PEC Industries A900 (Citronic Conquest V9) power amplifier

[richardash1981]

It says "PEC Industries A900" on the outside but inside the boards say Citronic and it is the same as a Conquest V9. I bought this amp second hand quite a few years ago and it has been working fine (after replacing the fans). For the last few years it has been doing about one gig per month, but different location every time, so lifted and shifted. Speakers are QTXsound QT10 10" + pizeo "disco boxes", 8 ohms rated at 200W peak, so the amp may drive full voltage swing but only half the design load current.

Last week it was out on a job when the sound went very quiet and distorted, the channel A fan switched up to full speed (which I have never know it do) and that side of the amp got very hot indeed. This was of course right at the end of sound check ...

I got the lid off, removed the main rail fuses from Channel A (main PSU is shared, but separately fused). Channel B then worked fine, and the heat and fan on the other side gone. I was able to drive both side speakers in parallel from Channel B (mixed in mono) to get the job done. In theory, no loss of power as the amp was now seeing 4 ohms load - smaller church didn't need flat out power anyway.

The schematic (which covers V12, V9, V4.5 models) can be found here https://www.eserviceinfo.com/downloadsm/167028/CITRONIC_Citronic Conquest 12 pwr sch.html I've now had a look inside an no obvious signs of what has gone wrong - nothing looks singed/burnt inside. Someone has carried out some repairs in the past, few components changed, but reasonably done.
Some photos here: https://www.flickr.com/photos/30661871@N03/albums/72177720317081608 

I'm not sure where to go next. From the schematic, the thermal protection and the output crowbar are completely separate. The crowbar (page 3 and top right of page 7) fires a BTA12 200 triac across the output terminals (T0220 package, looks fine in my third photo). I presume that this is meant to be followed by the PSU rail fuses blowing if there is an output device failure? The rail fuses were not blown, so assume that this wasn't what was going on.

Thermal protection is top left of page 7, CON1 pins 5 and 6 go to a 100 degrees C thermal switch on the heatsink, it looks to me like if this opens then the power to the input stage is shut off - which would make sense of low volume and distortion! I presume the expectation is that this would be caused by overdriving / blocked ventilation, and so muting the amp is the right thing to do. This also seems to be where the "thermal" LED on the front is driven from (I think this was lit up when I switched off from the fault, but can't be 100% sure).

So I suppose two questions:
1. Any ideas where to start on fault-finding to fix it?
2. Am I wasting my time and should replace either the amp or the system (including speakers)? I'm not 100% happy with the quality of the sound (it's powerful enough) and needs some fairly substantial EQ (which I'm sure is the speakers).

 

[J Pearce]

Going quiet and getting hot suggests a fault in the output stage.
If repairing the amp costs more than £30 I'd move on. You wouldn't need to spend a lot to achieve a significant upgrade. 

[themadhippy]

Quote

1. Any ideas where to start on fault-finding to fix it?

fit a dummy load ,fire in a test tone,crank the volume and follow the smoke,or  as its a dual channel device you could swap components   over side to side until the fault changes side.If that dont apeal time to  get the test gear out and start testing individual  transistors etc.

[richardash1981]

I spent today with signal generator and dummy loads. Needless to say, the only thing which could be persuaded to misbehave was the stereo/mono switch on the back (I know what to do about - but it means taking the main PCB out of the amplifier, which in turn means both heatsinks would have come come out, and most of the internal wiring loom (lots of spade connectors and headers). Anyway, some measurements for interest (all with 1kHz sine wave):

No drive, PSU rails +/- 88V DC (mains ~242V RMS).

1 channel driven into 8.25 ohms resistive load - 42V RMS = 225W (rated power), rails drop to 83V, no clip light. Ran for 10 minutes like this without the fans going into high speed (later found not long enough).

1 channel driven into 8.25 ohms resistive load - 52V RMS = 325W, rails dropped to 81V and just on the edge of the clip light. Scope on output indicates this is pretty much the clipping point. Ran for 5 minutes then stopped because the dummy load is only rated for 200W! Stayed on low fan again. Mains draw on this was 551W / 740 VA according to a plug-in mains meter (I think a Maplin one).

Soak test - both channels driven to 20V RMS = 50W. Took about 10 minutes for the fans to spin up into high speed (one channel took longer because the stereo/mono switch had dropped the signal 17 until I poked it). After that carried on for the rest of the 1 hour with no change (pretty warm, but case still touchable). After stopping the signal, fans spun back down to low 3 minutes later. At no point did the thermal indicators come on or the 100 degree heatsink temperature trips activate.

 

I also wondered if the amplifier had been oscillating (ultrasonically) and that might explain the symptoms. However despite some time playing with connecting one of the hot/cold input terminals etc. whilst checking the output on a scope, I was unable to get it to misbehave at all. This is frustrating because I am fairly sure I have heard the amp "chirp" when plugging/unplugging the input signal leads, and wonder if the two are connected.

So it has gone back together again, and we shall see how things go - I will be trying to get it out and run it again off-line before the next gig is due.