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160.33.161.176
Here we go again,
Hello inmates, before I ask for help on this one I want to thank all of you for your help in solving the last one. This problem is ironically similar, though a more complex unit.
The issue is a buzz/slight hum in the right channel. The left channel is dead quiet. Seems like I was just in this same spot with with my Scott 222 last month.
Here is a video of the sound and me turning the volume knob, you may have to turn it up to hear it:
https://drive.google.com/file/d/1QA30TyvdYCOrX93eMWz2YRX5sI2-WiKl/view?usp=drive_link
Schematic:https://www.drtube.com/schematics/mcintosh/c20-(1962).png
My theory is that it's either a ground issue, or RF interference?
I have just finished a very thorough restoration of this C20 Pre-amp which I was working on concurrently with some other audio projects. When I received the unit a couple years ago it did exhibit this buzz, though it was more pronounced than now post restoration. I was not entirely sure where it was coming from but figured my restoration would solve it.
All new PS with new volume pot control installed. All controls and contacts were cleaned. All the usual coupling caps have been replaced. The previous owner installed some very nice RCA's that appear to be good quality and installed well. Many of the resistors on the back panel look replaced. The power cord also was replaced by the previous owner.
Unit works perfectly with all voltages right on the nose. Tubes test good, I replaced the ones that were not up to par and cleaned the sockets. Unit sounds fantastic except for this buzz. It is silent when turned on, then at just about 11:00 on the volume I will get a buzz in the right channel. The buzz will increase slightly with volume, and at about max volume the buzz will dissipate a little.
No other controls will affect the buzz. Swapping tubes will not affect the buzz or move it to the other channel. Moving the shielded cables while on with a dowl will not affect the sound.
This is quite a complex unit and I need a plan for hunting down the cause. It's not as simple as the Scott which had this exact problem, buzz in one channel. In that case it was easy to trace the path and keep swapping the wires until isolated (it ended up being a poorly shielded cable) With this unit, it is not easy to find the correct points or swap the wires to isolate.
Is there a method for tracing these issues? -after trying all the "simple" stuff? I get so frustrated with these hum and buzz noise issues especially after you get so close!
Any help or advice appreciated, thanks!
Jeff
" at about max volume the buzz will dissipate a little. "This implies the hum is being picked up at or after the Volume control. It's a 1M pot, so the signal at the wiper is at the highest operating impedance (and therefore most susceptible to hum pickup) at mid position. One way to prove this would be to ground the "hot" side of the pot (which is connected to the wiper of the Loudness pot) and check whether the hum persists. Each channel of the Volume pot connects to a coupling cap that drives the cathode follower output stage. The terminal of the cap connected to the follower has a small amount of DC voltage on it; the side connected to the Volume control should not. It's worth measuring this to be sure the cap isn't leaking.
Edits: 05/21/25
Triode, just an update. I have narrowed the issue down to the last pot (the one adjacent to the barrel switch) Specifically the ground tap, on that last pot (which is the right channel where the buzz is) It sort of acts like a cold joint, though I re-flowed it.
If I add just a little pressure to that tap, I can make the hum much more pronounced, It will also produce loud static and popping. It will improve a bit with pressure, but that low level buzz is the best it will perform. Again the other channel remains perfect.
Here is a little video of it:
https://drive.google.com/file/d/1dpRb_5YGieGAjTc4lSOuN0isjIg1k95U/view?usp=drive_link
Pressing or adding pressure to any of the other lugs on the other pots will not produce or change the noise. The "hot" taps on that last pot might affect the buzz but only in that they will flex the ground tap I believe.
I guess I have a defective pot, though it's new?
I did send a message to the supplier "HiFi Audio" in NYC to see if they have any insights or have had other reports of this.
Jeff
Hi Triode,
thanks for taking a look. I first tested by putting the "hot" side of the volume pot to ground, the side connected to the loudness pot. The buzz still persists, though is just slightly weaker and completely goes away when nearing full volume.
I checked for voltage leaks on those caps. They were replaced, but figured I would check anyway. 19.5 volts at the pins and no voltage on the side of the .022 cap going to the wiper of the volume pot.
There really isn't much after this just the .47s which I replaced and the RCA's themselves.
The solder joints look good on the new volume pot, I'd be surprised if the issue was at the pot itself.
I noticed the original wires that run from v7 to the .47's is just plain wire that is twisted left and right side together. McIntosh did not use shielded wire here, would that have been better to use there? Unsure if RF is getting in there, but there isn't much between the volume control and the output connections.
I really appreciate the help
Jeff
Is the shell of the Volume control well grounded? The preamp's bottom cover is making good electrical contact? The connection at the grid of the output tube is extremely high impedance due to the bootstrapped cathode follower, probably about 10 Meg. The Volume control is the only thing pulling this area of the signal path toward ground, so all of this wiring is susceptible to hum pickup. I would be hesitant to use shielded wiring due to the likelihood of high frequency attenuation.
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