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Old 23rd Aug 2007, 12:55 pm   #1
Radio_Dave
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Default RAP 846 drawing too much current.

After checking and changing all the usual caps, etc I tried this radio for the first time today.

My series light limiter was useless (too bright) so I used a variac set to 200V and discovered the radio is drawing, nearly, 450mA and not sounding too good.

I've got no idea where I should start looking for the problem, but I must mention that the mains transformer has been replaced at some point... please don't say it's the transfomer!

David
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Old 23rd Aug 2007, 1:06 pm   #2
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Default Re: RAP 846 drawing too much current

Hi Dave,

How about disconnecting everything from the secondaries of the transformer, then seeing what happens?

I presume this set uses a mains isolating transformer, and not some odd autotransformer design.

Did you ever manage to get hold of a circuit diagram?

Nick.
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Old 23rd Aug 2007, 1:14 pm   #3
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Default Re: RAP 846 drawing too much current

Take the valves out - making sure you know where they go! - and see if the current drops. If it does not, then you're looking at the Transformer.

If it does, re-cap it and try it one valve at a time. There could be a duff valve or two.

Cheers,

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Old 23rd Aug 2007, 1:25 pm   #4
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Default Re: RAP 846 drawing too much current

With all the valves and dial lamps out the current drops to 63mA
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Old 23rd Aug 2007, 1:33 pm   #5
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Default Re: RAP 846 drawing too much current

The current must be going somewhere. I'd bet on leaky smoothing capacitors, but you've checked them. Try checking for voltage across all resistors. There should be none unless the set uses a potential divider to derive the screen voltages.
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Old 23rd Aug 2007, 2:07 pm   #6
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Default Re: RAP 846 drawing too much current

Unfortunately, with all the valves removed the transformers secondaries aren't connected to anything Is the transformer dead?
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Old 23rd Aug 2007, 2:10 pm   #7
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Default Re: RAP 846 drawing too much current

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With all the valves and dial lamps out the current drops to 63mA
Assuming the set has a valve rectifier, there shouldn't be any current drawn with all the valves out since there won't be anything connected to the transformer secondary. I would check very carefully around the transformer for any components like modulation hum caps and disconnect them. After that, check the resistance of all the transformer windings, and check for open circuit between all the windings and the chassis.

It does sound like a transformer fault or wiring short circuit.

Paul
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Old 23rd Aug 2007, 2:33 pm   #8
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Default Re: RAP 846 drawing too much current

Yes, I know what you mean. But not necessarily so. The primary will be across AC Mains, so it will form a circuit, so some current will flow. The issue is how much. Check for Hum Caps, etc though.

Put the valves in one at a time, and measure the current every time.

If it's had a transformer change in the past, is the new transformer right. I think the problem is the transformer too, but check everything else first! Then find a transformer that will take the load (or an origional transformer) or get yours rewound.

Cheers,

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Old 23rd Aug 2007, 2:39 pm   #9
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Default Re: RAP 846 drawing too much current

Where are you measuring the current? Is it AC Mains Current or DC HT current you're measuring?
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Old 23rd Aug 2007, 2:50 pm   #10
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Default Re: RAP 846 drawing too much current

63mA = 15W sounds like it might still be OK - I would not have expected it to have reduced from 450mA so much with shorted turns.

Faulty rectifier or electrolytics or maybe even a wiring error if the transformer has been replaced. Have you reformed the electrolytics and checked they are not leaking at the expected volts? Worth checking the wiring around the rectifier to make sure it makes sense - then power up using the variac without the output valve in place (assuming this is 6.3V heater set!) whilst monitoring the HT volts.
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Old 23rd Aug 2007, 2:57 pm   #11
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Default Re: RAP 846 drawing too much current

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Where are you measuring the current?
I'm measuring AC current from the variac.

I've disconnected all the secondaries and chassis connections but it's still drawing around 60mA.

I've checked for resistance between all the windings, but can't find anything

I'm, now, clinging to the slim hope that my AVO is broken. Would 60mA light up the 100W bulb in my series lamp limiter, because it doesn't

David
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Old 23rd Aug 2007, 3:03 pm   #12
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Default Re: RAP 846 drawing too much current

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I'm, now, clinging to the slim hope that my AVO is broken. Would 60mA light up the 100W bulb in my series lamp limiter, because it doesn't
No, 60mA wouldn't make the lamp glow visibly. It's more than I'd expect to be drawn by a completely off load mains transformer though, which is why I suspect a transformer problem. Of course, the radio may have multiple faults

Paul
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Old 23rd Aug 2007, 3:09 pm   #13
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Default Re: RAP 846 drawing too much current

There's still 390mA to be accounted for. What are the specified. heater consumptions of the valves
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Old 23rd Aug 2007, 3:20 pm   #14
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Default Re: RAP 846 drawing too much current

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What are the specified. heater consumptions of the valves
Sorry I don't know.

FWIW the valve line up is 6K7, 6K8, 6K7, 6Q7, 6J5, 6V6, 6V6, 5Z4. There's three heater windings, one for the 5Z4, one for the 6J5 and another for the rest and four dial lamps

David
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Old 23rd Aug 2007, 3:29 pm   #15
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Default Re: RAP 846 drawing too much current

450mA AC doesn't sound too unreasonable for a big set like that. A 100W light bulb will indeed drop too much voltage and glow brightly with that type of set, they're really only suitable for standard domestic superhets.

Are there any signs of distress when you bring it up (burning smells, smoke etc)?

Paul
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Old 23rd Aug 2007, 3:36 pm   #16
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Default Re: RAP 846 drawing too much current

6K7, 300mA
6K8, 300mA
6K7, 300mA
6Q7, 300mA
6J5, 300mA
6V6, 450mA
6V6, 450mA
5Z4, 2A

Excluding the rectifier that's 2.4 amps at 6.3V which would mean about 10mA being taken from the 240V Mains.

The rectifier would draw about 8mA from the mains.

So if my sums are right that's about 18mA. A long way short of 450-63=387mA
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Old 23rd Aug 2007, 3:38 pm   #17
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Default Re: RAP 846 drawing too much current

Well Paul, you've saved me looking up all the valves! As well as Paul's question, what exactly do you mean by 'Sounds Bad'.

Are those 6V6's Output Valves, and if so are they in Push-Pull ?

Cheers,

Steve P
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Old 23rd Aug 2007, 3:50 pm   #18
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Default Re: RAP 846 drawing too much current

There's no burning smells or smoke. Yes the 6V6 are push pull .

I suspect the radio sounds bad because I'm only running it on 200V, via a variac. The reason for not applying more voltage (and this thread) was because of the 450mA current drawn

Regards
David
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Old 23rd Aug 2007, 3:50 pm   #19
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Default Re: RAP 846 drawing too much current

Hi David, Since my RAP 646 was accessible , I measured the AC input current, With the set working normally at 240V I get 260mA, minus the rectifier, 120mA, minus all valves but with the 3 dial lamps, 90 mA. As the 846 has a push-pull output with extra valves and presumably a bigger mains transformer, your figures don't seem too unreasonable .

Reards, Mick. .
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Old 23rd Aug 2007, 3:50 pm   #20
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Default Re: RAP 846 drawing too much current

450mA at 200V equates to 90W, so presumably at full mains voltage, power consumption would be somewhat over 100W. Sounds fair enough to me, especially if the transformer runs fairly cool

Is there a ratings plate stating power consumption anywhere?

Nick.
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