Jeff Rowlands MC6 problems

[highviewvideo]

Hi all
This chip amp has an unusual problem. when in standby all the outputs have zero offset (correct and good). As soon as the standby button is pressed the circuit protectors trip open on ALL the power output supplies. The supplies are correct + and - 40v.
The input board is correctly receiving + and -12v from the regulators. All the boards are running in individual (not bridged) mode.
There are no speakers connected at this time. There is a little puff of smoke from the regulator bords on the smoothing caps.
To summarise: when the unit is taken out of standby it blows up!!
Any help will be much appreciated.
Remember I've been fixing amps for 35 years and this is the first time I am flumoxed !!!

[analog_sa]

Originally posted by highviewvideo
To summarise: when the unit is taken out of standby it blows up!!

What blows up? Sorry, but i couldn't make much out of your explanation. And please post some pictures.

[highviewvideo]

Only the circuit breakers pop open with that little puff from the regulator boards. There are no blown chips (it uses 6 x TDA7294 bridged in 4 channels and 12x TDA7294 bridged in each of the other two channels)
The unit has the capability of having any or all of the output channels in parallel (not quite bridge operation)
If the circuit breakers are reset the protection indicator LEDs all go out (correct). It is when it is brought out of standby and the mute lines to the TDA7294's are released that the breakers trip.
All the 48 output ic's are intact and check out OK.
Hmmmmm.

[analog_sa]

Originally posted by highviewvideo

All the 48 output ic's are intact and check out OK.
Hmmmmm.

This is the part i don't understand. Why are you certain the chips are ok? Can you really judge by their behaviour on stand-by? No idea what getting out of stand-by means, but likely it applies full bias to the output stage and if one of the chips is shaky you get some fault output condition briefly (dc?) and the protection cuts back to stand-by. Just a thought.

Thanks for the pic. It's really a jewel. Some chance of seeing more from the inside?

[highviewvideo]

The output amplifier boards are all identical with 12 chips on each.Two boards run the 12 chips as a 'bridged' mono amp
The other two boards are two channel with 6 chips per channel which can be externally switched to give various power output options. It is possible to have two high power outputs, 4 x lower power outputs or any combination by the switches.
Putting the machine out of standby lifts the mute on the chips . If there is any dc offset the fault inicator LED's light and the machine will not come out of standby. In standby they (the output ic's) have + and - 40v supply continuously fed via the circuit protectors (breakers).
There is no extra bias to the output stages when in operate mode. If one board is faulty then only its own breaker should trip.
Two or three breakers trip when it is brought out of s'by.
I'll photograph the inside and send some pics.

[troystg]

I had Model 2 and when I had a question about the remote turn on 5 pin DIN pinout I called the company in Colorado. I was connected directly to Mr. Rowland himself. He was SUPER nice and genuinely wanted to help me. He and another guy there (Rich I believe) are the tech support and are both FABULOUS!!!

Try calling or emailing to them directly.

[troystg]

I enjoyed looking it up too!

http://www.jeffrowland.com/Technology/MC6Tech.htm
http://www.jeffrowland.com/ClassicSpecs.htm

Here's a good one:

http://www.jeffrowland.com/Docs/MC-6Manual.pdf

Check out the wiring diagrams!... Jumpers to parallel the channels! Never saw that before.

[highviewvideo]

I wil give jeff Rowlands a call. They should be out of bed by now.
Here is a picture of the inside. It shows the four output boards and the signals board.

[troystg]

Beautiful. Brings a tear to my eye to see such a work of art!

I miss my Model 2 and Consonance pre-amp....

[highviewvideo]

Just had long chat with Jeff Rowlands.
As the TDA7294's are brought out of standby they turn on their output stages fully from the standby condition. It appears that there are several defective chips in this one (Maybe 15 to 20!!)
Anyway I think I've found a way to isolate them and check each board individually.
There is a mod by the manufacturers to turn the amp into a model 606. I am considering sending it back to USA for this mod as the improvements are manyfold.
I'll give it a few more hours on the bench but really don't want it to fail again in the future so the modified version may be the best solution.