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 a Pearl2, Aleph2 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?
Thank you
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 a Pearl2, Aleph2 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?
Thank you
Could be a cold solder joint or a failing transistor or mosfet.
Is yours one of the early ones with the IRF610 mosfet in the power supply (mounted to the heatsink) or was it upgraded to the 2sc4793? If its the mosfet then it may be failing.
Is yours one of the early ones with the IRF610 mosfet in the power supply (mounted to the heatsink) or was it upgraded to the 2sc4793? If its the mosfet then it may be failing.
Chamberman, no it was 2SC4793 on the heatsink.
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 next. Any suggestions where and how to point the freezer at, and what I expect to learn from this approach?
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 next. Any suggestions where and how to point the freezer at, and what I expect to learn from this approach?
An externally hosted image should be here but it was not working when we last tested it.
AFAICT from the schematic, REVERSE does not in fact invert phase but merely swaps the reft and light output channels. The problem should thus merely move to the left channel. If it truly does disappear, relay K114 and its solder joints would be your suspect... though I bet it actually doesn't and you just missed it.
So you might be back to square one basically. If it were a pot, it would be very scratchy as well... and then that still may not be the fault of the pot itself but rather a leaky (electrolytic) coupling capacitor, of which the unit sports 4 per channel, though C309 would be of particular interest.
Item #3 is explained by what appears to be a volume control following the gain stage, as found in some tube amps as well. Distortion is thus entirely dependent on input level and may not be entirely pretty at CD level with loudness war era material. Bit of a strange beast tbh. It should have very low noise, I'll give it that. Older Adcoms seem to have been just the opposite, low distortion with just average noise.
So you might be back to square one basically. If it were a pot, it would be very scratchy as well... and then that still may not be the fault of the pot itself but rather a leaky (electrolytic) coupling capacitor, of which the unit sports 4 per channel, though C309 would be of particular interest.
Item #3 is explained by what appears to be a volume control following the gain stage, as found in some tube amps as well. Distortion is thus entirely dependent on input level and may not be entirely pretty at CD level with loudness war era material. Bit of a strange beast tbh. It should have very low noise, I'll give it that. Older Adcoms seem to have been just the opposite, low distortion with just average noise.
@sgrossklass,
You were right, I missed hearing it it and the crackling noise moved to the left channel at "REVERSE"... doh! 🙁
So next I probed around the test voltages shown in the schematic and here's what I found:
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)
Picture of this schematic is attached.
So I am guessing it is either only the IRF9610, or all three of the MOSFETs circled in the schematics are bad?
Thank you
You were right, I missed hearing it it and the crackling noise moved to the left channel at "REVERSE"... doh! 🙁
So next I probed around the test voltages shown in the schematic and here's what I found:
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)
Picture of this schematic is attached.
So I am guessing it is either only the IRF9610, or all three of the MOSFETs circled in the schematics are bad?
Thank you
Attachments
You may want to look at Q234/246 as they set the gate voltages. As long as the base voltages for those two trannies is correct anyway and based on your measurements it is. What is your voltage drop across R248 & R208?
1) upload a readable full schematic
2) you will need troubleshooting or it´s just a guessing game, sorry.
So far we have reduced the search area to between input selector and volume control .... apparently .... but still lots of unknown variables within.
2) you will need troubleshooting or it´s just a guessing game, sorry.
So far we have reduced the search area to between input selector and volume control .... apparently .... but still lots of unknown variables within.
Disregard my last post. I was thinking you had multiple low gate readings. I guess I should read your post as well as look at the picture from now on. 🙂
I would check R313 vs. R273 and surrounding solder joints (also R327 - R322 junction).
Otherwise I suspect Q316 may be leaky. Its DC gain is lower, and Vgs comes out to a mere 2.3 V (vs. 4.5 V on the other side), yet Ids would seem to be the same as on the other side according to R323 voltage.
Mind you, it can't hurt to check whether temporarily disconnecting C311 changes anything.
Unfortunately, the potato quality schematic seems to be all there is.
Otherwise I suspect Q316 may be leaky. Its DC gain is lower, and Vgs comes out to a mere 2.3 V (vs. 4.5 V on the other side), yet Ids would seem to be the same as on the other side according to R323 voltage.
Mind you, it can't hurt to check whether temporarily disconnecting C311 changes anything.
Unfortunately, the potato quality schematic seems to be all there is.