Crown 800 CSL

[lowell]

Got this in with only one side working. I found 6 out of 8 power transistors (MJ15016, MJ15015) shorted, and 3 drivers, as well as some related caps and resistors. I've replaced all 8 of the power transistors with matched ones directly from Crown, per their recommendations. Also replaced the 3 drivers and the other components as well. Right now with my bulb limiter in circuit, I am getting +67v and -0v on my main rails. I'm sure I'm missing something simple, but can't seem to figure it out. Could use some help. It seems the ground/common reference is not right, as the entire power supply is across the NPN side of the power amp.

The working side is +/-33v with limiter in circuit. BTW, the bulb is glowing but not super bright.

I think this schematic is right, or close
http://bmamps.com/Schematics/Crown/C...ice_Manual.pdf

[lowell]

Right after posting I thought to check D309-312 diode string... they're ALL shorted.

[lowell]

Ok replaced those... still same result. All +66v across the NPN side.

[Enzo]

Look at the schematic, the amp is bridged. One side works conventional with output between the transistors. The other side - note the speaker negative is ground and the emitters on that side are also ground - is a flying rail type. You have 66 and zero instead of +/-33 because the amp on that side is skewed to one end. Just as if the other side was sitting at +33 instead of zero.

The manual has a fairly extensive theory of operation chapter.

Those shorted diodes would not cause ofset voltage, if they are all shorted, it just increases crossover distortion. However the fact they were shorted tells us excess current flowed through them from somewhere and to somewhere. Most likely shorted drivers.

Cheeck out the op amp and predrivers plus related resistors on that side.

[lowell]

Ah ok gotcha. So the two "sides" of the amp are different operation too. That's interesting, and surprising, but thanks for enlightening me on that. I'll check out the theory section.

[lowell]

So, where does the output go from the Low Side? According to their theory, the High Side output is fed to an opamp which inverts the signal and feeds the Low Side... but I don't see a "load" for the Low Side in their diagram. Excuse my ignorance.

Also, there are 2 channels and 2 outputs on the back. One works, the other, connected to the faulty side, has 65vDC across it. That is the "side" or "channel" that has +66 and -0v. So I'm not sure now if we're on the right path or not. Should both "channels" have +/-33? Or... not. haha There's a High Side and Low Side on BOTH channels of the unit correct? Finally, I have the switch in the STEREO position.

[Enzo]

The two channels of the amp are not bridged, the circuit of one chanel is two output circuits bridged.

[lowell]

Right ok. So that'd mean that my +66v reading is wrong then. There are 2 filter caps in here, 2 bridge rectifers. Each one feeds each channel. So I'd think they should both be +/-33v. Correct? And if so, any direction on how to proceed...thanks!

[Mark Black]

Here's a schematic that may be more accurate regarding part designations and such. The Power Base and CSL models were identical but with different model numbers.

What Enzo suggested about predrivers and such is spot-on. With an output failure in my experience there were usually a few shorted transistors along with some flameproof resistors (may not look bad) on the main PCB. Maybe some diodes too. Usually not opamps but one never knows...

Checking against the components on the other side with meter set to diode check for transistors/diodes and of course resistance for the resistors is almost straightforward with Q101 = Q201, Q102 = Q202, R101 = R201, etc. Too bad the PCB isn't physically laid out as a mirror-image so some hunting is required, and one needs to be careful about too much bending of the ribbon-cables as they are tinned and prone to breaking with a lot of movement.

Best of luck!

[lowell]

Anyone know what kind of transistor is mounted IN the heatsink for the bias servo? It's all broken apart. Looks like it says "mps" and then an "18". I'm thinking mpsa18. But isn't this just a general purpose transistor?

It was not reading right either before I pulled it... and AFTER I found 2 way over spec 22ohms. Also Q417 seemed a likely culprit to be putting +66v on Q401's base. Pulled it, tested shorted base to collector and replaced it.

Now have my bipolar supply! +/-33v

Pretty sure this is the last dead part...this servo transistor.

[Enzo]

What is in the other channel? What does the schematic say? And why does it have to be other than a common type?

MPSA18 is a low noise type.

[lowell]

I thought these servo transistors mounted to the heatsink were temperature controlled devices... I guess this must apply to all transistors - they conduct more when they get hot. ?

The schematic doesn't say, and I'm just being lazy and don't wanna pull the other channel out for fear of screwing it up. Plus I was curious about the temperature variable. So MPSA18 would work for such an application? (stupid question?)

[Mark Black]

The service manual does indeed list Q318/Q418 as "C 8813-5 Bias Servo MPSA18/MPS8097" under, oddly, "Mechanical (Non-Module) Parts".

Sorry, I should have included the SM with the schematic...

[lowell]

No worries and thanks Mark! I had 1 2n5088 in stock. Read it was a good sub. The servo voltages look good now. The amp is not drawing too much current so theres no dead short fault...however it's real distorted. I havent proceeded with any measurements yet but I'm sure I'll find something else. At least now the thing is running and putting out signal and no DC offset. Should be downhill from here.

[Mark Black]

Good deal - glad you are getting close.

Not to say they ARE the reason, but I have experienced distortion without high current draw from dirty or internally burned bias trimpots. Just another thing... :-)