Reverse engineering, have questions about circuit

[xtal_01]

So, this is off topic as it does not contain an Arduino. That said it on one wants to answer the questions here, can you point me to a site where I can get some help?

OK ... to start with, I am a machinist by trade ... but also build industrial control panels (starters, PLC's, relays, ..). I have always dabbled in electronics .... heck, I am so old (61) that I still have a tube tester and have used it recently. I am trying to teach myself about Arduino. got one ... programmed it and made some LED's flash. I actually have several home projects that I want to work on ... a PLC would be great but expensive ... I am hoping I can use an Arduino for most of them.

So, I was given a job to build a few simple control boxes ... nothing fancy ... they have a pressure switch and just alarm if the air pressure falls below a set point.

The circuit was designed and built years ago (my guess is the late 80's ... maybe the 90's) ... there is about 5 of these boxes and I was told they are "failing" ... not even sure how they failed but the throw them out thinking whey would just make new ones. I knew the guy who designed and built these ... he retired and has since passed .. so no asking him.

OK ... so I got on new box that was left in the store room ... and I got the schematic.

WOW ... no parts list. Worse the drawing does not match the board! The board says version 3 so I am guessing no one ever updated the drawings.

So, time to do some reverse engineering ... and to figure out how this worked (I like to understand what I am building / working on).

Most of it is straight forward ... transformer ... bridge rectifier set up ... 12 volt regulator ... the relay is always on so it alarms if the power fails ..... second regulator to charge the 9V NiMH battery that will alarm if the power fails.

Now I have a list of questions that I don't understand ... I am hoping someone can help. Some of the things I am asking about may have been leftovers from an earlier version and can just be eliminated (there was a 6 pin IC socket and a few jumpers not shown on the drawing and not use along with some long traces across the board that are not used).

1. I may have found the answer to this but am not sure ... the resistor R2 (I checked 1K) around the diode D5 ... why? The diode makes sense as it prevents the power LED from coming on if the power fails and you are battery. It says "optional" bit it is installed in all the units. Someone suggested it was a way of forcing the battery to trickle charge after it reaches it's "full" charge?

2. Why the diode D6? D7 is across the relay coil so that is the flyback diode. The press switch is in the same box so I don't think I need to think about stray voltages. Just don't see the purpose.

3. In red I drew R7. It does not show up on the drawing but is on the board ... and was meant to be here as there is silk screening on the board showing it. It is smaller than the other resistors (maybe 1/4 watt rather than 1/2 watt) ....Brown Red Black Brown Brown ... so 1.2K .... this is even odd as the other resistors on the board are 1K .. why not make them all the same if it is needed. Just completely lost with this one.

4. I think there may be a problem with the yellow power LED glowing when the power fails (it should be off). If I am right, power will feed from the battery though R4 ... then through R2 ... then R3 and give about 3 mA to the LED. Is there any simple solution to prevent this without effecting anything else. I might have a solution .... isolate the yellow power LED with the spare set of contacts on the relay ... but there might be a better solution.

5. My last question is on heat sinks. The original ones I think are obsolete (heck, even the fuse is obsolete). Box is a bit tight so I can't do my usual "go big" approach. Looked on DigiKey ... is 50 deg C @ 2 W better than 56 deg C @ 2W? Is 28 deg C / W the the same as 56 deg C @ 2 W? Don't think it is critical but I want the most cooling I can get in the little area and I don't understand the ratings on the heat sinks.

If anyone can explain this, I would truly appreciate it. Also, since I an cleaning up the board, if any one has any suggestions, feel free chime in.

Thanks !!!!!!!!!!!

[Paul_KD7HB]

Have you investigate services such as "this service?

[xtal_01]

I had never heard of them before. I want to take a shot at it myself first. I have already done a bunch of clean up ... there were just my last few questions I had.

[Paul_KD7HB]

My former electronic assembly company had a long-time customer whose major business was refurbishing dental office equipment. Most of that included replacing one or more printed circuit boards. We would not get involved in doing the copying process because of copyright laws, but always built the new boards they supplied. Very few boards had trace errors, but they did happen.