Mac 1900 problems

[macaltec]

OK, I have had this receiver for quite a while now. I have the service manual and schematics. It worked fine for several years and then it dropped a channel. It got new fuses and new output transistors. I am not very confident in the quality of the outputs I bought and it has been several years since I bought them. I have not used the unit in a year or so because it kept blowing fuses, not right away but after a bit of use each time. More like casual attempts at use never any extended periods of time. I am remembering that I was never able to really turn the volume up much before it would start to sound bad and/or blow a fuse since the initial "repair" of the outputs. I have several projects but this is the one that needs to be right. Should I look at a complete re-cap? Or just recap the amp section/pre-amp section?

All that said, I need some help.

Thanks

 

[macaltec]

I dug this thing out today. Did all the usual clean up stuff on the controls and contacts. Put some new fuses in and fired it up. Seems to be working fine on the bench for now. The left channel has slightly less volume than the right channel. I also noticed that it seems to be dependent on where the volume knob is at. It actually seems to even out after I get to 12 oclock and past. Anyone had any experience with this and possibly have a solution?

 

[HomerJSimpson]

I dug this thing out today. Did all the usual clean up stuff on the controls and contacts. Put some new fuses in and fired it up. Seems to be working fine on the bench for now. The left channel has slightly less volume than the right channel. I also noticed that it seems to be dependent on where the volume knob is at. It actually seems to even out after I get to 12 oclock and past. Anyone had any experience with this and possibly have a solution?

I'd think a recap may help fix that. Could also be some driver transistors.

 

[mnmmt]

Compare the resistance in each section of the volume pot at various points in the dial. You may find a big imbalance between the two sections. My mx110z pot is all over the place unfortunately, and the channels go in and out of balance as i turn the knob. . The volume pot may need a good cleaning or a full on replacement/rebuild.

 

[macaltec]

Thanks. I will start my journey there.

 

[c_dk]

Be careful what you clean it with......deoxit fader lube is the best choice. Improper solvent used in the past have caused many issues with these pots.

 

[macaltec]

Yep, nothing but De Oxit gets used here.

As a troubleshooting measure I bypassed the MAC preamp with my Carver pre and I am almost certain that there is no longer a difference in volume.

 

[HomerJSimpson]

Well at least you know its not in the amp section.

 

[macaltec]

Not so fast, it is still noticable just not as much. I swapped the speaker leads at the amp and the imbalance shifted to the opposite side. I will be getting a Mouser order together. Hopefully I can get a new volume pot from them as well.

 

[TSmith8605]

Yep, nothing but De Oxit gets used here . . .

For those who do not know, specifically use DeOxit F-series faderlube for cleaning pots. This is a good thread: http://audiokarma.org/forums/index.php?threads/the-idiots-guide-to-using-deoxit-revisited.207005/

Tracking problems with the MAC1900 and a few other older McIntosh preamps is common and I've been watching the threads about this for a long time and I've not seen anyone post that they've definitely found a good replacement pot. This is not just a volume pot, but it also includes the power switch and supports the loudness-contour function. Also physical space for aftermarket pots is limited. If you find a good one, please let everyone know.

Example thread that may help you: http://audiokarma.org/forums/index.php?threads/1900-on-off-volume-control-replacement.819889/

What I understand is that since the power switch is part of the volume control, every time a user turns the unit on or off, they move the volume control over the lower portion of its travel and cause excessive wear. On my 1900, which has some of these tracking and noise issues at lower volume levels but which is otherwise completely restored, I use a cheap power strip as a power switch. This means the volume control stays put except for actual volume adjustments and so additional wear is minimized. Also, putting some kind of audio attenuator (like an appropriate resistor) between the pre amp and amp, will force the use of the volume control in a more-rotated angle where the wear and mis-tracking are less evident.

 

[macaltec]

I vaguely remember hearing about this issue some time ago. It is a bit discouraging. I may end up getting the amp section up to snuff and use a nice tube pre amp with it. I'd be curious to see an exploded diagram and schematic of just the volume pot.

 

[macaltec]

Just a thought. How would a nice passive volume control work between the preamp and amp section of this unit? Then one could use a powerstrip of sorts to turn the unit on and off and thus leave the volume pot in one place. Thoughts????

 

[TSmith8605]

Just a thought. How would a nice passive volume control work between the preamp and amp section of this unit? Then one could use a powerstrip of sorts to turn the unit on and off and thus leave the volume pot in one place. Thoughts????

I think that would be a decent work-around allowing you to use the MAC 1900 preamp rather than another preamp although getting the thing to work as intended would be nicer.

 

[macaltec]

So what is the general consensus on the replacement volume pot from McIntosh? Is it good or not?