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I think my friend got a Ebay special..

My friend was a wining bidder on a guitar amp, it was posted as not working, stating that it blows the fuse instantly. So he knew that it was broke from the start, didn't pay much for it. THe seller even removed the shipping cost ($25) to be nice..

How ever, he asked me to look at it for him to see if it can be repaired. I don't know much about the cost of this type of equipment these days since I really don't buy it how ever, he stated it goes for $400 bucks new..

Any ways, I opened it up, noticed that it had already been looked at, cause some one was nice enough to mark the secondary xformer leads color on the circuit board. So I guess it's been unplugged from the board before I got there. I finally ended up doing the same to remove any doubt of shorts that could effect the loading that was taking place. As it turns out, even with all the secondaries disconnected, the xformer still loads up heavy.. This unit only extracts no more than 250 watts max from the line (2.5) amp fuse when working properly!

After performing some test on the primary of the xformer, I come up with Lx = 1.5 (4 mh) and 3.5 dc ohms.. Z = ~3.8 ohms. which = ~ 31 amperes at 120 volts, 60hz.. I guess this would stand the reason why it blows out a 2.5 amp fuse :)

When all said and done, I think this defect was already known about since the primary had already been disconnected and most likely a load test was performed to verify a bad xformer..

Ok, just thought I would pass that along if any one was interested. :)

Reply to
Jamie
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Hello Jamie:

That's a good story. Do you believe you can locate/install a suitable replacement transformer?

--
1PW
Reply to
1PW

Jamie, You can do this approximation with speakers because of the small number of windings having very low resistance and inductance. However, you can't depend on the DC resistance of a power transformer to resolve the current draw. The inductance of the power transformer winding is the limiting factor here. In order to calculate the normal unloaded primary current in your transformer, you need to measure the inductance at 60 Hz with all secondaries open. Calculate the total impedance using the inductance and DC resistance. Then you can calculate the current that the transformer should be drawing. If your transformer blows the line fuse with all secondaries open, that's a pretty good indication that there is an internal short. It might be in one of the secondaries. A shorted secondary winding will cause the primary current to go very high. You have two choices here... try to find a replacement transformer from the manufacturer (someone here might be able to help if you post the brand and model of the amp), or attempt disassembly of the transformer to try to locate the short, hoping that it's near the outside of the winding stack.

--
David
masondg44 at comcast dot net
Reply to
Dave M

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Probably bifilar winding so the failure at point of maximun voltage stress which is right in the middle of the winding on the inner layup, most likely.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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Reply to
N_Cook

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