RobertShaw HS780 Teardown

I recently replaced a old furnace with a 90+ for my father, I don't recall the make or model of the old furnace, but I salvaged some parts from it to look at. Today we have a Robertshaw HS780 Hot Surface ignition control. The furnace was working, so there wasn't a failure to fix, but it may be of some interest.

RobertShaw HS780

If you watch the video be warned, It's my first attempt at an ad-lib teardown. I kept talking as I figured out the board and was a bit off. It's not bad, hope to do better next time. I think the left small relay is the gas valve, the right one powers up this board when the thermostat closes.

Top

Bottom

By far the most common failure on HSI ignition systems is the ignitor (not this control). They are fairly fragile. Never touch the heating part, it could cause early failure.

Diagnosis would go like this:

• 120VAC present at terminals labeled 120 and NEUT?
• 24VAC present at TH and TR? (thermostat calling for heat?)
• When calling for heat is 120V present at  IGN terminals AND HSI not glowing? (if yes replace HSI, if no suspect large relay)
• HSI Glows and 24VAC present between VALVE and GND? (if yes, suspect gas valve / safety switches/wiring, if no suspect outer small relay)
• Failing all that test the inner small relay, inspect all solder joints for failure and caps for leakage. (if you can't find anything at this point consider replacing the control.)

Sometimes a relay will actually fail (shorted coil, open coil, or bad contacts) and they can often be sourced for $5-10 from digikey or a similar company.

Video Notes:

1. The fan/limit control won't come apart easily.
2. Typical setting are ~100 fan off, ~120 fan on, ~180 high limit.
3. You shouldn't be bouncing off the high limit, if you are you have an airflow problem, oversized furnace, etc.
4. Flame sense is through the ignitor.

ATX PSU Repair

As noted in the HTPC build I used a repaired Ultra D0408 ATX PSU. I was using it in the P4 and it would work fine for a long time, then just start randomly rebooting, then it would be fine again. At first I thought it was the motherboard or ram, but then discovered if i bumped the desk or case it would almost always start acting up. Of course that screams "Something's Loose!".

So I had the opportunity to remove it and inspect it as I did my upgrade. Here's what I found:

Caps all look ok, no bulges, etc.

Cover Off

Be very careful when taking these apart, Voltages may remain for a very long time. I held it by the wires to remove the board, flipped it over and checked those 2 big caps (upper left). They had less than 2VDC in them, but depending on the failure it could be over 300VDC.

Bottom of board

Now we can see something, I don't know if you can see it in the above picture but in the lower right corner there is a capacitor that has come loose from the solder. It is on the 12V rail. The heatsink on the left of the unit also has a bad solder joint, but it appears to be isolated.

Bad solder joint (about in center)

So I soldered it up and will try it out. These units are cheap, so it doesn't pay to spend much time fixing them, but sometimes it's something simple like this. It's also practice for things like the Parrallax. Time will tell if that was the problem or if it's something more serious.

Logitech Wave Missing Keystrokes & Teardown

My Logitech wireless keyboard started missing keystrokes. It's really irritating. I tried a few different things before I figured it out.

•  It was recommended to not have it's receiver plugged into a hub, so I plugged it directly into a usb port, that didn't fix it.
• I tried the "smash it in frustration" approach, which only succeeded in breaking one of the feet.
• It was so bad I decided to tear it apart and either finish it off or fix it. I didn't find anything to fix.
• I eventually discovered that it works great backwards, or if I hold my hands at a funny angle. it seems the PCB and antenna are under the wrist pad, and something is causing me to interfere with it.
• I dropped the receiver behind my desk almost at foot level and the keyboard seems to work great now. The mouse is another story...
• I'm contemplating running a new antenna around the perimeter, so part of it is not under my hands

Teardown
 It's all held together with philips screws, Thank you Logitech for not using some stupid tamper-resistant screws. Be careful, pulling the membrane off created quite a static charge, I hope I didn't do any ESD damage.

 The switch thingie is a 3 piece plastic thing with circuits and contacts on the top and bottom with a separator in the center with holes for each key

I wiped each piece down with rubbing alcohol.

 

Reassembly
 Put the zoom button and mute button on the lower part, all the other keys are snapped into the upper part.