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Old 15th September 2014, 02:19 PM   #1
Kei is offline Kei  Wales
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Default Inter-M R300 plus Transformer

I have said amp which has been used in a broadcast environment for the last 8 years without an issue. I have recently given it a basic service to replace the capxon 85 degree capacitors with something more robust. (was quite surprised that they had lasted so long considering their reputation)

Now that I've been using it for a while, I've noticed that the transformer is the only part that really gets hot. (very hot compared to the rest) It also vibrates a fair bit too, though not too bad that i can hear it above music. The amp is not being driven hard either. It runs a single pair of Spendor SP2/3e's or a single pair of Tannoy SRM-10B's.

I've had a good look but can't work out what rating it is. The service manual merely states T3.15AL/250V which I reckon just refers to the fusing. The actual transformer measures 90x115x75mm in size. I was looking at possibly replacing this with a 300VA unit from airlink or ILP. That is if the current one is weedier than it really ought to be.

This is what the amp looks like for reference.
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Old 15th September 2014, 02:44 PM   #2
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The transformer getting hot is probably caused by too high a capacitance as the main smoothing. It will be using so much energy keeping the oversized capacitors topped up. A common issue.
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Old 15th September 2014, 03:04 PM   #3
Kei is offline Kei  Wales
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I wondered that but the ones we have in work that are untouched also get rather toasty too. (though i will admit that mine probably gets warmer) Out of interest, what is considered to be the guideline capacitance per channel on a 2x 100W/8R amp and what size transformer?
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Old 15th September 2014, 03:39 PM   #4
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Did you replace one smoothing cap but not the other? That's a bad idea. Did you know that an electrolytic capacitor can lose 50% of its capacitance over its service life? They should always be replaced in pairs, and the same type should be used.

Making smoothing caps bigger can stress the transformer and rectifier. If you double the original cap size you should be OK.

Measure the transformer winding's resistance. Each leg of the secondary split winding should be reasonable close. Are both legs of the supply the same absolute value of voltage?

A bad smoothing cap can definitely damage a transformer.

You can often use a salvage transformer from a completely different unit as a replacement. Sometimes this can even provide an upgrade. But beware of putting bigger transformers in; you may have to do some other re-engineering, especially concerning the heat sinks.
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Old 15th September 2014, 04:21 PM   #5
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by JonSnell Electronic View Post
The transformer getting hot is probably caused by too high a capacitance as the main smoothing. It will be using so much energy keeping the oversized capacitors topped up. A common issue.
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Where did you get that weird idea from?

Oversized caps (which by the way being 10000uF per rail are not oversized at all) would at most have an initial longer charging surge/peak ... say 1 second instead of 500 milliseconds ... and then charge to peak voltage, period.

After that, any "topping" does not depend on the caps at all but on current pulled by the load.

In fact, larger caps will reduce ripple (which is the effect of "topping" 100/120 times a second) and supply peak demands (bass drum kicks, etc.) themselves, instead of instantly loading the power transformer down.

If that transformer really overheats (of which I'm not yet so sure), I'd attribute it more to, maybe, being designed for 60Hz and being used at 50 Hz than anything else.

The amp looks being very well made, Inter M Korea is a huge OEM supplier (among others, makes respected Hartke amps) , and that transformer looks huge.
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Old 15th September 2014, 04:37 PM   #6
Kei is offline Kei  Wales
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Fear not, that photo was taken part way through the work, hence why there are two different smoothing caps. (I didn't take a final shot) I've not done any measurements on the amp as yet as it is in use, but apart from the transformer running slightly hot, it seems to work perfectly. It's probably well within design parameters as top whack it's running at roughly 50 degrees. It just struck me as odd as none of my other power amps get as hot on the transformer. (harrison X150 and quad 240)

As to upgrading the transformer, I know that a 300VA is virtually a drop in fit only requiring a hole for the fixing. 500VA would be too big to fit without a lot of work to move the psu pcb further back. I wondered about it being designed for 60Hz rather than 50, but have no easy way of finding out.
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Old Yesterday, 03:05 PM   #7
Kei is offline Kei  Wales
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So the general consensus is that the transformer should be fine.

The only other query I had regarding the amp is that a small amount of mains hum making it to the speakers is there anything I can do to reduce/eliminate it? This noise occurs with or without an input. (has no effect)

I've attached the schematic if it helps.
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File Type: jpg InterM-R300-Schm.jpg (460.7 KB, 17 views)
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