Help troubleshooting crackling sound from one channel of Adcom GFP-750 preamp

[kop89]

I have an intermittent crackling sound from the right channel of my Adcom GFP750 preamp.

Characteristic of the problem:
- Crackling is intermittent, sometime quiet, sometime accompanied with a pop
- Only happens with preamp in "active" mode. In "passive" mode, the gain circuitry is bypassed via relay and only go through the volume pot.
- Crackling noise still happens even if the volume and balance pot is not turning.

I have built DIY preamp, amp, and DAC but I have no electronics troubleshooting experience and really have no clue where to start.

Thinking about giving the balance pot a contact cleaner spray, but the random crackling noise still happens even when the balance pot is not turned.

Anyone have any suggestions where to start looking? Service manual and schematics is attached.

Thank you

 

[zenmastering]

You're doing well with the sleuthing, despite what you say about your lack of experience.

With the preamp off, wiggle the volume and balance pots throughout their range, multiple times.

Swap cables (Left to Right) just to be sure it is the preamp's circuitry.

What you are describing could be anything from a dirty control to a bad connection to a dying semiconductor (transistor, IC, diode, etc.)

You can try cold-spray to isolate a potentially failing semiconductor. The cooling will reduce the crackling temporarily and often point to the exct component.

 

[RayDunzl]

If the knob crackles as you turn it...

Turn it back and forth all the way.

Again,,, and again... and again.

It should get quieter.

Keep turning.

If not, go with next idea.

 

[Specialcause]

If the knob crackles as you turn it...

Turn it back and forth all the way.

Again,,, and again... and again.

It should get quieter.

Keep turning.

If not, go with next idea.

Can you say what it is about moving knob back and forth that might make the crackles go away or become quieter?

E.g. Is there a build up of crackle-creating substances that the wiper clears away?

Would it be a good idea to move knob(s) back and forth periodically as per your instructions to keep knobs crackle-free?

 

[raindance]

It's likely to be either a bad capacitor in the unit, a dry joint, or a faulty switch. In the 80's, Luxman integrated amps suffered from tape monitor switches that would become noisy, for example. Doesn't sound like a pot issue.

 

[RayDunzl]

Can you say what it is about moving knob back and forth that might make the crackles go away or become quieter?

I don't have a science of potentiometers answer. Oxidation of contacts, dirt, aromatic deposits, whatever.

E.g. Is there a build up of crackle-creating substances that the wiper clears away?

That would be as good a guess as any. Ever cleaned something by rubbing it?

Would it be a good idea to move knob(s) back and forth periodically as per your instructions to keep knobs crackle-free?

That should be a harmless prophylactic.

---

https://support.newgatesimms.com/potentiometer-maintenance-and-lubrication-case-study/

 

[MRC01]

...
- Crackling noise still happens even if the volume and balance pot is not turning.
...

All this discussion of moving vol pots back & forth to clean them has merit in general. But the above observation makes me doubt whether it will help in this particular case.
Question: when it crackles, does the crackling increase or change in any way as you move the vol pot?
Note: many moons ago, I owned a solid state integrated amp that developed a low level crackle or buzz. Not related to the volume knob. I took it to the local shop and they found it had a bad transistor. Replaced that and it worked great for many more years.

 

[kop89]

Thank you gentlemen for the ongoing help and support.

I have just tried cleaning the balance pot with contact cleaner, and the problem still persists.

I have played around with the preamp again and characterized the problem thus far. All these observations are made in single-ended mode with the XLR pin 3 (negative) connected to pin 1 (ground):

1. Crackling is only on right channel
2. Crackling is only in "active" mode. The "passive" mode bypasses the gain circuitry and balance control, so the signal strictly only passes the volume pot.
3. The crackling gets louder with increased volume.
4. The crackling disappears when turning on the "Reverse" mode, which inverts the phase of the output signal
5. Crackling happens on any input selection, even when no source component is connected to the preamp

I think I can confidently say all these observations eliminate the volume and balance pots from the suspect list.

#4 above makes me think that the root cause of the crackling is a faulty part in the gain circuit, very possibly the positive leg of the balanced circuit (because the negative is connected to ground?).

Below is picture of only the right channel gain circuit. I am guessing the left side is maybe the positive leg, and the right side is the negative leg of the balanced pair, or vice versa.

I will try the freeze spray tomorrow. Any suggestions where and how to point the freezer at, and what I expect to learn from this approach?

 

[kop89]

I think I almost figured it out.

I checked the voltage test points on the service manual. Here are what I found different from the manual and the good devices from both channels:
Q308 (IRF9610): Drain voltage is low at 25V (39V on schematic, 36V on other good devices)
Q316 (IRF610): Gate voltage is low at 7V (12.7V on schematic, 12.3V on other good devices)
Q329 (IRF610): Drain voltage is low at 4.7V (8.7V on schematic, 8.2V on other good devices)

So I am guessing it is either only the IRF9610, or all three of the MOSFETs circled below are bad? Picture of schematic below:

 

[MusicNBeer]

Have you gotten any further with this? My suspect would be a faulty cap. I would measure DC resistance across all caps in the area. If one shorted out, it could screw up all the mosfet voltages.

FYI, reverse flips left and right channels. It doesn't invert phase. Looking at the schematic, I'm confused at why reverse would not just switch the crackle to the other channel. Have you checked voltages with reverse engaged? Also, did you check voltages with no load (amp connected) on outputs?

I just updated my GFP-750 with a new voltage regulator BJT. For some reason, my unit had the wrong transister in the service bulletin update, shown on last page of service manual.

 

[kop89]

I actually missed hearing it. Reverse did indeed move the crackle to the other channel. The voltages were checked with nothing connected to the preamp.

I will remove the PCB from the chassis this weekend and check for bad solder joints and any possible shorted caps.

Thanks!

 

[MusicNBeer]

Removing the PCB looks like a pain. I was going to do it but decided not to. Let us know how it goes. Good luck.

 

[zenmastering]

I would look at replacing C309 (100uf@100VDC) and C311 (10uf@50VDC) first. If that doesn't change your quiescent voltages, then replace Q316 (IRF610). You could also be correct that it's Q308 ;->

The thinking here is that the capacitors (either or both) could be 'leaky' and not blocking DC properly. Beyond that it is likely a bad MOSFET junction. Try the cold spray on both MOSFETs, one at a time, waiting a minute between sprays. It's best to do this when you can hear the crackle prior to spraying.

 

[zenmastering]

Also, check that Q314 (MPSA56) is actually an MPSA56 or close equivalent: On the PCB photo it looks a little different

 

[kop89]

I decided to update all the cheap electrolytics on the PCB with Nichicon and just as well order a bunch of MOSFETs, match them to hopefully within 10mV, and replace all the IRF610 and 9610 on the bad channel. Waiting for parts from Mouser to come in.

Also ordered the MPSA's just in case it is needed.

I figured this shotgun approach will still be a lot easier, faster, and cheaper than to have it professionally serviced. Knock on wood, hopefully this will fix it .

 

[zenmastering]

You're right on with the shotgun approach. I was a pro-audio repair-guy for a lot of years and that approach is often much cheaper for the client!

'Glad that you're replacing the electrolytics, too. Please also change all of the power supply caps, as well, not just the signal path devices.

 

[Chazincaz]

I decided to update all the cheap electrolytics on the PCB with Nichicon and just as well order a bunch of MOSFETs, match them to hopefully within 10mV, and replace all the IRF610 and 9610 on the bad channel. Waiting for parts from Mouser to come in.

Also ordered the MPSA's just in case it is needed.

I figured this shotgun approach will still be a lot easier, faster, and cheaper than to have it professionally serviced. Knock on wood, hopefully this will fix it .

Did you ever resolve it? I have one too doing that same thing!