[MCC]

Last year, I rebuilt a vintage McIntosh MAC1500 I found and restored it to beautiful operating condition- new caps, resistors, rectifiers, etc. It's worked wonderfully (albeit with some occasional bias adjustments) until last Friday, when it suddenly developed a humming noise mostly in the right channel. It all started in the middle of a song after the receiver had already been on for at least an hour.

One can cap is brand new, the other two are NOS directly from McIntosh. The issue persists when all the tubes are removed from the FM section, and I've tried switching each 7591 out individually with a known good backup tube. I've also tried switching the transistors from one channel to another in the preamp.

I'm currently running it with cheapo 12AX7 and 12AU7 tubes, which also do nothing to solve the problem that initially materialized with Telefunkens installed.

Visually, everything looks fine.

I put a trial of the Yoshimasa realtime analyzer on my computer and got this oscilloscope reading:

This waveform seems strange to me- does anyone have any idea what the issue could be? I've dug up a PDF copy of the service manual / schematic here.

Thanks in advance,

Michael

Edit: I forgot to mention that I replaced the rectifiers with new Fairchild Stealth 2 diodes a few days ago in hopes that this would solve the problem. At least I had an excuse to upgrade from the original Stealths.

 

[ziplock]

Hey there Michael, I remember following your thread on the restoration. Sorry to hear your having problems.

When does it hum? With just the speakers connected and nothing else? If not, try that first.

What does the hum sound like? Example, touching the center of an input jack will cause any amplifier to "HUM". Does it increase with volume?

Try moving the receiver to different outlets in different rooms. We want to rule out any odd ground loop issues. Bring a small set of speakers or use headphones if you don't want to lug around the larger speakers.

If you try all of this and the hum still persists. I would suspect an issue has developed in the power supply. A component may have failed that is allowing AC line noise (60Hz) to bleed through. Check all the usual test points for the proper voltage, including directly off the power supply (of course).

Still a stickler? You'll need to acquire a real oscilloscope and trace the audio path to determine the source. Perhaps in the pre-amp, assuming the MAC 1500 has one or in the main amplifier section.

Paul

 

[MCC]

Thanks Paul, your suggestions last year were invaluable to me. I now have a variac for testing and I've since installed 4 Thermometrics CL-80s in series to reduce power-on stress. I'm heading off to Iowa State for Cpr. E on the 19th, and I'm taking the Mac with me.

Quote:

Originally Posted by ziplock /img/forum/go_quote.gif When does it hum? With just the speakers connected and nothing else? If not, try that first.

The hum first appeared with headphones plugged in- my Energy speakers are actually in storage at the moment in preparation for my move to ISU. It persists with no inputs connected, and it's at about 1/2 - 2/3 listening volume. The majority of my listening is via headphones, so this is concerning.

I've moved it downstairs to Dad's speaker setup, and the hum is still there on speakers but almost unnoticeable from a distance. Headphones are the same as before. The audio performance is still great otherwise.
Quote:

Originally Posted by ziplock /img/forum/go_quote.gif What does the hum sound like? Example, touching the center of an input jack will cause any amplifier to "HUM". Does it increase with volume?

The hum is unaffected by any of the controls on the front of the receiver. It does sound similar to touching the center of an input jack, but more subdued.

I've studied the power section several times but haven't tested it part-by-part, as I haven't really had the time to do so yet.
Quote:

Originally Posted by ziplock /img/forum/go_quote.gif Check all the usual test points for the proper voltage, including directly off the power supply (of course).

Some of the voltages are actually lower than they should be, but they've always been that way. For example, the 460v supply actually measures at 410v but the 80v is actually ~81v. This is with 120v AC input as measured after the CL-80s.

Is this a real problem, or something that can be safely ignored?

Quote:

Originally Posted by ziplock /img/forum/go_quote.gif Still a stickler? You'll need to acquire a real oscilloscope and trace the audio path to determine the source. Perhaps in the pre-amp, assuming the MAC 1500 has one or in the main amplifier section.

Good suggestion- it looks like I may indeed need to invest in one. Do you think Ebay would be the best source, or is there another better place to get one?

-Michael

 

[ziplock]

Michael,

I'm looking at the service manual and noticed that you did not check the 7591s. These are the output tubes right before the L/R transformers. Try swapping the sets and see if the problem moves between channels.

If not these tubes, I'm pretty certain the issue still lies in this main amplifier section. You yourself already ruled out the pre-amp(s) by swapping the transistors. Further downstream we go...

Ebay would be the best choice for picking up an used oscilloscope. Do you ever plan to test anything other than audio? This will determine the maximum bandwidth you need. For most general purpose audio testing, a 20MHz scope with two channels will suffice. Techtronix is the best and you can pick up a tube based unit for a few hundred dollars. Try to find one that comes with probes or you'll be out of pocket an additional 50 - 100 dollars for a set. You can go much cheaper than this, but I wouldn't skimp on probes. A quality probe is important for accurate measurements.

Paul

Edit: About the power supply voltages, that would depend on what the tolerances are. Are these listed for the main power transformer? I've only been able to briefly glance at this. Although the 460V @ 410 does concern me. This seems out of spec. Look for voltage adjust pots?

 

[MCC]

I finally found the source of the hum- one of the can caps has a bad section, giving a reading in the range of nanofarads. The issue specifically was with the NOS Mallory 40-40-40uF cap I ordered from McIntosh last year. I never would have guessed that this would be the case since that voltage supply is for both channels, and there is next to no hum in the left channel. I'm still scratching my head over this one.

One last question- the capacitor I used was an 82uF 400v Nippon-Chemicon, brand new. I'm under the impression that you can never really have too much capacitance in a filter circuit. Is this correct?

I'll be sure to check out the low voltage issue too, probably next week when I have more time. There are no voltage pots for that circuit.

The oscilloscope will need to wait though. I don't feel that it would get enough use to justify the price.

Thanks again for your help Paul!