MTH PS1 Bottom Board Keeps Blowing

That is the battery circuit current limit resistor.  Do you have the battery connected opposite with the wrong harness.  PS-1 and PS-2 5V battery harness with molex plug are reverse wired.  So using wrong harness can do that.  You also can have a fault on the bottom board battery transistor or the 9V regulator going high voltage.   G

So the first thing to try (once I replace the resistor) is reversing the polarity of the battery harness?

All three of the stock harnesses I have are wired in the same polarity.  How could the battery get connected opposite?  Here's some pictures of the Class A board with the current limit resistor replaced that was replaced. There also appears to be some charring and dodgy solder around the solder points for what I believe is the 9v regulator, possibly due to a botched repair?

Well, that's one of the more ugly solder jobs I've seen if that's any indication.

Yeah, I'm not happy with the repair I paid one of OGR's forum members to do on this board.  Could this mess under the regulator be the cause of the problems?

It's hard to see if the solder is actually bridging, but the fact that it's bulging out like that suggests that it might be a cold solder joint to the board anyway.  I can't say if it's actually the source of the problem, but I'd certainly want to clean that up.

Just to be clear here, you had an engine that had problems, and you put a good board into it from another.

Now you have 2 engines with problems, right?

Yes.  Initially the Class A was having issues when moving in and out of neutral and blew that battery current limit resistor.  I sent it to a member to have the resistor replaced, and it came back with no better functionality, but a blobbed up 9v regulator as well.  In an attempt to troubleshoot, I swapped in a bottom board that had previously worked fine in an AC-6 (both engines are PS1), leading to the Class A to work properly forward and back, but with a loud buzzing from the speaker that wasn't there before.  When I put the bottom board back in the AC-6, it caused the buzzing to start in that engine as well, and then a few minutes later blew the battery current limiting resistor in that board.  So now I have the Class A board that acts erratically when going from forward to neutral etc, with a seemingly intact resistor, but this blobby mess on the 9V regulator, and the AC-6 board with a blown resistor.

OK, thanks.

Trouble shooting can be an art in itself. Not solid lines of right and wrong all the time. A few rules make sense and help from further trouble or making a situation worse.

I got the full history here now I believe. I'm not picking on you. I did something similar a year back or so. Another poster jumped on me assuming that I did something, or eluded to something.

I installed a good board set into a troubled engine. However I thoroughly tested all before the attempt. I checked all wiring and made sure I would not short anything out. I couldn't find any issue on the frame or wiring.

He assumed that I asked for more trouble. I am not a tech. I don't have the proper test rig. I risked a known level to figure out what was wrong.

I believe you did too. So learn from that going forward in the required fixing of these toys, and assume the associated level of risk.

Oops! Edited after reviewing old post of mine.

So going back to GGG's point, if I can cobble together an upper and lower board with intact components, I have to reverse the polarity (switch the wires) going from the battery to the battery socket on the board?  I don't get how the polarity required is the opposite of all my stock battery harnesses, including the one that came with this Class A.

The ridiculous solder joint that John points out can cause two problems first the solder can be bridging the gap and cause other problems but who ever did that could have fried the component with that much solder that close to the case. The heat can be conducted into the junction of the device. A good way to kill any solid state device.  I am assuming that you know for a fact that the two boards that you mated were good functional boards before you connected them but had a loud buzz in the speaker afterward. One question that has not been asked. "What are you using for a soldering iron" ?  Very many soldering irons can have a tip potential when plugged in and measured to a known  good ground. This tip potential can be as high as 40v.  So hook your VOM to a known good ground and then with the iron plugged in and the meter set on VAC  start measuring with your VOM set at something like 200v and if you see nothing turn it to the next lower range when you are finally on the lowest range and see something under 2VAC that iron is likely suitable for modern electronics though 0V is the goal.  Never use solder guns on these boards quick heating guns are induction devices and can have a tip potential as high as 40v.  John swears by his Hakko Iron I have a professional Weller esd safe iron that lives on my bench with a 0v tip potential and a couple of cheap chinese irons that read 0V as well in my tool boxes.  A bad iron can fry components faster than you can change them. Especially ICs and  FET transistors.                               j

Drive system problems may start with a motor that is not operating properly.  Even if it is working, (turning), it may be drawing a lot of current beyond the drive board's capacity.

@JohnActon posted:

John swears by his Hakko Iron I have a professional Weller esd safe iron that lives on my bench with a 0v tip potential and a couple of cheap chinese irons that read 0V as well in my tool boxes.

The Hakko irons are ESD safe models as well.

So I've considered just buying two PS1 lower boards and forgo dealing with trying to have these temperamental lower boards repaired or attempting to repair them myself.  Could there be something wrong with the upper board(s) that are causing the lower boards to fry?  The upper boards on both engines look perfectly clean and intact.  The forum seems slim on people willing to sell PS1 boards lately though...

Ok if these are original harness you are good. Now that I see that 9V solder job that could have damaged that regulator which also allows voltage to go high tracking with track voltage.  That causes the buzz can cause a loss of the resistor and can damage the top board.

If the top board got damaged that may have caused issues with the new bottom.  Or you damaged the regulator when swapping boards.

I have board I can sell.  You can send me the 2 bad board sets and I can test and make sure they are working fine.

When moving boards that insulator needs to be in place and you need to ensure the regulator does not get damaged in the movement.  G

@gunrunnerjohn posted:

The Hakko irons are ESD safe models as well.

Anyone on this forum more than a day knows that GRJ would have nothing less.           j

So what ultimately happened here was that somehow I got a 9V wire assembly that was wired backwards, so the 9V battery I attached to the board was feeding opposite polarity and blowing the board. I made the mistake of testing this on a second PS1 engine (the AC-6) and it broke that engine's board as well. GGG discovered this and the repair entailed replacing the board and the 9V battery connection wire on both engines.