SS Carvin problem

[drewl]

Got Carvin R600 SS bass amp that had ch2's output transistors blown.
I replaced them and the all the pre drivers, checked all the other components and all seems fine.
Ch2 now works, but the voltage across the 2 4.7ohm resistors on the pre drivers keeps rising...it's running about twice as high as Ch1.
I turn it off before it blows up....
what the heck am I missing here?
Any suggestions?
Thanks



http://www.carvinmuseum.com/pdf/amps...00%20REV-H.pdf

[Enzo]

That means current is increasing through them. Is the channel warming up too? Watch draw from the mains, does it climb with the voltage on the resistors?

If teh TIP31 bias transistor is leaky then it will drag the bases of the output xstrs together. That actually makes the amp run cooler, but with increased crossover distortion. But it also would increase the drop across those resistors.

Check the resistors and the bias pot all around that bias xstr.

I assume you verified the .22 ohm ballast resistors on the outputs.

Watch the drop- across D205,206. IS it stable or does it drift?

[drewl]

Ah Enzo to the rescue!
The TIP 31 was replaced as was all the pre drivers (damn thing was backoredered for weeks).
Bias pot had opened and it was replaced.
Heeding your advice from the past, I also replaced the diodes.
The .22's opened when the main xstrs died so alas, they too were replaced.
I have checked all the surrounding resistors in circuit, I may have to lift them to make sure......
seeing how this circuit was revised a few times, I'm thinking it's design ain't the greatest!
Thanks again E.

[Enzo]

The circuit looks conventional. Those revision snotes look pretty superficial to me. Moving jacks and LEDs then changing the pc board to accommodate.

Then those R1000 changes are just the notes for the higher power model. Same board, two models.

Whenever the bias xstr shorts, that pot is a weak link. If the output xstrs short to base, then much higher voltages/currents can flow through the bias circuit.

In the typical blow up, I check all the outputs and their ballasts, and the drivers. Important is to check all those resistors in the driver stage - in your case the 4.7 ohms - and then through the bias string.

And always check for open resistor in the stability network over next to the relay on the drawing. The amp will work without that circuit, but it can be unstable and go into 300 watts of RF without you knowing until it cooks itself. Rarely happens, but I also like to check the inductor by the speaker jack. If it breaks free, the output all runs through that 10 ohm resistor in parallel.

[drewl]

Okay, more findings.

The mains idle current is around 550ma and is rising slightly.

The current through D205 and 206 is stable.

The 5ohm stability network resistor is good, so is the output coil (zoebel network)
The 2- 4.7 ohm resistors also opened initially so they were already changed.

The odd thing is the bias current is stable, it's just the current through the 4.7ohmers that's rising.

One more odd thing, that current doesn't rise as fast with a signal applied as opposed to no signal.

[Westrek]

I know this is an old thread... What's the best way to adjust bias on the Carvin R600 ( or any other similar amp) if no specs are available?