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MAC 1500 drops out of FM stereo

TSmith8605

TSmith8605

Senile Member
Subscriber
Hey,
After about 10 minutes of operation or less of my MAC 1500, the red multiplex light fades and the FM tuner drops out of stereo. FM mono still sounds great. Other sources are fine.

Easy fix or does it need a trip up to L&M for an alignment of the multiplex section? New tube maybe?

Thanks,
 
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If the MPX section is tube it is worth changing them out, one at a time. Both my Mac tuners, MR67 and MX110, displayed similar symptoms as well as my Scott 340B. On both Macs it was the 6U8 which required replacement.
 
Even though I started selling Mac in 1978 I have only seen one MAC1500 and have never repaired one but...

On 1700 1900 and stereotech all have shown the same symtons you describe when a voltage regulator trans. in the power supply fails. Tis regulator sends a -dc voltage into the MPX circuit thru the mode selector switch muting the tuner when listening to other sources. I do not have a 1500 schematic but I would expect them to use this elegant solution to Tuner crosstalk on the 1500 also.
 
Destructor said:
If the MPX section is tube it is worth changing them out, one at a time. Both my Mac tuners, MR67 and MX110, displayed similar symptoms as well as my Scott 340B. On both Macs it was the 6U8 which required replacement.
Click to expand...

I'm going to second this suggestion to try changing the tubes before trying anything else. It's easy enough, and while tubes last a long time, they don't last forever, so they are a likely suspect. And since the problem occurs after a few minutes of operation, it may be heat-related, and tubes, of course, heat up.
 
In the AK database there is a MAC1500 schematic. The scan does not lend it self to enlarging but shows a few trans in the FM circuit that would produce the thermal issues you describe as they fail.
 
There are 2 technicians who know the MAC 1500 well enough in the USA that I totally trust to repair it's tuner. Audio Classics or Terry DeWick. L&M may be fine but check with them to verify they're comfortable with it. It's an older McIntosh piece which was low production.
 
KentTeffeteller said:
There are 2 technicians who know the MAC 1500 well enough in the USA that I totally trust to repair it's tuner. Audio Classics or Terry DeWick. L&M may be fine but check with them to verify they're comfortable with it. It's an older McIntosh piece which was low production.
Click to expand...

I prefer Terry for major work and audio upgrades. Terry did a complete overhaul, rebuilt the dead power supply and did audio upgrades of this MAC1500 in 2007. L&M does not do audio upgrades, but will bring the older units back to factory spec.

L&M (who are local to me so I don't have to ship) had no problem whipping one of my MR71s into shape a couple of years ago. I believe L&M is an authorized McIntosh repair station in San Francisco. Also, House of Music in San Francisco, probably the biggest McIntosh dealer in the Bay Area, recommends L&M for repair of old Macs.

I've been using L&M for years and my Teac A3100S has been through there a couple of times in it's 35 year life.

L&M should have no problem with the tuner section of the 1500 and they would be fine for this minor issue.

However, the problem seems to have been solved without a trip to either L&M or Terry. See my post above.

Thanks,
 
Thank you for chiming in about what solved your problem in case if I encounter one with issues. L&M's been around for a long time and does impeccable repairs. Have a superb week and Karma to you.
 
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