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Author Topic: SB 200 electric failure on power supply module  (Read 541 times)
IW2OIF
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« on: October 05, 2008, 01:29:43 AM »

Hi,
I got this amplifier from a radioham in Italy: it worked fine for one day but during operation the inside circuit breaker worked disconnecting the 220V. I’ve reset it but, in any case, no chance to switch it on: if I try to switch it on, the safety equipment of my home intervene (as if an electric shortcut would be present). I’ve disconnected the secondary (800 v) transformer from the power supply module where the big capacitors are installed and the unit was able to switch on (of course the tubes are not power supplied). If I reconnect the secondary to this module, no way to get the amplifier on (although I can see a big spark on the between the wire and the module).
Somebody as experienced this problem?
Do you think that the replacement of the power supply module with  a new one (like the PM-200 from Harback Electronics) could solve my problem? Or maybe the failure is located somewhere in the amplifier?

Thanks, IW2OIF, Pierluigi – Milan -  Italy
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W8JI
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« Reply #1 on: October 05, 2008, 02:25:20 AM »

You can make a simple test.

Find the two secondary HV wires on the high tension winding.

Disconnect both of them from the module and insulate them. Plug the amplifier in to power and turn it on.

Disconnect the amplifier from power. Now connect to the power supply ONLY one wire of the high tension. !!!Be sure the chassis is also grounded to your home electrical ground for safety!!! Now turn the amplifier on.

If it does not show a short, disconnect it and connect the other high tenison wire to the power supply as normal and the first one you tested is insulted. !!Be sure the chassis is grounded!!

Now test again to see if it shorts.

If the mains shows a short circuit any time only ONE of the high tension is connected you have a seconday to frame short on the power transformer.

You can also try removing the anode lead to the tubes, and testing the diodes, but many times the problem is the transformer with a secondary short to the frame of the transformer.

73 Tom



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IW2OIF
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« Reply #2 on: October 05, 2008, 08:04:56 AM »

Hi Tom,
Thanks for your hints.
I did the tests you suggested but in any of the two cases I had a short; I also measured the tension on the secondary and it was 800V, so I think that the secondary is ok.
The only way to have a short is to connect both the HV wires to the power supply board: in this case no way to get the amp working...
Any further suggestion is really welcome!!!

73 Piero
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W8JI
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« Reply #3 on: October 05, 2008, 02:25:29 PM »

If there was no short with only one of the two wires connected, the transformer is probably good.

Now you have to measure the diodes and capacitors.
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W8JI
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« Reply #4 on: October 05, 2008, 02:29:36 PM »

I re-read you message. I think I did not understand you at first.


If you had a short with ONLY one of the two wires connected, you absolutely have a bad transformer.

That means you have a secondary to ground or secondary to primary short.

NEVER measure the AC voltage unless you know exactly what you are doing and have many years experience working with HV and the proper meters!!!!

You can always tell anything you need without measuring HV .

Again, if you connect only ONE of the 800V wires and have a short,there is no question at all. You have a bad transformer.

73 Tom
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KB1GTX
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« Reply #5 on: October 05, 2008, 02:52:27 PM »

"The only way to have a short is to connect both the HV wires to the power supply board"

Not the transformer,,  check diodes and caps.

Do you have an autotransformer for lowering the input voltage?
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IW2OIF
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« Reply #6 on: October 05, 2008, 11:21:52 PM »

Again,
buy connecting one HV wire at time, I have no short: short just connecting both wires to the module.
Unfortunately I do not have any lower voltage transformer, but I think tat I'll change all the power supply module: it has many and many years of work....

73 Piero
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AD4U
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« Reply #7 on: October 06, 2008, 05:37:56 AM »

As I understand what you said (and I may not), I think your problem is on the diode / capacitor / bleeder resistor PC board.  The SB 200 is probably around 40 years old.  I would replace the filter capacitors and the diodes.

Dick  AD4U
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KA5N
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« Reply #8 on: October 06, 2008, 07:02:20 AM »

Sounds like 5 minutes with an Ohmeter (or DVM)would clear up this "puzzle".  
Replacing the diodes with half the number of 1N5408 rectifiers and the caps with 220 up to 470 ufd 450 volt modern caps with 105 degree ratings and either replacing the bleeders or at least making sure they are OK is a simple inexpensive task.  The other task usually needed is to replace the three 4.7 meg meter resistors with a string of high value (14 each 1 meg
1% resistors) is almost always needed.  There are a couple more electrolytics that may need replacement and the resistors in the bias/grid circuit as well.
Allen
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