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Negative voltage on the positive rail, what's the damage??
Negative voltage on the positive rail, what's the damage??
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Old 28th November 2020, 01:43 PM   #1
A 8 is offline A 8  Sweden
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Default Negative voltage on the positive rail, what's the damage??

Not sure if or how it happened but assuming I accidentally got negative voltage on the positive rail in a trad AB amp what would the damage be?
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Old 28th November 2020, 03:19 PM   #2
adason is offline adason  United States
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electrolytics?
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Old 28th November 2020, 07:07 PM   #3
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by A 8 View Post
Not sure if or how it happened but assuming I accidentally got negative voltage on the positive rail in a trad AB amp what would the damage be?
Insufficient data.

Is it a single or dual supply amp?

IF split supply, you inverted both rails or just connected negative supply to amp positive leaving amp negative disconnected?

Or any other combnation?

In any case, you have the amp in your hands, YOU tell us the damage you see.
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Old 28th November 2020, 07:38 PM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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If you connected neg rail to pos rail then probably would just blow fuses.
If rails got swapped then its like a few semiconductors would get taken out.
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Old 28th November 2020, 08:18 PM   #5
rayma is online now rayma  United States
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Post the schematic. Are you sure that the meter leads weren't just reversed?
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Old 28th November 2020, 08:58 PM   #6
A 8 is offline A 8  Sweden
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I doubled up on the transformers after the rectifiers to the same capacitor bank driving both channels. After paralleling the transformers on the dc side of the diod bridges and powering up the amp through 10 ohm resistors on each rail the amp seemingly shorted the positive rail on both channels burning the 10 ohm resistors.
Reversing the change still seemingly shorts the positive rails and on one channel puts a lot of DC on the output so something went wrong/broke.

The only thing "off" with my paralleling attempt was the phase being reversed before the rectifier on one of the four diod bridges but I did not think that would be a problem as I was using the dc side on the diod bridge. Obviously I missed something and when measuring the residual voltages on the rails I had a slight negative on the positive rails hence the assumption or the amp had gotten the wrong polarity on the plus rail even though I don't understand how.
The question/ask for help was to try and understand if a temporal negative voltage on the positive rail could explain the now shorting behavior of the positive rail.

Last edited by A 8; 28th November 2020 at 09:10 PM.
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Old 28th November 2020, 09:36 PM   #7
MarcelvdG is offline MarcelvdG  Netherlands
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Most integrated circuits get fried when you put more than one silicon diode forward voltage in the wrong direction between the positive and negative supply rails. In a discrete circuit, anything can happen depending on the exact circuit.
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Old 29th November 2020, 12:54 AM   #8
MarsBravo is offline MarsBravo  Netherlands
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Quote:
Originally Posted by A 8 View Post
...I missed something...
Yeah, water is wet, no matter if you believe it or not (at room temperature).
The thought is ok, double the power supply and more power is available for the amp, but that's not working.
I've the impression you've killed the supplies (maybe the transformers but surely the greatz-bridges and...) so better have the supply in good condition before connecting it to other circuits. As said above, all semi's & large caps are suspicious now. This is valuable learning money (be proud!). Welcome to the Incineration Club.
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Old 29th November 2020, 03:32 PM   #9
A 8 is offline A 8  Sweden
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Yes its probably fried even though the caps seem ok and at least 2 of the diod bridges.

I swapped the output transistors Sanken SC2922/SA1216 and things changed, now seemingly shorting both rails.

I still don't get what went wrong, the transformers and bridges were identical in terms of make and model and I've seen many designs that double transformer windings this way ie on the dc side of the diod bridges. In fact I have another one that runs this way albeit this one has two different voltages, on +-30 and the other -+40 but I don't see how that makes a difference as in my case one will be slightly lower than the other.
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Old 29th November 2020, 11:25 PM   #10
MarsBravo is offline MarsBravo  Netherlands
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That's 25% difference... fatal. Just 1% diff starts heating things up already.
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