MR80 display ranging

[kevzep]

Here is an update from Tim last night -


the maint/repair manual section 10 said short the input to the freq counter and observe the count; also remove one of the ICs (104) and observe the count.

It was late last night but I think the first test failed but the 2nd test worked. This suggested input components on the freq counter board. I considered injecting a sign gen source into the freq counter but didn’t find the specs for it until after the tests above suggested that the problem was u101 or u102.

Replacement u101 part will arrive Sat. The one in the unit is probably fine except in that socket it is likely to work its way out and become loose again. I didn’t pull it out to see if there was a bent or compromised pin but there may be; best to replace it (and it is a $<2 part).

Looks like good progress. The MR80 service manual is quite good, and has excellent circuit descriptions and test processes.
Whenever I replace IC's I always replace the socket or if it doesn't have one, fit a socket.
There is a type which have an improved and more reliable way of clamping onto the legs of the IC, they are like a round receptacle rather than a spade type socket. Your tech will know, we all use them.....much better and reliable connection.

 

[jbailey930]

I picked it up yesterday afternoon from Tim the tech. Been on now for several hours and the station's frequency maintains accurately on the display. I preset stations with the touch pads and it seems the fourth preset pad (furthest right) is not responding to touch......not a big deal as I listen to one or two favorites. And as I told @cawgijoe this isn't a car stereo. Eventually, I will get around to diagnosing that but for now happy its buttoned back up and behaving as expected. Will move it to the main system later this weekend.

Now may think about selling my MR77.....

 

[kevzep]

I picked it up yesterday afternoon from Tim the tech. Been on now for several hours and the station's frequency maintains accurately on the display. I preset stations with the touch pads and it seems the fourth preset pad (furthest right) is not responding to touch......not a big deal as I listen to one or two favorites. And as I told @cawgijoe this isn't a car stereo. Eventually, I will get around to diagnosing that but for now happy its buttoned back up and behaving as expected. Will move it to the main system later this weekend.

Now may think about selling my MR77.....

View attachment 2540741

Excellent to see this working now.

Now isn't the MR80 just one of the most amazing sounding tuners!! I just love mine, I even sold my MR78 to a friend recently because I thought I might replace it with another MR80!!
I can also recommend having it aligned, they respond extremely well to a good alignment, its a very long process on these units, I got mine exceeding factory specs!! I actually ran through the process twice, they are very sensitive to their adjustments.

Regarding the touch pad, it will be oxidation on the spring, I used some brass/silver cleaner on all my touch pads and they all work reliably. You have to take the front off, clean the contact part of the spring, and clean the contact part where the spring sits.
This is most certainly what the issue is, its very common.
Patrice over there in France has Gold Springs for the MR80 which make a more reliable contact, I was going to get a set, but I haven't had any issues since I cleaned all mine.

 

[jlovda]

I can also recommend having it aligned, they respond extremely well to a good alignment, its a very long process on these units, I got mine exceeding factory specs!! I actually ran through the process twice, they are very sensitive to their adjustments.

Never owned an MR80 but I believe they have ceramic filters in the IF. Wouldn't that make it simpler to align than an MR78?

 

[kevzep]

Never owned an MR80 but I believe they have ceramic filters in the IF. Wouldn't that make it simpler to align than an MR78?

The MR80 uses completely different topology than a MR78, I have aligned both, the MR78 is way easier and more straight forward to align...

 

[jbailey930]

Here is an update - the scrolling issue recurred after my last post. @cawgijoe neighbor took it back to review and found this -

Poking around the digital counter board, I see one unusual thing: there is a signal from one IC 103 to 105. It should be a nice square wave, from almost 0v to a little under 5v. But it is from about 2.5V to 5V. When IC105 is removed, the output from 103 looks good, so 105 looks like it is broken.
I’ve ordered 2 each, ic’s, about 4 different types, $25 or so, including the replacement for 105.
Not sure if this is a cause or a victim but the input of an IC should definitely not be pulling an input up.

Parts came today and I replaced two of them (I ordered 4 just in case) and now the freq display works fine. It sure is getting hard to find/order 14 and 16 bin through-hole parts!

Thrilled to see the frequency not roll all over the place. I’ve never seen LSTTL parts fail in this way; I hope there is not some intermittent power supply issue that takes a chip out every now and then. The power supply at the input to the counter board looked OK.

 

[twiiii]

About 10 years ago I had similar issues. I sent the tuner to Audio Classics because one of the designers of the 80 was on call if needed. As I remember in addition to cleaning and refurbishing the touch buttons. Two IC's were replaced along with two caps and three broken traces were repaired also. 133-081 & 133-078. Lamps were all replaced, too. The unit still wasn't right when it was received upon return so it was sent back. Another trace was repaired and a screw had come loose and became jammed against a circuit trace under a wire bundle.

 

[jbailey930]

About 10 years ago I had similar issues. I sent the tuner to Audio Classics because one of the designers of the 80 was on call if needed. As I remember in addition to cleaning and refurbishing the touch buttons. Two IC's were replaced along with two caps and three broken traces were repaired also. 133-081 & 133-078. Lamps were all replaced, too. The unit still wasn't right when it was received upon return so it was sent back. Another trace was repaired and a screw had come loose and became jammed against a circuit trace under a wire bundle.

I now think that the MR77/78 are better long term candidates. The digital components and ICs just may be too fragile and complex for long term. But for now I will be satisfied with my tuner pulling in the few FM stations worthy in this area. Cheers!

 

[kevzep]

I now think that the MR77/78 are better long term candidates. The digital components and ICs just may be too fragile and complex for long term. But for now I will be satisfied with my tuner pulling in the few FM stations worthy in this area. Cheers!

I've been using my MR80 as a daily driver since I repaired the display issue, its been a solid runner, not issues at all.
I think its the best sounding Mc tuner I have heard.

 

[twiiii]

I sold my 77 when the 78 came out and then heard the 80 and gave my 78 to my oldest kid. Both were re built 10 years ago for the 80 and a bit before that for the 78 at AC. Having heard the units side by side the 78 is better at pulling in very weak stations near strong stations and thats why I bought the 78. But when the two Universities up graded their transmitters I tried a 80 and for me it sounded more to my liking. I like the presets and the 80 doesn't drift like a 78 or 77. So I just push a button and listen and never have to worry as the units heats up and stabilizes like I did with the 77 and 78. The 80 has been with me 41 years has given me great service and many hours of enjoyment and now after rebuilding, maybe, I will get another 30 years, if I live that long. 10 down 20 to go.

 

[seacliffe301]

To those who have compared the 78 and 80 side by side and preferred the 80, I have a question. Were any of these 78’s used in the taste test a Modafferi modded unit? They had a slightly different, although subtle, characteristic about them. Just curious on how those units compared to the MR80. Thanks.