AR88LF blows mains fuse

[Keith]

Just as I was listening to an amateur on 80m saying "this old valve gear goes on forever", it all went dead - no dial lights - nothing! Now completely vaporises 1.25A fuses.

DC resistance measured across mains i/p is ~20ohms which I assume is far too low. Doesn't appear to be the on/off switch (replaced with toggle type) or the voltage range switch (wired out of circuit).

Is it worth trying to take the can off the transformer to see if there is a retrievable fault?

One other possible thought occurs to me. The primary seems to be split into two 117V windings presently connected in series. What are the chances of still being able to run on one winding from a 120V autotransformer?

Any advice welcomed,

Keith

[Station X]

I've just measured the resistance between the L and N pins of the mains plug of my AR88D and it measures 8.4 ohms. The fuse is probably 3 amps, but I haven't checked. The set works perfectly.

Remember that it's the inductance of the primary winding rather than its DC resistance which impedes current flow in the primary.

I'd be inclined to check for shorts across the secondary windings. You could pull out all the valves and see if that makes a difference to the fuse blowing. Likely suspects are the 4 uF smoothing capacitors which may have gone short-circuit.

[Station X]

The circuit is available here if you don't have it:-

http://bama.edebris.com/manuals/rca/ar88lf/

Don't try using half the winding on 120 Volts. The windings are meant to be connected in parallel when a 120 Volt supply is used.

[Mike Phelan]

Even if you could use half of the primary, there might be shorted turns in the other half, so it would still blow the fuse.
You may not find anything with resistance checks; a few s/c turns out of a few hundred would kill it with no discernible change in resistance.

[ppppenguin]

There's a good thread on finding shorted turns. You must disconnect all the transformer windings for this method to work.

https://www.vintage-radio.net/forum/...+shorted+turns

[Keith]

Thanks for the advice, chaps. I do appreciate that the dc resistance is not what determines the primary current but it did seem low. Clearly the primaries are wound with relatively heavy gauge wire on this one. I did try removing the rectifier but no change. so its not smoothing cap. Yes, I did wonder about shorted turns. I'll try removing all the valves first and then disconnect the secondaries.

[Station X]

Removing the plug from socket J1 provides an easy method of disconnecting the 6.3V heater winding from the valves and the mains supply from the primary. You could then check the resistance across the mains lead to see whether the fault is in the mains switch etc.

Another idea is to use a test lamp in series with the mains to limit the mains current and/or use a variac to bring the mains up slowly whilst measuring the secondary voltages.

[Station X]

For those of you following this story here's a rather poor copy of the PSU circuit.

[dsergeant]

You probably know already, but you need anti-surge fuses (ie 1.25AT) for inputs of mains transformers. The in-rush current will blow a quick blow fuse pretty damned fast. If the primary has fairly thick wire (as indicated by your DC tests) I would think 1.25A is pretty marginal anyway.

But I would still think your problem is shorted turns in the transformer.

Dave

[Station X]

If the worst comes to the worst Colomor are selling a new AR88D Mains Transformer via their eBay shop:-

http://cgi.ebay.co.uk/ws/eBayISAPI.d...m=270031885048

If it's mechanically similar to that in the AR88LF it could be used as a replacement.

[Mike Phelan]

AFAIK the LF and D versions are the same in that department - I have had both of them in the past. Probably what did my back in

[Keith]

Just to let you know, the problem was in the transformer. Having disconnected all secondaries and powered it up slowly, it got very hot with a smell of melting pitch. Didn't leave it on for long but found that although the LT windings were only down to about 75% there were virtually no volts on the HT winding. I wondered if this meant a short on this winding so tried dismantling. Nothing obvious yet, unfortunately, but haven't dug out all the pitch yet . Looks like its new transformer time! Thanks for all your help.

[Station X]

That's bad news Keith. I've never heard of an AR88 mains transformer going faulty before. The output transformers seem to fail regularly though. I suspect that buying a new mains tranformer will be cheaper than having the old one rewound and certainly cheaper than having a new one made.

At the risk of telling you what you already know, remember that mains transformers don't generally go faulty of their own accord. There's generally some external cause. Check the smoothing capacitors for leakage at their rated voltage. A DMM is no good for leakage tests unless the capacitor in question is extremely leaky.

[RobinBirch]

Colomor certainly used to have replacement smoothing capacitors. I bought one some years ago when mine started leaking.

If it is the capacitor that has caused the transformer to melt then be very careful if you decide to "repair" it by replacing the insides.

The smoothing cap is a multi element oil filled beast. The oil is nasty and I would only consider this if you had no other option.

Robin