Parasound HCA-1500A - I need some help (noob warning)

[Muddler]

I have a Parasound HCA-1500A that works great EXCEPT it will not turn off. As soon as it is plugged in it powers up. I assumed the power switch was bad, but it works just fine.

I also have an HCA-1000A, a smaller sibling that is working properly. The two amps appear very similar in design. On the HCA-1000A, the 12v trigger turns the amp on/off when the power switch is in the OFF position. You can force it ON manually by using the power switch (no 12V signal) but when 12V is going to the trigger it is ON regardless of the position of the power switch.

So, I am assuming something in the trigger mechanism of the HCA-1500A is the problem. There is a small circuit board (B2494A) right behind the power switch. 120V AC comes into the board and then back out, supposedly when the power switch is ON or trigger has 12V.

On this board are a few jumpers. One is for the power switch wires and another for 12 V trigger wires. I have unplugged both of these jumpers (switch and trigger) and the amp still powers up when plugged in. Kinda seems like the 12V trigger relay? is stuck in the ON position, but there is not continuity between the incoming and outgoing power lines when not powered up.

Has anyone ever seen this before? Obviously, I am a novice at best, with a multimeter and a schematic which I guess just makes me dangerous. I am at a loss for how to go about finding the bad part that needs replacing, nothing appears to be visibly damaged. Again, I do have a HCA-1000A that I could open up and use for comparison purposes.

Thanks in advance for any thoughts anyone may have or things I could try to help isolate the problem. Attached are some pics that folks wiser then me may find useful.

[john curl]

It is probably the relay. You have to check the drive voltage to the relay coils. If it changes with on-off switch, then the relay is stuck. If no change in voltage when the on-off switch, then it is in the relay drive circuits. You should really contact Tony@ Parasound.com for best info. He is the expert.

[Nrik]

I am a novice at best, with a multimeter and a schematic which I guess just makes me dangerous.

haha - excellent

[Muddler]

Wow, John thanks for your time and the quick reply. I do realize you are the designer (says so right on the board) and not a repair technician. Regardless, it's kind of like saying a prayer and God responding in person. I sent an email to Tony as you suggested, I appreciate the lead.

Now, I have no idea what you said in your post but I am sure it is profound. I assume the black box next to the transformer is the relay. It has four legs into the board. Both pairs have continuity when it is powered off. When powered up, the legs closest to the board edge measure 12V and the legs closer the transformer read 45mV. Power switch has no effect on these readings, they remain constant.

I am not sure if what I measured has any relation to what you suggested I check, but I do try.

Nrik - Floyd's Animals is one of my favorites. Cheers...

[Muddler]

Replaced the relay - No Luck. Relay Drive Transistor also tested fine. As did every component on the board.

My eyes just aint what they used be, I finally found a solder wisk on the bottom of the board oh so slightly bridging the power jumper pins. Not enough to register continuity when testing but obviously enough to conduct when plugged in.

Thanks again...