Robotron resetting; blowing fuse F5

TCinTEXAS

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My Robotron blew a fuse (F5 7ASB) a few weeks ago; first problem it's had in over a year. I replaced the fuse, checked the voltage. Ran it for a few more hours; everything seemed good.

A few weeks later, it starts resetting sporadically; every 20 to 30 minutes, then gradually gets more frequent, until it blew the fuse again. I checked everything out again, installed the 5V adjustment pot to fine tune the voltage. I'm getting about 5.05v to the CPU. I also reflowed the headers on the CPU PCB and P/S board. It seemed to run fine for a while, then started up again.

It blew the fuse again, and I did a little more research. Someone suggested a seemingly unrelated issue which supposedly fixed his Robotron, doing the same thing, so I tried disconnecting the coin lock coils. No change.

Voltages are all good. I powered it up and checked temperatures. The F5 fuse quickly rises up to about 190 degrees. Resistor R3 gets really hot; around 200 degrees. I don't really see any heat issues on the CPU PCB, except the CPU. The 6809E is getting up to about 120, which I think is a little hot.

Other than the resetting and the lines on the screen, which I believe are unrelated, the game plays great; nothing that would suggest any other problems.

I'm down to my last 7A fuse, so I'd like to fix this fairly quickly.

Should R3 get really hot? If not, does this indicate a bad resistor, or something else in the circuit causing it to overheat.

Should the CPU heat up to over 120 degrees? If not, same question as the resistor.

Thanks!
 
F5 is a +5 fuse.

has this power supply been worked on before? if not, that's probably half the problem. R3 is in the +5 circuit, if something's ratchet at the heatsink header that could be an issue.

the coin door lockouts are not even part of the +5 circuit, chances are your friend had some bad solder on a header somewhere and tinkering with the plugs got it just right.

the +5 adjustment is a pointless mod that people overemphasize. that's not going to really help your problem if the power supply is original.

the CPU, decoders, and 4116 rams run the hottest on the MPU. so that's normal.
 
F5 is a +5 fuse.

has this power supply been worked on before? if not, that's probably half the problem. R3 is in the +5 circuit, if something's ratchet at the heatsink header that could be an issue.

the coin door lockouts are not even part of the +5 circuit, chances are your friend had some bad solder on a header somewhere and tinkering with the plugs got it just right.

the +5 adjustment is a pointless mod that people overemphasize. that's not going to really help your problem if the power supply is original.

the CPU, decoders, and 4116 rams run the hottest on the MPU. so that's normal.

Sorry! I completely rebuilt the power supply a year ago. I installed the pot because the 5V was only 4.89V at the board.

By the way, the big 5V bridge rectifier is also getting really hot. I'm not sure how hot is normal.

What's "ratchet"?
 
I haven't had a chance to dig into this yet; wife wanted to go shopping for house stuff. Sheesh! :)

I see that F5 supports the +5v as well as the +12v Unregulated. Could the problem be with the +12v, and where would the most likely culprits be?

Side note (maybe): as I was digging through the cabinet with a flashlight, I noticed that the grounding strap is cut between the transformer and board set, and between the transformer and coin door. Could this be contributing to my problem?

With any luck, I'll have time to dig into it tomorrow.
 
I would unplug the sound board next. the MPU reset circuit and sound board are the only parts that use unregulated +12. if something is shorted on the sound board it will blow the fuse. otherwise you probably have a bad bridge rectifier BR1.
 
I would unplug the sound board next. the MPU reset circuit and sound board are the only parts that use unregulated +12. if something is shorted on the sound board it will blow the fuse. otherwise you probably have a bad bridge rectifier BR1.

I'm going to go out on a limb and say the problem is in the power supply PCB. I swapped it out with a good power supply from my Stargate, and it ran with no resets or blown fuses for over an hour.

I pulled BR1 and tested it against a new one, and it tested approximately the same. I also pulled one leg of R3 and tested it. Resistance drops to 0.0 after a while, but I tested a new 0.10 ohm, and it tests the same. I'm going to have to call that one good.

Thoughts?
 
My Joust started doing that and it was the power connection from the transformer to the power supply. If you look at the main +5v input, there are six pins for the full wave input to the bridge on the supply board (ACx2, GNDx2, ACx2), however only three are populated in the harness. Running additional wires from the transformer to these unused pins will help prevent burning of the contacts and pins on the supply. Once these start to burn, they must be replaced.

Replacing the round pin headers with square pin headers and new housings/contacts in the harness will help a lot. Especially on the ones that go to the transistors on the heat sink. All of the current goes through the transistor and those contacts.

Good Luck!

Kirk S.
 
My Joust started doing that and it was the power connection from the transformer to the power supply. If you look at the main +5v input, there are six pins for the full wave input to the bridge on the supply board (ACx2, GNDx2, ACx2), however only three are populated in the harness. Running additional wires from the transformer to these unused pins will help prevent burning of the contacts and pins on the supply. Once these start to burn, they must be replaced.

Replacing the round pin headers with square pin headers and new housings/contacts in the harness will help a lot. Especially on the ones that go to the transistors on the heat sink. All of the current goes through the transistor and those contacts.

Good Luck!

Kirk S.

Thanks. All of the headers and connectors have been replaced with Trifurcon a year ago. The "good" power supply PCB I tested still has original pins, so, unless the header solder joints are bad on the original P/S (and I don't think they are), I doubt that's the problem. I ran my little infrared thermometer over all of the connectors, and none of them are getting warm.

I appreciate the input.
 
to crazykirk's point, the 3 wires from the transformer to the power supply often burn up because the current becomes too much for them. I added additional 18 gauge wiring from the transformer tabs to the empty pins in the power supply AC plug, as they're duplicates (you can see with your meter) and this will help curb the high current issue.

if you replaced the plugs but retained the original headers, and this is especially so if your plug was burned up at any point, you will need new headers. additionally trifurcon pins will be effectively worthless with factory headers as they're skinny and round, and the U's on trifurcons won't be able to grab the headers effectively.

the AC voltages at what I will call the kirk plug for now pass through the big bridge rectifier BR1 to produce +/-12V to make up the unregulated +/-12 circuit. the +12 off that passes through some filter caps and such but make their way to the 6057 transistor, which is then output to the +5 voltage regulator to make up the regulated +5 circuit.

I gave you instructions on how to determine if your fault is in the sound board, which has its own bridge rectifier to create +5 to operate the sound logic from the +/-12 for the amplifier and sound output components. if the sound board's bridge rectifier is bad, it can blow the fuse. if the BR1 bridge rectifier on the power supply is bad, it will blow the fuse.

to the previous point about the coin door lockouts, a separate AC voltage is used to power BR2, the +/-27 circuit which is used for the lockouts and branches off to the 3055 transistor and is output to the +12 voltage regulator to make regulated +12. regulated +12 is only used for the 4116 rams, it's not to be confused with the unregulated +12. this is why your friend's coin door lockout nerf would have absolutely nothing to do with F5 blowing or not, as they're entirely different circuits on the power supply.

do what I suggested in my last post and report back.
 
to crazykirk's point, the 3 wires from the transformer to the power supply often burn up because the current becomes too much for them. I added additional 18 gauge wiring from the transformer tabs to the empty pins in the power supply AC plug, as they're duplicates (you can see with your meter) and this will help curb the high current issue.

if you replaced the plugs but retained the original headers, and this is especially so if your plug was burned up at any point, you will need new headers. additionally trifurcon pins will be effectively worthless with factory headers as they're skinny and round, and the U's on trifurcons won't be able to grab the headers effectively.

As I said, I replaced both connectors and headers over a year ago. My plugs are not burnt. The wires supplying A/C are not even warm.

I gave you instructions on how to determine if your fault is in the sound board, which has its own bridge rectifier to create +5 to operate the sound logic from the +/-12 for the amplifier and sound output components. if the sound board's bridge rectifier is bad, it can blow the fuse. if the BR1 bridge rectifier on the power supply is bad, it will blow the fuse.

I can check this, but as I stated, I swapped the power supply PCB with a known good power supply PCB from my Stargate, ran it for over an hour with no issues. The PCB in question would have begun resetting within 10 minutes, and blown the fuse within 20 minutes.

to the previous point about the coin door lockouts, a separate AC voltage is used to power BR2, the +/-27 circuit which is used for the lockouts and branches off to the 3055 transistor and is output to the +12 voltage regulator to make regulated +12. regulated +12 is only used for the 4116 rams, it's not to be confused with the unregulated +12. this is why your friend's coin door lockout nerf would have absolutely nothing to do with F5 blowing or not, as they're entirely different circuits on the power supply.

I think we all agree that had nothing to do with it. That's why I referred to it as "seemingly unrelated."

do what I suggested in my last post and report back.

I doubt the sound board is the problem, because I'm not blowing the fuse in the replacement power supply PCB. In fact, the parts aren't heating up at all. If you still think it's a helpful troubleshooting step, I can give it a try.
 
I missed the part where you used a different power supply. your bridge rectifier is bad then. those should always be replaced when rebuilding the power supply. 3055 is also encouraged, but only if you get them from a reputable source.
 
I missed the part where you used a different power supply. your bridge rectifier is bad then. those should always be replaced when rebuilding the power supply. 3055 is also encouraged, but only if you get them from a reputable source.

I replaced BRs, the diodes and 2 regulator ICs along with the caps, headers and connectors. That's not to say that it didn't go bad. The BR1 on the board, prior to the rebuild, was completely melted. I also replaced the transistors on the heat sink, which is the 3055, I believe.

Is there a way to test the BR1 to verify, or should I just swap it out? I pulled it and tested against another 35A 400V (spade connector, so won't work) with my DMM, and got roughly the same readings. I think the one in the rebuilt kit was a 35A 1000V, if I recall.

Thanks for your input.
 
If that fuse isnt being held by the fuse holder very well that could also cause problems. And check the fuse that handles the other side of the rectifier. Otherwise, I agree with Mecha, the bridge is probably bad. Threres not a lot of parts on that board that can absorb that kind of current load and not blow up. Its got to be something of size.

And this is why I add a switcher to support +5 only. That power supply runs bone cold after a switcher install.

No, I'm not saying replace the supply with a switcher and end up with CMOS errors, I'm saying ADD the switcher and wire +5 to it ONLY (well ground as well). Thats what I'm saying.

I also add the three missing wires from the transformer to the AC input header. And when the factory power supply HAS to stay there, I move the bridge to the transistor heatsink. I use 16ga wire ran twice from each connection on the supply board. The rectifier gets greased and mounted to the top left corner of the sink. Still runs pretty warm but no where near as warm as sitting there stewing all by its self.

Now I will wait for Mecha to give me grief. :)

Here are a few pics of an adapter i build for WMS games and a Defender that I moved the bridge on.

LOL! I just noticed that I got power for that switcher PRE ISO! I dont do that very often. :)
 

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If that fuse isnt being held by the fuse holder very well that could also cause problems. And check the fuse that handles the other side of the rectifier. Otherwise, I agree with Mecha, the bridge is probably bad. Threres not a lot of parts on that board that can absorb that kind of current load and not blow up. Its got to be something of size.

And this is why I add a switcher to support +5 only. That power supply runs bone cold after a switcher install.

No, I'm not saying replace the supply with a switcher and end up with CMOS errors, I'm saying ADD the switcher and wire +5 to it ONLY (well ground as well). Thats what I'm saying.

I also add the three missing wires from the transformer to the AC input header. And when the factory power supply HAS to stay there, I move the bridge to the transistor heatsink. I use 16ga wire ran twice from each connection on the supply board. The rectifier gets greased and mounted to the top left corner of the sink. Still runs pretty warm but no where near as warm as sitting there stewing all by its self.

Now I will wait for Mecha to give me grief. :)

Here are a few pics of an adapter i build for WMS games and a Defender that I moved the bridge on.

LOL! I just noticed that I got power for that switcher PRE ISO! I dont do that very often. :)

That's gnarly!!!! :) Actually, I could put in a switcher. I have one, with a Williams adapter. I've also added an NVRAM kit to replace the CMOS, so no problem there. I'm just stubborn, and want to run the linear.

After all of this discussion, the rectifier does make sense. It starts failing after about 10 to 20 minutes, so it's very likely heat related. I'm going to add the wires from the ISO to the P/S PCB, as Mecha (or was it CrazyKirk?) suggested.

The fuse holder is fine. I checked it, cleaned it and pinched it a little tighter, just in case. I had considered the heatsink idea. I've already ordered these, along with heat-conductive adhesive:

https://www.amazon.com/gp/product/B01KNV1WMK/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

If that works, it will be a bit less involved than your solution; not that yours isn't brilliant!
 
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That's gnarly!!!! :) Actually, I could put in a switcher. I have one, with a Williams adapter. I've also added an NVRAM kit to replace the CMOS, so no problem there. I'm just stubborn, and want to run the linear.

After all of this discussion, the rectifier does make sense. It starts failing after about 10 to 20 minutes, so it's very likely heat related. I'm going to add the wires from the ISO to the P/S PCB, as Mecha (or was it CrazyKirk?) suggested.

The fuse holder is fine. I checked it, cleaned it and pinched it a little tighter, just in case. I had considered the heatsink idea. I've already ordered these, along with heat-conductive adhesive:

https://www.amazon.com/gp/product/B01KNV1WMK/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

If that works, it will be a bit less involved than your solution; not that yours isn't brilliant!

Any sort of heatsink on that rectifier is better than nothing. I would have gone with something that had more vertical meat though.
 
Any sort of heatsink on that rectifier is better than nothing. I would have gone with something that had more vertical meat though.

Well, I got impatient (it's what I do) and mounted a new rectifier remotely, with a heatsink made from 3/4 inch aluminum channel, using thermal grease. I added a second wire from the A/C feed to +5v.

It's running now for about 20 minutes with no resets. Voltage at the CPU is +5.08v. The rectifier is warm, but nothing like it was. It's about 105 degrees, as well as the F5 fuse. Before, both were in excess of 200 degrees.

By golly, I think I may have fixed it!

CORRECTION: WE fixed it!

Thanks!
 

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I think they say to mount those bridges 1/4" off the board, but I don't. I go as high off as I can with the uncut legs lol. I also replaced all the ceramic resistors on the work games, mounting them off the board too. probably worth noting, there's a ceramic nearest to the DC output plug, that's for +5. those will often fail internally as I've seen a few times. it'll either cause intermittent +5 or you'll experience a little bit of loss.
 
I think they say to mount those bridges 1/4" off the board, but I don't. I go as high off as I can with the uncut legs lol. I also replaced all the ceramic resistors on the work games, mounting them off the board too. probably worth noting, there's a ceramic nearest to the DC output plug, that's for +5. those will often fail internally as I've seen a few times. it'll either cause intermittent +5 or you'll experience a little bit of loss.


+1 on this and that is why i brought in those bolt on heat sinks for those BR. they need all the help they can get.
 
I think they say to mount those bridges 1/4" off the board, but I don't. I go as high off as I can with the uncut legs lol. I also replaced all the ceramic resistors on the work games, mounting them off the board too. probably worth noting, there's a ceramic nearest to the DC output plug, that's for +5. those will often fail internally as I've seen a few times. it'll either cause intermittent +5 or you'll experience a little bit of loss.

Yeah. I mounted my BR high off the board when I rebuilt it a year ago, but it apparently succumbed to heat. I'm not sure about R3 resistor. It's a .12 ohm 5w, and reads between .1 and .0 on my DMM. I have a new .10 5w which reads the same, so...

I was going to replace it, but can't find .12 ohm 5w resistors.
 
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