Medieval Madness mid-playfield G.I. problems

[Longstreth]

Hello Everyone, I'm new to pinball ownership (~1 year) and I've run up
against my first repair that I haven't been able to figure out. I own
a Medieval Madness, and the mid-playfield G.I. circuit is giving me
some trouble.

If I put a fresh fuse into F113, the mid-playfield G.I.s will come on
just fine... for about ten minutes, then the fuse will blow again. At
first I tried replacing all of the #44 lamps with #47s to cool off the
circuit a bit (I also tried LEDs, but didn't like the look) but
eventually this leads to the same result. I checked all the wiring
below the playfield - everything seems solid, with no breaks in the
chain, no bad solder connections.

So then I thought It must be something else. In the manual, the mid-
playfield G.I. circuit is also associated with connector J-105 (which
has no heat damage, and all the header pins have continuity on both
sides of the board) and "transistor" Q4 which is actually a triac.

I'm not sure what the proper DMM readings are supposed to be, but when
I compared the Q4 triac with Q3 and Q5 (the top G.I. and bottom G.I.
circuits respectively) I noticed that the readings were inconsistent,
which I took to mean that Q4 had been fried. I went on Marco's to get
a replacement BT138E triac, and their system suggested a BTA12-600BW,
which it said was an equivalent part.

I watched the circuit board soldering demo in "This Old Pinball"
volume 3 about ten times, ordered a temperature variable soldering
station, did some test solders on some old broken circuit board junk
and then finally replaced Q4 (my first circuit board repair). As far
as I can tell, I didn't botch it (continuity on both sides of each
pin, plenty of silicone heat sink compound between the triac and the
heat sink, etc. etc.) The only problem was that the DMM readings
still came up weird, SPECIFICALLY: with the DMM set to diode setting
(checking for continuity) with the black lead on the tab and the red
on the center leg, there is NO continuity, which there IS on Q3 and
Q5.

I thought maybe the part was faulty, so I ordered a few more with my
next Marco order (successfully replaced the castle gate!), but when I
took the triacs out of the bag and tested them just on their own,
there was still no continuity between the tab and the center leg.
This makes me think that the BTA12-600BW actually does NOT behave the
same way as a BT138E. Can anyone confirm this?

Also, does anyone know what's going on here, or have any other ideas I
can try? I noticed that C42 and R255 are also in the circuit path
from the connector, to the fuses, to the triac. Should I try to
replace those as well? Any guidance would be very much appreciated!

Sorry, if this post is too long - it's my first time on here, so I'm
not sure what's appropriate. Thanks in advance for any help!

Best,
Alec Longstreth
http://droptargetzine.blogspot.com/

[Lloyd Olson]

I'd supect you might be going in the wrong direction.

You indicate F113 blows after about 10 minutes.

That sounds like an issue past the power driver board out into the game. A
short to GI wiring, a short in a socket.

Long tedious approach I think I'd pull out one bulb on that string at a
time, until F113 doesn't blow, then replace that socket. LTG :)

"Longstreth" <longs...@gmail.com> wrote in message
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[Longstreth]

Thanks for the tip Lloyd! Is there no way to test each of the sockets
with the DMM? If not I guess I'll stock up on fuses and give you
method a try.

Does anyone else have any other suggestions?

[cody chunn]

Inspect all the affected sockets and check for too-close proximity to a
grounded bracket or wire, or switch/solenoid lugs.

-cody

"Longstreth" wrote in message
news:1cb0dbbc-d31f-4dca...@x6g2000vbk.googlegroups.com...

[barakandl]

Suspect the issue to be on the playfield. Odd readings on the triac could be from shorts. Make sure no lamp sockets are mangled, touching anything.

GI runs in strings. Get the manual out and see if you can isolate the string that is giving you trouble and popping the fuse.

[Longstreth]

Will do, thanks Cody!

Can anyone weigh in on the Triac discussion above? Is it worth me
tracking down a proper BT138E to replace the BTA12-600BW that I have
now installed for Q4? Or is that part okay where it is? With it in,
the mid-playfield G.I.s still came on for their customary 10 minutes
before the fuse blew, so I'm assuming it's okay?

Thanks for the quick replies you guys, this is extremely helpful!

[Sk8boss]

If your gonna try pulling bulbs till the fuse doesnt blow try the
opposite. Pull all the bulbs first and see if the fuse blows: If the
fuse remains ok you know is on the string somewhere....put one bulb at
a time in. Wait between each bulb replacement. It will save you on the
fuses, but will take a little longer.

[Longstreth]

Nice! Thanks Sk8boss, that makes more sense to me. I'll try that
tonight.