Soundstream PCA1500D Qwerky

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Hello; This one is a bit strange. Let me know if anyone has seen this before...

When this Soundstream PCA1500D is powered up, it will instantly go into protect mode as soon as you connect the RCA's.
When I got it on the bench, I measured a 5volt offset voltage between the RCA shields and 12volt DC ground. ....huh?

Voltage rails were normal. No excessive current draw. Resistors, caps and op amps looked OK in the input section, so I called Soundstream and they told me to disregard the 5 volt offset and look for leakey mosfets(output or power supply)

I removed all the audio output mosfets. It still when into protect. I removed all the power supply mosfets, and installed 2 new ones
(one on each bank) just to run the amp. ....No go.

Then, I replaced all 4 rectifiers, ESR'd caps in the power supply and output rails. No change.

I'm getting ready to swap out the plus and minus 15V regulators next although, there is a clean +15 and -15 volts present.
I will forge ahead.
TIA for any advice scan
The signal source is NOT grounded to the ground terminal of the power supply.

When I plug the RCAs in 'before' I power it up, IT DOES go into protect Mode.

When I unplug the RCAs from the amp, the resistance between the amp's RCA shields and the chassis ground is 258.2K

Between the test deck's RCA shields and chassis ground is .888meg

(Both of these resistance readings took some "settle time"
I assume due to the capacitance in the circuits)

I really appreciate yout time. ....BTW, The amp didn't go into protect when I use a "home" cassette deck for a signal source.
DC issues or what ???
>>What happens if you ground the RCA shields of the amplifier to chassis ground?<<

If I ground the amp's RCA shields to the 12Volt supply negative, it immeadiately goes into protection. When I un-ground the shields. from the the supply negative, it comes out of protect and the green "happy" led lights up again

>>What are you using for a signal source (other than the home cassette deck)?<<

I am using a set of older Pioneer head units: either the DEH-205 or a DEH-10.

>>If it's a head unit, how does it operate without being grounded to the ground terminal of the 12v power supply?<<

I switched the DEH-205 to another (isolated) DC supply and the amp did not go into protect
So Perry and Scanlines what do you think was going on here? What can we learn from this? Do different amps behave differently with home equipment and car head units. i have used anything from home tuners to dj preamps to playstation one's and even car head units and tone generators for testing amps with varying results. some had more noise than others and some amps didn't seem to like my home preamp. what is the best signal source to use at home on my bench that will be compatible with most car amps out there? my guess would be a car head unit but what else works well and what should be grounded to what?

i must say my original sony ps one with removable rca's has a very powerfull and clean signal but it is very loud on most amps even with the gain all the way down and my wife doesn't go for that past 10pm.
The following voltages are from a working amp (not a PPI). Check the voltages with and without the shield grounded to the ground terminal of the amp to see what changes.

You may want to confirm that the negative speaker terminal is within ~220 ohms of the RCA shield.

I'm not sure what's going on here. The problem with the amp could be any number of things at this point.

The isolated signal sources are probably not causing it to go into protect because they're not dragging the shield ground to 0v (where it should be). When he plugs the signal source into the amp, the entire source unit's ground and power supply voltage are probably being shifted up 5v (with reference to the ground terminal of the amp).
that's exactly what i have noticed. i think i will dig out a decent old car head unit to use as my bench source. is that what you would reccomend or is there something more flexible? i also have a variable frequency sine wave generator that has a ground lug. should that be grounded to the chassis of each amp i test or directly to the power ground input?

Sorry for the thread jack Scanlines but it is along the same lines i think
A head unit will certainly work. I use an old home receiver as a signal source. The chassis ground of the signal source needs to be connected to the ground terminal of the power supply you use to power the amplifier. You should fuse the shield ground for the signal source with a 1 amp fuse. This will help protect the signal source from amplifiers with shorted transformers and for the times when you allow the RCA shield contact a power source.
great advice! i have had a couple of amps that sent a very loud feedback into my test speakers when hooked up to my old DJ pre-amp/mixer that was running to a home cd player. i did not have the chassis ground of the signal source connected to the ground terminal of the power supply I use. could it have been a shorted transformer or the ungrounded signal source? i think the amps in question were both older japan made kenwoods for some reason. luckily my setup had some king of protection circuit and once i reset everything it still played on other amps.

Using the working voltage model that you were so kind to provide, now, has got me scurring to try to find data on KPC127B chip (IC303). I believe it to be an opto isolator (8-pin dip) I just need to know the set up of that IC. The Soundstream schemo is kinda non-descript.

Here's what I found; Most of the voltage readings on my amp
are within .6 volt of the model except 2 ....the comp (pin 9)of IC301 ( PWM) chip and pin 6 of IC303.

The comp pin should be at 2.93v but, mine reads 5.64v the reading I should have for pin 6 of IC303 is 2.386v instead of 5.48v. looking at the schem. one of the collectors of the opto pak (pin 6) is keeping the comp pin up to 5v.

I'm trying to figure out which pins of the KPC127B (IC303D) is the led that's cutting off that transistor. this could lead me directly to the cause of all this misery. Datasheets for the KP.... seem a bit hard to find. Still looking.

BTW, the speaker output neg. is at 218.2 ohms of RCA shields.


Perry is right on the money.
I used to use only an isolated "home" cassette deck for a signal source. It was okay for most purposes but you could easily miss problems like the one I'm having with this 1500D. Or chase problems that really don't exist. That's why I now use mainly a head unit or a variety of sources if there are any questions.

I'm trying to get to the bottom of this on the Soundstream, (with much assistance from Perry). I did have another problem like this once. It was an MA Audio. Did the same thing. Turns out it was a bad drain insulator on one the power supply mosfets. Made the whole amp case +12 volt hot !
As far as I know, the dual opto-coupler in this datasheet is the same as the ones in the amp. Sheets/Lite-On PDFs/LTV-8x6.pdf

How can you have more than 5v on pin 9? Isn't it driven from the 5.1v reg of the 3525?

The 2.931v was probably only accurate for the amp I used. The ratio of windings on the transofrmer and the rail voltage would require different voltage on the comp pin.
Thanks for the datasheet on the opto.
I think either the config. is different on the KPC127 or it's bad.

The static test shows the diode between pins 3 and 4 might be leakey. It's associated collector goes to the comp. pin of the 2535
which IS clamped to the 5.1v refference, but through R309. I'm not sure why there is and extra .45 volts on the opto side of that resistor. Another reason I'm suspecting it. Next, I'm going to pull the opto and check it out of the circuit.

I'll let you know.
This KPC127 tests okay out of the circuit.

Furthermore, the mysterious ...the .45 above 5.1v reference voltage on the comp pin of the 2535 is still there with the KPC gone.

I began looking at IC307 (labeled IC305 on the pcboard)
I see its a programable zener reference that drives the LED on the KPC. it's cathode is sitting at -28.8v.

I don't know if that is where it should be.
Right now, I've got the suspect portion of the KPC out of the circuit and the amp behaves exactly the same.

I'm begining to get a feeling that my problem is somewhere between the power supply primary and secondary. Something off-set or leakey between 12V DC and +/- 56V and it's ground.

The RCA shields are where they should be, 220 ohms away from ground. I've checked all the RC pairs connecting the different grounds together. They're fine.

The guy who owns the amp says it worked fine in his other car.
He took it out, the amp laid around for several months and he had it installed in his new car it went into protect when RCA's are connected.

I'm going to examine these power supply voltages closely.

What is the purpose of the "VPPD" voltage ??
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