power amp turns on by its self out of standby mode? - Page 1
Forum section: electronics / projects

author: p5200
author_rank: Contributor
timestamp: March 02, 2014, 02:22:47 pm
content: This amp did this before, at first I could plug it in and unplug to use it then later, the standby relay and both speaker relays starting clicking in an out randomly. I got fed up and unplugged it for a few weeks. Then I decided to fiddle with it some more and plugged it in and it didn't come on? I pushed the power button and it came on normally and played fine every day all day for about a month. Then, when turned off/standby mode it started trying to turn it's self on then the channel indicators faded back off and it was back in standby with the standby led lit up as normal and these things also, were random. I checked the power relay several times with an ohm meter for stuck in closed position and it is fine. I replaced all the transistors, diodes, and the digital flip flop chip. I also checked the momentary on/off push button switch and no problems. Here is a schematic I received from parasound co. I also have two more but, I really think the problem is in this board but, I'm limited in my skills of understanding how things are controlled in schematic and troubleshooting in general. I appreciate all advice and help. Thanks!

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2014, 03:45:22 pm
content: Have you tried covering up the IR sensor to eliminate the possibility of stray signals from other remotes, sunlight, things like that?

author: fcb
author_rank: Super Contributor
timestamp: March 02, 2014, 04:05:53 pm
content: Quote from: IanB on March 02, 2014, 03:45:22 pmWhere is the IR sensor on the schematic?

author: p5200
author_rank: Contributor
timestamp: March 02, 2014, 04:09:37 pm
content: Quote from: IanB on March 02, 2014, 03:45:22 pmThis amp doesn't have remote control or a sensor that I can see unless I'm over looking it I bought the amp used it's about 4-5 years old I believe.

author: p5200
author_rank: Contributor
timestamp: March 02, 2014, 04:20:19 pm
content: Here are the other schematics I have. I suspected maybe the trigger board but while I had it out I plugged in the amp and it still, came on and all relays activated.

author: p5200
author_rank: Contributor
timestamp: March 02, 2014, 04:34:25 pm
content: Here are the general features of the amp.http://www.audioadvisor.com/Parasound-2125-125-Watt-THX-Ultra2-Two-Channel-Amplifier/productinfo/PAC2125/#.UxNcmOLhFkg

author: commongrounder
author_rank: Frequent Contributor
timestamp: March 02, 2014, 06:33:24 pm
content: Okay. If this were on my bench, the first thing I would do is make sure the standby power supply voltages are normal. You need stable +/- 15 volts for the chips to work properly. If those are okay, I would monitor the "mute" pin on the trigger board with a scope to see if there are any transient pulses showing up there. That could cause a false trigger of the power relay. This unit is meant to power up on the presence of either an audio signal, a 12 volt control voltage or just a manual button push, depending on the switch position. There shouldn't be any activity on the "mute" pin minus those signals. Be sure to check for loose or intermittent contacts on connectors, as well. You need to divide and conquer. A scope may be almost essential to see what is going on, but a fast responding multi-meter is better than nothing. Good Luck!

author: p5200
author_rank: Contributor
timestamp: March 02, 2014, 06:45:30 pm
content: Quote from: commongrounder on March 02, 2014, 06:33:24 pmHi commongrounder, I don't have a scope never even used one but, something I always wanted to get and learn how to use. I took the trigger board out yesterday checking components on it and thought, I think I'll plug it back in to see if the amp still came on and sure enough, it did that's when I was beginning to think ghosts or voodoo.Thanks!

author: p5200
author_rank: Contributor
timestamp: March 03, 2014, 12:20:13 am
content: I put the power/ standby board back in plugged the amp back in and came right on again. I felt the tops of all the transistors on that board and Q212 was very hot to the touch next I measured 14.84v. from D219 to ground but, got 0v from D220 to ground is that the proper readings I should be getting from these zeners? or is D220 faulty or maybe, another component dragging it down?

author: p5200
author_rank: Contributor
timestamp: March 03, 2014, 01:33:03 am
content: Just a thought maybe a dumb one but, would it be possible to convert to a standard on/off power switch and do away with the electronic on/off circuit without creating a harsh thump through my speakers at turn on and off? Just a thought in case I can't find the solution to this problem? I can't really afford a new amp and this one sounds great when its working.

author: commongrounder
author_rank: Frequent Contributor
timestamp: March 03, 2014, 03:06:08 pm
content: You should get a negative 15volt or so reading across that zener. With the power off, check and see if it is shorted using an ohmmeter or diode tester. Pull it and check again to confirm if it is zero ohms, since C220 could also be shorted. The pass transistor (Q212) would be hot because it would be forced to drop the entire supply voltage. It is possible there is an abnormal load on the -15v rail, but the zener should still have some voltage across it. My bet is the zener has gone short. Replace it with a 1 watt rated 15volt device.If you decided to bypass the entire standby circuit, you could break into the relay control by cutting the trace, or lifting the collector leg of Q217. Connect a switch between ground and R228 (that previously was connected to the afformentioned Q217 collector). When the switch is closed, that will activate the power relay driver circuit and turn on the amp. The standby lamp and push switch won't function anymore.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 03, 2014, 06:18:45 pm
content: You could hardwire the control to on and install a mains switch.

author: p5200
author_rank: Contributor
timestamp: March 03, 2014, 07:01:21 pm
content: Quote from: NiHaoMike on March 03, 2014, 06:18:45 pmThanks for the tip, not exactly sure how to go about it but will sure try to do that before I'll junk it.

author: p5200
author_rank: Contributor
timestamp: March 03, 2014, 07:09:34 pm
content: Quote from: commongrounder on March 03, 2014, 03:06:08 pmThat's great info, thank you very much!

author: p5200
author_rank: Contributor
timestamp: March 03, 2014, 08:20:05 pm
content: I checked D220 -15v zener again and there is a voltage but, it's only -3v. across it so I guess there is a drain or short somewhere.

author: commongrounder
author_rank: Frequent Contributor
timestamp: March 03, 2014, 09:44:40 pm
content: It could still be a bad zener, or the possible heavy load. The easiest way to check might be to pull the connector "CN20A" if that is physically possible. Do this with the AC power disconnected. Then power up the amp and measure the voltage at C228 (negative) and see if it springs back up to around 15 volts or so. If it is still low, check the voltage at C226 (negative terminal), the main input filter cap, and see if there is minus 24 volts there. If it is okay, then the problem is likely the zener, (or possibly R232, R226, C219, or C228). If it is low, either C226 has failed or bridge rectifier D222 has an open diode in it. A scope would show all of this very clearly, but in the absence of one, you have to infer from your meter measurements. You can also perform an AC voltage measurement across C226. If it has failed there would be very high DC ripple which has a strong AC component. It would measure in AC volts, not millivolts. If all of this power supply springs back to normal voltages, then you are off to the other board to check for failed (shorted) caps or other parts. I realize this is a lot at once, and will be tedious work, but at least you know the unit is misbehaving for a reason. If you are systematic, you will get to the bottom of it!

author: p5200
author_rank: Contributor
timestamp: March 03, 2014, 11:52:37 pm
content: I wondered if maybe the zener might be allowing too much voltage to pass through it. I can unplug the connector as you suggested and check the other components you mentioned. I'm slow due to health problems and have to do a little at a time. I really need to get another lighted magnifier with my Diabetes, Cataracts and Glaucoma I use a magnifying glass and a flash light to be safe from solder bridges and such.I'm going to do a little more today and see if I can find anything and I sure do appreciate your time and great suggestions!

author: commongrounder
author_rank: Frequent Contributor
timestamp: March 04, 2014, 12:51:11 am
content: No problem. Your comment about the zener is valid. If the zener changed value, or zener voltage, to something lower (like 3 volts instead of 15), that would definitely throw off the operation of the regulator circuit. Good luck with your investigations.

author: weirdal1968
author_rank: Newbie
timestamp: March 05, 2014, 12:26:58 am
content: Start a troubleshooting thread over atwww.audiokarma.orgLots of very experienced audio techs over there and hopefully somebody has seen a similar issue pass through their workshop. Include the model number in the thread title to get the attention of Parasound techs.