Silent amp, request for help

Hi! I have a broken amplifier (in a Wharfedale SW150 subwoofer).

I'm a relative novice when it comes to electronics, but I wanted to show willing before reaching out for help.

I think I've made some progress, but now at the limits of my understanding. I would appreciate some guidance. The experts here may even immediately recognise the fault.

Sorry for the long post; being new to this I don't know which information is important and which isn't. It's also been good as a way to make sure my own thought process is logical 🙂

Here's what happens on power on:

• power light comes on
• a small 'click' can be heard from the board approx. 1 second later
• but no audio -- complete silence.

Now I did have a way of getting some audio:

• turn the volume up higher and higher until the amp kicks in
• put the volume back where I want it
• unit works until power off.

But in the past few weeks this 'trick' has become increasingly ineffective, and now it no longer works at all.

This sounds likely to be very repairable and hopefully that 'trick' the key to what is wrong.

What I have done is:

• Got hold of a schematic from this forum (thanks!) It seems this is actually a "PC12 DX" and I cross referenced the schematic with the codes of some of the components and it seems to match.
  
• Visually inspected for any blown or bad components, but I can't see anything obvious.
  
• Looked out for any dry joints, and re-soldered quite a few of them just in case; this hasn't helped though.
  
• Verified that there is 15V and -15V from the PSU as expected, by applying a multimeter on CON202.

Looking at the schematic, I've tried to identify some parts. Remember I am fairly new to electronics, so very likely to be wrong and looking to be corrected 🙂

• Something that is possibly a second-order low pass filter, using pairs of resistors and capacitors.
  
• Op-amps which use some kind of feedback loop.
  
• A transformer that generates 250V AC and a bridge rectifier and possibly some kind of smoothing, but not sure how we get to 15V.
  
• A class AB amplifier, identified by the pair of transisters. Although the device seems to have 4 large transistors even though it's mono; is this a way to increase the power by summing two amplifier circuits in some way?
  
• A relay exactly on the output. I think this might be the 'click' I hear from the board on power on -- which I imagine is to avoid some kind of loud or nasty noise from the amplifier itself at power on.

Now I'm going to try and apply some logic:

• I can hear the relay 'click' and see 15V and the red power LED.
  
  So I think the power supply circuitry is in good health, and the unit is not being held back by a fault in the "auto power on" circuit (which I have switched to off anyway)
  
• The amp used to 'kick' in when I adjust the volume control -- VR200 I think.
  
  That would mean the fault is after the volume knob, and pretty much rules out everything left of CON202, including the low pass filter (and except for C207)
  
• I get silence, not a distorted sound.
  
  I think this means that no audio is making it to the amplifier, and not a fault in the amplifier itself -- because an intermittent fault in one of the amplifier components would lose a portion of the +ve or -ve and lead to distortion rather than silence.

So by best guess is some kind of fault exists between the volume knob and the amplifier and would appreciate people saying that I got it wrong, or suggesting what to look for next.

I have a multimeter, and also a budget electronic oscilloscope available to me. Though I've been reluctant to start attaching things until I have a clear plan of what I'm looking for and why; as I don't want to get mixed up in the high voltages and do damage to me or my tools 🙂

Any help appreciated -- many thanks.

Mark

Thanks for the suggestions.

Limited time tonight, but I took a look.

Ian, thanks for the note on there being two relays. I had not realised this. This device does actually have relay RL100, it's black about 2/3 size of the blue one, hidden below one of the large capacitors (the left hand one.)

I can see the logic of assuming fault in the 'auto power' circuit but there's one piece of evidence which suggests otherwise: it's connected to AT-SIGNAL which is much earlier in the signal chain. If this circuit were at fault then I think the volume 'trick' would never have worked since the two are operating on distinct parts of the system. Is this a correct assumption or did I miss something?

Because if I'm correct this rules out quite a lot of the system?

I did a couple more experiments. First I held a small electret microphone against each relay to try and work out which is making the noise. RL100 is the source of the click. RL101 appears to be silent at all times.

Also I tried disconnecting the TEMP_SW on CON102. This did not change the behaviour. I tested the disconnected component (labelled "KSD 01F H095C") with a multimeter and it is open.

If I had to decide myself what to do next it's to look at what's going on around C2. Right now my multimeter shows -45V (which is connected to the amplifier) but a moment ago it was zero -- that seems fishy to me, though perhaps it was just a poor connection to the multimeter. If I can conclude it has power at all times then we must know the audio is at fault. Unfortunately I can't listen to see if the whole sub is working while I test this because the cable for the driver is too short.

Anyway, it's too late and bedtime now. Any further advice appreciated. Thanks again.

I have no idea what country you are in as I would offer a repair service if you can't fix it.

Click to expand...

Thanks Jon. I am in the UK, but in London. I'm welcoming the opportunity to learn about amplifiers (which is long overdue!) so we'll see how we go. Thanks though -- we'll see if I have to admit defeat!

Hi, just a little update on my silent amp for completeness of the thread (and if anyone is interested)

After deciding that the voltage to the amp seemed to be reliable, I began tracing along the audio path using my budget electronic oscilloscope. What I found was a solder joint near the amp that did not look like it had nearly enough solder on it. I re-soldered this and a couple of nearby ones for luck and this seemed to make a difference.

So, my amp has been solid and reliable so far! This wasn't perhaps the adventurous repair I thought I might need, but it's a repair nevertheless.

Posting on the forum like this an excellent way to ensure it's bound to break again, just the moment I hit 'send'... 🙂 But I hope not.

Thanks again for your help

mk1 said:

Posting on the forum like this an excellent way to ensure it's bound to break again, just the moment I hit 'send'... 🙂 But I hope not.

Click to expand...

Gah, and it didn't happen a moment after, but it did take a week or so.

Now I'm in a position where the amp doesn't appear to work at all, unless I take the circuitry out, wave a screwdriver at it, put it back, power it on. Needless to say this is quite time consuming.

Back to the drawing board...