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Old 13th Nov 2019, 11:53 am   #1
ORAWA01
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Default Variac for powering up an old Transceiver?

I am planning to switch on a couple of old Valve Transceivers which are needing restoration and service. It looks it had not been powered on for many years. It has a separate PSU with speaker in it.

The transceiver is the National NCX-3 and NCX-5.

Do they have to be powered from the Variac to slowly bring up the voltage into the PSU?

Or is it OK to power on without it straight from the AC Mains 230V into the PSU?

If it needs one, then which type of Variac is good for the task? Thanks.

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Old 13th Nov 2019, 11:56 am   #2
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Default Re: Variac for powering up an old Transceiver?

I would suggest using a lamp limiter rather than a Variac.

https://www.vintage-radio.com/projec...p-limiter.html
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Old 13th Nov 2019, 12:01 pm   #3
Peter.N.
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Default Re: Variac for powering up an old Transceiver?

Personally I would just plug it in and see what happens although I am not recommending that course of action. If you have something like a 100 watt tungsten bulb try connecting that in series with it, the consumption on receive will not be a lot so it should show you if there is life in it.

Pretty well any variac will do the job, for receive test purposes 100 w should be enough but assuming it has a 100w transmitter you will likely need a 200w+ variac to test that.

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Old 13th Nov 2019, 12:28 pm   #4
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Default Re: Variac for powering up an old Transceiver?

It depends on how much risk you want to take. If you don't care about any of the components and are happy to replace anything that is destroyed with modern equivalents then just plug it in. It might be fine. If it's not then, well, you'll have a winter project.

At the other extreme, if you really don't want to trash anything that isn't dead already then you should reform the larger electrolytic capacitors using a capacitor reformer first. You should also open the boxes up and check visually for any signs of distress (overheated components or wiring, white-gettered or missing valves, cracked/bulging capacitors, anything loose rattling around).

A lamp limiter limits current. It doesn't protect items which might be destroyed by over-voltage (capacitors, for example) but once you have destroyed them it might save other parts of the kit. A variac limits the voltage. This is safer than just plugging the kit in, but in valve gear the rather nonlinear response to supply voltage means that a Variac is less protective than you might imagine.

Cheers,

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Old 13th Nov 2019, 1:39 pm   #5
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Quote:
Originally Posted by GrimJosef View Post
You should also open the boxes up and check visually for any signs of distress (overheated components or wiring, white-gettered or missing valves, cracked/bulging capacitors, anything loose rattling around).
Good advice: first, use the 'test kit' that Nature has given you: eyesight, sense of touch, nose. With something this old, a mechanical inspection first before applying power is always advisable.

Al.
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Old 13th Nov 2019, 1:46 pm   #6
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Default Re: Variac for powering up an old Transceiver?

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A lamp limiter limits current. It doesn't protect items which might be destroyed by over-voltage (capacitors, for example)

Cheers,

GJ
I wouldn't entirely agree. Current flowing through the lamp results in a volt drop across it. In the extreme case of a short circuit across the mains lead the lamp will light at full brightness and there'll be zero volts applied to the radio.

With other faults, and indeed with a fault free radio, the lamp will still glow to some extent and reduce the voltage applied to the radio.
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Old 13th Nov 2019, 2:32 pm   #7
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Default Re: Variac for powering up an old Transceiver?

A perfect current limiter would deliver a very narrow range of currents and be able to do this with a very wide range of output voltages. It's true that a lamp limiter is a long way from perfect. But its characteristic feature is that the lamp has relatively low resistance at 'normal' current but that this resistance rises substantially if the load tries to draw an over-current. That's really the sense in which I think of it as a current limiter.

Cheers,

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Old 14th Nov 2019, 9:34 am   #8
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Default Re: Variac for powering up an old Transceiver?

Great advice and info, thank you.

Will try with the lamp device, and see how they will go. It will save me the cost of buying variac, which is about 60 - 70£.
This is just one time project, so I will not be needing variac for any other things, I suppose.

Come to think of it, I have seen a USA Radio youtuber using a light bulb connected to the antenna switch using it as a dummy load testing all his valve transmitters output.

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Old 15th Nov 2019, 12:24 am   #9
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Default Re: Variac for powering up an old Transceiver?

I would also strongly urge the use of an RCD in the circuit, perhaps by means of a special 13 amp plug that incorporates an RCD.
This is not primarily to protect the equipment under test, but to protect YOU.

Use of same is by no means a cure all, but does reduce the risk of a fatal shock.
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Old 15th Nov 2019, 1:21 pm   #10
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Default Re: Variac for powering up an old Transceiver?

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I would also strongly urge the use of an RCD in the circuit, perhaps by means of a special 13 amp plug that incorporates an RCD.
This is not primarily to protect the equipment under test, but to protect YOU.

Use of same is by no means a cure all, but does reduce the risk of a fatal shock.
Thank you for your advice.

Yes, I do have a 13Amp RCD, which I used to use for the gardening tools such as power washer and garden shredder.

Will find it somewhere in the shed, and use it before the power on.
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Old 16th Nov 2019, 6:01 pm   #11
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Default Re: Variac for powering up an old Transceiver?

While I understand that you don't want to buy a Variac for just this one job, I think the biggest risk you face is the failure of large electrolytic capacitors if they get woken up with full voltage after years of being asleep.

I definitely wouldn't attempt it without a Variac. You need to find someone who has one who can help out - it's only going to take a couple of minutes to run it up to power.

I have an old Kenwood TS-530 (valve PA's and driver), which isn't used very often, and I always bring that up on the Variac when trying it out.

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Old 16th Nov 2019, 8:23 pm   #12
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Default Re: Variac for powering up an old Transceiver?

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Originally Posted by Bazz4CQJ View Post
While I understand that you don't want to buy a Variac for just this one job, I think the biggest risk you face is the failure of large electrolytic capacitors if they get woken up with full voltage after years of being asleep.

I definitely wouldn't attempt it without a Variac ...
I definitely wouldn't attempt it without a capacitor reformer. It's much cheaper than a Variac and, as a bonus, it will reform your capacitors .

Cheers,

GJ
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Old 16th Nov 2019, 10:12 pm   #13
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Default Re: Variac for powering up an old Transceiver?

I prefer the lamp limiter. A Variac will not produce any HT voltage if the rectifier is not heated sufficiently and a bit more voltage will suddenly produce a high current.
You could use a Variac and a lamp limiter or replace the rectifier with diodes to reform the capacitors.
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Old 16th Nov 2019, 11:57 pm   #14
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Default Re: Variac for powering up an old Transceiver?

You've hit the nail on the head when it comes to explaining why a Variac doesn't protect unreformed capacitors. But a lamp limiter will allow a near-normal current to flow into any circuit which doesn't have a fault in it. If there is a pre-existing over-current fault then a lamp limiter will indicate it, and will limit the current somewhat and, say, protect a mains transformer to a good extent. That's its job. It deals well with the situation of a pre-existing fault. But how does a lamp limiter stop the volts rising on a capacitor until the capacitor dielectric suddenly punches through, which it can if it's not been gently reformed ? The fact is it doesn't. Sometimes capacitors do survive a hard switch-on. But if they're not going to then a lamp limiter will barely help them.

We are faced with the problem of unreformed capacitors. I confess I'm struggling to understand why we don't address this problem with a capacitor reformer. It's the right tool for the job in the same way that a screwdriver is the right tool for driving screws. Or am I missing something ?

Cheers,

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Old 17th Nov 2019, 12:22 am   #15
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Default Re: Variac for powering up an old Transceiver?

The question about the HT depends on whether the rectifier is a valve or silicon; I'm guessing that with something of this vintage that it is silicon, so the Variac would be appropriate. Maybe ORAWA could let us know what rectifiers are in use?

However, I'd go along with the view that a capacitor reformer could be assembled for a good deal less money than a Variac costs, so that is a good way forward too.

The lamp limiter really serves to limit the severity of the damage which might occur. The Variac or the reformer try to stop any damage occurring in the first place.

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Old 17th Nov 2019, 11:39 am   #16
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Default Re: Variac for powering up an old Transceiver?

I think you may well be right about the silicon rectifiers. It seems National made a commercial power supply, the NCX-A, to go with the NCX-3 and the manual for it is on the BAMA site http://bama.edebris.com/manuals/national/ncx3/. That definitely had solid-state rectifiers in it. It looks like the transceiver was launched in 1963, so I guess the power supply was too. If someone had homebrewed a psu though then it might well still have used valves, at least for the 700V HT.

Cheers,

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Old 17th Nov 2019, 8:28 pm   #17
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Default Re: Variac for powering up an old Transceiver?

Here is a video explaining the lamp current limiter very well.

https://www.youtube.com/watch?v=1NoDmjjUtSg
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Old 18th Nov 2019, 10:50 am   #18
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Default Re: Variac for powering up an old Transceiver?

Hi the manual i have has 4x1n2359 in a bridge
Attached Files
File Type: pdf National NCX-A_manual.pdf (249.7 KB, 64 views)
File Type: pdf national_ncx-3_transceiver.pdf (2.70 MB, 68 views)
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Old 18th Nov 2019, 12:11 pm   #19
ORAWA01
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Default Re: Variac for powering up an old Transceiver?

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Originally Posted by mickm3for View Post
Hi the manual i have has 4x1n2359 in a bridge

Thank you for the manuals. That's great.

My NCX-3 is in very poor condition physically, so it will need good clean first. Dirts and rust and dust inside especially on the valve sockets and valve pins. Not sure if the valves are in working condition either.

I suppose all the paper caps. needing replacement too.
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