Car Audio Amp Troubleshooting Help

-
71340Duster

71340Duster

FABO Gold Member
FABO Gold Member
Joined
Apr 15, 2014
Messages
11,836
Reaction score
17,249
Location
Oak Harbor WA
I have a good old ADS PH15 6 channel amp that is cycling. I pulled it and starting probing around. I haven't been able to find a schematic yet so I'm just reading resistance, doing visual inspection, pulling plugs etc. It plays for 1 sec., I get audio in all 6 channels, then it senses an overcurrent condition and cycles off and back on. I've read and looked at a lot of components, haven't seen any shorts or differences between components. One thing I did do that made a difference was pull the plug I'm pointing to which powers the audio board, it does not cycle when that plug is pulled. I don't know if this is because there is too much load somewhere on the audio board or if that's because the power supply board is not loaded and once it is something on the P/S board breaks down. This amp sounds great, they don't really make them like this anymore so I'm going to do my best to fix it. Any overall tricks or suggestions would be appreciated. BTW I've looked at the backside of the P/S board but not the audio board yet, that's next.

IMG_3371.jpg
 
The radio in my truck does the same
Somewhere in the wiring to the drivers side front speaker there is a short

I couldn't be bothered to fix it, so I cut and capped those two wires right at the head unit and called it good

Point being, check those speaker wires
 
I have removed the amp from my suburban where it was powering 6 speakers. It's doing the same thing on my "bench" as I'm running it with a fairly heavy power supply that displays either voltage or current and with no speakers connected I can see it draw either 5 amps or come up to 12vdc then cycle. I really need a schematic, I read voltages coming out of the power supply to the 3 pin plug that goes over to the audio board. Red Black and Blue wires with 12v, 5v, and 30vdc. I'm kind of wondering if the black wire shouldn't be a ground, I need trace it back. The amp has dual 20 amp fuses with dual positive and ground inputs and the fuses are not blowing.

IMG_3372.jpg
 
I overlooked what could possibly be electrolyte at the base of these two 35v capacitors. I guess I thought it was conformal coating or flux from the soldering or maybe glue but a leaking cap makes more sense considering the age of this amp. Going to give it a shot, can't find anything else so far.

IMG_3374.jpg


IMG_3375.jpg
 
A swing and a miss! Those were glue, the cans weren't split or bulging anywhere in the base of the component. They also ohm'd identically so I soldered them back in but before I did I put the amp back together and it still cycled with them removed. I'm now looking at two large voltage regulators (I think that's what they are). They are on the input and I've seen before things fail on the input and the output more than the middle sometimes.
 
Took this into the shop to work on if I'm in there during lunch break, co- worker was poking around in it getting ready to signal trace with an O'scope when ZAP it stopped powering up. We had the small connector that was the overcurrent sense wire lifted, not smart to run for very long like that, he did, not thinking about it. We'll get it, found out that the final amps are still made, but they are looking fine so far. I am going to take diymirage's advice though and read the speaker wires both between for 4-8 ohms and each lead to ground. That would point me at the right output of the 6 channels if one is hurt but not really showing resistance issues.
 
Got it back to cyling although it cycles faster after a suspect transistor was replaced with one that wasn't quite the same spec so Monday he'll swap it back. Good news for the day is found schematics online, although they are for a PQ-10, they are almost identical except PH15 has more channels. It's a switched power supply and we found an IC that's a generator that does not have the proper voltage being supplied once the turn on, or remote voltage is applied, so that's the area we're in now.
 
Today we split the audio board and power supply board by providing external voltages to the audio board. The positive 25vdc blew a 4 amp fuse when we triggered the remote directly on the chip so we took another look at the finals. Although I thought they read the same resistance wise, there was one channel that acted a little differently so I removed both transistors and the amp played audio. Put it all back together with the two components missing, it played for awhile and now the power supply (switched) has died so back to it when we have time, probably not this week. I have yet to read the speaker wires in the rig but will take a hard look at CH 1 when I do.

IMG_3430.jpg
 
The pre amp for CH 1 got very hot without the finals soldered in. The spec sheet I found on DIY Audio has you solder in a couple of resistors in their place for troubleshooting purposes so we did that. I also saw a "condition unknown" ADS PH15 on Ebay so bought it for parts, should be here in a week or so.
 
Without "at the least" a diagram on something like this, I'm lost, other than looking for obvious.......shorted transistors and caps, etc
 
67Dart273 said:
Without "at the least" a diagram on something like this, I'm lost, other than looking for obvious.......shorted transistors and caps, etc
Click to expand...

Yeah, I don't have a schematic either, although I do have one from an ADS PQ-10 which is very similar but a 4 channel amp. It's fairly basic because it's old school, switched power supply that gives a plus and minus 25vdc with an overcurrent, mute circuit. And there's the audio section, 6 channels of pre amp with 6 finals. Right now we have an external p/s jumped in and have audio playing except channel 1 which likely was the first thing that crapped. May still be the CH 1 preamp stage that's tripping the overcurrent, and the p/s could be fine, time will tell.
 
"Feel test" for hot parts? Infrared thermometer?
 
67Dart273 said:
"Feel test" for hot parts? Infrared thermometer?
Click to expand...

That's a pretty good idea, I don't have one but could get one. Both me and my coworker used to shoot Freon on the transistors, IC's etc., can't really do that anymore : ) Also not at our disposal is a Huntron Tracker although we do have an O'scope and I did find a home grown Octopus kit.
 
scan0002.jpg
scan0003.jpg
scan0004.jpg
Here's some schematics of the PQ-10, PH-15 very similar, same vintage. Poor quality from ADS so don't know how'll they'll post.
 
The connection code diagram shows pinouts of diodes and transistors. A multi-meter in diode setting is often used to check junctions and shorts. Study up how to do that if you do not know. It is easy, but sometimes a pin or two might need to be lifted to open circuit.

Test loading of the +,- 25V supply will help isolate if the problem is in the power supply or amp channels. Since channels are the same, if any work, spot comparing non-working will help trouble shoot, also disconnecting channel power feeds to take bad off line, placing on good one at a time.

Most common failures, electrolytic capacitors shorts or reduced values, shorted semiconductors, or open power resistors, bad solder joints or shorted thermal insulators on transistor collectors.

Much better amps today that are greatly more power efficient, and compact. .....
 
Last edited:
Yes, we were looking around for the correct value/high enough wattage resistors to do a test load on the power supply. There are no shorted or leaking caps that I can found and all PN junctions have been looked out using the diode checker scale, forward and reverse.....that is how we found the hurt channel one finals. I have an amp inbound from Ebay, condition unknown, we will likely run it and see how it's P/S is doing, we can then rob a few components here and there from the audio section if they ring out good. Also, I've looked at the boards both sides under magnification, no bad joints that I can see. Not sure if I agree that the amps today are much better, at least for me. No PPO power ratings and BS distortion numbers on this old amp.....output devices are rated at 8 amps each etc....
 
A class D (digital) amp is very efficient, the class A is linear with transistors in the active mode. In the active mode the transistors are biased and consume current even with zero input signal.

The problem your are having might be related to bias or dc offset issue. It might be age related component drift, or leaky capacitors. The triangular symbols in the schematic is the opamp gain stage. With zero signal, the place where the emitters of the power transistors join with resistors should be near zero. A bias problem will make an offset, and if large enough, will hit power supply limit, or burn speaker coil.

I have developed both linear and digital amps, but not for audio. They were used in precision motion controls.
 
Thanks for the pointers, there are bias issues in the amplifier stage. I will check the points you mention and see if something's showing up there. It's been awhile since my schools and since I've played with older class A audio amps (back when I was working on it in the Navy).
I understand class D is extremely efficient, but I don't consider them a "digital" amp as that class of operation can occur with an analog or digital input. Even with the great enemy of heat and as inefficient as a class A is they still have low distortion so they're still the best for my ears.
I'm glad you mentioned the possible burnt speaker coil, I know I had an issue with my Channel 1 and I need to check the resistance of the speakers across the coil and to ground.
 
So here's my spare parts amp. There were some huge critter legs and a torso of some sort, probably a cockroach but I don't know, amp was shipped from MO. A couple of missing screws and the input DIN connector has been home fabricated (common). Will wait until I have time to lift the boards and take a better look, plus some static resistance checks before I let the magic smoke out : )

IMG_3471.jpg
 
Running an amp with no load is not good. Get yourself one those cheap component checkers, they are soooo cool! My amp will go into 'Protect' if one of the speakers is shorted. but wont cycle, it will just sit there with a red LED waiting for a reset.
electronic component tester
 
I like that tester, be interesting to see what it can do, thanks. Today we used an old school audio generator to do some signal tracing. The "parts" amp that I got off ebay was in fairly bad shape audio board wise, the power supply seems good. The amps only draw a little over an amp when revving in neutral, we don't put any input, music or signal on them unless we have a speaker on the output.
One thing I didn't notice right away last night was the previous owner put the amp back together with the insulating on top of the power supply vice under the audio board.

IMG_3473 (1).jpg


IMG_3474.jpg


IMG_3471 (1).jpg
 
71340Duster said:
I understand class D is extremely efficient, but I don't consider them a "digital" amp as that class of operation can occur with an analog or digital input. Even with the great enemy of heat and as inefficient as a class A is they still have low distortion so they're still the best for my ears.
Click to expand...
They are digital, because the output is pulse on and off, never in the active mode. The switching happens at high frequencies beyond hearing, and filtered. Signal processing for equalizer, noise canceling and other effects done in digital realm. Also nearly all music today stored digital format.
 
Okay, I understand the class of operation, I understand that they achieve about 90 percent efficiency. The first class D amp was built in 1961 I believe, sorry, I still believe that it is a misconception to call a class D amp a "digital" amp. Thanks again for your help with troubleshooting, today was mostly checking out my new parts amp and didn't have much time to check biasing. One thing I did get done was patch the new amps power supply over to the original amps audio board (after changing out two output trannys on CH1. There wasn't anything on the base of the replaced output trans, power supply is cycling so more troubleshooting in the future.
 
The power supply function is to boost the DC voltage. It does that by using transistors to chop the DC into AC, then use transformer. The system has feedback for regulation. If you do not understand D, digital switch mode control, you are limiting yourself. The power supply is similar .... same with regulators used in alternators. They are not linear, they switch at a fast rate, the inductance of rotor averages current.
 
No workie on the amp today, they took a substation offline and it put 4 local communication sites on battery (-48vdc) so I had my NOC bugging me about the little red lights they were seeing. Between that and the phone I was working on at the top of the dam, no time for the fun stuff!
 
You must log in or register to reply here.
-

Back
Top