McIntosh MR80 Stereo Mode not working.

[Drummerboy2]

It's been awhile since I last posted here about my MR80 after I recapped it in the Power Supply and Audio section.

Ever since I bought it and it was told to me by the seller the Stereo mode didn't work. After I got it and noticed it had been recapped but some if the caps were not correct so with Service Manual in hand I went back and recapped the PS and Audio sections with the proper values and better parts. Everything worked except the normal issues with the Touch Preset and Scan buttons which I don't care about, I just want the Stereo mode to work. I'm able to pulling stations from 120-125 miles away clear unless weather throughs a wrench into it.

All stations come in clear and have strong signal, the stations are dead on with the only think I notice that the Upper End and Lower End stations are slightly out of the range. All functions work as they should as well.

I read awhile back here that D229 and D230 may cause this and that the person fixed stereo issue by replacing those two Diodes.

Does anyone have any suggestions other than sending it in for a full service (short on funds for awhile)? Any help advice will be appreciated.

I found the Thread where this guy said how he fixed this. This was over on the FM Tuners Group.

"Well, after lots of studies, parts replaced tests, asking for advice and so on, I finally solved the problem. The above mentioned issues got suddenly solved once I replaced the two detector diodes D229 and D230 at the end of <3> RF/IF stage. These are 1N5711... The old ones tested ok but they let a +2.1 V DC get into the multiplex board <4> that trowed off the IC 905 Deviation detector making the circuit think it is always off-station and blocking the above.."

His Stereo symptoms were the same as mine.

 

[w3jn]

Most likely the stereo decoder is running off frequency. Put a freq counter or scope at the circled point labeled 228 KHz on the schematic below. Adjust R908 carefully for exactly 228 KHz. The 228 KHz is divided down to 19 KHz, where the decoder phase locks to the signal's stereo pilot. It needs to be fairly close for it to lock in. As I recall that adjustment is fairly touchy.

Otherwise, check the voltage at pin 3 of IC901 when exactly tuned to a station (or no station/no antenna). As the other person found, if there's a DC offset it'll upset that whole circuit. Besides the bad diodes, the discriminator could be out of alignment.

Finally, check the voltage at pin 9 of IC902. If that pin is at ground potential the IC is being forced into mono mode.

 

[Drummerboy2]

Most likely the stereo decoder is running off frequency. Put a freq counter or scope at the circled point labeled 228 KHz on the schematic below. Adjust R908 carefully for exactly 228 KHz. The 228 KHz is divided down to 19 KHz, where the decoder phase locks to the signal's stereo pilot. It needs to be fairly close for it to lock in. As I recall that adjustment is fairly touchy.

Otherwise, check the voltage at pin 3 of IC901 when exactly tuned to a station (or no station/no antenna). As the other person found, if there's a DC offset it'll upset that whole circuit. Besides the bad diodes, the discriminator could be out of alignment.

Finally, check the voltage at pin 9 of IC902. If that pin is at ground potential the IC is being forced into mono mode.

View attachment 3094699

Thank You for the reply and info. I will look into these points and will get back. I have a Scope so I should be able to get a accurate measurement. I will also order those 1N5711 Schottky Diodes for the D229 and D230.

I did forget to mention that the Stereo like comes on every so often but rarely. I have gotten the Stereo light to come on with stations about 121 miles away in Super Narrow mode and antenna with rotor.

Hopefully it's just an adjustment and or bad diodes. Like I said everything about the stations works great and has strong signal reception with stations stable with no creeping movement. If I do push the Lock button it will skip to the next strongest station. Filter, Muting, and Narrow/Super Narrow all work fine.

 

[w3jn]

Don’t replace anything unless you prove it to be bad, or evidence such as voltage or scope readings point in that direction. Just because some guy had that problem doesn’t at all mean yours does.

I had a MR78 with a bad IC equivalent to IC901. That manifested itself in no stereo and a low audio output.

None of this means that’s your problem. That you have a scope will make this immeasurably easier.

 

[Punker X]

Going to echo this and, try the adjustment first. Sometimes you don't even need a counter to get it working. Just a small tweak will bring it in. Said it before and now again. Probably 95% or more of no stereo problems are not a failed component, but is an alignment issue. MR80 was top of the line when new and probably deserves a proper alignment after all these years, in addition to the new components that have been introduced.

 

[Drummerboy2]

Don’t replace anything unless you prove it to be bad, or evidence such as voltage or scope readings point in that direction. Just because some guy had that problem doesn’t at all mean yours does.

I had a MR78 with a bad IC equivalent to IC901. That manifested itself in no stereo and a low audio output.

None of this means that’s your problem. That you have a scope will make this immeasurably easier.

No, wasn't going to replace anything blindly, I was going to order parts for another project and was going to order those diode just cause since I was ordering already. I was going to do those things you mentioned then report back my results.

I was going over the SM last night on those areas you mention and the instructions.

Also I did notice after reading the SM about the Upper End and Lower End max's and mine is a little off. I will not mess with this until I do the other steps first. Don't want create any more issues on top of what I already have going on, I will deal with those issues first before moving on to others.

I will get into this tomorrow when I have the time alone to concentrate on this.

Thanks again.

 

[Drummerboy2]

Going to echo this and, try the adjustment first. Sometimes you don't even need a counter to get it working. Just a small tweak will bring it in. Said it before and now again. Probably 95% or more of no stereo problems are not a failed component, but is an alignment issue. MR80 was top of the line when new and probably deserves a proper alignment after all these years, in addition to the new components that have been introduced.

Hi Punker X, Yes, I agree. I feel from how the MR80 is performing that it must be something slightly off because the Stereo light, Stereo mode, and Auto filter work some times. Also as I'm manually Scrolling through the stations from one end to the other end I see the Stereo light come on a few times.

I was listening to KUSC College Classical station that is about 125 miles from me and the stereo light came on for quite awhile while in the Super Narrow Mode and positioning the Antenna on the roof with the rotor to a certain point

Also I did notice after reading the SM about the Upper End and Lower end max's and mine is a little off.

Like said above I will do those steps first and what results I get.

Thanks.

 

[w3jn]

The MR80 is one of the more complex tuners I've aligned. Since you're getting stereo sometimes, and in super narrow, the signal level to the demultiplexer chip might be low enough it's not locking on the 19 KHz pilot. Try adjusting R905 (it's on the stereo demod board, near where you're gonna test for 228 KHz). Note its initial position so you can return it to its original spot if that doesn't help. You don't want that level too high, the demod chip wants about 1V of signal and distortion rises either side of that.

Setting the band limits isn't hard, but since that voltage tracks the front end it may need an entire alignment. The front end uses varactors to resonate the RF amp, LO, and mixer input and if the voltage is off they won't track correctly. Whether they are tracking correctly now or will track correctly if you reset the band limits is anyone's guess. Just a caution before you dive into that.

 

[Drummerboy2]

The MR80 is one of the more complex tuners I've aligned. Since you're getting stereo sometimes, and in super narrow, the signal level to the demultiplexer chip might be low enough it's not locking on the 19 KHz pilot. Try adjusting R905 (it's on the stereo demod board, near where you're gonna test for 228 KHz). Note its initial position so you can return it to its original spot if that doesn't help. You don't want that level too high, the demod chip wants about 1V of signal and distortion rises either side of that.

Setting the band limits isn't hard, but since that voltage tracks the front end it may need an entire alignment. The front end uses varactors to resonate the RF amp, LO, and mixer input and if the voltage is off they won't track correctly. Whether they are tracking correctly now or will track correctly if you reset the band limits is anyone's guess. Just a caution before you dive into that.

Ok, cool! This why I asked about the Upper Lower end limits. I had a feeling that would most likely mess things up if I adjusted them.

I will do the 228Khz check/adjustment on R908 and then take a look at the R905 adjustment next. Noted to the initial position before I move them.

I will post before any adjustment pics and then the after the adjustment pics with scope and front panel of the tuner.

Does it matter if it's in Narrow or Super Narrow and Lock button IN or OUT while looking at those TP you instructed me to look at? I know you said just to tune into a station to check, just want to make sure I'm not missing anything.

Thanks Again.

 

[w3jn]

One thought - if your scope has X/Y capability you can connect the TP1 and TP2 jacks on the back of the MR80 to the scope and set it for X/Y, about 1V/division. Center the dot on the scope before you connect the cables. This will give you an idea of the IF passband and if it's centered on the discriminator. You should see a symmetric arc on a nice strong signal, when it's center tuned the arc will be symmetric either side of the center line. This however is presuming you're exactly tuned on frequency.

I stole this image from someone's for sale ad (and the scope is a Mac MPI-4 but it will be exactly the same if you set your scope up right) to give an indication of what you'll see. This one's just a little bit tuned off center.

 

[Drummerboy2]

One thought - if your scope has X/Y capability you can connect the TP1 and TP2 jacks on the back of the MR80 to the scope and set it for X/Y, about 1V/division. Center the dot on the scope before you connect the cables. This will give you an idea of the IF passband and if it's centered on the discriminator. You should see a symmetric arc on a nice strong signal, when it's center tuned the arc will be symmetric either side of the center line. This however is presuming you're exactly tuned on frequency.

I stole this image from someone's for sale ad (and the scope is a Mac MPI-4 but it will be exactly the same if you set your scope up right) to give an indication of what you'll see. This one's just a little bit tuned off center.

View attachment 3095759

Yes, I have a 100Mhz Rigol Scope with X/ Y functions. when I mentioned TP in my post I was referring to Test Point you marked on the schematic. I could try the X/ Y function, I tried this function awhile back on a another tuner. The display wasn't as clear as the one in the pic when I was looking at it when trying to use like a TUBE Scope on the Marantz tuner, could have been because the display was on Coarse instead of Fine. I was going to buy one of these Mac MPI-4 but were hard to find at the time.

I will try this tomorrow as well.

 

[Drummerboy2]

So as I was watching a movie I had the MR80 on tuned into that KUSC Classical station in LA. The Stereo light was off most of the day but it turned on for about 15 mins until the signal got low on the indicator and went OFF and is still off now. It sound like it was in stereo, then when the Stereo light went off the soundstage shrunk.

I snapped a pic while it was on.

 

[w3jn]

Yep, separate test points, TP1 and TP2 are RCA jacks on the rear of the MR80; the 228 KHz test point you'll have to find, I think I had to put the probe on one of those resistors in the circuit as theres no test point on the circuit board.

 

[w3jn]

Whoops, neglected to answer your questions.

The LOCK will engage the AFC; that is, it will adjust the local oscillator a bit such that the discriminator output is 0V. The SUPER NARROW position will improve the sensitivity somewhat, at the expense of tuning criticality. So with those two points in mind, they way I'd approach this is:

1, disconnect the antenna and tune somewhere there's no signal at all. Check the voltage at pin 3 of IC901. Should be zero or very close to it. If not, the discriminator is out of alignment or there's a bias issue/bad diodes in the discriminator. But before you go wholesale replacing components -

2, Connect an antenna, connect your scope to TP1 and TP2, tune to a nice strong station. Center the arc using the MR80's tuning knob. Engage LOCK and NARROW. Hopefully the arc will be nicely centered and symmetric - when you engage LOCK does the arc shift left or right? Now try SUPER NARROW. The width of the arc should change as well as its shape but it still should be centered. If neither are the case again I'd suspect discriminator misalignment. If the LOCK doesn't work right and/or there's a considerable voltage at pin 3 of IC901, perhaps those diodes are bad.

3, All that said you might be able to work your way around this without re-aligning the discriminator by adjusting the two pots we discussed previously - do so with LOCK and NARROW engaged. Note that if you adjust R905 for more gain this will also increase the DC error output from the discriminator that goes to the AFC circuit and will possibly create issues with the LOCK function.

Do you have a good, accurate signal generator capable of 10.7 MHz and 75 KHz deviation? If not DO NOT mess with the discriminator transformer, chances are you'll just make it worse.

 

[Drummerboy2]

Whoops, neglected to answer your questions.

The LOCK will engage the AFC; that is, it will adjust the local oscillator a bit such that the discriminator output is 0V. The SUPER NARROW position will improve the sensitivity somewhat, at the expense of tuning criticality. So with those two points in mind, they way I'd approach this is:

1, disconnect the antenna and tune somewhere there's no signal at all. Check the voltage at pin 3 of IC901. Should be zero or very close to it. If not, the discriminator is out of alignment or there's a bias issue/bad diodes in the discriminator. But before you go wholesale replacing components -

2, Connect an antenna, connect your scope to TP1 and TP2, tune to a nice strong station. Center the arc using the MR80's tuning knob. Engage LOCK and NARROW. Hopefully the arc will be nicely centered and symmetric - when you engage LOCK does the arc shift left or right? Now try SUPER NARROW. The width of the arc should change as well as its shape but it still should be centered. If neither are the case again I'd suspect discriminator misalignment. If the LOCK doesn't work right and/or there's a considerable voltage at pin 3 of IC901, perhaps those diodes are bad.

3, All that said you might be able to work your way around this without re-aligning the discriminator by adjusting the two pots we discussed previously - do so with LOCK and NARROW engaged. Note that if you adjust R905 for more gain this will also increase the DC error output from the discriminator that goes to the AFC circuit and will possibly create issues with the LOCK function.

Do you have a good, accurate signal generator capable of 10.7 MHz and 75 KHz deviation? If not DO NOT mess with the discriminator transformer, chances are you'll just make it worse.

I saw this too late, I already adjusted the 228Khz and that did the trick. I know have stereo on all the stations that I should have. You weren't lying its a little tricky but I zoned in on it.

Thank You again for all the help and Patience with my questions.

 

[Drummerboy2]

Whoops, neglected to answer your questions.

The LOCK will engage the AFC; that is, it will adjust the local oscillator a bit such that the discriminator output is 0V. The SUPER NARROW position will improve the sensitivity somewhat, at the expense of tuning criticality. So with those two points in mind, they way I'd approach this is:

1, disconnect the antenna and tune somewhere there's no signal at all. Check the voltage at pin 3 of IC901. Should be zero or very close to it. If not, the discriminator is out of alignment or there's a bias issue/bad diodes in the discriminator. But before you go wholesale replacing components -

2, Connect an antenna, connect your scope to TP1 and TP2, tune to a nice strong station. Center the arc using the MR80's tuning knob. Engage LOCK and NARROW. Hopefully the arc will be nicely centered and symmetric - when you engage LOCK does the arc shift left or right? Now try SUPER NARROW. The width of the arc should change as well as its shape but it still should be centered. If neither are the case again I'd suspect discriminator misalignment. If the LOCK doesn't work right and/or there's a considerable voltage at pin 3 of IC901, perhaps those diodes are bad.

3, All that said you might be able to work your way around this without re-aligning the discriminator by adjusting the two pots we discussed previously - do so with LOCK and NARROW engaged. Note that if you adjust R905 for more gain this will also increase the DC error output from the discriminator that goes to the AFC circuit and will possibly create issues with the LOCK function.

Do you have a good, accurate signal generator capable of 10.7 MHz and 75 KHz deviation? If not DO NOT mess with the discriminator transformer, chances are you'll just make it worse.

I do have a nice Kenwood Signal Generator that I think is capable of 10.7, don't know about the 75KHz deviation though. I will look into the voltage on IC901, should be ok since I'm getting Stereo now but when I engage the Lock function, it went to the right/Up onto the next station Up from where it was at. I will check this on all stations, may have just been because the next station up was stronger in Signal?

Those diode you mention that may be bad IF I have those conditions you mentioned, are those the D229 and D230 that I posted about?

 

[w3jn]

Excellent! Yeah, upon further reflection I guess I shoulda recommended you start with the simple stuff first.

In any case I wouldn't mess any further with it, unless you wanna hook up TP1 and TP2 to your scope and check out the multipath display. That's a really cool feature that Mac added to most of their tuners and it's useful in both tuning in a station as well as alignment.

The AFC range is really wide on the MR80 and it could be you didn't have the weaker station centered and when you hit the LOCK it zipped to the next station up (or down) that was a lot stronger. I've noticed this on my MR80 and MR78 (both carefully aligned by me) so it's not really indicative of a problem.

If the stereo is working fine, you achieved your goal and unless the audio is very distorted, the diodes are fine. Further messing about might make it worse.

 

[Drummerboy2]

Excellent! Yeah, upon further reflection I guess I shoulda recommended you start with the simple stuff first.

In any case I wouldn't mess any further with it, unless you wanna hook up TP1 and TP2 to your scope and check out the multipath display. That's a really cool feature that Mac added to most of their tuners and it's useful in both tuning in a station as well as alignment.

The AFC range is really wide on the MR80 and it could be you didn't have the weaker station centered and when you hit the LOCK it zipped to the next station up (or down) that was a lot stronger. I've noticed this on my MR80 and MR78 (both carefully aligned by me) so it's not really indicative of a problem.

If the stereo is working fine, you achieved your goal and unless the audio is very distorted, the diodes are fine. Further messing about might make it worse.

The frequency was off and measuring 225KHz. Took a few attempts but got into spec. Hopefully it holds there. I was remembering when I was adjusting that R908, that someone mentioned a long time ago on a thread to look into that Pot but didn't give any specifics and I didn't want to touch anything without know what I was doing. I took the measurements right at where the Jumper goes to the edge of the board.

Also, I remember replacing that cap right next to that jumper wire when I recapped the tuner after finding some caps not correct with the SM.

Audio is clear and sounds wonderful. I really don't care about the touch buttons for the Presets and Scan Up and Down.

I will play with the tuner for the rest of the days and see if I find any other issues but I'm a happy camper right now.

Here's a pic.

 

[w3jn]

The touch buttons and scan is a real digital can of worms. Could be simple, could be complex, but I guarantee you if that board needs to come out that holds all the pots for the tuning presets it's a giant pain in the ass. Even adjusting the up/down band limits is a bit of a challenge as the trim pots aren't very easy to access as I recall. I had to replace the tuning pot on my MR80 because some knucklehead sprayed some kind of cleaner into it that removed all the carbon deposited on the phenolic piece inside the pot. Those ball-bearing reduction pots are hard to find so I swapped one of the preset pots with the tuning pot, and removed the phenolic carbon element from a junkbox pot. So DON'T spray anything into those tuning pots unless you want a multi-day jigsaw puzzle!

That said, the scan depends on the stereo working so you may try it now and see what it does.

But congrats on owning one of the finest consumer tuners ever made. It may not be #1 of all time but it's way up there, and in my opinion one of the best looking tuners ever.

 

[Drummerboy2]

I never would spray contact cleaner into any tuning pot, I would just replace it.

I have read so many Horror stories on others trying to fix those touch buttons and then to have all kinds of issues when they go back to put everything back together. No Thanks!

My MR80 sat idle in it box after I did the PS and Audio PCB upgrades. I was going to sent it to Terry Dewick but he stopped working on them. Then I was going to send it to Mike Z. here in San Diego but got a new job and was way to busy with that and other stuff. So had time so I pulled it out last week and let in break in for the week before I posted my Issue here. I just had a gut feeling it may be something simple as the stations were very strong and now issues pulling in distant stations.

This is one of the best Tuners and best looking. I happy now to have back in my main system and enjoying it right now.

Thank You again for all the help!