Crown MT 2400 ch 1 no output

[pontiacpete]

I have a question I'm hoping someone here can answer.
I can power up the amp on a 150w limiter. Ch 1 has no output and the voltage on the body of the 8188s in 0v and on the 8718s there's 112vs. On the other working channel both PNP and NPN have +/-58v. Usually on the limiter, if the output transistors are shorted the light will indicate a short, does this mean there isn't a short?

There is no DC on the output and no hum.

here's a schematic. Thanks for your time and help.
pete

[MarkusBass]

Pete, I think that the first step in this case is to understand how the amplifier works. Crown uses special amp topology called "Grounded Bridge". You can read about it here: http://www.crownaudio.com/pdf/amps/grbgpapr.pdf (page 7). The power supply is not grounded and low side output transistors have emitter grounded. This is not typical topology so your question "Usually on the limiter ...." is confusing. It may be true for typical amp topology but not in this case. I would say that you have shorted output transistors or drivers. I think that this can be easily checked, especially that you have other channel working correctly. You may also check the ground connection.

Mark

[pontiacpete]

Thanks for the link, Mark. That was very helplful.
I called crown and found that their transistors sell for $19.00 each. Is there a less expensive equivalent or is it best to buy from them?

[MarkusBass]

I called crown and found that their transistors sell for $19.00 each. Is there a less expensive equivalent or is it best to buy from them?

I think that 8188 and 8718 are symbols used internally by Crown for SJ7147 and SJ7148 by On Semiconductor (hard to buy). Possibly a good replacement (can anyone confirm this?) are MJ15025 and MJ15024 also by On Semiconductor (about $5 each). But if they have originals, why not use them?
Edit: SJ7147 was PNP 250V 16A TO-3 - looks exactly like MJ15025 :-).

Mark

[Mark Black]

Last time I talked to a Crown tech he told me the 8187 & 8188 transistors were basically graded MJ15024/25, but that one of the part numbers (don't remember which one) had been moved into the MJ21193/94 series pending exhaustion of previous stock for the other one. The key here is that Crown sorts & grades for matching and that is important since there are only three paralleled transistors handling any given rail function.

Safest bet is to just get the Crown parts.

Testing for bad ones is usually pretty simple with a meter set to "diode test" and comparing to the other (working) channel.

Unfortunately when that style of Crown amp loses output transistors there are usually some associated component failures, sometimes up on the main board in addition to the output module. You will also want to be sure and stabilize the ribbon cables where they exit the main board with some hot glue or RTV (if that hasn't already been done) to prevent conductor breaks when moving things around during troubleshooting.

Best of luck!

[95strat]

I worked at Crown for over 14 Years, as a Tech and I can tell ya, DONT USE UNGRADED OUTPUTS, your amp will never work right! Buy the Crown part if you can still get it for the un - Godly price. There is a High side and low side to these amps, and they both usually go out, you are going to end up replacing at the min 6, thre NPN, Three PNP

[Jazz P Bass]

I worked at Crown for over 14 Years, as a Tech and I can tell ya, DONT USE UNGRADED OUTPUTS

Interesting.
Do you know which function they are graded for?

[95strat]

Interesting.
Do you know which function they are graded for?

gain if I remember right.

[Jazz P Bass]

Thanks.