(Topic ID: 294653)

Relay and coil resistance question

By marcburn

4 months ago



Topic Stats

  • 9 posts
  • 6 Pinsiders participating
  • Latest reply 4 months ago by marcburn
  • Topic is favorited by 2 Pinsiders

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Sing Along R relay (resized).jpg

#1 4 months ago

Hello,

I’m a newbie but have recently acquired a dead Sing Along. I’ve spent the past month familiarizing myself with the machine, cleaning, re-wiring the power and all the pre-power up stuff. Last week I powered it up For the first time and the motor began spinning and I got G.I. which made me me very happy. However the reset sequence does not complete and the motor continues to run. I’ve traced it down to two problems where my question really lies. The huge coil that pulls in a reset bar to reset the control relays is not firing. I’ve reset the relays manually with the game off and the bar does move freely. The R relay which is the hold 115v relay is also not latching. The resistances of the reset coil was a little under but around 30 ohm instead of 31. The R relay read .6 vs Expected ~30ohms and looks black so I figured it was for sure bad. So I’m in the process of replacing both in hopes that gets me further down the road to having reset working. Now when my new r relay came in I tested it on the bench and it shows 33 ohms but after soldering it in its now showing .4 ohms. I put the old r relay on the bench and it’s showing 32 ohms Where it was .6 installed which led me to suspect it was bad in the first place. I’ve noticed one or two other coils that are doing similar. On the machine A given relay may show a much lower ohms reading than on the bench. Is this expected or is it an indication of a different problem that I should be looking for? Most of the resistances of coils/relays test on the machine very close to expected value but a handful do not. Appreciate any thought or tips here. Thanks in advance,

Marc

#2 4 months ago

Ive had good luck testing relay coils with a 9v battery pushed against the coil lugs. Most times this enough power to activate the relay.

#3 4 months ago

Measuring a coil resistance is a GUIDE ONLY! Coils are actually measured for inductance NOT resistance but most people don't have an inductance meter and resistance is usually a good enough ball park.

If someone says a coil should measure 27ohms expect that a reading of anywhere 24~30 would be normal.

When ANY electronic component is in circuit (including coils) it is usually connected to many other components and they will often affect readings. The only sure fire way to measure resistance of any part (coils included) is to remove the wires from one terminal so the coil is then out of circuit.

Same thing applies when checking fuses - always lift one end to make 100% sure you get the correct reading.

This isn't always required but it is something you must keep in mind if you get a suspicious reading - disconnect one terminal before condemning the part.

#4 4 months ago
Quoted from marcburn:

I tested it on the bench and it shows 33 ohms but after soldering it in its now showing .4 ohms.

Welcome to Pinside. This is a common mistake. To understand your measurements it helps to look at the schematic:
Sing Along R relay (resized).jpg
If you clipped your meter across the R relay coil (at the two red terminals for example) your instinct might tell you that the meter would report the resistance of the relay coil. But the meter actually sees two different paths between those points. One path is through the R relay coil as you'd expect and is probably 30 ohms. But the other path the meter detects is the path through the transformer and the fuse which has a much lower resistance, probably close to the .4 ohms you measured.

That's why you can't reliably measure the resistance of a device in circuit unless you understand the rest of the circuit. Consider for example the same test on the Ball Return Kicker solenoid just above the R relay coil. In that case the O relay and Motor 3B switches will most likely be open when you take your measurement so you will get an accurate measurement of the resistance of that solenoid.

/Mark

#5 4 months ago

Yes, other components connected to certain coils can cause "false" readings with an ohm meter. Always best to disconnect one wire from a suspect coil to test it properly "out of the circuit".

#6 4 months ago

Thanks for all the great responses. I will try the replacement relay and coil that hopefully fix my problem. Since they both read proper resistance out of circuit I’m less sure now that they are the culprit to my reset problem. When I pulled the R relay out I noticed that someone had put in an A9735 while the schematic calls for an A9738 so that could contributing to the problem. The SB arm switch is closing when the SB relay fires so Seq control coil should be getting power to fire in order to complete the reset. I’m cleaning all the housing mounts for it today and will try it tonight.

#7 4 months ago
Quoted from marcburn:

the reset sequence does not complete and the motor continues to run. I’ve traced it down to two problems where my question really lies. The huge coil that pulls in a reset bar to reset the control relays is not firing.

If (1) the score reels reset to zero and (2) the DS Reset relay on the control bank trips,

Inspect and diagnose this circuit with Alligator clip jumper wires
https://pinside.com/pinball/forum/topic/gottlieb-big-shot-repair#post-6305153
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features
Example of a pinsider actually doing this https://pinside.com/pinball/forum/topic/red-baron-tech-question#post-5858156
Be careful because this circuit runs on 120 volts

pasted_image (resized).png
#8 4 months ago

Thanks HowardR,

I will check out those links. In my case using the circuit you provided, the sb relay stays latched so the sb armature switch stays closed. Even though R relay is getting power it never closes the R switch in that 115v circuit. I’ve removed the playfield to try to simplify the reset process so DS never fires since the playfield isn’t there. I’ve manually set the score reels to zero and checked that the switches are clean and closed for each reel. Motor continues to run indefinitely. Let me review those links to make sure I’m understanding everything. Thanks again!

#9 4 months ago

Ok. Update to my problem above. Quick recap of what I’m trying to solve is getting my sing along pinball to get through the reset process to be able to begin a game. When I set the motor in the number 4 position the control bank setup fires exactly once even though the motor continuously turns and each time 3c Comes around the sequence bank set up Coil fires and Is followed by the DS relay immediately latching. This latching DC latching is causing that DS switch to be open and the sequence bank to never reset the SB relay. I have reset all the score reels to zero and verified the runout switches are all closed which is causing the DS relay to latch. If I start it with motor in 1st position, then the control bank coil never fires. I don’t know what else to try but it seems like some sort of timing issue. Any help or suggestion on what to else to try is welcomed greatly. Thanks, Marc

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