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TNC113 went dead. Where to start?

adh2000

Titanium
Joined
Dec 21, 2005
Location
Waukesha, WI
Displays flashed on and off for about 30 seconds, then the unit went dark. Opened the unit, nothing obvious. What to do now? I have a meter, scope, etc. and reasonably good at using them.
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Agree with Ross, check the power supply. In my limited experience the 3-pin regulators tend to die.

On the VRZ models the striped yellow/white wire is unregulated 18VDC which feeds to a 3-pin regulator and the gray coming out is regulated 12VDC, the striped green/white wire is unregulated 9VDC which feeds to a 3-pin regulator and red coming out is regulated 5VDC. It might be similar in yours.

PS: another member here has recently sent me a set of TNC schematics. Not sure if they cover your model but even if not, the power supply might be similar. Send me a PM if you want copies.
 
Very helpful Ballen. I’m not measuring any voltage anywhere so back to more basic stuff. The power switch is a two pole switch. I measure 105 VAC from one pole to ground zero on the other. Why is this a two pole switch? The TNC unit uses two of the phases? I need to take another look in the main cabinet. These and other questions are probably answered in the schematics you sent (thanks for that) but we had storms through last night and the power has been off at the office so haven’t been able to open those.


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I’m not measuring any voltage anywhere so back to more basic stuff. The power switch is a two pole switch. Why is this a two pole switch?

For safety. Since the standard German line plug has no polarization, either of the two lines might be live. So both lines are switched, to ensure that when the switch is off, nothing downstream of the switch is live.

In principle this is not needed in the USA, since there, the standard three pin connector, and the standard two pin polarized connector (one pin broader than the other) can only go one way: Leads Direct | Wiring an American Plug
However in practice it is still a good idea, since there are many unpolarized plugs in use and many mispolarized sockets in place.

I’m not measuring any voltage anywhere so back to more basic stuff.

In that spirit, first check the fuse, and then check the switch that is used to select 110 or 220V.
 
OK. Neutral is switched. So my voltage readings are correct then? 105 VAC on one side, zero on the other. Fuse is good. It would not be switched to the incorrect supply voltage. The unit worked for years. In fact I was using the machine when it died.



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My 151 died a few months ago, power supply

I had a spare
But since it is also 30years old, I went to digikey and bought every single electrolytic cap on the board.
In my 'spare time' I will replace all the caps in a backup supply

One of the caps was blown apart BTW, I don't see on in your pic, but it is likely
 
OK. Neutral is switched. So my voltage readings are correct then? 105 VAC on one side, zero on the other. Fuse is good.

If the primary of the transformer has ~110 VAC across it, all is good. Next check the secondary voltages.

It would not be switched to the incorrect supply voltage. The unit worked for years. In fact I was using the machine when it died.

My experience in fixing electronics is that one should never assume anything. Just trace the circuit and keep checking. Perhaps the supply voltage switch had a contact that was oxidizing, and one day it just arced and burned. It worked right for years, but no connection anymore. Such things do happen.

Anyhow, the two primaries should be in parallel, and each should have line voltage across them. If that's the case, all is good on the primary side, move to the secondaries.[/QUOTE]
 
Not seeing anything that looks like a transformer, unless it’s the square object with the blue label. Must be. No access to the wires going in or coming out. The power switch is to the far right for reference.
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Yes, that blue thing is the transformer. Probably power comes in through that little board with the corner cut off and a pair of capacitors and inductors on it. That's a line filter. Then the line power passes through the voltage selection switching and to the primary transformer connections coming out on the right side of the blue box (in the photo). Secondary connections are coming out to the left, and will be to the sets of four diodes, each of which forms a full-wave bridge rectifier. Suggest you look a the voltages across each of the large filter caps, which will be at the output of the full-wave rectifiers. If all is well, you should see mostly DC with a bit of AC ripple.
 
Suggest you look a the voltages across each of the large filter caps, which will be at the output of the full-wave rectifiers. If all is well, you should see mostly DC with a bit of AC ripple.

I’ve got zero across both sets of caps, no DC, no AC. I need to get some sharper pointed probes. With that I should be able to get at the transformer connections. It is looking like the transformer is bad. Looks like a very custom probably totally unavailable part.


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What you're describing is exactly what happened with mine. Worked fine, then the display started flashing, then went off totally. I'm not confident enough with a tester to go digging so I've been thinking I'd have to send it to Singer for an exchange unit. Would love to hear of you can find a solution!
 
I’ve got zero across both sets of caps, no DC, no AC. I need to get some sharper pointed probes. With that I should be able to get at the transformer connections. It is looking like the transformer is bad. Looks like a very custom probably totally unavailable part.

A quick Google search turned up what might be a service manual for this Please Wait... | Cloudflare. No schematics, but there is a connection diagram.
I see some fuses and a bunch of switching to select for input voltage. Your problem is more likely a fuse or a bad contact than the transformer IMHO.


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Thanks for that. It does show fuses all over the place most I can’t find. However turns out the voltage selector switch doubles as a fuse holder. Inside is a 250V 400mA fuse which is blown. Progress.


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Don't know yet. I don't know of anyplace local I can walk in and buy this fuse. So I order it from McMaster, but I leave in the morning for a week so won't get back to this project until the end of next week.
 
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This is the back of the voltage selector switch. The leads around the periphery go to different taps on the transformer primary. The center tap, brown wire, goes to the right side of the small circuit board shown, next to the lighter yellow device. What is the function of this small board?

I could switch the voltage selector to another voltage, say 200V instead of 100V where it is now. The unit wouldn’t work that way but if the fuse doesn’t blow it might suggest the problem is in the transformer primary rather than the small circuit board. What do you guys think? I wish I could more easily disconnect the transformer. It looks really difficult but I think I’m going to have to disconnect the transformer secondary and see if the fuse still blows.

I only ordered 5 fuses. I better order more.

Alan



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