Crown XLS 802 with vexing power amp issue

[asfi]

Stereo power amp, 800W/channel, all linear circuitry, emitter-follower amps, shouldn't that hard to fix, right?

Amp comes into my hands, one channel is blown. Find that someone has already serviced this beast, one of the output transistors has been replaced. It went, and a whole bunch of others with it, so I replace all of them in that channel. FWIW, I use the same On Semi parts used originally. Bought 'em from Mouser, no BS, no fakes.

I checked Q112 and Q113 before putting the output transistors back in.....seemed fine, not shorted. Visual checks on everything are OK. Should be good to go, yeah? But when I apply power through a variac, as I turn up the volts, the amp supply stays at +/- 1 volt. Powered off and rechecked.....yup, no shorts. But something is definitely turning all the transistors.

What have I missed? And how can I check it without much in the way of supply voltage?

[Leo_Gnardo]

Your attachments (schematics?) aren't showing up.

What's the function of Q112 & Q113?

How are all the emitter resistors in the output circuit? At their proper fraction-of-an-ohm values? Not likely all of 'em went open circuit, but if they did, you'd have almost no output.

I'm thinking, if the output fault caused limiter resistors to short, that also might cause the symptom you're seeing.

[Enzo]

Whenever transistors blow, resistors are often damaged or opened.

[asfi]

For some reason, the schematics appear as clickable links instead of embedded thumbnails, and there's an option to save or open them. At least in my browser, that is....


Q112 and Q113 are the drivers.


Regarding the amp, I did check all the emitter resistors before installing the big mess of TO-3's, and they were fine. Double-checked them now, they're all still fine. Re-checked the output transistors, they're still fine. Obviously something's amiss somewhere upstream. Rats, was hoping to avoid having to brute-force my way through this beast component by component......

[The Dude]

Schematic Link:

http://rdn.harmanpro.com/product_doc...n_original.pdf

Edit: After looking at the schematic, It may not be the right one even though I cobbed it directly from Crown. Designations don't seem to match up?

[asfi]

Yeah, that's the earlier version. Trying again to attach the files....

Except I don't see any means of attaching a PDF in this comment.

[asfi]

Digging back into it, and checking every component in the output stage: yeah, the drivers were fine, but the pre-drivers were turned into weak shorts, as was one of the transistors in the current limiter. Replaced pre-drivers, current limiters, and (while I was at it) the drivers too. Now it amplifies! But consuming too much power, the heatsinks get hot even with no load on the o/p. There's no bias adjust in this circuit.....so now what? Not seeing any HF oscillation on the scope.

138373-7_2 XLS D Schematic.pdf138381-5_2 XLS D Schematic.pdf

[Jazz P Bass]

How about a few voltage measurements.

The output bases will tell you how hard the devices are turned on at idle.

Make sure Q1 & it's resistors are good.

[asfi]

Aaargh, been barking up the wrong tree.

The channel I repaired seems to be working brilliantly. Approximately 0.2 mV across the emitter resistors, and an output DC offset of around 44 mV.

It's the OTHER channel that's got approx 160 mV across the emitter resistors. Time for some A/B measurements and thinky bits. If I can't solve this on my own I'll post some voltages.

[The Dude]

Be sure to explain to the customer how bridge mode works. If both channels were bad, it's likely they don't understand it.

[asfi]

Here's what I've measured. Obviously Q2 isn't shorted, but I haven't (yet) removed it from the board to test out-of-circuit as it's necessary to dismantle half the heatsink assembly to get at it.

[asfi]

Bit the bullet and extracted Q2. HFE= 600, Vbe = 625 mV. Yeah, it's fine.

And just for giggles, I pulled Q1 and tested it. Dead, base shorted to collector. Remember, this is from the channel that seems to be working properly. Followed the traces back and yep, I am not hallucinating.

Douglas Self notes the absence of the supernatural in his work. I wish I could say the same.

[asfi]

And Q2 is BC184L which is obsolete. How critical is this device? The BC547C has similar gain, can it be subbed in?