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Old 07-06-2017, 01:08 PM   #61
5inc
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Default Re: Crest CPX1500

just got back from the shops where none had a single J174 (Q100,101) the closest I found was a J175 in a bigger enclosure...
http://www.datasheetcatalog.com/data...1/2SJ175.shtml

I'll have to order some J174s online...
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Old 07-06-2017, 02:41 PM   #62
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Default Re: Crest CPX1500

The other end of R100 goes to GND, so you will hear nothing there. Don't worry about that.

The signal should keep going through R101 and R102, but will be shunted to ground also if Q100\Q101 are switched on. They are controlled by the DELAYA signal. If the DELAYA signal is not working correctly, then Q101 and Q100 may never be getting turned off.

You need to check what the DELAYA signal is doing and what voltage is on the gates of Q100 and Q101 before you replace them. They may not be faulty.

Another problem could be if CR100 or CR101 are shorted\leaky. They are clamping diodes to protect the input of U100 but could cause a problem with the audio signal if they are faulty.

The same goes for if U100 has a shorted input, or if U101 is faulty or being incorrectly driven.
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Old 07-07-2017, 01:32 PM   #63
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Default Re: Crest CPX1500

I just checked voltage on gates of Q100/101 and have 13.9vdc. I then turned the tracer on and checked the both ends of R106 assuming that would be DELAYA, I hear nothing. Is that what you ment by "check what the DELAYA signal is doing"?

CR100/101 are unlikely shorted or leaky, I allready shotgunned them... and the same goes for U100/101. Well, appart from the fact that I fiddled with them with the tracer and thats when I blew the fuses. Could they have gone bad by that?

And when you say U101 could be incorrectly driven, are you talking about wrong voltage or wrong/no signal?
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Old 07-07-2017, 03:11 PM   #64
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Default Re: Crest CPX1500

Q100\101 are P-Channel JFETs so that 13.9v on the gates should be turning them off, assuming they are not faulty (shorted).

To troubleshoot further, you could try the following:

Check R101 and R102 to be sure they are not open-circuit or have gone high-ohms.
If R101 and R102 are OK, then temporarily remove Q100 and Q101 from the circuit and see if the signal then gets through to Pin 3 of U100.
If it still does not, disconnect Pin 3 of U100 and see if the signal is now present at R102\CR101.
If there is still no signal, disconnect Pin 6 & 1 of U101.
If the signal is still not getting through R102, the only other possibility is that C103 has shorted and the signal is getting pulled to GND through it and R107.

DELAYA is a control line, not an audio signal line. It controls the power-up sequence, likely to prevent switch-on thumping of the speakers.
I meant for you to check what voltage it is at. Measure the voltage on DELAYA (at R106) while the amplifier starts up. When you first turn on the amp, DELAYA should be at 0v or close to it. After a second or two, the voltage should rise to that 13.9V.
If the voltage on DELAYA goes high immediately after you turn the amplifier on, or if it stayed at 0v, it would indicate a fault with the DELAYA signal generation circuitry.

Yes, it's possible you blew U100/101 again. I don't know what or how you probed where, but maybe you caused them to short out and that's what blew the fuses.

Yes, U101 could be getting some incorrect signal or no signal from another part of the circuit. It could also be faulty.

Last edited by Agent24; 07-07-2017 at 03:19 PM..
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Old 07-08-2017, 12:28 PM   #65
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Default Re: Crest CPX1500

Done, R101/102 are ok. Resistance Check reveals a short of 40 ohms, probably Q100/101.

Took out Q100/101 and trace probed pin3 of U100 to hear fizz. (J174's have a 40ohm short from source to drain)

Tried to unsolder pin3 of U100 but ended up taking the trace with me. It is now disconnected.

Probed R102/CR101 and finally got signal.


Does that mean U100 is faulty, appart from Q100/101?(these transistors and the ones on the other channel are equally shorted, I had seen it and thought it was a circuit matter)


I'm ordering a pack of J174s, and will change U100 and fix the broken line when those arrive. Do you recomend using a couple of strands of copper wire to do so?
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Old 07-08-2017, 03:20 PM   #66
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Default Re: Crest CPX1500

The transistors may still be OK, a JFET will show some conduction between drain\source because there is no PN-junction there, and being a P-channel there is no voltage on the gate to shut it off while you are testing it.

Put the transistors back, one at a time, and with U100 still disconnected, do you still get signal?

If U100 had a shorted input on Pin 3, it would cause the signal to be lost, but also, if there is some problem with the feedback signal going to U101, then U101 could have its output of Pin 6 pulled low and be sinking the signal to ground. Is the DDT LED for that channel constantly on? That could indicate such a problem.

With U100's Pin 3 still disconnected, what resistance do you measure between Pin 3 and the power pins, 4 or 6? That could indicate a shorted input if you measure a low resistance.

Yes, you can repair the trace with a piece of wire. Just be sure you don't make any solder bridges!
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Old 07-11-2017, 01:19 PM   #67
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Default Re: Crest CPX1500

Thanks for the note on the JFET's

DDT led on that channel IS constantly on. I didn't find any low resistance between pin 3 and 4,6 or 8 (something like 200-400K).

I connected Q100 and Q101 one by one and still have signal on R102/CR101.
Will now proceed to replace U100.
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Old 07-11-2017, 05:23 PM   #68
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Default Re: Crest CPX1500

I just replaced U100 and of course, it wasn't shorted. It still shows the same symptoms, so I have to go one step back.
-"Yes, U101 could be getting some incorrect signal or no signal from another part of the circuit. It could also be faulty."

I don't fully understand the feedback thing, so I followed BDFED (hoping it would stand for feedback) on P101 and ended up at the bridge/stereo switch. I saw it was set to bridged so switched it back to stereo, and fired the amp but everything is the same. I may be on the wrong foot but that's all I could figure out for now.

I have been checking the delay signal on R106 and it does rise after the delay startup time(I forgot to mention earlier) Should I understand that this indicates DELAYA signal is correct?

I also just checked voltage on R102/CR101 and it recieves voltage (-11,8VDC) from the second I turn on the device...
R105 does the same.
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Old 07-13-2017, 11:04 PM   #69
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Default Re: Crest CPX1500

Yes it sounds like DELAYA signal is working properly. So that's good...


But, the DDT LED indicates that the audio signal is clipping (or it thinks it is) and that the amplifier is running in limiting mode.

Since I assume you are not overdriving anything during testing, then there seems to be a fault somewhere which is causing the limiter to want to drive the JFETs off, either because of a fault in the limiter itself or in the signal path before it.

If you toggle the GCL Enable\Defeat switch on the rear of the amp, can it turn the DDT LED off?


Also, have you checked or changed or done anything else with U102 yet?
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Old 07-14-2017, 11:23 AM   #70
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Default Re: Crest CPX1500

Yay, DELAYA seems ok!

I think I'm not overdriving anything, there's no load on the outputs, but I have pulled the volume knob all the way up to get the signal through the amp for tracing.

I just toggled GCL enable switch while the amp was on, it made no difference.

About U102, no, I still havn't changed it. I have tested it on diode mode on the DMM but didn't find any shorts. I could give it a shot, after checking resistors around.
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Old 07-14-2017, 01:46 PM   #71
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Default Re: Crest CPX1500

I just replaced U102. I must have done something really wrong in the process because now it blinks again(both DDT leds and Channel A yellow led).

Found a short between pins 3-4 of U101.
R194 has raised to 76ohms

Gonna replace them both tomorrow evening unless you think there might be something that caused this to short again (obviously appart from me)
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Old 07-17-2017, 03:50 AM   #72
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Default Re: Crest CPX1500

Sorry the short on U101 wasn't on pins 3-4 but on 7-8, that both go to 15V+IA.

That, and having a burned R194 seems to point that U100 was bad again instead. Replaced both, and now everything looks stable:

I fire it up and first both DDT leds turn on, then they go off for a glimpse to turn back on together with green power leds for a few seconds. Finally, after another second or two, DDT leds go off and only power leds remain on.

After seeing all this I plugged in my testing speaker, saw it made no difference in fan speed, and tested the volume, but no luck, will try to trace and see where it leads.
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Old 07-17-2017, 03:39 PM   #73
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Default Re: Crest CPX1500

Sounds like you're making some progress!

The fact that both channels do the same thing now is probably a good sign.
Though, hopefully the user manual could tell you if the LED sequence you're seeing is the correct one.

I assume you have tried the test speaker on both channels?
And just a thought, but maybe it needs two connected at the same time?
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Old Today, 07:02 AM   #74
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Default Re: Crest CPX1500

you see, channel B has been dissconnected at TX15/16/17 from almost the beggining.

Perhaps that has something to do with the fact the signal won't arrive to the output? I wanted to get channel A running before jumping on to B, for the sake of simplicity and a better learning experience.

And since everything looks stable now (no shorts) it could be the time to start laying hands on channel B...

But on the meantime, I started signal tracing, and could only hear up to R101/R102. Nothing around U100/101/102.

and about U101... I'm a bit confused with this one. I've been seeing different things that seem weird, but I'm not sure. Appart from the fact that I don't know nothing about feedback. And appart from the fact DDT led finally goes off, I have no other constance that that is ok (so I'm not certain yet).
Then, between pins 3-6 I have a 40 ohm short. It took me 2 pieces to realize the short wasn't on the IC but on the track.
Between pins 2-3 I have 100ohm (as expected), but pins 1-2 show 140ohms. Also on the track.
The ICs out of circuit read fine.
I also made one test with U101 out of circuit and signal arrived to the output! But as soon as I put it back on I lost all signal again to return to a constant pinkish noise.

Do you know how I can test feedback signal?
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