Overheating Problem with a DC80-D Tweezer-Weld

"The manufacturer went out of business."

How about this: AlphaTron Industries DC-80 Weld Controller

Need a sketch of the circuit to diagnose this problem.

Your charge /discharge circuit is faulty.

The 7.5Ω resistor is intended to discharge the capacitors when they are already charged and you adjust the knob to a lower Watt-second setting. It must be connected across the capacitor bank only long enough to discharge to the desired level. This connection must be either via a relay or a solid state device (SCR, Triac, Transistor, etc.), which in turn is controlled by the pot connected to the knob.

Apparently the 7.5Ω resistor is remaining connected when it should not be.

If the controlling device is a relay, it has sticking or stuck contacts. Either clean the contacts or replace the relay.

If the controlling device is solid state, then it may have blown to a short. Replace the device. You mention having replaced an SCR. Commonly an SCR connects the capacitor bank to the output leads. This SCR remains conducting until the bank is completely discharged. Since an SCR remains conducting until the supply voltage is nearly zero, some other device must be used to discharge the bank partially. I would suspect a power transistor, most likely in a TO3 package. Find and replace it.

Of course it is also possible for there to be a problem in the controlling circuitry, such as a poor or broken contact to the pot, or even a dirty pot, but these problems are less likely. There must also be some form of a comparator to measure the voltage of the bank, and tell the transistor when to stop discharging. This is likely to be an IC, possibly 8-pin, but it could also be discrete components.

The 25kΩ resistor is presumably to discharge the capacitor bank when not in use. It would not cause these symptoms; if it were to fail, the only problem would be danger to a service person (like yourself).

If you can't find a circuit diagram, a series of photographs posted here could help...

Let us know what you find!

It's a relay that controls the charge / discharge of the capacitors. I have witnessed the relay changing state back and forth during each cycle. I have just replaced it, but I still have the two separate problems.

The minor problem is the partial discharge of the capacitors while making the weld. The operator is still getting acceptable welds with partial discharge, but I know that the capacitors are supposed to discharge completely before recharging.

The major problem is the occasional complete discharge of the capacitors with no recharge. This is when the 25kΩ, 50W and 7.5Ω, 50W resistors get hot. They haven't actually popped. I've measured their "out of circuit" resistances after cooling back down, and they measure within tolerance. The operator wasn't paying enough attention to the meter to notice either symptom until she noticed the smell of the insulation starting to burn away from the leads of the resistors.

The cathode of the SCR is connected to the low side of the capacitors. The anode of the SCR is connected to one primary lead of the output transformer.

I'm going to try taking an AutoCAD LT 97 file of the circuit that I had to trace out myself and post it here.

It sounds to me like you have narrowed it down to the trigger circuitry. Follow the leads from the microswitch (either in a foot pedal or in the tweezer itself), presumably through the plug in the back of the unit, to find that section of the circuitry. Make sure the microswitch itself is opening and closing reliably, and especially look for frayed wires that could be touching. There might be a small resistor to pull the trigger voltage either up (more likely) or down when the microswitch is open. Most of these trigger switches work by pulling a low voltage down to zero. A poor solder joint or a broken resistor could cause intermittent operation or lockup of the SCR. There is probably also a transistor or IC to convert the downward voltage of the trigger switch into an upward voltage to trigger the SCR.

I'm sorry, but I haven't been able to get my AutoCAD LT 97 file to be cut/pasted or downloaded to this message. Any hints?

Export it as a JPEG or PNG file, and then you will be able to post it, using the camera icon in the editor toolbar. I wish I could tell you to use PDF, but unfortunately CR4 does not allow that.

If the AutoCAD LT 97 does not allow export to JPEG, then adjust to the largest image that will fit your screen, then use a screen capture (Shift-Command-4 on a Mac) to produce the image.

That's a start, but even after enlarging several times, I still can't really read it. Try posting the circuit in three sections. Also, after the image appears in the editor, drag the lower-right corner of each image over to the right side of the text to enlarge it. Don't drag beyond the text, or it screws it up for some viewers.

OK, now we're getting somewhere!

So the foot pedal turns on Control Relay 1, which turns on Control relay 2, the discharge relay.

As Welderman thought, the 7.5Ω resistor is a charge current limiting resistor, and it will indeed overheat if the SCR fails to turn off. My original thought, that the 7.5Ω resistor was used to discharge the capacitor bank, was clearly wrong. There is no provision in this unit to lower the charge in the already charged capacitors, so if you have it set to a high Ws value and then lower the setting, you either have to discharge through scrap once, or just wait 'till the 25kΩ/zener combination pull it down to the desired level. Your meter will indicate the actual level, not necessarily the level set by the control knob/autotransformer.

My best guess now is that the contacts 2 and 8 of Control Relay 1, which must be normally closed, are not making good contact, so the trigger of the SCR is left floating. That trigger must be pulled low by those contacts and R6 in order to reliably stay in the off condition. With the unit turned off and the capacitors discharged (wait 'till the meter indicates zero), you can measure the resistance across those two contacts; it should be under one Ohm. Clean the contacts or replace the relay. If that does not fix the problem then look for loose wires or bad solder joints.

A pretty good job of tracing the circuit! I know you spent several hours doing that... There is at least one error: each of the two connections from the autotransformer must connect to one of the two AC terminals of the diode bridge. I see one side of the transformer connected to both AC terminals.

Good luck!

Thanks dkwarner!

I did change Relay 2, but it didn't stop the problems I was having. The contacts between 2 and 8 on Relay 1 are clean. The resistance was under 1Ω. I started fishing around again, checking voltages at different points during weld cycles. At one point I measured the voltage from the gate to the cathode of the SCR during a weld cycle. I measured 50V, which was more than I expected, but the needle on the meter started acting properly again. I ran several more cycles while measuring voltage between the gate and cathode and measured 20V. This is the voltage I was expecting, and the needle on the meter was still acting normally. I'm not sure what measuring the gate/cathode voltage did, but the welder is doing what it's supposed to now consistently. Neither of the symptoms I had has come back.

I knew there was something odd about how the AC side of the rectifier was wired. I couldn't figure out how the rectifier could work like that. I've traced that part of the circuit again and again in the past, coming back the same each time. I am probably missing something, so I'll look again until it makes sense.

Thank you for your help, dkwarner.

CuriousCat

Thanks for getting back!

With that 50V between gate and cathode, the gate was almost certainly floating (not connected to ground through R6 and relay 1). My guess is that you moved enough solder around while measuring to bridge a poor solder joint at the gate contact. I'd remove that connection, clean both the wire and the terminal, and re-solder. If I read correctly that R6 is 1Ω, then with power off and discharged. the DC resistance between gate and cathode should measure under 2Ω.

Here's my guess of the connection between bridge and autotransformer:

In case you can't see it, one of R4 and R5 goes to one end of the autotransformer and the other goes to the slide.

You're Welcome!

Dick

After looking again, I see I should have said that one of R4 and R5 goes through the relay to one side (the right side in your drawing) of the autotransformer, and the other one goes to the slider of the autotransformer.

It sounds like the device that discharges the capacitors, is remaining on. It should be shutting off after the weld is made to allow the capacitors to recharge. It may be the discharge device (likely an SCR), or it might be receiving a false trigger signal from defective circuitry. I would disconnect the discharge device from the circuit and see if the capacitors charge to the proper voltage and the excessive heating stops. Then determine why the unit continuously discharges.

That is conceivable, but then the output transformer would be continuously connected across the capacitor bank, so the bank could never charge. In that case the discharge resistor would not overheat. Now it is possible that the 7.5Ω resistor is a charging current limiter, rather than the discharger that I described. In that case, it very likely would overheat.

That is relatively easy to check: If it is a discharger, as I assumed, then when connected by its switching device, it will be in parallel with the capacitor bank. If it is a charge current limiter, it will be connected in series between the high voltage supply and the high side of the capacitor bank.

CuriousCat indicated that two resistors were overheating. It is easy to understand that the 7.5Ω resistor could overheat in either of the 2 possible applications, but for the 25kΩ resistor to exceed 50 Watts would require over 1100 volts. I seriously doubt if there is that much HV in an 80Ws welder.

CuriousCat, please check these things and get back to us!

I agree, but OP describes two symptoms. I was addressing the second one where the capacitors discharge, don't recharge, and the resistor overheats. I suspect the 7.5 ohm resistor is a charging current limiter and the higher value resistor is the bleeder resistor. In that case, a shorted discharge device would cause symptom #2.

You got me to looking a little more, and to reread the OP. He does indeed describe two totally different symptoms.

I was not able to find a circuit diagram for any Alphatron Welder. I am most familiar with larger welders, but I do have one old Unitek 80Ws capacitive discharge welder. It uses a relay to connect the capacitor bank to the transformer. If the Alphatron also uses a relay, it could explain both symptoms. Dirty/worn contacts could sometimes not make a good connection (symptom #1), and could sometimes stick (symptom #2).

To CuriousCat: Look for a relay connecting the capacitor bank to the output transformer. If you find one, clean the contacts. On the other hand, you mentioned an SCR, which is also a common discharge device. In my limited experience, SCRs either work or fail completely - I've never seen one that sometimes worked and sometimes did not. If it is an SCR and is intermittent, then I would suspect the controlling circuitry.

BTW, I just noticed that this was your first post. Welcome to CR4, and please do get back to us...

Hi DK,

I wondered where this thread had got to all the posts had gone to my spam bin. Thunder bird has a mind of its own.

I can understand the faults if the SCR keeps conducting, the 7.5 ohm resis would overheat and the voltmeter would not manage to climb very high.

What is curious is why R3 25k ohm is a 50Watt resistor , it is only there to charge C6 to 20Volt set by the Zener diode to trigger the scr gate. Also why would this resistor get hot and smoke , even if it was looking into a short it would only pass a few milliamps and there are only 2 components to short out that is the zener and C6. Is it 25K Ohm?? I note the OP read 20Volt at the gate when the welder started to work ok,Although this would have only been for a brief time.

I agree with you scrs iether work or fail compleatly iether shorted or open circuit, the fact thet theOP fiddelled with somthing around the 2CR and it came good would indicate a problem in this area, there would not be a contactor from the capacitor bank to the outputtransformer the scr fullfills this function.

How are things still going would the OP respond?

Checking some figures I think R3 is a 5watt resis.

I wondered the same about R3. Assuming the maximum voltage on the capacitor bank is 400V (leaving a 50V safety factor), that leaves 380V across R3 and 20V across the zener. I=380/25k=15.2mA. P=(15.2mA)^2*25k=5.7W. Since the unit could be left on with the capacitor bank fully charged for long periods, your 5W sounds a little low, but then 50W sounds pretty high...

You are definitely right about the SCR being the discharge device. I called Relay 2 the discharge relay only because it is what initiates the discharge. I don't understand why they used two relays, since neither one has to carry the discharge current, and CR2 does both the disconnect from the power source and the connect of the trigger voltage. Something is fishy about that NC-HO designation at the footswitch.

I too would like to know if the unit has continued to function correctly.

Looking at it again,

I had thought the duty cycle of R3 to be low but it is the only element in the circuit to pass a a constant low discharge to enable setting the power on the capacitors to different levels. So the higher wattage is justified.Probably a bit on the high side but is a good safety factor, So I still do not see why this resistor would overheat even fully shorted.

Switching sequence:-

Foot switch closes-- energises 1CR

This opens 1CR-2 and closes !CR-1

CR1 energises 2CR this opens 2CR-1 and switches over 2CR-3 to trigger SCR gate.

The 2 relays sequence ensure power disconnect before trigger takes place.

One question where is 2CR-2??

Not sure what NC - HO means unless it is a double function ie: when pressed down it locks in the HOld position and has to be pressed again to release it thus ensuring a full cycle as if it was let off very quickly the charge circuit could kick in while the SCR was still discharging and would not shut down. Maybe that was part of the fault condition? Once the weld had taken place the capacitors would not build up to maximum setting due to the constant drain. Bad contacts in the foot switch could give this problem.

It is a very clumsy method of control.

Hello dkwarner and garth,

Yes, the welder is working just fine now.

The footswitch with the label NC-HO is how it is operated inside the welding head. It is actually a lever arm microswitch using the NC set of contacts. The contact is held open with a cam until the footswitch attached mechanically by a chain to the mechanisms inside the weld head cause the microswitch to be driven down, releasing the lever arm from the cam and allowing the contacts to close.

The R3 resistor is 25K Ω, 50W. When the malfunction occurred the temperatures of R3 and R2 measured with an IR thermometer climbed into the 280 F range by the time I got the unit turned off.

Thanks for getting back again!

Ah! so NC-HO means Normally Closed, Held Open. There must have been a mechanical reason for doing that! It's not really a footswitch, but a foot actuator of a switch located in the unit.

Hello all, I realize this is an old thread. We at Aviation Technologies are starting to repair TR (Thrust Reverser) Blankets and need to repair the Tweezer DC-80 we have been using, it keeps blowing fuses and sometimes feeds more amperage to the electrode than needed, I was wondering if the complete schematic was available for this unit so I could possibly troubleshoot the problem. I don't see it in the comments where it had been posted, thanks Jeff.

Hello Jeff

It sounds like the same or a similar problem to CuriousCat's original post.

As I look at his thread today (2/20/16) all the places where there should have been illustrations in posts by CuriousCat are just white spaces. When I clicked on the link in post #1, I get a "Page not found" error.

That white-on-black circuit in my post #18 is very difficult to read, and I don't remember whare it came from...

If you think it would be of any use, I do have a complete circuit (which I traced myself) of a Unitek 1026C capacitive discharge welder. This is an old small welder (100 Watt-second, I think) using vacuum tube technology, with a relay to connect the capacitor bank to the weld transformer. I see I created this drawing in 2012, after the original section of this thread terminated. I acquired the unit in non-functioning condition, and repaired it. If you are interested, Send me a private message (click on my avatar) with your email address, and I'll send a PDF of it.

I believe the DC80 is solid state rather than vacuum tube technology, but many of the concepts still apply, so it might be useful. FWIW, we're currently working on thrust reversers as well...

I had the same result the pics are missing.

But I did a bit of searching and picture conversions and managed to get a poor quality black on white of the original pics.

I will manually enhance them and scan back into my computer if still required.

I have the circuit for my unit, so don't waste your time unless Jeff comes back or PM's you. On the other hand, If you do it, I'd like to add it/them to my collection of diagrams.

I'm really curious to know where you searched to find the originals!

Search here,

https://www.google.com.au/search?q=Tweezer+DC-80&tbm=isch&imgil=pm7EPU8pPQ8RdM%253A%253BvutZxSKlyscOTM%253Bhttp%25253A%25252F%25252Fwww.liveauctionworld.com%25252FAIDLIN-AUTOMATION-DC80-C-TWEEZER-WELD_i20987164&source=iu&pf=m&fir=pm7EPU8pPQ8RdM%253A%252CvutZxSKlyscOTM%252C_&usg=__cxdwJo0vqFM-i965qsmvVaIS97I%3D&biw=1150&bih=778&ved=0ahUKEwjG7qSlnIXLAhVDFJQKHUp9ALUQyjcIVA&ei=dMXHVsbkD8Oo0ATK-oGoCw#imgrc=vPbkgUwyrYm4zM%3A

You can see the circuits in the general miniature pics

Very good! I can see the miniatures, but when I click on them to enlarge them, I just get a white block. I'm using a Mac, which does not have Adobe flash installed, so perhaps that prevents me from seeing them.

If I could get good copies of those images, I'd definitely reverse colors; I find that white on black much more difficult to read...

Just before I started to reply to this, I saw that Jeff did come back, so do what you can! If you create PDFs, I'd like a copy. I'll PM my email. If you can't reverse the colors, I'm pretty sure I can...

Thanks dkwarner and Garth, I am sorry I have not gotten back with you until now. I am very interested in what you both have to offer, as I think taking bits and pieces from both will help me in understanding what these relays do and the trigger mechanism for troubleshooting the one I have. At this point as I have read every post on this thread, it could be the points in the relay or a bad solder joint some where. I have in the past taken the cover off a relay and sanded the points with 600 grit sand paper, so I think I will try that again. The cover on these relays are clear and one has a burnt spot right where the points are located. I plan to check resistance after I take the covers off, slightly press the points closed (manually) and check resistance, this should tell me their condition as I do this test, might find a loose connection or a bad solder joint as I remove these relays as well. I will PM you dkwarner, thank you so much for youalls reply's, Jeff. PS: yes I am from Texas.

I'm waiting for the PM... My drawings are created in Vectorworks, which you are not likely to have available, so I'll have to export them for you. Do you have a preference of JPEG, PNG, PDF, or something else? I have quite a few options available...

PDF would be wonderful, thanks dk.

Two were sent at 8:18 AM PT

I received them dk, they are expertly done, thanks so much. Now the work begins, he he.