Monitor Audio RSW-12 Plate Amp Popping/Crackling

[bmor]

Hello all.

I need some help fixing this amp, I'm at the limits of my knowledge. The amp developed a hum a while back so I decided to change out the electrolytic caps which seemed to be a known issue. That worked well, got rid of the hum, but I now have popping/crackling when the amp board powers on, with no signal input to the amp.

I've checked for bad solder joints and fixed those I thought were suspect, pulled and tested the transistors from the AC-Shift board, pulled resistors where needed to test those in circuit better, measured other things everywhere that I could and cannot fix this thing.

I'm not sure where to go from here. Any thoughts would be much appreciated. I posted the schematics for this amp a few days ago.

Thanks,
Bruce Morton

[redwire]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

If a new problem shows up after replacing capacitors, it means I've made a mistake doing that.
I would double-check the replacement electrolytics are installed good, no solder bridging or reverse polarity. You can easily measure DC on each leg to confirm none are backwards.
Audio crackling and noise can be from a semiconductor- or one that is getting excess DC from a bad blocking capacitor. Also check the power supply rails +/-12V and the +15V AC shift, that transistor Q11 on heatsink looks like it ran too hot and might be faded near dead.

If you have a signal tracer or some audio amp that you can probe the signal in a few places, to narrow it down.
The preamp limiter C27/Q2 I would make sure are U5A,B should be off so -ve V at the gate.
You can listen to the preamp output CON1 SG to see if it is where the noise is. Next would be muting, on the amp board U10B Q10/C10.

I would first suspect the new parts have a solder bridge somewhere. Use an old toothbrush to get solder balls and specs off the boards.

Schematics were posted here.

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Hi Redwire, I agree with your first statement. I had already gone back and pulled them out and re-installed the caps on the amp board after I realized I didn't pay attention to it being a double sided board. I will check everything again.

The Q10 (mislabeled as Q11 on the schematic) does run very hot. I wasn't sure if the board dis-colouration was normal for the age of it. I had pulled it and tested it with diode tester on multi-meter and HFE and it seemed good, but I can get a replacement easily this weekend so I'll change it.

I don't have a signal tracer but will look into getting one. My problem is I don't have the experience to know how to probe circuit boards like this, especially without a service manual. I'm comfortable around electricity being a retired electrician/designer but have never really worked with electronics in detail, though am trying to learn. My first question is, bear with me please, where should I check the 12V and 15v. I'm not sure of what I should be seeing where.

Meanwhile, I will check all the new work again.

[lotas]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

You need to check these tracks, they may be broken (this happens over time after the Chinese glue, even metal (copper) starts to corrode under it).

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Lotas, will do. Looks like I removed dried glue from those areas a while ago and maybe caused some damage.

....I've checked the traces, they seem to be ok.

[petehall347]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Popping/Crackling is usually a transistor .. the transistor will test good with a tester but fail under normal use .
try heating areas and cooling transistors .

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Petehall347, I'll pickup some freeze spray on the weekend and read up on testing that way.

[redwire]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Q10 if it tests OK I would leave alone then. It supplies 15V for the power-on circuit and relay on that board, I don't see it powering anything in the audio sections.
The voltage regulators appear to be U15,U16,U17 I did not see them on any posted schematics but +/-12V and another voltage comes from those three TO-220 packaged IC's.
I could be wrong, but that is a place to check the audio power supply voltages are OK.

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

I was wondering the same thing about the AC-Shift board and whether anything on there could cause the noise.
The voltage regulators are shown on dwg. AMP-041229, there’s 2 pages. Connector P12 assoc. with U15 and U16 connects to P10 on the AC-Shift board.
I’ll check the voltages tomorrow morning.

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

The outputs from regulators U15,16,17 all appear to be good.
I can pickup a signal tracer tomorrow but have never used one before. May need some help with that.

[momaka]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Capacitor C14 on the power shift board, right next to that hot transistor Q10, seems to still be original. It may not be the problem, but I strongly suggest changing it. After so many years, it more than likely has gone bad, especially with all of the heat around it.

Also, this: \/

You need to check these tracks, they may be broken (this happens over time after the Chinese glue, even metal (copper) starts to corrode under it).

Don't just check the tracks, but remove their solder mask on top and coat with fresh solder. This is the only way to know you have a good connection.

On that note, see the multi-colored set of wires going to that board and the brown glue on them - remove all of that glue and check the tracks/traces under it. This brown glue is absolutely terrible and so it's best to remove absolutely every single little piece of it there is.

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Momaka, C14 is new, I replaced all the caps, but I will check it and all the others with the LCR meter. I also picked up a replacement for Q10 and all the other transistors on the board but have yet to install them.

I’ll also remove the last of the glue and check all the tracks. What’s the best way to remove solder mask, just scrape it gently with a blade?

I used freeze spray on the transistors and most other items on the AC-Shift board with no results. I then used it on the AMP board concentrating on the SMD’s. There was no immediate improvement but it did cause the relay K10 to shut off a couple of times. Then after I had finished spraying most areas and left everything energized for a while it went quiet with no static! There was just a slight hum with my ear against the speaker. This lasted for a day or two but now it’s back to making noise so I’m scratching my head again.

I’m in the process of putting together a signal tracer but never used one before. I'll start with what Redwire was suggesting in an earlier post.

[redwire]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

This Badcaps thread (post #10 schematic) on making a signal tracer using a cheap LM386 board from eBay or Ali. Just add a cap, volume control likely there already (hopefully higher than 10k ohms) and the two clamp diodes.
You 'll have just a single probe (and common GND) to touch around and listen for the signal or noise. It's protected to not cause damage to anything and worth the few bucks to have one.

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

Redwire, thanks for the tracer link. I decided just to buy a kit, Velleman K7000, and have it together now. Yesterday I checked the things you mentioned earlier:

- “The preamp limiter C27/Q2 I would make sure are U5A,B should be off so -ve V at the gate.” I measured -5V at Q2 gate and the function of Q2 checked out ok with diode tester.

- “ You can listen to the preamp output CON1 SG to see if it is where the noise is. Next would be muting, on the amp board U10B Q10/C10.” No noise at these points and clear through C15 & C16 to #1 & 2 input of U11.

When probing I'm connected to the ground plane on the board. I'm not sure how to safely proceed beyond this point.

[bmor]

Re: Monitor Audio RSW-12 Plate Amp Popping/Crackling

I think I found my problem. SMD resistor R31 is reading 56K instead of 16K. I replaced it with a 1/4W resistor I had on hand and the noise has cleared up. I was able to use the signal tracer to narrow down the problem to somewhere between the master PWM module U11 and the mosfets.
Thanks to those who helped me with their suggestions, I learned a lot working on this.