Hi Folks,
I am a newbie to electronics please take it easy on me since I believe these questions must be dumb to some.
I have an old Fadal probe controller board. basically, it switches Renishaw probes for the CNC machine from the table to the spindle when called by the control. This is an old board that no longer is made circa 1987, hence it needs to be repaired.
I took a look at the board the 35v 3300uF capacitor was popped so I was hoping by changing it the board would live again but no it popped once more
I removed it and took some readings from the power transformer (PC-16-1500, Single 115 V 6 Pin ) per the datasheet the size is 24VA, in series 16 VCT @ 1.50 A and parallel 8 V @ 3.0 A. I measure the input to the transformer correctly at 115v however on the output coil 1 and 2 were each at 10v and in series at 20.5. The power at the capacitor was 38.6v.
I am looking at the obvious the transformer is not putting the right voltage out, am I correct? Or there is a certain level of error that is acceptable? Or perhaps there is a component that is regulating the power that is not working right.
38.6v for a capacitor rated at 35v is past its design?
Is there somewhere else on the board I should be looking at?
I know, I know these are obvious questions but then again I am no specialist in electronics, hoping to hear from people that know much more than me. Thank you very much in advance!

 

Welcome to AAC.

Some things that will help:
A picture
A schematic (if available)
Transformer voltages: They are what they are. But when rectified and filtered they are outputting a higher voltage. For instance, a 12 volt transformer is putting out 12 volts RMS. When rectified it puts out 12 volts pulsed. When filtered it puts out over 16 volts. So your 10V reading (assuming AC) would be about 13.5 volts DC. (10 X 1.414 - 0.6 = 13.54). During the engineering of the board this was taken into account. So you may read 10 volts, but after rectification and filtration it will be higher.

You said your secondary is a center tap. That's useful information for us, but there still a lot we need to know and see. So a picture will help.

 

Very seldom a transformer will put out the wrong voltage without some more symptoms, they are very basic devices.
Especially outputting a higher voltage on both secondaries.
According to the AC voltages you measured, the voltage across the capacitor should not be 38.v ?
I would do a little more reverse-engineering to discover more details on the P.S.. circuit.
Is the transformer running fairly cool?

 

Thank you, gentleman! I think Tonyr1084 is nailing it in the head I am attaching a picture of the board at the top in that picture is the datasheet info for the regulator but after reading Tony's post I think he is right the regulator might be ok.
The board is simple there are not too many components, the picture shows them all.
There is a Positive Voltage Regulator, Could that be bad? I am going to google how to test it.
MaxHeadRoom, yes all voltages I took are AC, I am unsure how many volts should be across the capacitator but I do know is rated for 35v 3300uF.
I do not have schematics for the board unfortunately, this thing is ancient!

 

MaxHeadRoom, I forgot to reply to your las questions, yes it is running cool.

 

I'm old enough to remember that some time in the last decades the grid voltages were raised from 110 to 115 V resp. 220 to 230 V.
Provided the capacitor had little headroom in '87, with the raised input voltage it might be beyond spec. Why not try a 50 V type of the same capacitance, hoping that the subsequent circuit will work under the changed conditions.

 

I agree with @du00000001 , you are most likely seeing voltages that are the result of a corresponding higher primary voltage input.

 

The PC-16-1500 is (somewhat miraculously) still being manufactured by Bel Signal Transformer. Mouser doesn't have any in stock but they are expecting to have 8 of them in by July 16. Better get your order in now! Then again you still need to figure out what failed before you attempt the actual repair, anyway: https://www.mouser.com/ProductDetail/Bel-Signal-Transformer/PC-16-1500?qs=/wXgF/fIho%2BdQHLosTLLhQ==

 

The PC-16-1500 is (somewhat miraculously) still being manufactured by Bel Signal Transformer. Mouser doesn't have any in stock but they are expecting to have 8 of them in by July 16. Better get your order in now! Then again you still need to figure out what failed before you attempt the actual repair, anyway: https://www.mouser.com/ProductDetail/Bel-Signal-Transformer/PC-16-1500?qs=/wXgF/fIho%2BdQHLosTLLhQ==

I am not convinced yet it is a bad transformer.

 

I'm not "convinced" either but this is happening in a machine shop, where machines get moved around and reconnected and motors replaced and rewired constantly. The odds would be pretty good that without an adequate amount of supervision the "incoming 115" right now happens to be more like 208 (not saying I know that's what happened, just postulating), if that happened it would probably need replacing.

 

Thank you everyone for your replies!
The CNC machine has its own 3 phase transformer that gives a constant 115 to the electronics, I do not think is an input power issue, but I could be wrong.
I am thinking perhaps I got a bad cap since I did not test it before I put it on the board, and it did look a bit beat up.
If that does not work perhaps I give a try at a 50v cap like du00000001 mention.
As a side note do voltage regulators go bad? I saw there is one on the board I just have not gotten around googling how to test it.

 

Thank you everyone for your replies!
The CNC machine has its own 3 phase transformer that gives a constant 115 to the electronics, I do not think is an input power issue, but I could be wrong.

3 phase CNC machines usually have a single phase transformer fed from two phases.
Renishaw is generally high quality well made units.
Have you actually done any measuring of the '115v' ?

 

MaxHeadRoom: this transformer takes in 3 phases and it feeds 115v 2 phase to the electronics and 3 phase 220v to the spindle.
Yes, I have taken reading from the transformer the power is as it should at the machine.
I did put in the new cap on the board and gave it a try still it failed, I could feel the cap overheating, I did pull the plug before it popped I also noticed that the components in the picture I am attaching did get very hot, what are those? Could those be faulty?
Perhaps I should give du00000001 idea a try with a higher voltage cap, the factory one was a 35v, I did read 38.6v when I measure without it, should I give a 50v a try or perhaps a 40v?
Then again I am unsure since I did notice those other components overheating. Anyhow, thank you for your help.

 

Not sure that some 40 V rating would be available for Ta caps. Was somewhat surprised that a 50 V rating exists. If you cannot get a Ta cap with that voltage/capacitance rating, use an aluminum cap instead. (Might require some fumbling as Al caps are usually larger, but should be possible.)
At least Al caps are a bit more tolerant wrt overvoltage. I'm skmewhat surprised that none of the Ta caps burst in flames as this is their intrinsic failure mode.

 

I did put in the new cap on the board and gave it a try still it failed, I could feel the cap overheating, I did pull the plug before it popped I also noticed that the components in the picture I am attaching did get very hot, what are those? Could those be faulty?

That is the four diodes for the bridge rectifier, check them all, you may have to lift them of the foil to check.
Use a diode check if you have it on your meter.

 

Once more thank you for your help gents
du00000001: I did find the 50v caps but you are right there is no 40v. I am a bit hesitant to put the 50v I am afraid I will fry something else but it is an option.
anniel747: Yes I double-checked that.
MaxHeadRoom: I did check them on the board and they seem ok, I just find out that you should remove components for testing. I will remove them and check them again. They are dirt cheap I bought some to replace them with new ones.
Anyhow, I'll post how it goes. Thank you once more everyone for your help.

 

Once more thank you for your help gents
du00000001: I did find the 50v caps but you are right there is no 40v. I am a bit hesitant to put the 50v I am afraid I will fry something else but it is an option.
anniel747: Yes I double-checked that.
MaxHeadRoom: I did check them on the board and they seem ok, I just find out that you should remove components for testing. I will remove them and check them again. They are dirt cheap I bought some to replace them with new ones.
Anyhow, I'll post how it goes. Thank you once more everyone for your help.

Oh - a very beginner (I already assumed so much.)
The capacitor rating is just about what the capacitor is able to "survive". A capacitor is not a battery - it's just buffering the voltage that it "sees" at its poles. so no need for fear about an overvoltage caused by the capacitor

About the hot diodes:
If the Ta capacitor fails (due to overvoltage or reverse polarity), it "mutates" into a low resistance (aka short circuit). In this situation, the transformer will its very best... Thus the diodes have to dissipate more power than in the normal operation state. Anyway: diodes usually "survive" massive maltreatments (up to 10 times the nominal current, 10 times the normal power dissipation - provided the maltreatment does not last too long). So I do not expect the diodes suffered fatally.

 

With the 50v cap in circuit, check the voltage across it, if it survives, if not there is something drastically wrong going on there.

 

du00000001, MaxHeadRoom: Thank you very much for the replies! I order the 50v capacitor, I will give it a try first
With that said I see just a few simple components on the board, I think the relays look more complicated than the board itself. I did remove the relays to test the board thinking there might be something wrong with them but still the same issue.
I'll let you folks know how it goes after I replace the cap with the 50v one, it should get here in a couple of days. I am keeping my fingers crossed!

 

Hi fellas, I did replace the cap today with a 50v and it was an epic failure it did not get as hot as the 35v but after I say about 10-20 seconds it popped.
What would you folks suggest is the next thing to test? Thank you!