems a MOV (metal oxide varistor) is missing (vaporized in fact), but that wouldn't prevent it from working, it just shorts the ac line together when they reach a certain voltage, with it gone it should power up, unless something else is broken. Sure enough the line filter next to it had melted a wire clean off, I didn't see it at first because the MOV had left a bunch of residue that hid it.
The other black spot on the circuit board (far left) is under the output resistors, they work fine, but must run a bit hot under prolonged load.

I bridged the broken wire on the filter, sheilded my face and plugged it in. It powered up nicely, made a moderately loud buzzing noise (no load), so I unplugged it and connected it to a 12V battery. Absolutely quiet now, but putting an amp or so into my battery. The battery is connected to an inverter I use for emergency power, so I plugged a toaster in and it jumped right to 45A. I ran a couple toaster cycles then let it fully charge my battery and all seems well.

The Repair:
I couldn't find an exact replacement for the filter, so I had to make a few mods to the board for the one I found. I had to guess on the MOV, I used a 175V MOV (datasheet said 150VAC max) I still have no idea what size would have been right, I even spent a couple hours trying to find how to choose the voltage and still didn't come up with an answer.

But a new MOV and line filter later it's back in service. The good news it the MOV did it's job, protecting all the electronics after it, it should have had a fuse in the AC line and the MOV and line filter may have survived. There was some minor damage to the traces around the line filter and MOV, but nothing too bad. The owner was instructed "Don't hook it to 240 again!", but I think he already knew that.

The other side of the board, nothing wrong here, just in case you want to see it:


I picked up a broken 55A version of this unit. As far as I can tell it's exactly the same board. I'm going to do a post and probably a video of diagnosing and repairing it, but that may be a while. 
I was going to post the correct value of the MOV, but it doesn't have one. The MOV footprint is not populated. I have the numbers off the filter, but google hasn't helped my find anything about it, so it may be a wild goose chase. In case it helps someone:

06847475-000 843TC6
AXM EIA-17-0315-1A

Monday, January 16, 2012


If you read Part 2 of my Sanguinololu Build you saw my arduino ISP (in Circuit Programmer) built on a breadboard to burn bootloaders my 1284p's. It worked really good, but I had to be really careful not to bump any wires loose or pull them out accidentally.
Some day I may buy a proper ISP, but until then it would be nice to have something that would work on the rather rare occasion I actually need one. So I decided to make something more permanent but not require a dedicated Arduino either.

The Project:
  • Connect an Arduino to any 6 pin AVRISP header
  • Include status LED's
  • Be completely removable (mounted on headers)
There you have it, 3 led's (Pulse, Error, Data?) Schematic was figured from the Burning the Sanguino Bootloader using Arduino as ISP page.  LED's are dead bugged on the header, not sure if you even need the cap. just plug in the 2 headers, the 6 pin ISP plug and burn away!

Seems to work so far, Tried it on my other arduino, not really extensive testing, but seems to work.I'd like to make a couple pcb's for various chips with the 6 pin ISP header on them, but that's all for now.

Just built a second one, decided to move the ground from the power side to the io pin side, that allows selection of 5v or 3.3v on the VCC wire. or you can leave it disconnected if your target is self powered.