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CTS1200 Problem

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Hello!

I have a CTS-1200 Amplifier that gone into fault mode.

When powering up, the FRONT two RED leds starts flashing. and all the below resistors starts SMOKING!!

R935

R936

R937

R938

R939

R940

R960

R961

Any ideas where to start?

Or what is the thing that causes this problem??

Regards

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Hello FS,

These resistors are the load resistors of the crowbar circuit. The crowbar circuit 'shorts' out the rail voltages in case of a 'catastrophic' fault in the power amplifier.

This circuit is not exactly part of the SMPS circuit but is located in the Fault detection and protection of the amplifier stages.

The task of the SCR is to effectively short out the rail voltages to protect the amplifier section from high current flows when a fault is detected. When the crowbar circuit is triggered or enabled the energy accumulated in the DC power caps are shorted out through the path comprised of the SCR and resistors; It is possible that the resistors will heat up and smoke a bit. Once the crowbar is triggered, the short is detected by the SMPS protection circuitry and the SMPS modulator is taken off line (no power to the amplifier section). Note : Once the SCR has been triggered, it will remain enabled until power is cycled.

So, you probably may have some sort of short or major fault in the amplifier section. Remove all loads from the amplifier before powerering up. You may have to remove the two (large) ferrite beads, located near the filter caps and in line with the rail voltages, to verify if the problem is in the amplifer section. Of course, verify the SCR!

Good luck!

Alain

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Hello Alian.

thanks for the replay. I removed the two beads. And the power supply just worked.

i am facing new problem in the power supply. Voltage is not equal on outputs. Is this a regulated power supply??

Still i am trying to find out the problem in the amplifier section. I removed all of the power transistors. But still the smps cannot start if i reconnect the amp section without the power transistors.

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Hello FS,

The CTS power supply is of the SMPS type. So yes, this type of ps is... challenging!

As you noticed, once you removed the ferrite beads the smps comes back up. This indicates that the fault lies in the power supply section. Most probably, the protection circuit detected some DC at one of the output stages.

When you remove the o/p devices you can notice that the voltage measured at the output may be off. From my experience, the output with voltage is the problematic one. The voltage that you're measuring comes form the low voltage side! If you continue to investigate the output section, verify the capacitor in the CB driver stage (220pF/500V). I have found some caps that became resistive! Verify also the solder connections on these drivers and of the regulators, located under the output section's [heatsink] Silpad.

Alain

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Another old thread. But this same amp came to me by a friend. The power supply isnt challenging, but protection circuit is.

At the point, I am not worrying about protection. So I removed Q25, which send power to the SCR. Removed all the 21194, and MJE1503x. Powering up the amp, I see voltage on the rails were about +-111, and the low voltages were +22&-24, going through 7815&7915, gives +-15. Everything seemed good, nothing is burnt. But the green lights still didn't come on. In frustration, I checked everything single diodes, caps, transistors, op-amps, comparators, and resistors on the dam board.

It sure wasn't helping, until I got my hand on the schematic. Found out that the "Current Monitor" circuit had some defective resistors. It wasn't like the resistors weren't working, but the resistance values were slightly off, within tolerance. Ultimately causing the U112 to be unbalanced and output high. In result, setting U111 and U211 to high, which are responsible for putting the amp in some Stand by(Not ready) mode.

As a Trial and Error procedure, I eliminated R538 & R638, and grounded R547 & R647, which disable the Current Monitor circuit. Powered up the amp, and the green lights came on. Turned it off, I then put back all the 21194, and MJE1503x. Without Q25, I turned the amp again, but this time, I bravely added my speaker without checking the outputs. Then came the rock music came on loud and clear.

Clearly either the power supply high rails are a bit off or one/more of these resistors(R533, R534, R535, R536, R633, R634, R635, R636) went slightly off causing U112 to drive high.

I am thinking of leaving monitoring circuit out of the picture for now, test the baby for a while.

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Good job! Can you tell me about the[LVA] transistors located at the right of the main, behind the Ch1 heat-sink, seen from the front. You will notice a string of transistors. If these transistors are MPS42 or MPS92 (no 'W'!), replace these transistors for MPSW42 and MPSW92; Also, replace R428 and R466 with new resistor, 649 Ohm. If these 'W' transistors have been properly installed, then no need for replacement.

Please advise.

A.

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Coming back to this old thread.

Any one found the problem?

I found nothing faulty in the power amplifier section, this amplifier is driving me crazy.

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Hello

After hard work, I managed to make this amplifier running.

Now, I have a new problem, that the repaired channel, is heating very quickly, even if the amplifier is cold power it up, wait 5 seconds the FANS will run at maximum speed.

I trien to find out how to adjust the BASE, but no details on how to do that in the service manual.

Any clue?

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The bias adjustment measurement is taken off TP x22 and TP x24 (located a little above the heatsinks); The bias voltage should read between 400 and 450mV. Also, verify the drive transistors; Check the R68 emitter and collector load resistors. There may be a bypass cap in the drive section (C-B) that may have been damaged. The bias transistor is located beneath the silpad heatsink and may have been damaged; Verify the regulators on the other side of the driver (under the silpad heatsink).

Hope this helps!

A.

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Hello

I have just found that the BIAS transistor is damaged, it measures two diodes, thanks to the transistor analyzer , these test tolls are priceless.

I will change it tomorrow and I will post my updates

Thanks

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Note : When I mentioned : 'Check the R68 emitter and collector load resistors', I meant the 0.68 ohms or R68 ohms resistors; It was not a designation identifier!

A.

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Hello

I changed the damaged BASE transistor, the channel is not heating.

But a new problem is showing now, when connecting a 4 OHMS load at the repaired channel, I am only getting 35V with strange signal

The good channel is able to deliver 65V nice sine wave

Any ideas?

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Do you get full signal 'without' a load? When loaded, does the signal appear as a half-wave rectifier? I would verify, as I mentioned above, the emitter and collector .68 ohms resistors.

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Hello

Yes, without load I am getting around 80V, same as the good channel.

No load

http://postimg.org/image/5zwc1pj07/

I checked all of the 0.68R resistors, All are good.

I am getting LOWER output voltage when i connect the load around 40V, see how the wave looks like with load

Loaded

http://postimg.org/image/7nhala319/

http://postimg.org/image/7nhala319/

The working channle gives around 67V at the same load clean sine

Hope i can solve it

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Checked with Crown :

Verify the JTS temperature circuit : Locate and remove the two Ch1 JTS pass resistors R587 and R588 (51R1, 51.1 ohms) and the two Ch2 JTS pass resistors R687 and R688.

Then, compare both sides; It is possible that the issue is with the differential current amplifier.

Hope this helps!

A.

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For an easier troubleshooting experience! My hypothesis is that the Ch1 JTS circuit is affecting the signal. By removing the JTS circuit from the two channels, you'll be able to concur if the JTS is the circuit affecting the problematic channel.

My suggestion is to decouple the JTS circuit from the main circuit. By removing the coupling resistors you basically remove the JTS circuit from influencing the main circuit. By altering Ch2 you bring Ch1 to the same level as Ch2.

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I am not getting a better result by removing those resistors, except that the wave forms looks better, only clippling, no strange wave as before

Also, I am unable to get more than 35V at the amplifier output, See image for best wave clipping

http://postimg.org/image/wv6qijftz/

Another image when channel starts clipping

http://postimg.org/image/6mxl3kzuv/

Image at TP111 , looks strange

http://postimg.org/image/ip3zoimxx/

Image at TP211 looks good

http://postimg.org/image/wxtp4v78r/

This amplifier is driving me crazy, it gives 80V without load

Hope I get some more help

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I got an update

I noticed that the same channel when loaded, feeding sine wave, I noticed that the THERM protection LED turns on quick, even the heat sink is not so hot, I even have the fans running on top of the heat sink for testing purposes.

I conducted a test on the working channel, forcing it to over heat, no fans used, THEN!!!! the working channel started to show the same effect!!! as of the faulty channel!!!

Image from the WORKING channel when loaded and forced to over heat

http://postimg.org/image/6i1l5ttq1/

in the image above, the voltage was 65V, when the channel over heated, voltage dropped to 35V, and the wave started to get bad.

Now I think that we know the problem is in the thermal control section?

Please advice

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Hello FoxSky, Yes, the two images from TP111 and TP211 do appear strange! Do the signals stay the same with or without load? (I'm suspecting current limiting...)

First of all, you have to be `careful` when injecting sine wave and loading the amp; The smps doesn't handle well sine waves at full output. Preferably, use program material and avoid clipping!

I sent you a PM.

Alain

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