McIntosh C20 Noise Issue

[Sgrobstein]

I have a McIntosh C20 that has been completely restored (All caps and resistors replaced).

There is an inconsistent noise coming from the unit. Noise is louder in the phono section. Noise appears about 1x per second but is inconsistent when it shows up. There is a consistent sounding noise that is very low with a louder portion showing up every second or so.

Oscilloscope probe was on C1A for pictures. I have attempted to isolate the issue with the following steps:

- Replaced rectifier tube (Same Issue)
- Replaced final output tube (Same Issue)
- Removed all tubes aside from the 6X4 (Same Issue)
- Replaced 1st capacitor stage (C1A) with different capacitor (Same Issue)
- Connected R111 directly to C2A, Disconnected C2A from 250V source and C2B (Same Issue)
- Disconnected filament transformer wires (Same Issue)
- Cleaned light dimming swith (Same Issue)
- Removed all lamps (Same Issue)
- Disconnected C1A from C1B leaving C1A the only part connect (Same Issue)

Pictures of the bottom/top of the 60Hz cycle is attached (notice the inconsistent peaks)
Pictures of the oscilloscope running at a very low pulse cycle showing the inconsistent signal.

Any help would be much appreciated.

 

[adnick]

Do you have an analog multimeter that you could hook up to the output? Initial thought is a power supply issue and an analog meter would jump slightly concurrently with the noise.

Keep in mind—pure speculation (-;

Someone with more knowledge and experience will come along...

Best of luck,
Andy

 

[Sgrobstein]

Andy,

I only have digital multimeter's at the moment. Thank you for the idea.

Stephen

 

[adnick]

No problem, it’s an old trick that I learned from one of my mentors...obviously much smarter than myself.


VR
Andy

 

[Mr Boochie]

You wouldn't happen to be close to any digital units? Phones? Anything like a computer plugged into the same line as the pre-amp?

 

[adnick]

Should add— B+ or - might also show those minor needle fluctuations....

 

[Sgrobstein]

Mr. Boochie,

Nothing is close to the amplifier. My phone glances by it occasionally when speaking with my mentor but not regularly, i.e. the issue is present when no other electronics are around aside from the oscilloscope, variac and other testing gear. Oscilliscope and variac are the only items "on" while testing.

Stephen

 

[Mr Boochie]

Stephen, couple other questions. Did you use this great pre-amp, prior to the caps being changed? Were there noise issues, prior to the change-out? And did the tech really replace all the resistors?

Thanks!

 

[Sgrobstein]

Mr Boochie,

I, myself have replaced every resistor and capacitor on the unit aside from the ones on the bass and treble controls. Everything else is new.

I did not have this unit working prior to restoration. I purchased it from an original owner and figured it should be restored prior to use as the unit had been sitting on a shelf for over a decade.

Thank you for working through this with me. All the help is very much appreciated.

Stephen

 

[Mr Boochie]

Rhythmic noise sometimes is a low frequency positive feedback or parasitic oscillation issue, between sections. I'd go over the each main filter section (main electrolytic caps and resistors) in your power supply. Check for good solder points and good grounding points. Sometimes, the pi-filter sections don't sufficiently filter one section from another --- for whatever reason. Then input and gain stages can couple, oscillate, and induce these kinds of pulsing noises. Good soldering and wiring is critical for a healthy & clean power supply.

Good luck, with the fix!

https://en.wikipedia.org/wiki/Motorboating_(electronics)

 

[Buck Rogers]

I use one of these. Vacuum tube signal tracer.

https://i.ytimg.com/vi/W5x35TmTKr0/maxresdefault.jpg

Lets you listen to caps, resistors, tubes and high voltage power supplies.

Will let you hear noise that cannot be seen on a scope.

Don't leave home without one!

Buck

 

[Sgrobstein]

Buck,

Seems like quite the handy device. I'll watch out for one.

Mr Boochie,

I disconnected all of the power supply caps down to the first one with the issue persisting throughout each one being removed. I then replaced the first cap in the stage and the issue continued. That, to me, makes the caps in the power supply not the issue.

Stephen