NCX-5 powered by NCX-A keeps blowing fuse

[stuarth]

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

[ORAWA01]

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.

[ORAWA01]

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?

[snowman_al]

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...

[Station X]

Originally Posted by ORAWA01

Yes, 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.

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?

[ORAWA01]

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.

[HamishBoxer]

Again are those diodes ok?

[ORAWA01]

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.

[snowman_al]

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.

[HamishBoxer]

Correct Al.

[stuarth]

If the lamp still glows brightly with D1 to D4 disconnected, then one or both diodes in the bias supply (D5 & D6) may be faulty, so disconnect those.

S

[TrevorG3VLF]

Do not change any component just for the fun of it. Find the fault first.
Diodes chould have one end disconnected to test them although a dead short can be detected without disconnecting.

C1 and C2 are connected in series with R1 to R4 providing balance. Check these resistors without disconnecting.

Work your way from input to output. Repair and test each section separately before moving on. Remember that intercomponent wiring can be faulty.

I have a PSU here which was operating poorly. Someone had changed components and reinstalled the rectifiers to give half wave rectification instead of full wave. Obviously a compulsive changer.

[ORAWA01]

Thank you for the great advices and information. Will keep them in mind, and keep on working on the PSU.

[ORAWA01]

Originally Posted by stuarth

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

My DMMs are cheap ones from Amazon, and I checked their Maximum ACV handling and reading capability in the spec., and they are rated at 1000 ACV. I wonder, if these DMMs are not going to be able to handle the high voltages in the PSU. The highest ACV I see in the circuit diagram is the pin out 11, which is +700V.

I also have a Fluke DMM, but it is a vintage Fluke 25 from 1980s. I have no information on its Spec.

[ORAWA01]

I have managed to replace the 4x rectifying diodes in the PSU. The PSU is now outputting voltages.
Using the DIY lamp current limiter, powered on the PSU. Voltage measurements with the limiter in-line were,

PIN 3 = -63V DC
PIN 4 = 5V AC
PIN 9 = 196V DC
PIN 11 = 367V DC

However, when the lamp current limiter was taken out, and straight AC Mains fed into the PSU, the PIN 11 voltage went up to 900V.
It is supposed to be 700V in the diagram. It is not blowing any fuse, but there is slight buzzing noise from the PSU, and I feel it is far too high voltage than the normal for the PSU.

What could be wrong here, and where do I need to look for the cause and fix of the problem? What would be the best way to reduce the pin 11 voltage to 700V?

Thanks

[ORAWA01]

Another thing I noticed from the PSU was, there are 4x 7500 ohms resistors in series in the diagram. But in the PSU, there are 6x of them and 2x smaller green ones on the top as well.

I checked the ohms of each of the resistors in the PSU, and they seemed reasonably within the range, but one or two was out of range. The total resistance at the end point to the end to end of the resistors in series was 45k ohms.

Is it not supposed to be 30k ohms? Why more resistors than the circuit diagram were added into the PSU? That question rose to me. Any ideas? Thanks.

[snowman_al]

Originally Posted by ORAWA01

... However, when the lamp current limiter was taken out, and straight AC Mains fed into the PSU, the PIN 11 voltage went up to 900V.
It is supposed to be 700V in the diagram. It is not blowing any fuse, but there is slight buzzing noise from the PSU, and I feel it is far too high voltage than the normal for the PSU.

What could be wrong here, and where do I need to look for the cause and fix of the problem? What would be the best way to reduce the pin 11 voltage to 700V?

Voltages are quoted 'under load'. If there is no load, the voltage will be approximately 1.4 times the AC voltage from the transformer secondary.
Check the AC voltage from the transformer secondary without the lamp limiter. Tell us what it is. (I would guess 640 ish?) Also check you have 12 volts AC on pin 4 without the lamp limiter.

Originally Posted by ORAWA01

... Another thing I noticed from the PSU was, there are 4x 7500 ohms resistors in series in the diagram. But in the PSU, there are 6x of them and 2x smaller green ones on the top as well.

I checked the ohms of each of the resistors in the PSU, and they seemed reasonably within the range, but one or two was out of range. The total resistance at the end point to the end to end of the resistors in series was 45k ohms.

Is it not supposed to be 30k ohms? Why more resistors than the circuit diagram were added into the PSU?

The grey 7.5k ones look original. So likely a factory mod. They are there to balance the voltage between the two capacitors C1 and C2 and to 'bleed' (discharge) the capacitors if the unit is not under load. So their value is not too important as long as each half is fairly equal in resistance.
I would guess the green pair are R8 the 30k 'bleed' resistor for C3?

[ORAWA01]

Ahhhh, not under load state it was, when the readings done.
Will read the transformer voltage and also the 12V AC voltage sometime today, and update here.

Also, I connected the NCX-5 transceiver to the PSU, and switched it on with the lamp limiter inline. But there was absolutely nothing happening with the rig. Not even the pilot bulb came on, and none of the valves lit.

I could understand that because the PSU is not powering enough current with the lamp limiter, but I would have thought the pilot lamp and a few valves might have lit.
I was suspecting if the rig itself might need to be recapped, because all the parts inside looks original.
But maybe the rig should have been switched on without the lamp limiter? I was too cautious in case it will be damaged by over voltage.

Thank you.

[ORAWA01]

Originally Posted by snowman_al

I would guess the green pair are R8 the 30k 'bleed' resistor for C3?

I was under impression those green pair were just added ones to the great ones to increase resistance last night, because they seemed having continuity all together with the grey ones.
But will try to clarify on that too today or when next time I could look into it.

[Station X]

Check that the DC voltages across C1 and C2 are approximately equal.

Check the values of the green resistors and trace where they're wired to.

When it comes to connecting to the transceiver the first thing to check is that the transceiver is actually turning on the PSU. It does this by shorting pins 1 and 2.

Then check that 12.6 VAC is being fed to pins 4 and 6 of the transceiver's connector, from where it will go to the heaters. 12.6 VAC suggests 6.3V heaters wired in series, so one blown heater will result in no power to another heater.