NCX-5 powered by NCX-A keeps blowing fuse

[ORAWA01]

Originally Posted by HamishBoxer

Have the bridge rectifier diodes been checked ok yet?

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

[stuarth]

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

[ORAWA01]

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.

[Station X]

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.

[ORAWA01]

Originally Posted by Station X

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.

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

[Station X]

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

[ORAWA01]

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.

[Station X]

Originally Posted by ORAWA01

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.

Thanks.

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

[ORAWA01]

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

Sure, great info. Thanks.

[ORAWA01]

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.

[Station X]

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.

[ORAWA01]

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

[ORAWA01]

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.

[Station X]

Test them out of circuit and test them BOTH ways.

[HamishBoxer]

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

[ORAWA01]

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.

[stuarth]

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

[ORAWA01]

Great advice, all noted. Thanks.

[HamishBoxer]

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

[TrevorG3VLF]

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.