Fun with MR71 detector: Replacing those Internal Capacitors!!

[pathologymd]

Hello everyone,

Like many people who own tube McIntosh tuners, I have been plagued with problems in the internal capacitors in IF can transformers and detectors. In my case, I have had problems with detectors. My MR67 had intermittent problems with shifting into an apparent narrow bandwidth mode where tuning would become critical, both with listening and the center tune meter. Terry Dewick suggested I remove the transformer and soak the base in De-Oxit. I did, and it has worked since.

Now, My MR71. For a while, periodic “thumps” would occur in the music along with a little deviation of the center tune meter. Putting the tuner on the bench revealed the detector quite sensitive to movement, so I felt I had to go in and do the same thing as my MR67. The schematic shows three internal capacitors. One across the primary, one across the secondary, and one from one side of the primary to the secondary center tap. The capacitor across the secondary is a ceramic disk soldered to the two terminals. The other two are the internal mica capacitors. I removed a wire from the primary side so I could measure the capacitance. It measured something like 3 to 5 pF. The other internal one measured about the same. I gently sprayed De-Oxit between the two layers of the base, and it wicked up into the narrow void. I twisted the two halves of the base a little and remeasured: a solid 33 pF on each.

To remove excess De-Oxit, I sprayed canned air between the layers of the base, and voila! Lots of silvery speckles came flying out: MICA! I had inadvertently “blown away” much of the internal mica. Remeasuring revealed basically no capacitance. This mistake was actually great, because now I could replace those pesky internal capacitors with new ones without trying to take the transformer base apart! I replaced the one across the primary across the internal pins, and the one from primary to secondary center tap once I got the transformer installed. The two 6.8 pF disk capacitors in the detector circuit each measured about 25-29 pF, so I replaced them as well.

After realigning the detector, distortion was 0.2-0.3% in mono and 0.4-0.5% in stereo with 38 db of separation—better than this tuner had ever performed. Sounds awesome too! I have completely updated this tuner in Modafferi fashion and it is now perfect.

I know this has been covered before, but I thought my mistake may be an easier solution than taking the base apart or drilling in from the side to “destroy” the pesky capacitors before replacement.

Sorry for being long-winded!!

William

 

[skippy_ps]

Nice work Bill. So, you left the silver mica thingie in the base of the transformer?

As I'm sure you remember, I took that transformer apart on my '67 and broke one of the windings and ended up having to re-wind it. Not very much fun.

Murray

 

[pathologymd]

Hi Murray,

Yes I remember!! I did everything I could to make sure I didn't have to do any rewinding! That fine wire and I don't get along.

Well, thanks to the power of the canned air I used to get rid of the excess Deoxit, it also got rid of the mica as well. I didn't intend to do that, but it worked out just great. You can see the absent mica when you view the bottom of the transformer. This may be a less invasive way to get rid of the internal mica capacitors so you don't have to take the bottom apart or drill from the side to destroy the aberrant capacitor. I've been listening all night and it sounds great and is rock stable.

This is not something I enjoyed doing, but it really is a must if the problem exists.

Good to hear from you, Murray--

Bill

 

[skippy_ps]

Who woulda thunk it - that you could disperse the silver with canned air. Mine was fairly intact and I could even get a pf reading from the "chip" with it removed although it was behaving similarly to yours - a little wiggle on the top of transformer and the volume would pop up and down.

Once the brass rivet is cut and pushed up, the whole base sorta flops around with the windings attached and while I was "fixing" it, one wire broke. :tears:

Anyway, nice goin' and

Murray

 

[dshoaf]

This is happening to one of my MX-110s and I've been putting off the project due to other ones being in the way.

Thanks to the both of you, its popping back up to the top again.

There's nothing like having a 'Lewis and Clark' going ahead to map the trail........

Cheers,

David

 

[dewickt]

I have a very thin tip I use in my soldering iron to melt away the plastic at the fingers that go to the mica, then I pry the fingers up and cut them off. This way I do not drill out the rivet and the base stays intact and I can avoid Murray's problem of a broken wire.

 

[Gary Follett]

Hello everyone,

Like many people who own tube McIntosh tuners, I have been plagued with problems in the internal capacitors in IF can transformers and detectors. In my case, I have had problems with detectors. My MR67 had intermittent problems with shifting into an apparent narrow bandwidth mode where tuning would become critical, both with listening and the center tune meter. Terry Dewick suggested I remove the transformer and soak the base in De-Oxit. I did, and it has worked since.

Now, My MR71. For a while, periodic “thumps” would occur in the music along with a little deviation of the center tune meter. Putting the tuner on the bench revealed the detector quite sensitive to movement, so I felt I had to go in and do the same thing as my MR67. The schematic shows three internal capacitors. One across the primary, one across the secondary, and one from one side of the primary to the secondary center tap. The capacitor across the secondary is a ceramic disk soldered to the two terminals. The other two are the internal mica capacitors. I removed a wire from the primary side so I could measure the capacitance. It measured something like 3 to 5 pF. The other internal one measured about the same. I gently sprayed De-Oxit between the two layers of the base, and it wicked up into the narrow void. I twisted the two halves of the base a little and remeasured: a solid 33 pF on each.

To remove excess De-Oxit, I sprayed canned air between the layers of the base, and voila! Lots of silvery speckles came flying out: MICA! I had inadvertently “blown away” much of the internal mica. Remeasuring revealed basically no capacitance. This mistake was actually great, because now I could replace those pesky internal capacitors with new ones without trying to take the transformer base apart! I replaced the one across the primary across the internal pins, and the one from primary to secondary center tap once I got the transformer installed. The two 6.8 pF disk capacitors in the detector circuit each measured about 25-29 pF, so I replaced them as well.

After realigning the detector, distortion was 0.2-0.3% in mono and 0.4-0.5% in stereo with 38 db of separation—better than this tuner had ever performed. Sounds awesome too! I have completely updated this tuner in Modafferi fashion and it is now perfect.

I know this has been covered before, but I thought my mistake may be an easier solution than taking the base apart or drilling in from the side to “destroy” the pesky capacitors before replacement.

Sorry for being long-winded!!

William

Hello everyone,

Like many people who own tube McIntosh tuners, I have been plagued with problems in the internal capacitors in IF can transformers and detectors. In my case, I have had problems with detectors. My MR67 had intermittent problems with shifting into an apparent narrow bandwidth mode where tuning would become critical, both with listening and the center tune meter. Terry Dewick suggested I remove the transformer and soak the base in De-Oxit. I did, and it has worked since.

Now, My MR71. For a while, periodic “thumps” would occur in the music along with a little deviation of the center tune meter. Putting the tuner on the bench revealed the detector quite sensitive to movement, so I felt I had to go in and do the same thing as my MR67. The schematic shows three internal capacitors. One across the primary, one across the secondary, and one from one side of the primary to the secondary center tap. The capacitor across the secondary is a ceramic disk soldered to the two terminals. The other two are the internal mica capacitors. I removed a wire from the primary side so I could measure the capacitance. It measured something like 3 to 5 pF. The other internal one measured about the same. I gently sprayed De-Oxit between the two layers of the base, and it wicked up into the narrow void. I twisted the two halves of the base a little and remeasured: a solid 33 pF on each.

To remove excess De-Oxit, I sprayed canned air between the layers of the base, and voila! Lots of silvery speckles came flying out: MICA! I had inadvertently “blown away” much of the internal mica. Remeasuring revealed basically no capacitance. This mistake was actually great, because now I could replace those pesky internal capacitors with new ones without trying to take the transformer base apart! I replaced the one across the primary across the internal pins, and the one from primary to secondary center tap once I got the transformer installed. The two 6.8 pF disk capacitors in the detector circuit each measured about 25-29 pF, so I replaced them as well.

After realigning the detector, distortion was 0.2-0.3% in mono and 0.4-0.5% in stereo with 38 db of separation—better than this tuner had ever performed. Sounds awesome too! I have completely updated this tuner in Modafferi fashion and it is now perfect.

I know this has been covered before, but I thought my mistake may be an easier solution than taking the base apart or drilling in from the side to “destroy” the pesky capacitors before replacement.

Sorry for being long-winded!!

William

Greeting!

New member here. I know this is an old thread but here goes anyway"
I have completed the task of removing the mica capacitors from the base of my MR71 discriminator can. I read the discussion regarding this topic but only got an inference that the values of the capacitors is around 30 pF. Is this the case? If not, what would the values be?

Gary

 

[clinic-audio]

Welcome on AK

 

[backmd]

Hi, hoping to help someone here is the story of a happy Mcintosh MR71 repair:
Mcintosh MR71 1-2-21

No dangerous shorts, infinite ohms, needs 1 amp slow blow. I think the tubes are appropriate. Have service manual from hifiengine, replaced with proper 1 amp slow blow. 7.6 ohms and no dangerous shorts to chassis. Power up on variac now. 25 volts 0.5 amps, starting at 8pm. 35 volts at 0.5 amps at 805pm. 35 volts at 0.5 amps at 845pm. 17.5 watts, and there is no tube rectifier, so the capacitors are seeing the full voltage now. At 40 volts we see unstable spikes in current, but stabilizing at 0.55 amps for 22 watts. 93 degrees at the capacitor now. 70 watts is the spec. so that would be 0.7 amps at 100 volts. 50 volts at 0.7 amps at 917pm. Transformer and capacitor are 103 degrees. 12 tubes with heaters. 55 volts 0.75 amps burning 41.25 watts. R lower quadrant of transformer is 112 degrees while the can capacitor is also 112 degrees. Burning 44 watts now 55 volts at 0.8 amps. I went up to 0.9 amps and things started to become unstable in that Mallory filter capacitor can. Stay at 0.8 amps and 55 volts. Now it is 0.75 amps at 55 volts, time to watch the news. 130 degrees on the can. 55 volts at 0.95 amps is 52 watts at 160 degrees. Take it way down now. Down to 0.3 amps overnight at 25 volts. Nah shut her down and let it cool.


Serial number 85B13. how to remove panlocks. One of the selenium diodes is shorted! The one closest to the front! Both diodes replaced with 10 amp 1000 volt silicon diodes and soldered in and connections verified. All four can capacitors measure well and good. One Sprague 40uf capacitor measures 50uf out of circuit and reinserted into the circuit. 2200 ohm resistor which was close to 3000 ohms replaced.


75 volts 0.25 amps, much more like it, 18.75 watt! 100 volts at 0.36 amps, and tubes are starting to light. 120 volts at 0.45 amps. 54 watts! Time to hookup to antenna and Pioneer SX535 test receiver and taste the soup. We have tunes in mono and about 86 stations. Stereo lamp lights. The dial string is very loose, took off the top and now the slider moves nicely on the slide rule dial.


No stereo but the multiplex tubes are both good. 12AU7 measures 90 and 95. 6U8A measures 93 and 96.


L transformer is 19k and R transformer is 38k. The special yellow alignment tool fits nicely. 19Khz transformer is seized so added WD40, and now it turns. No stereo so far. Put both transformers back at the initial settings which were written in magic marker.

19Khz phase control one complete turn clockwise and one complete turn counter clockwise and no change in the mono signal, but all indicators are primed for stereo multipath, signal strength and stereo indicators are where they are supposed to be. Cathodes of V15 at pins 3 and 8 are important as they interface with what appears to be an optocoupler. Put a 100 ohm resistor across the cell and see if it goes into stereo!


I removed the super tall 6BN8 and opened the bottom of the chassis, and she is laying on her back. I found the four wire lamp/photocell. Lamp side is 4.5 ohms so it may work, the photocell side has a proper ground and also. I found a ¼ watt 100 ohm resistor, and will solder it in.

The cathodes of the 12AU7 should drop to about 200 ohms when tuning a stereo signal, lamp in opto should come on and CDS cell drop to low resistance. The check for the cell is put a 100 ohm resistor across the cell and see if the 12AU7 goes into oscillation at 38K, should then be in stereo. Optocoupler bypassed with 100 ohm resistor soldered in and connections verified. Thank you Larry Deruin sp?


We have stereo now but is sounds funny, must hookup scopes and generators.


Start MPX alignment with Sencore SG165 stereo signal generator.

Step 1 turn the MPX light adjustment completely clockwise. Check.

Step 2 hookup is fine and have both Fluke output and oscilloscope waveform. Ok I have a 100Mhz stereo signal going in and the voltage on the fluke was in volts, but now it is in millivolts. Ok but the peak was quite broad. Hold on now the 100pf capacitor disc is shorted. I do have a 100pf capacitor labeled 101. That is 10 followed by one zero.

Cut the legs close to the bad cap. Need long leads so used the old leads. Soldered in and connections verified. Still measures a short, but let’s repeat step 2. Ok it does not tune, so I put everything back to the way it was as best as I can tell.

Step 3 adjusts with a slot alignment tool. No tuning even after using WD40 twice. Tune for maximum amplitude 14.89 volts on the Fluke.

Step 4 Ok I just realized that the Sencore was not set to MPX signal. Start over.

Step 2 hookup is fine and have both Fluke output and oscilloscope waveform. Ok I have a 100Mhz stereo signal going in and the voltage on the fluke was in volts, but now it is 63 millivolts. Ok but the peak was quite broad.

Step 3 adjusts with a slot alignment tool. No tuning even after using WD40 twice. 15.11 volts on the Fluke.

Center of channel meter is illuminated but does not swing normally. It is centered though.

Go to FM alignment steps 4 and 5.

Step 4, needed WD40 to free up slug, and adjusted for maximum negative voltage.

Step 5, was right on zero, no adjustment needed. Put cover back on and reevaluate.

72 stations and ringing (distortion) is worse than ever.

Perusing through the internet trying to find a similar MPX distortion issue:
On the Fisher 400 checkout the C210 capacitor by the four matrix diode. It is connected to the four matrix diodes via 100k ohm resistors. Also R215 should be centered. This is the separation potentiometer. Also check C41 on the discriminator circuit. V7 the 6au6 has pins 2 and 3 connected to a .005 capacitor which is definitely shorted. Grid 3 is shorted to the heater, effectively shorting out the discriminator. Hold on now, pin 2 is grounded and pin 3 goes to the heater which is close to ground. So this makes a short less likely.

Back to the McIntosh MR71 with distorted stereo that did not improve with the alignment per the service manual. Maybe there are out of spec parts on the MPX board.
However the 330pf capacitors may be shorted; C85 and C86! Let’s look at the oscillator area. Yes the 100k resistor measures like 138.5k ohms, replace that primarily and retest. Also the rainbow capacitor measures short in circuit. Either 360,000 ohms in parallel according to the parallel resistor calculator or 100k ohms. I have a 98k ohm resistor. V squared divided by R is 0.25 solve for v. 0.25 x 100k is voltage squared. 158 volts is the 0.25 watt dissipation. It goes between the two different coils in the 19kHz oscillator transformer. Maximum voltage on schematic is less voltage at 146 volts and 96 volts in the general vicinity. Soldered in and connections verified. Now can I be courageous and flip the unit right side up without shorting anything? No that would result in a physical crunch of the MPX board.60 stations most in stereo. Still have distortion! Must pull board if twiddling the phase does not help. Must pull board and can do so without desoldering wires, since the wires are long enough! I do have the separation modification for the McIntosh MR71! I may have a later model since there are 10uf capacitors in the output. Screws for the MPX board are on top and right by both 19Khz and 38Khz transformers.


Inspecting the board all four diode matrix diodes are good. 2nd and 4th resistor that are marked 47k are actually 30k ohms, that would be R74 and R76. Actually they now measure 46.5k and 46.4k ohms with the Fluke leads reversed. All well and good. 22k resistor measures 26 ohms in circuit. That is fine since it is in parallel with a coil.

R77, 1 megohm resistor measures 532k. That is fine since basically it is in parallel with R78. 150k resistors measure 154k and 159k ohms. 1megohm resistors measure 1.05 and 1.02 megs and these are silver 5 percent tolerance resistors. 4800 ohms measures 4966 ohms. R57 100k ohm resistor measures 73k ohms both ways. 103.6k out of circuit. This is quite suspicious since C57 100pf is in parallel with such and measures 260k ohms. Out of ciruit the capacitor is infinite ohms. Is this a smoking gun? No after replacement of capacitor we still measure 74.4k ohms or so.

R58 which should be 220k measures 78k both ways! Mesures 252.k ohms out of circuit! Bad by nearly 15 percent. Is this also a smoking gun? Probably. But there is like 2.2 megohm resistance now, less likely a bad cap?And it measures 0.047uf and the spec is .05uf, so leave it in and find a 220k ohm resistor. Soldered in and connections verified.

R60 is 1.05 megohms within 5 percent. 4800 ohm resistor is 4900 ohms. R61 470 ohms resistor is 504 ohms within 7 percent. Replacement measures 460 ohms. R62 2700 ohm resistor is 2639 ohms within 2.2 percent. 100 megohms measures infinity and that probably is OK and beyond the range of the Fluke.

We have measured all the resistors in circuit, now we need to clip a lead and measure the four questionable ones out of circuit. R57 and R58 are so superficial I may desolder just one lead. However this solder joint is at least 50 years old, so I may have to add new solder to desolder these joints. This completes all the resistors.


Since C68 is 258k ohms will replace with a 101 capacitor which is 100pf. 101 cap is physically larger than the 102 cap, which is in two bins!


Let’s now measure 14 capacitors!

Capacitor Spec Measured

C81 5nf 7nf Right on the resonant circuit diodes

C80 5nf 3nf Right on the resonant circuit diodes

C86 5nf 6.7 Right on the resonant circuit diodes
C83 5nf 3.0 Right on the resonant circuit diodes

Measuring again and switching polarites I can get both 3 and 7nf so the average of such is 5nf right on spec. Leave the existing ceramic caps in circuit.

C89 20nf 18.5

C90 20nf 18.5
C84 330pf shorted in circuit! Measures 335 out of circuit reinstall!
C88 430pf shorted in circuit! Measures 435 out of circuit reinstall. Resoldered in and connections verified
C87 430pf shorted in circuit! Symmetric resistance so likely fine
C85 330pf shorted in circuit! Symmetric resistance so likely fine
C73 1nf 1.7nf vs replace, choose replace. 102 measures 1000pf

closest match after measuring 7 of these soldered in and connections verified!

C74 1nf 1.83nf replace original or replace. 102 capacitor measures 973pf within 2.7 percent; perfect install this one. Soldered in and connections verified, one end may go to coax and end at pin 7 of tube 6BN8, true and connections verified.


New soldering iron tip, the old one had loss of tip.

C68 already analyzed and replaced

C69 already analyzed and it is good.

This concludes the capacitor analysis replacement on the MPX board.

Test transistor no leads to hookup 3 leads. So must test with the primitive multimeter vs the Sencore TF26. The 132-042 crosses to an NTE123AP a general purpose NPN silicon transistor. Both junctions test out properly as good diodes! Remount the MPX board vs just test it.


On the 19khz oscillator transformer, C79 the 4.7nf mica capacitor is exactly 4.7nf. Resoldered in and connections verified. C78 the 4.7nf mica capacitor is exactly 4.77nf.

Carefully adjusting the 38k Hz oscillator slug changes the frequency of the ringing but it is still there, darn I cannot find my hex alignment tools. The phase adjustment does not matter, and neither does the 19Khz oscillator, but the 38Khz oscillator is very important. Carefully adjusting the top and the bottom slug of the 38Khz oscillator worked magic! We have terrific stereo separation now, without audible distortion. Power down and reinstall cover.

 

[backmd]

Hi, hoping to help someone here is the story of a happy Mcintosh MR71 repair:
Mcintosh MR71 1-2-21

No dangerous shorts, infinite ohms, needs 1 amp slow blow. I think the tubes are appropriate. Have service manual from hifiengine, replaced with proper 1 amp slow blow. 7.6 ohms and no dangerous shorts to chassis. Power up on variac now. 25 volts 0.5 amps, starting at 8pm. 35 volts at 0.5 amps at 805pm. 35 volts at 0.5 amps at 845pm. 17.5 watts, and there is no tube rectifier, so the capacitors are seeing the full voltage now. At 40 volts we see unstable spikes in current, but stabilizing at 0.55 amps for 22 watts. 93 degrees at the capacitor now. 70 watts is the spec. so that would be 0.7 amps at 100 volts. 50 volts at 0.7 amps at 917pm. Transformer and capacitor are 103 degrees. 12 tubes with heaters. 55 volts 0.75 amps burning 41.25 watts. R lower quadrant of transformer is 112 degrees while the can capacitor is also 112 degrees. Burning 44 watts now 55 volts at 0.8 amps. I went up to 0.9 amps and things started to become unstable in that Mallory filter capacitor can. Stay at 0.8 amps and 55 volts. Now it is 0.75 amps at 55 volts, time to watch the news. 130 degrees on the can. 55 volts at 0.95 amps is 52 watts at 160 degrees. Take it way down now. Down to 0.3 amps overnight at 25 volts. Nah shut her down and let it cool.


Serial number 85B13. how to remove panlocks. One of the selenium diodes is shorted! The one closest to the front! Both diodes replaced with 10 amp 1000 volt silicon diodes and soldered in and connections verified. All four can capacitors measure well and good. One Sprague 40uf capacitor measures 50uf out of circuit and reinserted into the circuit. 2200 ohm resistor which was close to 3000 ohms replaced.


75 volts 0.25 amps, much more like it, 18.75 watt! 100 volts at 0.36 amps, and tubes are starting to light. 120 volts at 0.45 amps. 54 watts! Time to hookup to antenna and Pioneer SX535 test receiver and taste the soup. We have tunes in mono and about 86 stations. Stereo lamp lights. The dial string is very loose, took off the top and now the slider moves nicely on the slide rule dial.


No stereo but the multiplex tubes are both good. 12AU7 measures 90 and 95. 6U8A measures 93 and 96.


L transformer is 19k and R transformer is 38k. The special yellow alignment tool fits nicely. 19Khz transformer is seized so added WD40, and now it turns. No stereo so far. Put both transformers back at the initial settings which were written in magic marker.

19Khz phase control one complete turn clockwise and one complete turn counter clockwise and no change in the mono signal, but all indicators are primed for stereo multipath, signal strength and stereo indicators are where they are supposed to be. Cathodes of V15 at pins 3 and 8 are important as they interface with what appears to be an optocoupler. Put a 100 ohm resistor across the cell and see if it goes into stereo!


I removed the super tall 6BN8 and opened the bottom of the chassis, and she is laying on her back. I found the four wire lamp/photocell. Lamp side is 4.5 ohms so it may work, the photocell side has a proper ground and also. I found a ¼ watt 100 ohm resistor, and will solder it in.

The cathodes of the 12AU7 should drop to about 200 ohms when tuning a stereo signal, lamp in opto should come on and CDS cell drop to low resistance. The check for the cell is put a 100 ohm resistor across the cell and see if the 12AU7 goes into oscillation at 38K, should then be in stereo. Optocoupler bypassed with 100 ohm resistor soldered in and connections verified. Thank you Terry Dewick!!!


We have stereo now but is sounds funny, must hookup scopes and generators.


Start MPX alignment with Sencore SG165 stereo signal generator.

Step 1 turn the MPX light adjustment completely clockwise. Check.

Step 2 hookup is fine and have both Fluke output and oscilloscope waveform. Ok I have a 100Mhz stereo signal going in and the voltage on the fluke was in volts, but now it is in millivolts. Ok but the peak was quite broad. Hold on now the 100pf capacitor disc is shorted. I do have a 100pf capacitor labeled 101. That is 10 followed by one zero.

Cut the legs close to the bad cap. Need long leads so used the old leads. Soldered in and connections verified. Still measures a short, but let’s repeat step 2. Ok it does not tune, so I put everything back to the way it was as best as I can tell.

Step 3 adjusts with a slot alignment tool. No tuning even after using WD40 twice. Tune for maximum amplitude 14.89 volts on the Fluke.

Step 4 Ok I just realized that the Sencore was not set to MPX signal. Start over.

Step 2 hookup is fine and have both Fluke output and oscilloscope waveform. Ok I have a 100Mhz stereo signal going in and the voltage on the fluke was in volts, but now it is 63 millivolts. Ok but the peak was quite broad.

Step 3 adjusts with a slot alignment tool. No tuning even after using WD40 twice. 15.11 volts on the Fluke.

Center of channel meter is illuminated but does not swing normally. It is centered though.

Go to FM alignment steps 4 and 5.

Step 4, needed WD40 to free up slug, and adjusted for maximum negative voltage.

Step 5, was right on zero, no adjustment needed. Put cover back on and reevaluate.

72 stations and ringing (distortion) is worse than ever.

Perusing through the internet trying to find a similar MPX distortion issue:
On the Fisher 400 checkout the C210 capacitor by the four matrix diode. It is connected to the four matrix diodes via 100k ohm resistors. Also R215 should be centered. This is the separation potentiometer. Also check C41 on the discriminator circuit. V7 the 6au6 has pins 2 and 3 connected to a .005 capacitor which is definitely shorted. Grid 3 is shorted to the heater, effectively shorting out the discriminator. Hold on now, pin 2 is grounded and pin 3 goes to the heater which is close to ground. So this makes a short less likely.

Back to the McIntosh MR71 with distorted stereo that did not improve with the alignment per the service manual. Maybe there are out of spec parts on the MPX board.
However the 330pf capacitors may be shorted; C85 and C86! Let’s look at the oscillator area. Yes the 100k resistor measures like 138.5k ohms, replace that primarily and retest. Also the rainbow capacitor measures short in circuit. Either 360,000 ohms in parallel according to the parallel resistor calculator or 100k ohms. I have a 98k ohm resistor. V squared divided by R is 0.25 solve for v. 0.25 x 100k is voltage squared. 158 volts is the 0.25 watt dissipation. It goes between the two different coils in the 19kHz oscillator transformer. Maximum voltage on schematic is less voltage at 146 volts and 96 volts in the general vicinity. Soldered in and connections verified. Now can I be courageous and flip the unit right side up without shorting anything? No that would result in a physical crunch of the MPX board.60 stations most in stereo. Still have distortion! Must pull board if twiddling the phase does not help. Must pull board and can do so without desoldering wires, since the wires are long enough! I do have the separation modification for the McIntosh MR71! I may have a later model since there are 10uf capacitors in the output. Screws for the MPX board are on top and right by both 19Khz and 38Khz transformers.


Inspecting the board all four diode matrix diodes are good. 2nd and 4th resistor that are marked 47k are actually 30k ohms, that would be R74 and R76. Actually they now measure 46.5k and 46.4k ohms with the Fluke leads reversed. All well and good. 22k resistor measures 26 ohms in circuit. That is fine since it is in parallel with a coil.

R77, 1 megohm resistor measures 532k. That is fine since basically it is in parallel with R78. 150k resistors measure 154k and 159k ohms. 1megohm resistors measure 1.05 and 1.02 megs and these are silver 5 percent tolerance resistors. 4800 ohms measures 4966 ohms. R57 100k ohm resistor measures 73k ohms both ways. 103.6k out of circuit. This is quite suspicious since C57 100pf is in parallel with such and measures 260k ohms. Out of ciruit the capacitor is infinite ohms. Is this a smoking gun? No after replacement of capacitor we still measure 74.4k ohms or so.

R58 which should be 220k measures 78k both ways! Mesures 252.k ohms out of circuit! Bad by nearly 15 percent. Is this also a smoking gun? Probably. But there is like 2.2 megohm resistance now, less likely a bad cap?And it measures 0.047uf and the spec is .05uf, so leave it in and find a 220k ohm resistor. Soldered in and connections verified.

R60 is 1.05 megohms within 5 percent. 4800 ohm resistor is 4900 ohms. R61 470 ohms resistor is 504 ohms within 7 percent. Replacement measures 460 ohms. R62 2700 ohm resistor is 2639 ohms within 2.2 percent. 100 megohms measures infinity and that probably is OK and beyond the range of the Fluke.

We have measured all the resistors in circuit, now we need to clip a lead and measure the four questionable ones out of circuit. R57 and R58 are so superficial I may desolder just one lead. However this solder joint is at least 50 years old, so I may have to add new solder to desolder these joints. This completes all the resistors.


Since C68 is 258k ohms will replace with a 101 capacitor which is 100pf. 101 cap is physically larger than the 102 cap, which is in two bins!


Let’s now measure 14 capacitors!

Capacitor Spec Measured

C81 5nf 7nf Right on the resonant circuit diodes

C80 5nf 3nf Right on the resonant circuit diodes

C86 5nf 6.7 Right on the resonant circuit diodes
C83 5nf 3.0 Right on the resonant circuit diodes

Measuring again and switching polarites I can get both 3 and 7nf so the average of such is 5nf right on spec. Leave the existing ceramic caps in circuit.

C89 20nf 18.5

C90 20nf 18.5
C84 330pf shorted in circuit! Measures 335 out of circuit reinstall!
C88 430pf shorted in circuit! Measures 435 out of circuit reinstall. Resoldered in and connections verified
C87 430pf shorted in circuit! Symmetric resistance so likely fine
C85 330pf shorted in circuit! Symmetric resistance so likely fine
C73 1nf 1.7nf vs replace, choose replace. 102 measures 1000pf

closest match after measuring 7 of these soldered in and connections verified!

C74 1nf 1.83nf replace original or replace. 102 capacitor measures 973pf within 2.7 percent; perfect install this one. Soldered in and connections verified, one end may go to coax and end at pin 7 of tube 6BN8, true and connections verified.


New soldering iron tip, the old one had loss of tip.

C68 already analyzed and replaced

C69 already analyzed and it is good.

This concludes the capacitor analysis replacement on the MPX board.

Test transistor no leads to hookup 3 leads. So must test with the primitive multimeter vs the Sencore TF26. The 132-042 crosses to an NTE123AP a general purpose NPN silicon transistor. Both junctions test out properly as good diodes! Remount the MPX board vs just test it.


On the 19khz oscillator transformer, C79 the 4.7nf mica capacitor is exactly 4.7nf. Resoldered in and connections verified. C78 the 4.7nf mica capacitor is exactly 4.77nf.

Carefully adjusting the 38k Hz oscillator slug changes the frequency of the ringing but it is still there, darn I cannot find my hex alignment tools. The phase adjustment does not matter, and neither does the 19Khz oscillator, but the 38Khz oscillator is very important. Carefully adjusting the top and the bottom slug of the 38Khz oscillator worked magic! We have terrific stereo separation now, without audible distortion. Power down and reinstall cover.

 

[backmd]

Bummer just turned it on and it does not work. So now I get to use the Sencore SG165 MPX generator and the Tektronix 150Mhz four channel oscilloscope. Kind of looking forward to the diagnostic challenge.

 

[backmd]

6DS4 bad rf tube, there are like two stations now, and the signal strength is zero! Ordered one from that auction site

 

[backmd]

6DS4 bad rf tube, there are like two stations now, and the signal strength is zero! Ordered one from that auction site

Nope, six bad shorted ceramic capacitors, just in the RF section of the McIntosh MR71. After 50 years even ceramics go bad.

 

[Pio1980]

I generally avoid receivers and FM tuners of this period for precisely these reasons, the inevitable can problems are a royal pain in the ass to deal with, I don't have the resources to align them afterward, and radio in general is no longer worth the bother in many areas.

 

[Destructor]

Time to put my MX110 and 67 out on the curb.

 

[dcgillespie]

What's your address?

 

[clinic-audio]

Yes !
Tell us where your curb is

 

[Destructor]

It's a long drive.

 

[Pio1980]

Time to put my MX110 and 67 out on the curb.

You don't have FM stations worth the bother up there in Massachestnuts?

 

[Destructor]

Actually I do and I love my tube tuners. The 110 needs an alignment. I also have two nice tube receivers. A Scott 340B and a Fisher 400. I currently use a Pioneer SX828 as a tuner. I hardly ever use my Mac tuners anymore. I used to use my 110 with an MA230 that I modified as a stand alone tube amp. It was a great combination. I need to concentrate on restoring the 230, another unfinished project.