NCX-5 powered by NCX-A keeps blowing fuse
 

Hi All

I powered on the NCX-5 hf rig connected to the NCX-A psu, and it keeps blowing the fuse. It has a glass fuse at the back rated at 4A.

I replaced the fuse to a fresh new one, and connect the NCX-A into the mains which is a 110V step down transformer. The fuse is not blowing in the NCX-A at this moment.

It is when the NCX-5 is powered on, at that moment, the fuse blows. The blown fuse shows a burn all around its glass.

What could the cause for this? I was thinking that the NCX-A psu is OK (it has been fully recapped recently with high quality parts), but the rig itself NCX-5 has problem. But what do you think? Any ideas?

Thanks

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Whereabouts in the circuit is the fuse located? In the mains supply to the PSU, in the output from the PSU, or in the HF rig?

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

It is in the mains supply to the PSU.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Possibly a short rectifier diode, if it has them. Worst possible mains transformer. I am assuming that powering on the rig switches the mains to the power supply.

Should not be difficult to trace.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Circuit courtesy of BAMA.

Full manual available here:-

http://bama.edebris.com/manuals/national/ncx3/

Switching on the transceiver shorts pins 1 and 2. I'd try disconnecting the PSU from the rig and shorting pins 1 and 2 to see if the fuse blows. Be very careful though. You don't want a 700 VDC shock!

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Thank you for your info & advice on this problem. :thumbsup:

All the fuses in my tool tray has run out this afternoon, as the NCXes kept blowing them off when powered on, and I kept on trying with different settings on the rig. I placed an order for 10 more fuses this evening. When they arrive, I will have a go again.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Has the PSU ever worked since it was recapped?

Fred

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Diodes worth a check first, then make sure that cap is correct way around at bottom of circuit.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

When you get new fuses, may be worth a try with a lamp limiter if dc resistance checks are inconclusive.
Rob

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I think we've been there:-

https://www.vintage-radio.net/forum/...d.php?t=161363

Good idea nevertheless.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Ok Graham, my memory is not as good as yours!
Rob

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Don't you believe it. I have "Bottom of the stairs" moments as well as "Top of the stairs" ones!

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I am not sure. It had been recapped by the previous owner.
I am trying to power it on for the 1st time since I got them long time ago.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

When the PSU arrived to me, the fuse was already burnt.
The PSU is marked as 110V. Is there any possibility that someone might have had plugged it into 230V accidentally, and burnt the fuse?

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Still didn't manage to get / make lamp current limiter. But would circuit breaker and fuses do the same job? = if over voltage or over current flows, then they will trip, or burn / blow?

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

100 percent chance I'd say, but the fuse probably saved it.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

"Still didn't manage to get / make lamp current limiter. But would circuit breaker and fuses do the same job? = if over voltage or over current flows, then they will trip, or burn / blow?"

Without technical info to back this up, if it were me I would expect the fuse to blow and the trip maybe trip as well? If I had a limited number of fuses, then I would use the lamp limiter for certain.
All depends on just how many fuses you wish to sacrifice!
Rob

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Letting the equipment just blow fuses is harsh.

The current required to blow a fuse as you have described runs the risk of damaging something else.

A lamp limiter allows you to see faulty behaviour without risking hard to replace parts.

Whilst trying to find the cause of the fuse blowing you need to reduce the available current to more manageable levels.

Time to get your limiter built and your meter out.

Cheers


Mike T

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Have the bridge rectifier diodes been checked ok yet?

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I had 2x fuses, which both blown. I put another one in, but it is underrated one at 0.5A.

The books says, it takes 3.7A for transmit, and 1.5A to receive. So when the rig is being powered on, it will draw at least 1.5A.

So, I am not powering it on now, but when the PSU is plugged into the mains, it is live, when checked with NCV DMM. So, the PSU itself is not blowing the fuse, when live. It is only when the rig gets switched on, it blows the fuse.

Now and for a few days, I have other things to clear, and organise, so they are put away for now. But will get them out and have a look again. If I could get more ideas where to look and check, that would be great. Thanks.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I shall check them, next time when looking at it.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

You don’t have to “build” a lamp limiter, a table lamp with a 100W incandescent bulb will do the job. Just wire it in series with the mains supply which you need “limiting”.

In your case, you could wire the lamp across the holder where the fuse keeps blowing (leaving the fuse out of course). Then if you have a short circuit somewhere, instead of blowing the fuse, you will just have the table lamp light up.

But your unit is 110V, so you really need a 110V bulb, a 240V bulb will be rather dim. Or you could wire the table lamp, with its 240V bulb, in series with the input to your step down transformer.

You mention checking the power supply to be live with an NVC DVM. If you mean a non-contact voltmeter, this may only be telling you that you that there is mains getting to the box, not that it is being applied across the transformer primary. To check that the power supply is really on, and that the fault lies with the hf rig itself, you need to check for something coming out of the supply, I would suggest the 12V AC supply for the valve heaters.

Be VERY careful probing inside the supply, there are lethal voltages in there, and no amount of RCD or isolation transformer protection will save you. Some of the voltages are high enough to destroy most DVMs. To play it safe, I would wire a 12V bulb across the heater supply (with the unit disconnected from the mains), then plug it in and see if the bulb lights with the rig switched off. If the circuit is as shown in an earlier thread, it will not.

Ps - my suspicion, based on the description of the blown fuses and the fact that is a 110V unit in a land of 240V supplies, is that the HV diode bridge is blown, and hopefully fuse(s) saved everything else.

Pps - an apology to Fluke owners - you meters should survive.

Stuart

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Thank you for your advice, Stuart.

Yes, I am aware of the danger in the PSU with the high voltage. I was going to wear surgical rubber gloves when working on these things for protection from the voltage too.

The ordered fuses have not arrived, and I realised the seller is in China. It will take weeks to arrive.

I had to order another batch of the fuses, making sure the seller is in UK, and hopefully it will arrive this week.

I need to take out the PSU from the shelf, open it up, and check the Diodes. With the most caps in the PSU replaced with shiny new ones, and good clean state of the rig's internal parts, I am hoping that it will power on with the new fuse.

I was looking for 100W bulbs in the shops, but couldn't find one. What I have found was 100W replacement LED bulbs. I am not sure if they are the same rating. The bulb is very bright like standard 100W one, but LED bulb?

I have a circuit breaker, and will use it when plugging in the PSU to the mains, and the mains plug of the PSU has a fuse in it, which is 5 AMP, and at the back of the PSU has fuse holder which takes 3-4 amps slow blowing glass fuse.

So I am hoping the circuit breaker and the 2x fuses will protect the PSU and the rig, if something is not right, and draws over current when powering on.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

An LED lamp is not suitable for a lamp limiter.

You need an old fashioned lamp with just a filament and no electronics. These are sold as Rough Service (RS) lamps and this is an example. Other suppliers are available.

https://www.bltdirect.com/240-volt-1...-service-pearl

Before you even think about applying power check that the new capacitors in the PSU have been fitted the right way round and test the diodes using the diode test function of your DMM.

You should test the PSU without it being connected to the transceiver. You can do this by shorting pins 1 and 2 of the plug.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

What should be the result of shorting the pin 1 and 2 ? Do you mean with DMM for reading VDC output?

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

If you look at the diagram attached to post #5 you will see that shorting pins 1 and 2 of the plug will apply 120 VAC mains to the primary winding of the PSU's mains transformer.

Normally this short is applied when you switch on the attached transceiver, but it is better to test the PSU on its own, so hence the need to short pins 1 and 2.

DMM = Digital Multi Meter. You can use its diode test facilty to test the diodes in the PSU as described in post #1 in this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=42194

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Aha got you. So, the ac mains input is going into the primary coil via the pin 1 and 2, which ups the voltage via 2ndary coils in the psu generating the high voltages and feed into the rig via other pins?
And the high voltage will only be present on other pins to check, when the pin 1and 2 are short.

Yes, I do have 3x DMMs and use them normally for checking voltages and ohms and connectivity, but have not checked diodes for long time. And the link is very useful refreshing my memory on how to and what to look for in checking diodes.

Thanks.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Correct. There is also a low voltage secondary winding feeding the transceiver's heaters.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Sure, great info. Thanks.:thumbsup:

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

The fuses arrived. I put the new fuse in, and just did stand alone PSU test by shorting the pin 1 and 2, as suggested by Graham.

And the fuse blew. So it seems clear that there are major faults with the PSU.
Will open it up, and check the rectifying diodes, and the replaced caps.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Well at least you've proved that the fault is in the PSU and not in the associated transceiver.

I doubt the fault is major unless you've been unfortunate enough to suffer a burned out mains transformer. All the other components with the exception of the smoothing chokes can easily be replaced, but choke faults won't blow the fuse.

Use a test lamp connected instead of the fuse next time you power it up.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I hope it is not major fault, as you say, such as the burnt transformer.

I checked the replaced caps, and they are fine.
But the Diodes seems problem. There are 4 rectifying diodes, and each reads different values, and the last one reading changes from 0.07 - 2V constantly.

D1 = 0.08V
D2 = 2.8V
D3 = 0.04V
D4 = 0.07 - 2V changes

There are 2x other diodes D5 and D6 ones for -80V pin 3, and their reading seems clear and constant 2.5V

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I have checked the diodes again using my MK-328 TR/LCR/ESR Tester, and it picks out 3x of the diodes as faulty from the 4x rectifying Diodes.

D1 is coming up as a capacitor
D2 is damaged or faulty
D3 is damaged or faulty
D4 is coming up as a diode with 1.25V and working properly

D5 is coming up as a capacitor
D6 is damage or faulty.

But not sure, if the diodes are to be tested in circuit, or should they be taken off for checking?
I know capacitors could be giving totally different reading when they are in circuit. They appear as resistors in circuit in the readings.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Test them out of circuit and test them BOTH ways.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Normally they show up either way if short in a bridge circuit and of course replace all 4.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Yup, will take them out, and replace them if in doubt.

Where is the best place to buy the rectifying diodes, and are they readily available for ordering up?

In the circuit diagram, they appear as, Silicon 1600PIV similar to IN2359, and
Silicon 500PIV similar to IN534.

And, I shall rescue a lamp from the shed, and put alligator clips on each mains wire, to which it could clip the fuse holder both ends each for testing, because blowing every fuses I had with the faulty PSU, like elephant gobbling up biscuits were no fun at all.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

You should aim to not blow any more fuses! As Cobolt blue said in post #18, it’s harsh on the equipment. A fuse does not limit the current, it only limits the time the over current lasts, and hopefully prevents fires. A blackened fuse indicates a fault current well in excess of the fuse’s ratings. Use a lamp limiter, it will limit the fault current.

Note that under fault conditions, semiconductors, in your case the rectifier diodes, will be destroyed faster than a fuse will blow.

The main HT bridge diodes need to be high spec devices, you need a reliable source of parts not cheap fakes. For the -80V supply, the diodes (if they were 500V devices) should have survived having 240V into the unit (if that’s what happened), but they need taking out and checking.

I don’t think you should be measuring the HT supply, or the HT winding on the transformer on a typical auto ranging DMM, you will exceed its maximum voltage rating. However, the supply includes a handy voltage divider chain, R1 to R4. You should check that each resistor is OK. Then when you have replaced all the faulty diodes, you can power up the unit (still through the lamp limiter) and measure the voltage across one of these resistors, it will be one quarter of the full HT.

Oh, and in case it hasn’t been mentioned before, use a lamp limiter.

Stuart

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Great advice, all noted. Thanks.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Radiospares or CPC Farnell for diodes etc. Min order for free delivery only £8 I believe.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I would get a handful of 1N4007 diodes (1000V 1A). Two in series would replace each 1600V diode and one would replace each 500V diode.

If you have a 120V supply you could run up the power supply on the 240V tapping so measured voltages would be half that in normal operation and thus measureable.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

If you use 2 diodes in series for each of the 4 diodes in the bridge, you should probably put high value resistors across each one to equalise the reverse voltage, these resistors must withstand high voltages, and it’s all built on tagstrips. For the sake of 4 high voltage diodes, I wouldn’t compromise on a high power unit such as this. Note that if any of the diodes are faulty, the others will have suffered a degree af abuse. Even if they appear OK on test, they may fail with full power applied. Just replace them all.

Yes, you can connect your lamp across the fuse holder instead of the fuse. It’ll be a 240V bulb on a 110V supply, so even with a dead short, it won’t be very bright, but it will limit the current nicely, at least while the transceiver part is not connected.

If you power up the supply with the diodes removed, the lamp should glow very dimly or be completely dark. Measuring the voltage on the 12V heater winding should show if the transformer is doing something. Do not measure the HV secondary with your DMM. (This is where an old AVO comes into its own, but I guess you don’t have access to one.). The transformer should run cool, if it heats up, it may be faulty. (If it’s designed for 60Hz, it might run a bit warm even with no load). If the 12V is present, and the transformer doesn’t get hot, it’s probably OK.

Then try again with good diodes fitted. You should now be able to measure the voltage across one of those big resistors, probably around 200V at no load, corresponding to a no load HT of 800V. The limiting lamp should still be lit either very dimly or not at all.

With the transceiver connected and warmed up, the power consumption is too high for a lamp limiter, but if you move your lamp to the primary of your step down transformer then you should see the lamp slowly getting brighter as the valves warm up. If this happens, then there are probably no catastrophic fuse blowing faults, and can apply full power.

Another safety note - from the schematic attached earlier, there is only one earth pin in the multi-way connector between the power supply and the transceiver. If as you engage or disengage that connector the HT pin was connected but the earth pin was not, then you could get a high voltage between the chassis of the power supply and that of the transceiver, a shocking state of affairs. You should unplug the power supply from the mains before you connect or disconnect the two units.

Stuart

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Great detailed advice and info, thanks.

Yes, I am going to order a full set of 1600V rectifying diodes of 1A each, and replace them all soon.

For lamp limiter, I have an old lamp in the shed. I will attach alligator clips to each mains wire off the lamp, and clip them onto the fuse holder, so the bulb in the lamp will be as a fuse in the circuit, when powering on the old devices for the first time after long storage.

This NCX-A PSU has 3 wire mains cable, and the earth wire had been screwed into the chassis. I am not sure if the rig needs some sort of grounding like that, and if it has it. Not sure the now.

Re: Using lamp current limiter
 

Yes, I do have a security light in the shed has incandescent bulb of 120W. Maybe just replace with that?

I managed to find an old fireplace bulb of 60W. I put that on to the LCL, and connected up the problematic PSU. The LCL bulb came on, and it is very very bright.

So, it must be drawing excess current? It has been keep blowing the fuses, but now with the Lamp Limiter, it didn't. The fuse is fine.

It's been quickly taken off, after confirming the bright glowing bulb of 60W. Should I test with 120W halogen bulb of the security light? or just conclude the PSU has fault somewhere?

Re: Using lamp current limiter
 

Yes, you have proved there is a fault with the power supply... when using the lamp limiter the fuse in the NCX-A will not blow. You are limiting the current through the power supply to the current the bulb takes. So for a 240 volt 60W bulb that is 0.25A. The power supply fuse is 5A. A bigger bulb will tell you nothing you do not already know.

The bulb tester here can do two things.
1, prevent further damage (or blown fuses) to a faulty 'part or unit'. In which case the lamp lights brightly and does not change over time, say 20 or 30 minutes.
2, help re-form dormant electrolytic capacitors. Now the lamp lights brightly at first, then slowly dims over the first 20 to 30 minutes. How much it dims depends on the condition of the capacitors.

As there are so few parts in the NCX-A it would be best to first prove the transformer is good. Disconnect the bridge (D1,2,3,4) and try the lamp limiter again. If it lights and stays bright still there is a fault within the transformer. If it lights and dims very quickly at switch on then replace the rectifiers and caps...

Re: Using lamp current limiter
 

I assume this is with pins 1 and 2 shorted to connect the mains supply through to the transformer's primary winding and that you were powering the PSU from a 120VAC or thereabouts supply?

Where was the lamp limiter connected?

Re: Using lamp current limiter
 

Yes, the NCX-A pin 1 and 2 connected, and plugged into the LCL.

The LCL is plugged into a 120V step down transformer, because the NCX-A is 120V mains only.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Again are those diodes ok?

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

I have ordered a new set of diodes rated at 1600 PIV and 1A, and they will arrive in few days. Will take out the old diodes, check and replace them.
But wondering if there are other possible problematic parts causing keep blowing fuse.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

If you disconnect the diodes D1-D4 you will isolate the diodes and smoothing caps from the transformer.
If you use the lamp tester with no diodes connected and the lamp still lights brightly all the time you need to check the transformer or shorts in the mains wiring.
If the lamp flares brightly then dims significantly, replace the diodes and caps with new ones. A faulty diode will damage an electrolytic cap.

Re: NCX-5 powered by NCX-A keeps blowing fuse
 

Correct Al.