NCX-5 powered by NCX-A keeps blowing fuse

[ORAWA01]

The DC voltages at C1 is 900VDC. But C2 is 475VDC.
Also when from lamp limiter inline to when it is being switched off (letting the full AC flow into the PSU), there seem a bit of arching noise in the lamp limiter plug (maybe from the fuse?). I thought the fuse is blown, but it wasn't.

Anyhow when being switched over to full AC current flow, there is some violent thump noise. And the C2 voltage is only 475V.

The resistor values all seem OK. They are all correctly wired up.

There is a good 12.5VAC from the pin 4 and 6. The PIN out number 11 is 900 VDC.

[ORAWA01]

Originally Posted by snowman_al

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?

The green pair seems the R9 660 ohms 10W (2x 360 ohms in series). Under the green pair are two grey pair of 330 ohms which are R5 and R6.
The diagram has note saying they should be taken out, if the PSU is to power NCX-3.

[snowman_al]

Originally Posted by ORAWA01

... 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.

I assume the lamp limiter lights brightly!
Remember the rig takes 140 watts in receive and 360 watts in transmit. Last count you had a 60 watt bulb in the limiter. Chances are the bulb is getting nearly all the volts and the PSU practically zero... and that is how it is supposed to work, it limits the current to that of the bulb.

You have a dilemma now the lamp limiter is no longer going to help.
Unless you have access to a variac you might have to go down the plug it in and see what happens route.
If it does not blow the fuse straight away, leave it switched on for 10 - 20 seconds and switch off again. Leave it for a couple of minutes.
Switch on again for 20 - 30 seconds and switch off again. Wait another couple of minutes. Repeat for 30 - 60 seconds.
Now switch off and leave it for 10 minutes. Check any electrolytic caps in the rig are not hot to touch.
Now extend the switched on time to 5, 10 and 15 minutes. Switch off and leave it a couple of minutes each time still.
If you smell or see smoke switch off straight away!
After that you should be able to make voltage measurements in the rig with reference to the manual. Good luck!

[ORAWA01]

Yes, it did. Very very bright.

I am thinking of getting a variac although they are expensive but maybe worth it for protecting the vintage radios and bring them back to life? Thanks.

[Station X]

I assume that the lamp glows brightly as soon as you switch the rig on? Or does it happen with the rig either not connected to the PSU, or switched off?

If the latter there may be a problem with the PSU's smoothing caps.

[ORAWA01]

Yes, it was. The psu now seems working.

I followed the instruction of switching on and off with the intervals, and the rig came on. I was excited with the rig receiving quite well.

https://youtu.be/QwHv3fXb2B0

Then after about an hour there was some smoke from the rig. I opened the top, and looked closely into where the smoke was coming from. It was the wafer switch of the band selector. There was small arching and sparking from the wafer switch with smoke.

There are some resistors and caps on the wafer switch. They looked burnt.

Will need to strip the NCX-5, and give it a good clean especially the wafer switches, replace the damaged resistors and caps around the wafer switch.

[HamishBoxer]

Hope the switch is ok,could just be a leaky cap and associated reses.

[TrevorG3VLF]

With 110V floating around and high voltages obtained, I would suspect that you are feeding with too high a voltage. This would also cause thumps when a transformer saturates.

[ORAWA01]

The PSU problems seems have been sorted out. It was receiving really nice for about an hour before the band selection wafer switch was arching with smoke.

The rig needs good through clean with switch cleaner on all the controls and switches and maybe a few burnt / leaky caps and resistors need replaced.

I feel it is getting somewhere now. The P.A. looked very healthy, when in TUNE mode, it was growling with some serious RF power. The life is there in the TX.

Thank you so much for all the info and advice, which were vital to bring the rig back to life. I was not sure if it will even power on ever, first time when it arrived. It looked unused for about 30-40years at least.

And it sat in the shed for the whole year forgotten until I saw it again after much rain and some dampness got into. Rescued them and dried with towel, and then in dry warm air for a few days, and started working on it.

Yes, it is running on a 110V step down transformer. That thump noise in the start has disappeared now. It seems the PSU HV problems has settled down itself. Now the rig itself needs get sorted.

[ORAWA01]

I was keep checking the POWER SUPPLY, NCX-5 after replacing the refiier diodes, and output voltages seem way too high than the circuit diagram figures.

The Circuit Diagram
PIN 3 = -80V
PIN 4-6 (Heater) = 12.6ACV
PIN 9 = 280V
PIN 11 = 700V

The NCX-A Set voltage readings with my DMM
PIN 3 = -106V
PIN 4-6 (Heater) = 13.4ACV
PIN 9 = 484V
PIN 11 = 900V

Is this normal after replacement of the diodes? Would it be OK to run the rig with the higher voltages?

Should it be only used after the voltage is adjusted to the book recommendation? Or would it be OK to run because the book's voltages are for under no load readings?

Thanks

PS: What are the PIN 3 and 9 for? What do they power in the rig?

[snowman_al]

ORAWA01 I am a little concerned by your question, ''What are the PIN 3 and 9 for? What do they power in the rig?''

Pin 9 is the main 280 volt DC supply to the whole rig.
Basically it powers all of the receiver stages and most of the transmitter stages.

Pin 11 is 700 volts to power the transmitter 'final' 6JB6 valves. These need a negative 'bias' on their grids to operate, so

Pin 3 is -80 volts to provide the negative voltage for the 6JB6 valves. Above.

As mentioned before... the voltages in the manual are with the rig connected to the power supply so 'on load'. If you are measuring the voltages without the rig plugged into the power supply, the voltages will be higher.

Please be careful if you are not sure what is what?

I have included a user manual which has the schematic so you can trace out the various voltages.

[Station X]

First check the input voltage to the PSU. The diagram says 117 VAC (Nominal 120VAC), but the voltage delivered from your 240V/120V transformer may be higher than that, particularly if the mains voltage is high in your area.

Voltages quoted will be on load. Check that all valves are lighting up and fully loading the LT supply.

I suspect that the 700 VDC supply is only used on transmit, so must be measured with the transmitter keyed up and operating correctly.

EDIT. Crossed with snowman al.

[ORAWA01]

Thank you for your clarification, snowman and Graham.
I should have read the manual for the info. I was being lazy. Sorry.

Yes, the PSU output voltage seems all around very high, and indeed last night the rig went mute. I mean no sound on reception. Not even hash noise. Just total silence. All the valves are lighting.

The Power Supply has dual voltage switch 230V and 115V. It had been used with 230V, and our mains voltage is 237V.

I am trying to reduce the pin3 voltage -107V to nominal -80V by inserting a resistor inline.

From ohms law, V=IR, current in the pin is 10mA, so this is how I was thinking, could you please confirm it is right? Or totally wrong?

The amount voltage I want reduce is, about 27V from 107 - 80 = 27V. Let say 30V. From ohms law, V=IR.

30= 0.01A (the pin 3 current is 10mA) x R
R = 30 / 0.01
R = 300
300 ohms?

Would 300 ohms resistor serially inline to the pin3, bring the voltage to -80V?

Thanks

[HamishBoxer]

Are these voltages taken off load I wonder? in which case possibly not too far out, or have I again missed a previous post?

[ORAWA01]

Yes, they are taken off load with the rig disconnected from the PSU. I am trying to troubleshoot why the rig has gone mute, no sound on receive.
The manual says, that when the BIAS pin3 voltage is too high, the VOX can over hang.

Just found another problem in the PSU. The R5 and R6. They are about 9M ohms each, where they should be 330 ohms.

[Station X]

According to post #61 the resistor values seem OK. 18M is virtually open circuit! I'm surprised that you can measure 280 VDC off load even with a DMM. You are measuring on the socket side of the resistors?

Assuming your meter is OK, I'd replace these resistors before you go any further.

[ORAWA01]

One was 18M ohm and the other one was 365 ohms. So discarded the 18M ohm one, and replaced with a new one.

The rig has no audio from last night, no volumn on reception. There is a very weak hum when volumn up at max, so trying to sort the problem.

Thanks.

[HamishBoxer]

Are these the burnt reses near the switch?

[ORAWA01]

No they were in the power supply.

[ORAWA01]

Yeah, it is now sorted. Replaced the burnt resistor and another couple of capacitors, and the rig came back to life.

Thank you for all your info, and help. cheers.