Have you connected the siegcraft boards to the controlled power lamp braid on the BG lamp panel?
That power wire is daisy chained between the siegcraft boards and it sounds like there's a broken connection in the daisy chain.

BTW, the Solenoid Expander Board is controlled by a feature lamp so may explain why it's misbehaving.

2.56VDC is a problem, you should be seeing around 6.5VDC on the feature lamp power braid.

What voltage do you measure at TP1 (feature lamp power) on the rectifier board, and also on the braid wire at the feature lamps on the playfield?

Lastly, measure at the point on the three siegcraft boards as shown below - they should all measure the same voltage of 6.5VDC

Seigcraft_Power.jpg

Sounds like your BR1 bridge rectifier on the rectifier board has an internal diode that's gone open circuit and it's only providing approx half voltage.

Just confirm that with your meter set to DC, black meter lead on the GND test point on the rectifier board and the red meter lead on the "+" point of the BR1 rectifier you're only getting 2.8V instead of 6.5V

Also measure the DC voltage on TP3 of the rectifier board (12-14VDC) just to make sure your meter is giving somewhat accurate readings
.

If you measure AC with the black meter lead on ground, you will get an incorrect reading in this scenario. You need to measure across the two squiggly line marks "~" on BR1 to measure the transformer voltage at that bridge rectifier. i.e. one meter lead on one squiggly line point of BR1, the other meter lead on the other squiggly line point (opposite corner) of BR1.

Cool.
Ok, so what's happened with the feature controlled lamp problems?

Can you post pics of the problematic lamps? Knowing exactly which lamps are playing up will help map where the problem is. The solenoid expander is actually controlled by the lamp board so first thing is to get the lamps working properly.

The feature lamps are powered by rectified but unregulated DC - i.e. the DC is not flat and is rippling in semi sine waves. When measuring AC on this type of supply rail, your meter will tell you how much ripple voltage there is.

The +5V power to the MPU board is regulated DC - i.e. the current is near flat line like a battery and should have no ripple so measuring AC on this type of supply rail should result in a near zero reading.

From the diagram below - the blue line is zero volts reference.
> The AC waveform is what power looks like out of the transformer and into Bridge rectifiers.
> The Rectified DC waveform is what power looks like on the output of bridge rectifiers.
> The Filtered (via capacitor) DC waveform is what happens when you add a large capacitor to a rectified DC supply rail - think of the 12VDC supply on these games due to the 11700uf capacitor at C23 on the SDB.
> The Regulated DC waveform is what the output of the +5V regulator on the large heatsink of the SDB looks like.
> The Semi rectified DC voltage is what was coming out of your faulty BR1 bridge rectifier - note half of the waveform is missing hence you'll measure about half voltage.
.

Voltage_waveformsA.PNG

The lamps that are stuck on are all over the shop.

Is there any animation in any of the lamps during attract mode?

What happens if you remove all the siegcraft boards from the equation? Do the lamps still stay stuck on?

Have you got a logic probe?

Ok, with the lamp board reconnected, hookup your logic probe and probe the following pins on the lamp board:
U1 pin 1 =
U1 pin 2 =
U1 pin 3 =
U1 pin 21 =
U1 Pin 22 =

U1 Pin 23 =
U2 Pin 23 =
U3 Pin 23 =
U4 Pin 23 =

These pins should all be pulsing. Any that your probe indicates are stuck high means you have a connector issue between the MPU board and the lamp board - most likely at the MPU side since the Aux lamp board is also affected with stuck on lamps.

Follow this one back to the MPU board. It comes from MPU J1 pin 11 and it's the "Lamp Strobe #1" signal.
I would have expected that most of the lamps would have been flashing rather than just stuck on solid with this signal missing at the lamp board.
Anyway, to the very right of MPU J1 pin 11 is resistor R78. First probe the left leg of R78 which goes to J1 pin 11.
Then probe MPU J1 pin 11.

Don't burn yourself

The F3 fuse is for the 12V power supply rail to the SDB - that supply rail does not go to the playfield.
Have you repinned any of the connectors between the cabinet and the head? Maybe accidentally put a wire in the wrong position or plugged a connector into the wrong socket/location?

The lamp strobe #1 signal goes from MPU J1 pin 11 to the Lamp board J4 pin 13. Check/reterminate the wire terminals at both of those connector locations.

Can you disconnect all three connectors fro the Auxiliary lamp driver board.
Does it change the behavior of the playfield lamps and if yes, which lamps?

Next, put the Bally lamp driver board back in.
With the machine still OFF connect a jumper wire to TP2 on the lamp driver board which is ground.
Connect the other end of the wire to pin 1 on either of U1, U2, U3 or U4 (the four chips have the same signal on pin 1) or an easier location on the same signal is the bottom leg of resistor R75 on the lamp board (R75 is top left corner area).
Does shorting this signal to ground change the behavior of the playfield lamps and if yes, which lamps?

In that case U1, U2, U3 and/or U4 on the lamp board may be faulty. Grounding pin 1 at these chips stops their ability to listen to the MPU board lamp select requests. In this state, most if not all of those stuck on lamps should have extinguished.

1. Are these lamps all stuck on from the moment you power on the machine or do they all come on once attract mode starts?

2. What voltage are you now measuring at the base of the lamp feature lamp sockets on the playfield, i.e. the power braid connecting to each controlled feature lamps.

3. Are any of those U1, U2, U3 and/or U4 hot to touch?

Jumper that Lamp Strobe #1 signal to ground again (i.e. hook up your jumper wire between TP2 and the lower leg of resistor R75 on the lamp driver board).
Grab your logic probe and probe the pins at U2 as highlighted in the diagram below:
Only *one* of those 16 pins is allowed to read high. They should otherwise all indicate low. If 2 or more of those 16 pins indicates high with your logic probe, that chip is likely faulty. You can do the same with the other 3 chips to test them. let us know the result.

Also try this if you have time: Remove the pin 7, 6, 5, and 4 wires from the J4 connector at the lamp board and leave them floating. NOTE, the lamp board connector pins are numbered upside down, i.e. pin 1 at the connectors is at the bottom and counts upward.
This should make all those stuck on lamps stay off (well at least the ones not driven by the Aux lamp board).

LDB_4514_Ouputs.jpg

Can you post a photo of this blue wire and lamp socket? I'm curious how it could have affected so many lamps.

Post some photos of the connections and new switches.

Does this also happen when ALL drop targets are in the raised position, or only when certain drop targets are down like the rear #7 drop target (rightmost) and rear #2 drop target?

Were both blue wires not connected or only one? Just trying to understand the logic in the stuck lamp symptom because it currently makes no sense.

Have you done the basic stuff and run solenoid test mode and switch test mode to see what anomalies you're getting?

Sounds like the zipper flipper mechanism might be stuck and the solenoids are struggling to move it. Check those solenoids manually to see if they can freely pull in.

Top middle back target. Is it stuck?

Is the solenoid expander working?

I believe this is normal. From memory, my game does it too including the displays briefly going off and on.

Sort out the stuck switch 30, then in switch test mode activate each switch starting from 48 and count downwards to switch 1. Note down if/when you get incorrect switch numbers showing.

If the top right slingshot and bottom right pop bumper activate independently and correctly in solenoid test mode, then this is likely a switch matrix issue.

You've most likely shorted one of the switch strobe wires to ground or another strobe line.

Install the J2 connector on an angle so the bottom half is making contact but the top half isn't and see if the game boots. Then try the other way so the top half of the J2 connector is making contact but the bottom half isn't.
This will help narrow down the playfield switch wires to half that need to be checked.
But I suspect it will be one/two of the strobe switch wires on the playfield where you were making adjustments (the strobe switch wires are the first 5 wires coming from J2).

The switch lines are fed to/from the U10 PIA chip on the MPU board. You've shorted something in the switch wires when adjusting the switches that's causing the U10 PIA ports to fail the power on test. You need to go over your switch work looking for shorted/pinched wires.

Can't quite tell from the third picture but so long as the banded side of the diode is away from the grey-yellow wire, it should be ok.

The problem is that you might be seeing ghosting effects in the matrix.

Does the switch 18 rebound still play up in game mode when all drop targets are raised?
When all drop targets are raised and the ball is removed from the outhole, does switch test mode report "0" closed switches?

Strobe line 5 (orange-blue wire) is supposed to be connected to a resistor on the playfield. The resistor should have a grey-brown wire on the other leg. Do you remember installing this resistor on the playfield?

Yep that looks like it

Maybe the green capacitor on that lower right pop bumper switch is faulty/shorted. Clip one of the capacitor leads close to the switch lug and bend the capacitor away. See if that fixes the stuck switch 39.

The built in test modes are your friend.

Run the solenoid test mode and note down any coils not activating or activating at the wrong time. Only then can we build a map of what's logically going wrong.

When you have switch issues, you need to make sure you're starting from a blank canvas with switch test mode reporting "0" closed switches. As above, if a switch is being reported as closed but it isn't, if that switch has a capacitor then temporarily clip one of the capacitor leads.

Once you get "0" closed switches, begin activating them starting from the highest number switch working your way down as per the switch numbers listed in the manual. It's important you do it in highest to lowest order on these old Ballys/Sterns because switch test mode only reports the lowest number switch it finds as closed even when there are many closed switches.

If a switch reports the wrong number and you have drop targets down, raise all the drop targets, and drop one target at a time to see which drop target is affecting the wrong number switch being reported. That drop target may have a shorted or backwards soldered diode on it.

The Knocker (#1) and Open Zipper Flipper (#17) solenoids are driven by the same wire from the SDB, but are differentiated by the solenoid expander.
Both of these solenoids must be the three lug variety and wired as follows:

Knocker:
- Grey-Red wire on centre lug,
- Orange-Green wire on the end lug with non-banded side of other diode
Open Zipper Flipper: (this is the small coil on the zipper flipper assembly)
- Brown-Orange wire on centre lug,
- Orange-Green wire on the end lug with non-banded side of other diode

No Shield of God button next to the right flipper button? That sucks..

Looks to me like you might have those rear standup targets and rear drop target wires crossed over.
Confirm the following wires:

Left rear standup target:
- White-Yellow
- Brown
Middle rear standup target:
- White-Yellow
- White-Brown
Right rear standup target:
- White-Yellow
- White-Orange

Rear Drop Target #2 (2nd from left):
- Orange-Blue
- Brown-White
Rear Drop Target #4 (middle):
- Orange-Blue
- Brown
Rear Drop Target #6 (2nd from right):
- Orange-Blue
- White-Orange

I was about to agree with you on that until I checked my machine and it's the same. Electrically it's functionally equivalent but wired in a roundabout way that makes it look odd in comparison to how most other switches are wired and doesn't purely match the schematic.
Each switch has a "strobe" signal wire and a "return" signal wire. Most switches have the strobe signal soldered on the side of the assembly with the non-banded side of the diode. On this switch Bally has soldered the wires backwards and flipped the diode accordingly.

It looks like all four gate switches are wired backwards with the white-yellow strobe wire on the tip instead of the side. Not sure if there's a good reason behind it. It's certainly easy to get confused by this. Makes me want to rewire them on my machine so they look right.

Got any pictures of those three rear standup target and rear drop target switch wirings?

Do you understand how to read wire colors on the wiring diagrams/schematics?

Any chance you can repost those pictures in high resolution - it would really help if I could zoom in.

By the way, did you know those three standup targets behind the rear drop targets are a special type with transparent faces? Notice those targets have lamps directly behind them?

https://pinside.com/pinball/forum/topic/bally-transparent-targets-what-machines-used-them/page/3#post-2551919

I can't see any errors in the wiring.

Lets clarify the switch behavior in switch test mode. Please answer all six points.

1. With all drop targets raised and the ball removed from the outhole you get "0" closed switches right?
2. Drop the leftmost upper drop target. What number appears as closed?
3. With the leftmost drop target still down, drop the second left upper drop target. What number appears as closed?
4. With the two leftmost upper drop targets still down, drop the third left upper drop target. What number appears as closed?
5. Raise the two leftmost upper drop targets, leave the third left upper drop target down. What number appears as closed?
6. Since the third left drop target is still down, if it now reports the correct number, find out which of the two leftmost drop targets causes that third target to misreport.

If you have a logic probe, probe the common upper drop targets strobe wire (orange-blue) and also the common standup target strobe wire (white-yellow). Does the probe show similar indications on both wires?

And is switch #43 (2nd rightmost rear drop target) closed?

Have you tested the diodes on those rear drop targets and three standup targets behind those rear drops?

This will happen on any machine because you are completing a circuit path through your body. The sweatier your hand, the more sensitive it will be.

Nope, switch #43 is the second drop target from the right on the rear bank.

MedusaDropTargetSwitch.jpg
MedusaSwitches.jpg

Does this mean you went through and activated each single switch and they all registered correctly according to the switch numbers in the manual?
If you did test every switch, did you leave drop targets down or raise them after each drop target test? In your case it's best to leave them down as you're testing. As previously mentioned you must test from highest number switch and work your way down the list.

When you're checking the diodes on the drop targets, are you measuring the diode with the drop target down or up? Make sure they're up when you're measuring so the switch is open and no other circuitry in the matrix can affect the diode you're testing.

Does this problem still occur if you manually raise all rear drop targets?
What about the switch on the zipper flipper mechanism - does it being closed help induce the problem?

Post a clear picture of the switch. I've seen switches from factory incorrectly put together so it's not inconceivable.

By the way, check the coin switches on the coin door - there's been many switch problems posted around here that were caused by coin switches wired wrong or stuck.

Good.

Revisiting this, your next step is to isolate that diode. Unsolder one leg of the diode and bend the leg out so it's not touching anything. Measure the diode with your meter in both directions.
If is measures shorted then replace it again.
If it tests ok, then measure across the two lugs on the switch that the diode connect to. If the two lugs measure shorted then pull the switch out and inspect why the lugs are shorted - it may be that the lug leafs were incorrectly installed and is missing a separator.

I had this exact problem on a Fathom I worked on a few months ago - a drop target switch had the separator out of place and the diode appeared shorted. Did your switch look like it came from factory like this?

See post #55 above.

When starting a game, only the single middle top target should remain up. I don't see any options in the manual to change this.
Do the top targets all drop properly one by one in sequence during solenoid test mode?

The problem is related to your solenoid expander board. Does the lamp next to it flash during lamp test mode?

Ok, so do you hear the relay on the solenoid expander click, when that lamp switches on and off?
Take the small solenoid expander board out and check the soldering on its pin header - the soldering does fracture/crack over time on these boards. Resolder the header if needed.

Ok, so grab your multimeter and set it on DC voltage. Connect the black meter lead to ground.

The Grey-Red wire from the solenoid expander powers the default solenoids. By default solenoids I mean solenoids that are powered when the solenoid expander is OFF. You should measure 43VDC at pin 7 of the solenoid expander connector and at any of the playfield solenoids that have a Grey-Red power wire.

Go into lamp test mode. When the lamp next to the solenoid expander flashes, the relay on the solenoid expander switches 43VDC from the Grey-Red wire over to the Brown-Orange wire which powers all the non-default (alternate) solenoids that have the Brown-Orange power wire. Check that pin 5 on the solenoid expander connector jumps to 43VDC when the lamp illuminates, and also check that power is reaching the Brown-Orange wire at those solenoids that are not working.

Medusa Solenoid Expander wiring

If you don't read 43VDC at the Grey-Red wire then the rear drop targets 1,3,5,7, saucer and both open/close zipper flipper coils wouldn't work. You probably didn't make proper contact with your meter probes when measuring since you've made no mention of these solenoids not working.

Find the break in the daisy chain of that Brown-Orange wire between the working and non-working solenoids.

What have you tried so far to diagnose this?

If you run a jumper wire from ground to any of these points, does either lamp illuminate?

From the Bally lamp driver board schematics:
SCR Q40 controls the Same Player Shoots Again lamp.
SCR Q51 controls the Saucer 200k lamp.

Note: the following is in reference to the Alltek lamp driver board only.

If you ground the top metal tab on either of these SCRs, the respective lamp should light. This only tests that wiring from the SCR through the J1/J2/J3 board connectors to the lamps are good or not. This does not test the SCRs.

If the above tests work, then you can manually test activate the SCRs.
Grab a jumper wire and connect/touch one end on test point TP3 of the lamp driver board.
For the smaller SCRs touch the other end of the wire on the RIGHT leg of SCR Q51 to switch it on.
For the larger SCRs touch the other end of the wire on the LEFT leg of SCR Q40 to switch it on.

Looks like we missed the first basic test of ground testing the lamp sockets - have you done this yet?

The base of the feature lamp sockets connected by the bare braid wire is 6.5V power, not ground - I hope you didn't blow the F1 fuse on the rectifier board.
You need to jumper the lamp socket lug with the colored wire coming from the lamp board to ground to test the lamp/socket.

You know what to do then
The fault is local to the lamp/socket.

Sounds like the lamp power wire to your LED adapter boards on the Aux lamp board isn't hooked up properly.

Sorry, this instruction doesn't ring any bells. The grey wire is supposed to hookup to the 6.5V feature lamp power braid on the back of the display/lamp door.

Best if you post some pics of these adapter boards and where the grey wires are connected.

Temporarily install standard incandescent lamps in those two non-working lamp locations.
Go into display test mode (i.e. so the game is not activating any feature lamps).
In this state, the base and also the colored wire lug of the lamp sockets must both measure 6.5VDC. Is this what you're measuring?

Well, that's good. The incandescent globe is a very low resistance which is why you get about the same voltage on both lugs in this test.
If you ground the colored wire on that 200k lamp it should work.

The Shoot Again colored wire goes back to pin 22 of J3 at the main lamp board so disconnect J3 from the lamp board and remeasure the colored wire at the lamp again (so the lamp board is out of the equation).

Is this still happening?

Resolder the 6.5V braid wire on the lamp socket base - maybe the solder didn't stick to the base.

Unsolder the colored wire and remeasure the voltage on that socket lug. If it now goes to 7.34V then that colored wire is shorted somewhere.

When this sort of thing happens, give us as much detail as possible. Which lamps, which solenoids, when exactly they're triggered, etc, so we can try to logically map out what's going wrong.

Sounds like you need to carefully go over that overcrowded under playfield looking for shorts.
At least go into switch test mode and do the tests again from highest to lowest switch as per earlier in this thread.

So a few posts ago you mentioned this lamp now works when you ground it. Go back upstream to the lamp driver board and perform the SCRs tests previously done.

Try another incandescent lamp first.

Can you try wiggling J1 on the MPU board and J4 on the lamp board during the display test - any difference? Sounds a bit like a poor connection on the Lamp Strobe #1 signal from MPU to lamp board.

Does grounding the 200k lamp wire at the lamp board connector illuminate the lamp?

You're looking at basic electrical issues with both these lamps that are local to the sockets.
Base of the lamp sockets should have braided lamp power (6.5V). Manually grounding the socket tip lug completes the circuit and the lamp should fully light. Doing this test completely eliminates the MPU/lamp board from the equation.
If they don't light, inspect the lamp/socket/connections on the socket.
If they do light then go upstream to the respective colored wire at the lamp board connector and ground it in the connector then based on the result look for an open circuit on the wire to the lamp, or go further upstream to grounding the metal tab of the respective SCR.
Essentially the SCRs on the lamp board switch the colored wire connection at the lamp to ground under control from the MPU.

Is there any abnormal behavior during game play?
If you go into display test mode again and disconnect J2 and J3 (playfield and cabinet switch harnesses respectively) from the MPU board, does it make any difference?

Great! Time to manually test the SCRs
Jumper a wire from test point TP3 on the lamp driver board to the "Gate" pin of the respective SCRs. If the lamp lights, the SCR is good. If it doesn't light then based on the test results in your previous post, the SCR is bad.

If you look carefully at the lamp driver board, some of the SCRs have "C", "A", and "G" printed on the board near the SCR legs.
The "A" leg is the "Anode" leg that's also the SCRs metal tab - it connects to the lamp.
The "C" leg is the Cathode leg - it's soldered to ground.
The "G" leg is the Gate leg. This leg switches on the SCR and essentially connects the A leg to the C leg - i.e, provides ground to the lamp.

SCR Q40 controls the Same Player Shoots Again lamp.
SCR Q51 controls the Saucer 200k lamp.

If you're still not sure which is the Gate pin of the SCRs, refer to the last part of post #89.
As always, try the TP3 to Gate jumper test with working lamps to make sure you've got the process right.

Does grounding the "A" leg of those SCRs light the lamps? Note the leg layout of the small and large SCRs is different.

Can you do me a favor and post a picture of the Alltek installation stating this instruction?

The wire needs to be connected to the braid on the lamp/display door that powers the feature controlled lamps which is either 5.4VDC or 6.5VDC depending on the generation of classic Bally/Stern.
On Medusa, the very bottom braid wire on the lamp/display door is connected to a thick blue wire and this powers the "Game Over", Tilt" and "Same Player Shoots Again" lamps. Note the thick blue wire also goes to the braid powering the "High Score To Date", "Ball In Play" and "Match" lamps.

The wire from the Alltek board needs to connect to the braid at these lamps where either of these thick blue wires connects.

Yep, that's how it should connect.
Same if you're using the Siegecraft flicker free boards on your Aux Lamp board.

TP3 should be jumpered to the "G" leg of the SCRs, not ground.
TP3 has the correct voltage used to manually test activate SCRs.

If you ground the metal tab of Q40 and the Same Player Shoots Again lamp lights, but jumpering TP3 to the left leg of Q40 doesn't, then Q40 is faulty. It might have a short on the "Gate" (left) leg of the SCR meaning installing a SCR over the top will never work. If you want to butcher it, cut the left and right legs of the original Q40 to disconnect it from circuit and install your replacement SCR in the through holes. This location requires a MCR106 SCR.

The Same Player Shoots Again lamp does not light in attract mode. As per your Paragon project, not all playfield lamps light in attract mode. Use lamp test mode for testing.

I have them under the slingshots and the side plastics that are screwed to the rails - they're positioned where the top locking nuts on the posts are for those side plastics.

Do you work in pyrotechnics? You seem to like blowing things up )

If a wire fell and shorted on the knocker then find out what that wire is for. Was there any smoke or burning smell?
Is your J1 connector at the MPU good? Both lamp and display info come from there.

Do any of the problems go away when you disconnect:
Both J2 and J3 from the MPU board?
Or the lamp driver boards (J4 from the lamp board, J1 from the aux lamp board)?
Or all displays?
Or connect one display at a time - try different displays.

MPU J4 activates solenoids, so your issue goes beyond J1 on the MPU board.

Have you got a logic probe?

What's the display high voltage at now?

MPU J2 is the more important one to disconnect since it's the playfield (where you've done most of the work) switch harness that partly shares the same data bus as the displays and lamps.

So you can start and play a game, just no displays?

Pick any display - are you getting pulses at all these pins on the displays connector:
4 -> enable first digit (ones)
5 -> enable second digit (tens)
6 -> enable third digit (hundreds)
7 -> enable fourth digit (thousands)
8 -> enable fifth digit (ten thousands)
9 -> enable sixth digit (hundred thousands)

15 -> display select

16, 17, 18, 19 -> four pins make up a binary coded decimal number for actual number to display

Do you measure 5VDC at TP1 on the display board?

Wiggle the connectors to see which is affecting the displays.

Pin 9 at the solenoid expander connector (yellow wire) is the +43VDC solenoid power line. It's likely that yellow wire comes from the nearest coil so trace it.

1) Does it light when you ground the Brown-Blue wire at the lamp driver board connector J3-22?

2) Does it light when you ground the metal tab of SCR Q40 on the lamp driver board?

3) Does it light when you run a jumper from test point TP3 on the lamp driver board, to the left "gate" leg of SCR Q40?

Ugh, sorry I said in the previous post the same player shoot again lamp wire is brown-blue. Not sure how I made that mistake but it's a Blue-Yellow wire. Is this the color you're seeing on that wire at the lamp socket and lamp board connector J3 pin 22?

There's no intermediate connector on that lamp wire either - it's direct from the lamp driver board to the lamp socket.

We're here for the long haul, can't have this game not working properly

The second and third drop target (from left) are both activated by the same driver transistor on the SDB. What happens when you ground the metal tab of transistor Q6 on the SDB?

Medusa_DT2_DT3_PF.jpg
Medusa_DT2_DT3_SDB.jpg

Has this been resolved?

The switch issues are separate to the solenoid issues. When the game is resetting drop targets, it doesn't consider the state of the drop target switches.
Check that you have zero ohms continuity from the top drop target switches to the MPU board as per below:

Medusa_SwitchMatrix_PF.jpg

Your solenoid issues are likely due to switches not registering.

Switches 44, 36, 20, 12, and 4 are all on the same row in the switch matrix. This row also has switch 28 but you didn't mention that it's not working. What's the story with switch 28?
Have you made sure there's continuity on that switch row signal back to the MPU board?

A few posts ago you mentioned switch 48 and 46 not working. Is switch 46 working now?
Is switch 48 making proper contact when closed?

When you fix things, can you tell us what the problem was so we can understand where the source of the issues were?

This sounds suspiciously like you've still got switch issues with those top drop targets. If they're not resetting after you hit the round two sequence, one of those drop switches probably isn't registering properly.
When it happens in game, lift each of the top targets one by one and drop them back down to see if the game scores/registers them.

This ^^^

The flipper switches are not part of the switch matrix. What happens when you bang the playfield with your fist?
.

There's three #18 (10 point) switches, not two. Check the third one.

Medusa_10PointSwitches.jpg

Cool.

To be honest, I would replace the capacitors - when you have fast hits on standup targets and they're not recognised, that can get annoying.

Have you cleaned the switch contacts?
.

I don't know if these pictures help.
Maybe you're missing the spring on the back of the metal tab that latches the plastic hook.

Zippers mechanism latch open
Zippers mechanism latch touching
Zippers mechanism closed
Zippers mechanism metal latch spring

Do other coils have 43V solenoid power? If yes, your daisy chained 43V solenoid power (yellow wire) might have lost connection somewhere on the playfield at one of the coils.

You've had a few bad coil connections now - you might want to check that they're all solid by pulling on them - resolder any that look flaky.

Agreed it's a pain. The wood rails the playfield sat on and was maneuverable upright on the previous Ballys was much better.

As usual, ground the colored control wire at the lamp socket. If the lamp stays dim, it's the socket or lamp. If the lamp goes bright it's likely a bad connection back to the lamp board on the colored control wire.

Make sure you measure 6.5V - 7.0V AC across the two lamp socket connections - don't use ground as a reference. After that change the lamps and/or sockets as necessary.

Ok, this time put the black meter lead on a ground point and measure both pins at the lamp sockets with the meter still set on AC voltage. What voltage do you get on each colored wire at the lamp sockets?

Well, the daisy chain usually starts at the rear of the playfield. Find other GI lamps with orange and green wires and check continuity on the orange wire.
That orange wire comes from the rectifier board at J1 pin 1. Check continuity from the orange wire at a dim lamp to that pin on the rectifier board, and then further upstream past the J1 connector at test point TP1 on the rectifier board.

No. There's two sets of GI lamp wires from the rectifier board to the playfield. You have the orange and green set, and then the red and white set. Red and orange are connected together back at the rectifier board. Green and white are connected together back at the rectifier board and these are also ground.

Note, the schematic below lists the orange wire going to the zipper flippers EOS switch stack to power the lamps under the main lower red flippers. Check there for a bad connection.

Medusa_Playfield_GI.jpg

Grounding the anode leg/tab of the SCR doesn't actually test the SCR. It just tells you whether you have good continuity from the SCR to the respective lamp.
To test the SCR, you need to run a jumper from test point TP3 on the lamp driver board to the left "gate" leg of SCR Q40.

Actually it was previously determined that Q40 was faulty. How did you repair it? Didn't you mention you piggy backed a new SCR on top of the faulty one? Can you post some clear pictures?

BTW, the lamp board doesn't have "transistors" - they are "Silicon Controlled Rectifiers" (SCRs) also known as Thyristors. They may look like transistors, but SCRs have a different function

Oh good, you removed the faulty onboard SCR.
Ok, the reason it's not working is because the replacement SCR is on backwards. You need to flip it around so that the metal tab faces outwards

The question is whether that replacement SCR is now damaged.

This is how I test SCRs with a multimeter:
Set your multimeter to diode mode.
With the particular replacement SCR you're using (MCR106 or C106) test all combinations of the meter leads on all three SCR legs, including swapping the meter leads around.
With the metal tab on the backside, you should only get a diode type reading with the black meter lead on the left SCR leg and the red meter lead on the right leg.
Now.. put the black meter lead on the left leg and the red meter lead on the middle leg. You should get no reading. Carefully slide the red meter probe on the middle leg so it bridges across to the right leg and shorts the two legs, then slide it back so it's only touching the middle leg. The meter should now show a diode reading between the left and middle leg.

I hope this makes sense.

Is this an electrical/electronic issue or mechanical?

Is the issue that drop target #3 from the left sometimes doesn't respond (switch not recognised/no score) when it's knocked down, or that it's recognised but after the whole bank is reset, drop target #3 doesn't drop when its individual coil activates to drop it?