PS section WORKS fine (draws 1.5A, produce rail voltages but not the right ones due to the missing BANK 8/ T8), IF there are no mosfets in BANK 8 or if the transformer 8 (T8) is out of the board.
Fitting both Transformer 8 (T8) with T8s mosfets in Bank 8 causes excessive current and mosfet overheat in bank 8, which left unattended and without current limit leads to smoke and fun.
And don't get me wrong, i've switched the mosfets many times, even took out working ones from the other banks, issue persists, and it's not MOSFET related. It's like bank 8 tries to drive a short.
The gate drive is OK i've even posted pictures of it. Power supply caps, gate resistors and pull-down ones, drivers and etc have been ALL replaced.
I'm attaching a part of a similar schematics (not exactly the same) of the power section.
Fitting both Transformer 8 (T8) with T8s mosfets in Bank 8 causes excessive current and mosfet overheat in bank 8, which left unattended and without current limit leads to smoke and fun.
And don't get me wrong, i've switched the mosfets many times, even took out working ones from the other banks, issue persists, and it's not MOSFET related. It's like bank 8 tries to drive a short.
The gate drive is OK i've even posted pictures of it. Power supply caps, gate resistors and pull-down ones, drivers and etc have been ALL replaced.
I'm attaching a part of a similar schematics (not exactly the same) of the power section.
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OK, I understand. Is it possible to test the xformer without the MOSFETs attached? So you have no short but can verify which windings are OK?
Jan
Jan
A turn to turn short in a transformer is almost impossible to detect.
Winding to winding shorts are easy to detect.
If the transformer has been working OK it is probably OK unless it has been toasted.
Winding to winding shorts are easy to detect.
If the transformer has been working OK it is probably OK unless it has been toasted.
Interestingly enough when i've received the amp the damage was only in banks 7 and 8, respectively T7 and T8 (check schematics i've posted earlier for referance)
Other 1~6 banks weren't even slightly affected. I suppose when the short happened, the auxilary voltages were not present anymore and this triggered the protection circuit. So that's how the amp arrived at my door.
So only banks 7 and 8 were bad - mosfets, gate resistors and drivers. Bank 7 is working flawlesly after the rebuild but Bank 8 is still problematic.
"Is it possible to test the xformer without the MOSFETs attached?" - this is actually the heading of my thread...if i knew how to do it, i would not be wasting anyone's time...
Other 1~6 banks weren't even slightly affected. I suppose when the short happened, the auxilary voltages were not present anymore and this triggered the protection circuit. So that's how the amp arrived at my door.
So only banks 7 and 8 were bad - mosfets, gate resistors and drivers. Bank 7 is working flawlesly after the rebuild but Bank 8 is still problematic.
"Is it possible to test the xformer without the MOSFETs attached?" - this is actually the heading of my thread...if i knew how to do it, i would not be wasting anyone's time...
Sorry for the long delay.
I think maybe i found something. Took the MOSFET out of bank 8 where Transformer 8 sits.
The first picture is what T7 secondary winding "sends" to secondary of the T8. It's a small wave with amplitude of 3.44vrms.
The second picture is what I see at the primary winding of T8(drain pads on the Mosfets at bank 8). It's not even a regular wave.
As the rest of the transformers T1~T6 are working perfectly fine I looked at their readings and waves.... and here it is - they are in series but what T1 sends to T2, T3 --> T4, T5-->T6 is the same wave as at picture one but amplutite is like 11 times MORE. Its 3.85vrms but with the probe at 10X, not 1X. Can be seen at picture 3.
Why does T7 secondary winding produces only 3.45v instead of like 38.5v ?
(this is the winding connected to T8, the other secondary one is directly connected with a cable to the rectifier and produces the RIGHT voltage) The wave look the same, but amplitude is not.
I think maybe i found something. Took the MOSFET out of bank 8 where Transformer 8 sits.
The first picture is what T7 secondary winding "sends" to secondary of the T8. It's a small wave with amplitude of 3.44vrms.
The second picture is what I see at the primary winding of T8(drain pads on the Mosfets at bank 8). It's not even a regular wave.
As the rest of the transformers T1~T6 are working perfectly fine I looked at their readings and waves.... and here it is - they are in series but what T1 sends to T2, T3 --> T4, T5-->T6 is the same wave as at picture one but amplutite is like 11 times MORE. Its 3.85vrms but with the probe at 10X, not 1X. Can be seen at picture 3.
Why does T7 secondary winding produces only 3.45v instead of like 38.5v ?
(this is the winding connected to T8, the other secondary one is directly connected with a cable to the rectifier and produces the RIGHT voltage) The wave look the same, but amplitude is not.
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So its likely either a damaged transformer or something is loading transformer badly like a shorted cap or rectifier gone.
Caps and diodes are all in parallel so how would they affect only one of the transformer but not all of them ? Now I think T7 is the one which has gone bad, not the T8...
With or without MOSFETS in T7 bank ?
That's picture number one in my previous post. This is what T7 feeds to T8.
If you pull the secondary windings out of the board for T7 and T8, can you install the FETs for T8 without excessive current draw?