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Old 27th March 2013, 03:00 PM   #11
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Email me.
babin_perry@yahoo.com
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Old 28th March 2013, 05:06 AM   #12
SQLGuy is offline SQLGuy  United States
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I traced further into the circuit this evening. Here's what I'm seeing: rail voltage is fed through the Zeners (supposed to drop 12V) then through the 330Ohm load resistors, then NOT TO GROUND, but instead off the driver board and eventually to a 100uF cap in parallel with a couple of rectifiers, making a little power supply that's fed by a small winding off the main toroid whose output appears to be coupled by a 10uF cap.

I'm not really sure what they're trying to do here, though I get the impression that the power supply of this amp can run at different voltages depending on load. Is this system supposed to provide variable regulation for the driver stage as the power supply voltage changes? If so, this part of it is not working right. The rails for both sides are consistent at 45V. (Same as the working amp.)

Any suggestions from here?

Thanks,
Paul
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Old 28th March 2013, 09:26 AM   #13
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FETs are commonly driven with 10v (Vgs) to ensure that they're switched on completely. They will turn on at about 3.5v but won't completely switch on until the drive voltage is a bit higher.

To get a 10v signal, they need 12v of supply (you lose some drive voltage across the transistors). If they have significantly more than 12v, it will cause the drivers to run hotter (and to some degree will have more trouble getting the FETs to switch off efficiently).

If the voltage is too low for the drivers, the FETs will typically run hot because they're not being switched on completely.

The power supply is likely regulated for the rail voltage. The other windings will produce an output that will vary with the pulse width and the 12v input voltage.

They use the resistors and the 12v Zeners to maintain a constant 12v from a not-so-stable voltage from those windings.

If this amp has windings dedicated to this purpose, that's why the voltage-dropping resistors have a much lower value than the JBL amp. Instead of dropping from rail, they're dropping from a much lower voltage.
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Old 28th March 2013, 02:29 PM   #14
SQLGuy is offline SQLGuy  United States
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Hi Perry,

I think I didn't explain this clearly. The supply for the drivers is dropping FROM the rails, but it's not dropping to ground. The bottom side of the regulator circuit is to this cap-coupled dedicated supply.

On the working amp, the dedicated supply is providing about 22V or -22V on the bottom end of the 330Ohm resistors. On the non-working one it's providing 22V and -30V.

So, on the six working paths (counting both amps), voltage values are:
Rail: +/- 45V
Zener: Drops 12V to +/-33V
330hm: Drops 11V to +/- 22V
Dedicated supply: +/-22V

On the two non-working paths:
Rail: -45V
Zener: Drops 9V to -36V (since there's not enough current across them to regulate)
330Ohm: Drops 6V to -30V
Dedicated supply: -30V

Or maybe it is a bad Zener? They are effectively in parallel.... on the other hand, with the 330Ohm in there too, I would have expected to see some difference in the voltage drop of a bad one versus a good one; they are in fact dropping exactly the same voltages.

The biggest pain with this amp is not being able to run it without clamping everything down. Pretty tedious... Next step will probably be to reassemble it again, then scope out the A/C from the dedicated supplies... maybe that coupling cap or the filter cap are dry...
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Old 28th March 2013, 03:01 PM   #15
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The Zener voltage is what's critical.
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Old 28th March 2013, 03:04 PM   #16
SQLGuy is offline SQLGuy  United States
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Clearly, but Zener's without enough current through them will not regulate.
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Old 28th March 2013, 04:06 PM   #17
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I missed the part where you mentioned that you were referring to two amplifiers, not one.
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Old 29th March 2013, 05:54 AM   #18
SQLGuy is offline SQLGuy  United States
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A bit more digging this evening, but no solution:

1. I replaced the 10uF 50V caps that couple the transformer pulse to the rectifiers of the "ground" circuit for the 330Ohm resistors. No change.

2. I replaced the Zeners on the incorrect-voltage sides of the two driver boards. No change.

I also made some measurements. On the good side, the dedicated winding from the toroid is generating 16VAC. On the bad side it's 8VAC. All other outputs from both toroids match.

The windings on the bad side are loose as they wrap around the toroid core. On the good side they're lacquered down. Moving the loose windings around has no effect on the voltage they're producing. Seems odd, though, that the voltage is exactly half on the bad side. At this point I'm leaning towards the idea that it's actually a bad transformer, much as I think that's unlikely.

Don't suppose these transformers are available anywhere?

I may swap one from the good amp to verify whether that is in fact the issue.
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Old 29th March 2013, 11:17 AM   #19
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Do the large primary and secondary windings of the two transformers have the same amplitude square wave on them?

If you insert a low value resistor in series with the two small secondary windings (one from each transformer), you can measure the voltage across the resistors to see if the current passing through each side is the same.
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Old 29th March 2013, 07:43 PM   #20
SQLGuy is offline SQLGuy  United States
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I scoped the two transformers and the square waves on the supply windings and those on the higher-voltage secondaries of both were identical. It was only the small winding that provides regulation for the driver circuits that was different. (20V p-p on the bad side, and closer to 40V p-p on the good side).

Still looking like a bad transformer....
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