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Author Topic: How to test AL 811 Power Supply Transformer?  (Read 987 times)
VE3XKD
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Posts: 51




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« on: September 07, 2008, 05:55:09 AM »

Hi

I received a used, and somewhat abused AL-811 for repair. The owner said it 'kept blowing fuses'. I bought it from him as an 'interesting project' to work on. I wanted to test the power transformer in the power supply on the bench before powering it up. Does anyone have any ideas how I could go about safely testing the Power transformer? I assume that unsoldering and checking the primary and secondary windings for resistance and/or open before I apply power is a good idea? How many ohms resistance should a typical Linear amp transformer exhibit?

Does anyone have any thoughts/ideas of other things I might try?

Regards

John
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VK2ACM
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Posts: 102


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« Reply #1 on: September 07, 2008, 08:29:42 AM »

John,
If the main fuses are blowing as soon as you turn on the amp you might want to check your tubes as it sounds like you have a bad tube or tubes.
I had a similar problem and with alot of research and assistance from some experienced hams found a tube was creating the problem, you can also check the 50ohm resistors towards the left hand side of the amp when open.
John please make sure power is uplugged from the amp when working inside the unit.

I hope this has helped you out...73 Clint vk2acm/w5
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KA5N
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Posts: 4380




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« Reply #2 on: September 07, 2008, 10:24:56 AM »

The most likely reason for blowing fuses when power is applied is a short circuit.  As mentioned tubes can be shorted, but it is more likely one or more shorted silicon rectifiers, one or more shorted electrolytic capacitors, or other shorted components.
The transformer might have shorted windings or a winding might be shorted to chassis (ground).
With power off amp unplugged and electolytics discharged check for shorts of the items listed above with an ohmeter.  If item(s)are found to be shorted replace them.  If no shorts are found, then post on eHam for details on transformer troubleshooting.  The likelyhood of a bad transformer are slight.
Good Luck
Allen
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K4DPK
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« Reply #3 on: September 07, 2008, 01:02:47 PM »

John,

If you already have the secondary unhooked from the rectifier board and filter caps, you can easily tell if the transformer has a short simply by plugging the primary in to the AC.  Be careful and make sure the loose ends of the secondary aren't touching anything before you do this.  If it blows a fuse, the xfmr most likely has a problem, but as previously mentioned it is most likely a tube, diode string or filter bank.  You can isolate which by the process of elimination.

Regarding the possibility of an open circuit in the secondary....it wouldn't be blowing fuses, would it?

In any event, be careful.  Dead usually lasts an awful long time.

Phil C. Sr.
k4dpk

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W8JI
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« Reply #4 on: September 07, 2008, 05:02:25 PM »

John,

The single most common cause of blown fuses in the 811 amp, by a very large margin, is bad tubes.

You can pull the plate caps off the tubes to see if one is shorted.

If you want to test the transformer, you cannot really do it just by pulling the HV windings. You would have to pull the HV winding wires ONE at a time, and leave the other one connected. The reason you have to do this is the most common defect in a HV transformer is a secondary to core or primary short.

If you pull both secondary windings, it will not check for the most common failure mode.

If you do ANY of this, be sure you use a grounded outlet and ground the chassis. Be SURE the cover is on and has some screws holding it, and that whatever you pull does not hit something else.

Odd's are virtually certain you have a bad tube, or much less likely a bad transformer. Diode failures are pretty rare but I suppose that does happen. They are pretty tough diodes.

73 Tom

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