MR80 Resurrection Update
The half-dead 80 has been fully revived and is now working great!
Here's the resuscitation rundown:
The automatic scanning, presets and digital display features of this tuner were very sophisticated for its time and took many boards and numerous ICs to provide this functionality. This proved to be a challenge in multiple ways because I had to go through the boolean logic and theory of operation to understand how the push button circuits controlled the tuner and indication leds. It also didn't help that there were MANY mistakes in the service manual and on the schematics that came with the tuner. Things like incorrect voltages, wrongly labeled push button inputs to ICs, incorrect connector numbering, wrong high/low logic states and so on made for much frustration and longer hours than necessary. Oh well...such is life right! :yes:
Pre-set buttons didn't respond and preset 3 & 4 LEDs.
These 2 problems were interrelated. IC 301 on board 8 is a NOR R-S latch that determines which preset button has been pressed and feeds this state to downstream circuits that select the preset station circuit and turn on the appropriate LED. The IC had a short on preset 4 causing the BCD Decoder (Board 6 - IC 503) to misinterpret the status of the pushbuttons, this meant none of the downstream preset functions would work.
FIX - Picked up a NOS MC14043B off eBay from a US surplus supplier. All preset circuits and LED indicators work.
Up and Down Scan push buttons and LEDs don't work
After repairing the preset problem the Up & Down scan function came partially back to life. But things weren't quite right as no matter which button I pushed it would always scan down and the Down LED stayed on all the time. When the UP button was held the UP led would flicker but the tuner would still scan down.
This turned out to be another shorted chip, IC601 which is used as a voltage comparator to determine when the tuning circuit reaches the top of the FM band. When in automatic scan mode the tuner reverses its scan direction when it reaches up or down limits. So when it hits the top it goes back down and vice versa. The short caused dependent circuits to interpret that the tuner was always at the top of the band.
FIX - Picked up a NOS LF-356 JFET op amp off eBay from a US surplus supplier. Replaced chip and adjusted Scan Up & Down limit pots. All scan functions and LEDs work.
Missing Stereo and Lock bulbs.
I was somewhat concerned that there might be problems with these circuits as someone had taken the time to cut the bulbs out but not replace them.
Fix - Replaced with JLK Components 7381 long life 50,000 hr bulbs from Mouser. This was a bit of a pain as it is next to impossible to remove the board. So I left it in place and used an iron on the back and solder sucker from the front to remove most of the solder and then used a sharp bamboo toothpick to enlarge the holes. New lamps work great, circuits function properly.
I cleaned the end of the problematic contact springs and back of the pushbuttons with Deoxit and adjusted the touch sensitivity (R509) pot. I found that the service manual sensitivity adjustment did not work and instead adjusted it empirically by turning it in small amounts until all of the buttons responded properly. The buttons now work perfectly on the first press.
My guess is that cleaning the contacts and adjusting the sensitivity using my method will solve the majority of intermittent button issues many have experienced.
All in all it was rewarding to bring this old girl back to life. For less than $10 in parts and a bunch of heartbeats I now have a beautiful, wonderful sounding, high performance McIntosh tuner! My rooftop Antennacraft 70 Fm Triple Driven-Directional Antenna will be here this week, but that's story for another time.