Need Diagnostic help with McIntosh MR 7082 Tuner

[Timetrial]

I'm looking for help diagnosing a problem with my tuner. Appreciate any guidance or referrals if a professional repair is indicated. I acquired it along with a C31V preamp and MC7270 Amplifier.

The units were dormant for several years. Most have come to life. The tuner only works with AM.

The NiCad batteries were removed and minor corrosion cleaned from the clips securing the batteries. The AM functions appear to work properly. The FM does not.

Operation AM

AM button pressed. AM light illuminates, Mono light illuminates.

Press SCAN UP or SCAN DOWN . The station frequency display increments/decrements, signal strength bars illuminate based on signal strength, tuner will lock on stations.

Press SEARCH. Light illuminates above preset station button. Scrolls to next preset every few seconds until reaching 7 unless SEARCH is pressed again to remain on a preset.

Press MANUAL. Light illuminates above the Tuning knob. Rotating the Tuning knob clockwise/counterclockwise increments/decrements the station frequency display, signal strength bars illuminate based on signal strength.

Operation FM

FM button pressed. FM light illuminates, STEREO light illuminates, Station frequency displays 189.3, right Tuning Indicator arrow illuminates. Static heard.

Press SCAN UP or SCAN DOWN . No static. FM light remains illuminated, STEREO light remains illuminated, Station frequency displays 189.3, right Tuning Indicator arrow remains illuminated.

Press SEARCH. No static. Light illuminates above preset station button. Scrolls quickly to next preset, does not stop at 7, continues to cycle. Station frequency display constantly changing in range 21.6 – 23.0. Pressing SEARCH again has no impact.

Press MANUAL. Light illuminates above the Tuning knob. Static heard. Station frequency displays 189.3. Rotating the Tuning knob clockwise/counterclockwise has no impact. Station frequency remains at 189.3.

Any ideas?

Thank you,

Michael

 

[c_dk]

What part of the country did it sit dormant?

 

[Timetrial]

The equipment was inside a French wardrobe that had been converted to house an entertainment system. This wardrobe was always inside a home in Northern California. Temperature range would have been 50 - 90 Fahrenheit with low humidity. The tuner is in mint cosmetic condition inside and out and shows no signs of damage from rough handling, temperature extremes, or moisture.

 

[c_dk]

It is not uncommon for the push button controls to electrically lock up because of circuit board contaminants. Coastal areas with salt air do it all the time, other high humidity areas are also candidates.

Circuit board cleanliness is next to godliness with all these type units. Can drive a tech crazy if he hasn't seen it before.

 

[Timetrial]

Thank you for your suggestion c_dk. I can check the circuit board behind the front panel for dust/debris. Since the tuning knob and push button switches are fully functional for AM, and have the same function when FM is selected, my assumption has been the root cause is elsewhere. The FM button itself also seems to function. When pressed, the display lights for FM, STEREO, TUNING, and radio frequency [displaying 189.3] all activate. When AM is pressed, the lights for AM, MONO, SIGNAL, and radio frequency all activate.

But as you infer, the link between faults and resulting behavior are not always obvious or linear.

Any other thoughts from those following the thread?

Thank you,
Michael

 

[JlAnsford]

Did anyone solve this problem of AM working perfectly but FM stuck at 189.3?

 

[GabrielS]

I've the same issue, the AM works but no sound from FM. Opening the unit and doing measurements on components still mounted on the circuit board, I found R20, 80ohm resister burned and C21, 100uf/16v shorted on the FM tuner board. Replaced both components yet the issue of no FM sound persists. Will have to do more testing but found also the voltages on most of the IC's except for one are askew not matching the values on the schematic when powered on. Suspect a power supply problem or a transistor may have failed but will have to remove them from the FM board to get accurate results as they're packed in there tight. Perhaps the FM also is in need of an alignment but haven't gotten to that stage yet.

 

[GabrielS]

Did anyone solve this problem of AM working perfectly but FM stuck at 189.3?

I found while pulling on wires to see what effect it would have on my issue with Am functioning ok but no FM sound, that disconnecting one of the connectors leading to the display board gave a reading of 189.3. Perhaps one of the connectors in your unit became loose and just needs to be reconnected. As far as the no FM sound, it unfortunately still persists on mine.