Help fixing Crown power amplifier

[BlackUnikorn]

Plz help me out: I got a Crown Power Base 2 amplifier for free and its not working. I'm trying to figure out what's wrong with it.

It appears that the previous owner connected the outputs to the inputs or something along those lines : (


So, basically I replaced/repaired a couple of blown small signal transistors(which have the same function for either channel), some pcb traces on the input ground of the amp, two or three blown resistors in the "voltage translator stage" and a couple more resistors on the output board near the output transistors along with some blown diodes.

I should have known better cause when I powered up the amp two of the resistors blew up again. I'm starting to think that some/all of the output resistors are shorted, but the strange thing is that its not blowing the fuses and apparently the protection circuit is not doing its job. What should I check for? Is it worth fixing?(They're about $250-$350 on ebay)

I called Crown and they say that the power transistors can only be bought from them since they are "screened" for specific characteristics, they are like $7 each (there's 16 of 'em $$). Furthermore I searched all over the internet for replacements and can't find any info on the output transistors. All I know is that they are Motorola and are C6492-0, and C4751-1, pnp and npn respectively.

BTW the schematic can be found at http://www.crownaudio.com/pdf/amps/P...0J0446-5_A.pdf

The parts which I found out to be bad from just looking at them are:
Q101
Q201
R162
R240
R162
R180 and c147(maybe)
R01
D07
D08
D06 (along with pcb trace)
R09
R10
R08
C09 (maybe)

There are probably more bad parts, like the output transistors, and that's what I want to find out. I'm pretty sure someone who has worked a lot on power amps can tell whats wrong by looking at the schematic and the syptoms. BTW, how can I check the output transistors without a transistor checker? Can I use a diode checker?

Thanks in advance...

[rkc7]

Well, I'm not real experienced, but I've checked transistors. There are 3 pins on it, and the simplest way to check (not to test if it is working perfectly, though) is to use an ohmmeter or diode check (or transistor check if you have it). I initially check them in-cirucuit by the following method:

1. put red lead to left-most pin (from front), black on right lead.
2. keep red on left, put black on middle
3. move red to middle, black to right.
4. put black on left, red on right
5. keep black on left, right in middle
6. black on middle, red on right.

If you get almost 0 ohms (or diode check beeps) in one configuration, or usually more, then you should remove from the board and retest it. But, make sure the resistance or beep remains constant, as it may initially read short or beep if there's a capacitor between pins and will produce such results until it charges enough (usually less than a second to a few seconds). If you get the same results out-of-circuit, it's bad.

You should also test for consistancy between same model transistors (of course, the circuit may interfere and drastic differences should be confirmed by out-of-circuit testing).

Since transistors are like diodes, one polarity configuration between two pins should give some resistance (or foward voltage with diode check), but nothing in the opposite polarity (but can also be influenced by circuit).

In the end, I just want to say, you can do initial checking in-circuit and usually get an idea if a transistor is bad.... BUT! make sure you take it out of circuit and test it before you believe it's bad.

Good luck!

[djk]

If you expect that amp to work you will need to replace all 16 outputs, 4 drivers, and 4 predrivers, emitter resistors, the bias diodes and the bias transistor as a MINIMUM.

Check the current limiter transistors, they are probably bad.

Check the last voltage amp (high side), it is probably
OK. If bad, replace the whole front end.

The protection circuit only works in a properly working amplifier.

During testing use a 100W lightbulb in series with the line cord, play for 3 or 4 hours before going direct.

Outputs
MJ15022/23
Drivers
MJE15030/31
Predrivers
MPSA42/92

Keep in mind a couple of things:

You can buy a new POS 500W amp for $300, but how long will it last?

The Crown circuit board and heatsink is drilled for another set of outputs (used in the PB3, MA2400, etc), add them to the PB2 and it will handle low impedance loads much better (change all the emitter resistors to 0R27).

[anatech]

Hi BlackUnikorn,
djk is right. I have repaired these before too. These are not pretty when blown. Be aware that this is not a "normal" amplifier to repair. I'd feel better about you repairing this one if you had experience and better test equipment. If you do proceed, keep your eyes open and use common sense. If you can get someone who knows these to help, do it.
Use new mica insulators and proper heatsink grease. Do not overtighten the transistors. Please clean all the old grease off, it will cause you grief if you don't.
-Chris

[imix500]

Those are great little amps and I completely understand wanting to fix it yourself, but sending it back to crown might not be as devastating on the pocket book as you might think. I've had many crown's repaired and the most expensive was an old mt600 with both channels blown up by a rental. It was $181 +shipping to bring it back to factory specs. (your results may vary of course) Cheaper than buying another amp though.

[djk]

"sending it back to crown might not be as devastating on the pocket book as you might think"

Crown has BOHICA pricing on repairs.

A friend returned a working MA600 that had the fan air flow reversed. Crown replaced the fan (rather than reversing the blade), replaced every non-Crown part with CPN (keep in mind the amplifier worked when returned), and sent it back with a bill for over $300.

If your pockets are that deep, just buy a new amp.

"Use new mica insulators "

On what?

There are none on the outputs or drivers, the heatsinks have voltage on them and are isolated from the chassis.

[BlackUnikorn]

Wow, thanks for the help guys. I got around testing the output transistors and most of them were bad. I figure its best to replace them all with new ones, but both output transistors and drivers are almost $100 !!, plus I'm missing the top and bottom panels(necessary to make the chassis pressurized/cooling). I see these on ebay for around 300 every once in a while so I guess its better to buy a new/used amp.

Is this a decent sounding amplifier for HIFI? What other amps would you recommend? I'm looking at least 250-300 watts/ch of clean power. BTW, my speakers are around 8 ohms and very inneficient (82 dB).

[imix500]

Wow, I guess I've been lucky guys. I did have a techron DO2000 repaired once come to think of it, that was very expensive as I recall.
The panels are pretty critical for proper cooling on these amps. Especially if it's got a fan (don't remember if the pb2 does). These are really great amplifiers though, any of the amps with the construction of the ma/mt series, which the pb2 has, are rock solid as long as their not abused- as with any good amp. Their specs are pretty good and not "fudged" at all and right up there with the rest of the tour worthy amps of its time. Crown even gives you the specs of YOUR amp when you buy one new. The main drawback is the limited output impedance range of the pb series, but if you're gonna do 4 or 8 ohms, it's no problem.

[anatech]

Hi djk,

"Use new mica insulators " On what?

-Yup, brain fart, just general advice. And you are right, this amp doesn't use 'em.
I was just thinking of how many amps for service that just came from other shops I've seen. Many with the old grease, insulators and new blown outputs. Kinda makes you sad, don't you think?

Thanks for pointing that out.

-Chris

[bassrogue]

Sorry to bring up a dead subject but I can get hold of a power base 2
Has anyone tried one of these as a high fidelity home amp? Would it be worth the time to bring it home?
Thanks!