MA 6100 60hz hum

I have Macintosh ma 6100 and I have been using it for a while combined with a dac through aux input. It always had a low, barely noticeable 60hz hum through both speakers. I just got a turn table and unsurprisingly the hum is more noticeable through phono input. The hum is present without any wires plugged in. I searched these forums and the common advice is to check for ground problems. What would my first troubleshooting steps be in relation to a potential ground problem?

 

No knowledge of the MA 6100, working off service manual, others may have
better/first hand experiance.

Hum can be caused by many factors so they are difficult to track down.
Some causes are,
- bad connection/break in the ground circuit causing one part of the GND
circuit to be at a higher voltage than the rest. Suggest resistance check
key points in the GND circuit to ensure continuity/0-1 ohms. Normally
results in significant hum.
- Tired main filter caps. Increased voltage ripple (120Hz) finds its way
into the audio path via the output transistors. Problem noticable in power
amp stage mostly. Expect minor hum unless the caps or possibly rectifiers
are dead.
- Failure in regulated voltage supply. Again increased voltage ripple (120Hz)
is fed into preamp/phono stage, caused by failed component diode, transistor,,,
- non-ideal grounding design. A star configuration is widely accepted as the best,
with separate paths for analogue/digital GND or divided how you wish. A lot of
the old designs have a contiuous GND circuit from RCA/s through preamp then power
amp, psu terminating at the main filters. Minor improvements can be made by
converting, to some extent, to a star configuration
- Incorrectly grounded source, problem evident only when source connected via RCA's
- environmental/external interference/noisy power supply
- and, and,,,


Many units do have some hum and it's considered "normal", you have to decide
if it's excessive and requires your attention.

First step would be to check all ground connections with a multimeter, amp powered off/unplugged.
Expect maybe 1-2 ohms (leed resistance...). You should visually check solder/wire connections.
Reflow if in doubt.

If hum present when no wires (RCA's/speakers,,,) connected then rule out external source.

Try and test amp as separate, ie, standalone PRE then as a power amp (use ipod as pre?)
Is hum present in both pre and power, also on both channels?

The schematic is not real good. It provides a logical connection diagram. The board
layout/interconnection needs to be tested.

 

Our last McIntosh MA6100 sold was our sales floor demonstration unit in 1978 so that would make all MA6100 s 19 years past due for a complete restoration recap...just based on general electrotlytic service life expectancies.

Also over the years a galvanic interaction has been causing issues within the ferrules used to crimp the ground drains to the circuit board leads. A proper restoration will involve slitting the heat shrink wrap, soldering the ferrule, and reinsulating the connection.

There has been much discussion about these procedures in the AK McIntosh forum, and by the way the computer company Is Macintosh, the Binghamton NY. stereo company is Mc.

Welcome to AK.

 

I was going to point you to the mcintosh forum and specifically c_dk
Too late.

 

Thank you all for your help! Per mbz I'm getting a standalone pre-amp to help diagnose the pre-amp stage in my McIntosh and bound the problem somewhat. Due to a room configuration I can't place the turn table too close to the McIntosh anyway, so a separate pre-amp appears to be the way to go for a longer RCA. Such set up might even allow me to keep ignoring the phono problem for a while. I understand that my unit is long due for a refurb. I looked at McIntosh forums and, frankly, I'm a bit intimidated. I play around with robotics as a hobby and I can put together a perfoboard circuit. Signal processing was some course in college years ago. I am probably out of my depth here. From your experience how hard is it to do a full diy recap? Should I just send it somewhere to be refurbished?

 

Sounds like you have a suitable background to do the recap/refurb however if you have any doubt then send it to a tech, the Mc is too nice an amp to stuff up.
The recap is quite easy, just repetative and being methodical. From c-dk above, the hum appears to be a known issue with the ferrules, the fix is quite easy
(technically) but sounds tedious.

Plenty of people can help you put together a recap list, only look at replacing the electrolytics.
However suggest you do it in stages starting with the ferrules, maybe take a look and understand whats needed and if you are comfortable doing it.

 

Is the hum 60 or 120Hz?

Confirming that narrows the problem. 60 Hz points to ground problems, 120 Hz points to tired capacitors or PS problems.