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SX-980 Power Transformer Shorted

samc777

samc777

Active Member
I picked up a SX-980 for an almost give away price knowing it would not "Light Up".

Unfortunately troubleshooting was easy. The 6A power transformer fuse (FU1)was missing from the board. I knew then I was in trouble. A couple of minutes of testing and blowing a fuse then cutting the 120VAC wire to the transformer confirmed my suspicion that the power transformer is shorted on the AC side.

So, here I am looking on the net for a replacement transformer which I cannot find.

I know nothing about transformer replacements or if they can be rewound? Can anybody shed some light on this issue? Are replacements available or do I look for a donor? Can I rewind it myself?

Ref from the SX-980 Service Manual:
Power Supply ATT-449

Markings of the transformer
Bando
BE91V-0001
ATT-449
B-BE11-ZEH

Thanks
Dave
 
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i don't understand how cutting the primary side confirms a shorted transformer .surely you need to cut the secondary side ?
 
petehall347 said:
i don't understand how cutting the primary side confirms a shorted transformer .surely you need to cut the secondary side ?
Click to expand...

Hmmm...

Maybe I do? Maybe I'm all wet in my thinking here?

Is it possible to have a problem in the secondary that draws a too high current and blows the fuse for the AC primary? I'm just going by the schematic that shows the FU1 fuse protecting the 120VAC wiring side only.

I would think that a secondary short will cause one of the secondary fuses to blow? However this is why I love AK. I'm here to learn and if you are correct I'll keep on digging.

I must admit that I'm a little foggy on AC power transformers and if you a sure that this can happen I'll continue to troubleshoot. Back to the task at hand. I'll post what I find.

Dave
 
cutting the primary will cut power to transformer ..
a short on the secondary will blow the main fuse .
i will look at the diagram now so i can give you some pointers
 
Disconnect the secondary side for sure. Measure resistance to ground across the filter capacitors. Bet you find a dead short. Suspect the bridge rectifer or output transistors.
 
petehall347 said:
i don't understand how cutting the primary side confirms a shorted transformer .surely you need to cut the secondary side ?
Click to expand...

Well now you just lost me:scratch2:, how does first cutting either side really solve anything?

Would it not be best to measure for a short? :banana:

Then, :yes: disconect the secondary and see if the fuse still blows. If not then measure the voltages on the secondary... ?
 
Couple of things can cause this- 1- shorted outputs, 2- shorted rectifier. I have a hard time believing the transformer is shot, transformers rarely go bad. Ohm out the primary, I'm betting it's not shorted. Ohming means measuring the 2 wires going into the transformer with a DMM set to ohms. Starting point- check the rectifier using a DMM , if it passes pull the outputs and test them as well.

To avoid blowing fuses and if you plan on getting into the hobby build a dim bulb tester.
 
It is an approach, perhaps not the best one. If the circuity after the secondary is shorted the fuse will not blow when the secondary is disconnected. The very simple way is to simply measure resistance across each filter capacitor. If you see a dead short start troubleshooting on the secondary side.,Again suspect output transistors and the bridge rectifier.
 
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its possibly shorted diode/s unless other fuses have been replaced with the wrong ones .. d11 d12 d13 d14 .
if diodes are good then its more likely one or more output transistors are shorted
 
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Sam,

I'd wager a couple of beverages that the transformer is intact, and that load is taking the protection out. You MAY be able to diagnose that using an impedance bridge or high current ohmmeter, but those are pretty rare in most shops. A "normal" resistance check with a DMM will likely show only a few tenths of an ohm - Completely normal.

These guys are probably steering you right.
 
petehall347 said:
its possibly shorted diode/s unless other fuses have been replaced with the wrong ones .. d11 d12 d13 d14 .
if diodes are good then its more likely one or more output transistors are shorted
Click to expand...

I would like to thank all of you that weighed in here. A great afternoon, I have learnt a ton with your help.

So, this is where I am at now. With reference to the SX-980 full schematic at the top of position 7B (available in the AK document website) I have cut wires "red" and "yellow" going to diodes D12 D13 D14 D15. The main fuse FU1 6A does not blow with these two wire out of the circuit. Of course nothing works either but progress is being made.

I chose those two wires first due to those locations NOT having a dedicated fuse. I note that the top diode pack does have dedicated fuses. I wonder why it was designed that way?

I also eliminated the other fuse paths one at a time. They checked out Ok. So I am assuming that Pete Hall has me on the right track and I need to follow this path and see what I find.

All comments welcome.

Dave
 
Watthour said:
Sam,

I'd wager a couple of beverages that the transformer is intact, and that load is taking the protection out. You MAY be able to diagnose that using an impedance bridge or high current ohmmeter, but those are pretty rare in most shops. A "normal" resistance check with a DMM will likely show only a few tenths of an ohm - Completely normal.

These guys are probably steering you right.
Click to expand...

You win! I'll be happy to buy one of these years. :thmbsp:
 
no real need to cut wires . but still . i should have said earlier ..
check each of them diodes for short first . leave them in circuit for now . if any look shorted pull them and test again . if they are ok check the output transistors for emitter to collector shorts .again if any appear shorted pull em and test again .
 
petehall347 said:
no real need to cut wires . but still . i should have said earlier ..
check each of them diodes for short first . leave them in circuit for now . if any look shorted pull them and test again . if they are ok check the output transistors for emitter to collector shorts .again if any appear shorted pull em and test again .
Click to expand...

That was it exactly! The D14 diode was shorted. I was able to search AK and found a thread that MTF had advised a replacement for the original SR3AM-8 diode. My local Radio Shack had 4 so all 4 were replaced.

Now we have music. :music:

The controls are a little scratchy but that is enough for today. I have DeOxIt on hand and will continue with cleaning the switches tomorrow.

Once again thanks for everyone's help.

Dave
 
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