Class AB amp in protect when bridged

[Lenin89]

Hi guys

Amp going in protect when bridged. Both channels works flawlessly if tested in regular stereo mode, puts out full power per channel.

Does not protect in bridge mode if there is no load connected. I can see nice big sinewave on the scope.

When using 4 or 8 ohm load in bridge mode goes straight into protect.

Pre-amp section seems fine with stable +-15v.

Any ideas ?

Schematic is attached.

 

[Perry Babin]

Which pin (1 or 7) of IC22 is going high when it goes into protect?

 

[Lenin89]

Pin 1 goes high. I do have like 5 seconds window where the amps works in bridge mode before it protects, but that's it.

 

[Perry Babin]

Can you alternately remove Q125 and Q225 (one at a time) to see if one channel is the problem or if either channel will trigger over-current in bridged mode?

Are all of the emitter resistors within tolerance?

 

[Lenin89]

Something else i've just noticed just before pulling out Q125 and Q225.

In the schematics there are D110 +D210 (1N4148), but in the amp i'm missing those, there are jumpers installed at those places and if i try to measure the voltage at this point with MM, amp goes in protect no matter which ground i take for reference.

 

[Perry Babin]

I've never seen this type of protection without those diodes (or diodes bypassed).

 

[Maxikill97]

Sometimes I see amps for sale which has been modified for extra sound quality, as they describe.
Sometimes they say that because of the modification the amplifier can't play bridged anymore. Maybe this has something to do with that? Because of the missing diodes which are bypassed by a wire.

 

[Lenin89]

I've fitted the diodes back as in schematics - nothing changed.
Removing Q125 makes the amp do full power in bridge mode.
Tested Q125 out of the board on a LCR meter GM328 - showed just fine.

But I'm going to replace it with a new one just in case it's leaking.

 

[Lenin89]

Same with a new transistor Q125.
There is 1v difference between the primary and secondary ground.
I've just also noticed that the output transistors are heating up unevenly.
The resistors at the bases of the outputs are being 4.7 ohms instead of the 68ohms as in the schematics. Actually a lot of resistors are different values than the amp in front of me. I think there are different revisions for the same amp...

 

[Perry Babin]

Are the values in this diagram closer?

Did you very carefully check the value of all of the emitter resistors?

 

[Lenin89]

Are the values in this diagram closer - YES
Did you very carefully check the value of all of the emitter resistors - my DMM reads to only about 0.1ohms. This is same value as my leads resistance ~ 0.1ohms.

So if there is anything below 0.1ohm how can i check it precisely ?

 

[Perry Babin]

If you read 0.1 ohms it's likely that they're OK.

To get a higher resolution, you can do what your meter is doing by driving more current through the resistor. The series resistor isn't critical. Your 4 ohm dummy load will work. The applied voltage isn't critical. You'll calculate the resistor voltage with the formula R=V/I.