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In Reply to: RE: These look decent posted by Caucasian Blackplate on July 24, 2007 at 21:39:02
CB is on to something here. Those caps in the bias supply are there to reduce noise from the diode switching. You actually can run the speakers without the caps. IIRC, the stator voltage is about 5,000 volts total, but I've never measured the voltage. The key is to have a little capacitance across the diode string. If you know the voltage you can use a larger cap with sufficient voltage across the entire string , too.
Stu
Follow Ups:
Thanks for the info guys.
I contacted the russian supplier re shipping; those do look good.
I have limited technical knowledge, but willing to do some soldering (very carefully). I am experiencing very reduced output in one speaker. I swapped the power supplies, and confirmed that the problem is not the panel. The most common probable cause is the caps, according to my research. So, I thought I would start by replacing those. None of the components in the ps show any obvious signs of distress. I welcome any comments an/or suggestions.
Thanks again.
Lino
If you suspect the caps, you can performa simple ohm meter test on them to check for leakage and thus isolate the defective cap. I worked on a F-81 with a defective cap and simply removed it and it seemed to work fine. I later replaced it with a ceramic , IIRC.
Stu
Thanks Stu, I appreciate the help. So, I used my multimeter (ohm) to check the caps in each speaker. I get no reading at all from the caps except for one in the speaker with the reduced volume. Is this what I should expect? Is that the bad cap? By the way, I noticed that in the good speaker, one of the caps had already been replaced with a cap (physically different)with a lower V rating; 1600 vs 2500. Should I replace that one as well?
Thanks again for walking me through this.
Lino
BTW, when you worked on the F-81 that you mentioned, was the problem reduced output as well?
The issue was a lowered volume.
You may want to check the ratings of the diodes. Most 'stats use a regular 1n4007, a 1000 piv unit. The lower voltage rating should be OK if so, as you will still have a 60% overvoltage protection. How many diodes are there in the string?
The Stax units were quite expensive and are relatively scarce, but since I have seen the problem and apparently you have a pair with similar issues, the caps look to be a recurring problem.
And yes, you are supposed to get no reading as the cap is supposed to block DC voltage. If you do get a reading, then that cap is leaking, and will probably leak a lot more at the higher voltages.
Good luck:
Stu
PS: I did like the sound of those units, and they are well worth repairing.
Thanks again, Stu. I appreciate you taking the time. How do I test the diodes using a simple multimeter? What do I test for?
BTW, any harm in using a .018uf 2500VDC instead of a .01uf 2500VDC to replace the bad cap?
Thanks a million.
Lino
I believe you were already doing the right thing: take a resistance reading positive probe to the 'top' of the diode/cap and the negative to the 'lower' lead (voltage wise. The cap should read open.
The slightly larger cap should be no issue. As used as a snubber, it is supposed to shunt off any diode noise. For most 'stats, the stator panel is normally at a constant voltage and the transformer, hooked up to the speaker leads, provides the changing voltage to move the panel (stator panel). I've wondered what much larger caps might do to the sound, but never had a chance to experiment as I had only one side to repair.
Good luck,
Stu
Hi Stu
Again, thank you for all your help. I replaced the defective cap, and I can proudly state that my Stax's are up and running again. They have been missed.
About two years ago, the speaker in question stopped working properly, as described earlier. I purchased a pair of Paragon Regents (repair of which, you also helped me with), to "hold me over" until I got the Stax's working again. I liked the Paragons enough that the Stax's stayed in a closet all this time. When the tweeter in the Paragons needed replacement, I got on a roll, and decided to fix the Stax's as well.
I would like to restore the power supplies in the Stax's, now that I am more familiar with their inner workings. My technical knowledge is limited, as I said before, but I am willing to learn, and I think I am pretty good with a soldering iron, and can use a fair amount of logic. I would like to ask you to indulge me, and let me ask a couple of additional questions, and please forgive my lack of expertise:
-there are eight diodes in each bias supply. How exactly do I test them? If I put a probe from my multimeter on each of the diode's leads, what should I be checking for? What reading should I be getting?
-how do I determine the values of the diodes? The F-81 schematic that I have (it is available in the FAQ page of Audio Asylum), doesn't state the value for the diodes.
-I often read about the benefits of replacing diodes in audio gear with "high speed" or "fast recovery" diodes. I would like to do this if it is beneficial. What brand would be appropriate?
Any other tips would be most appreciated.
Best regards.
Lino
If they're diode snubbers, go ahead and use ceramic disc, that's totally ok... but I still like the Russian Military stuff!
-Paul
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