MR80 display ranging
- Thread starter Gregory
- Start date
kevzep
Its all about the Music
Funny you should post about this now, I just had to pull the MR80 out of service and slip the MR78 in the rack as mine has started the continuous scrolling of the display again, I haven't had a look at it yet as I have another job on the bench at the moment and I also have the engine out of my car, so a lot going on here, but when I get to it I'll share what I find.
There is obviously some sort of failure in the pre-scaler I would guess.....
Gregory
Soundco Kid
This issue seems to be common for the 35+ y/o MR80. Some tech shops certainly know the cause and fix, but are keeping it to themselves.
Another possibility: below is what Terry DeWick said in post #50 of this thread.
"You are down to needing a Scope to check the level of the signal going to the counter assembly, possibly the input buffer in the counter is weak or the signal low from the RF module."
I've counted (at least) three AKers that have a ranging MR80 display in the posts here. It's time to reach out for answers.
-Greg
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kevzep
Its all about the Music
Indeed, and when I get some time I will be looking into it, I know I am going to have to have it on the bench for quite some time, so I will clear these jobs, get my engine sorted and back in the car, then I will look at the MR80....
I will share what I find so others can have some insight.
Gregory
Soundco Kid
Did that in Jan 2013.
https://audiokarma.org/forums/index.php?threads/mr80-power-supply-recap.492147/
-Greg
kevzep
Its all about the Music
*******UPDATE*******
Now that the engine is back in the car (well, has been since last year), I have had time to put the MR80 back on the bench.
New Zealand is locked down hard due to Covid, so today two weeks in, it was a day dedicated to the MR80.
To re-cap this problem.
The display will randomly scroll very fast, it has a fair amount of instability. The actual tuner will stay on the station its tuned to.
Here's what I did today.
I checked all the power supplies focussing on the +/- 5VDC which runs the Frequency Counter assembly.
I could not find any issues here. I even ran the Frequency Counter Assembly off my very clean bench supply to fully eliminate a supply issue.
I then checked the 200khz Oscillator, then followed the frequency dividers, base calculators and so forth, I just could not find any issues here.
As I started to run through the rest of the testing procedure for the Frequency Counter assembly. I could not find anything wrong, yet there it was, the display unstable, and then randomly scrolling.
How the frequency counter driver works is it receives the Carrier Frequency + 10.7mhz (Intermediate Frequency). This is from the Oscillator + Buffer with carrier wave. It then goes through a 3 stage buffer and then into some dividers and multipliers and other mysterious cmos chips and then drives the display to read the right number.
Part of the testing procedure is to unplug the RCA plug which comes from the Oscillator in the Front end circuit and short the plug out. This then makes the display read a nominal number they call the test pre-load number.
Now, as soon as I disconnected that plug, the display stopped scrolling and being unstable.
It read the 89.3 as it was supposed to, and solid as a rock.
The next few processes came up fine as well.
Then I read that to test the display you could inject a Carrier Wave into the input of the Frequency counter board and it should read your carrier wave "XXmhz" - 10.7mhz.
So I tried this, and at first it wasn't working, because I didn't really know how much level it was going to take to drive the circuit.
This is where it gets interesting.
As I increased level it got to a point where it started influencing the display, but it wasn't accurate, and the display was unstable, and I thought for a minute it might have been the triple line receiver chip they use as a buffer, but I thought, what the heck, I started increasing the level, and it got to a point of threshold where all of a sudden the display snapped into the number it was supposed to be reading.
I was injecting 100.7mhz to get a reading of 90mhz on the display, and it was reading 90.0 mhz solid. I had my spectrum analyser on the input of the Frequency counter driver circuit, and I took this and verified the three gain stages in the triple line receiver and this was all stable and working correctly.
I then swept through the entire range the frequency readout can do, and it was all working nicely.
I thought, right, it's not the Frequency Counter circuit. We need to possibly work back into the IF stage and check this oscillator output.
As soon as I plugged the oscillator back in, the display went crazy again. I wanted to look at this on the scope, but its not really possible to do this as the scope loads the circuit and pulls the signal right down, however I could see it on my spectrum analyser, but it was't able to show me how "noisy" the signal was, it only really shows there is a signal present.
I wasn't able to ascertain if the signal was clean or not, so I decided to replace the oscillator transistor. As luck would have it, I work on a lot of tuners and I had a suitable replacement, it had more HFE, but I thought that shouldn't be an issue, which is wasn't, its just an oscillator.
So I pulled the original transistor and put it on the tester and it came up quite a bit lower in HFE than it is spec'd.
Anyway I fitted the replacement, and WOO HOO, IMMEDIATELY it started working properly!!
I then decided to do a full alignment on the MR80, so that took about 4 hours, there is a LOT to aligning a MR80, during the entire time, not one glitch or anything dodgy happening with the display.
I have been listening to it for over an hour and a half, its still fine. The Auto scan works, the main tuning works, but I have some issue with preset 2 and 4, but I am not concerned with that right now, I will diagnose that tomorrow, but I think I might need some of Patrice's gold springs.
But the presets that work and hold the frequencies now, and they never used to.
So I am going to leave the MR80 running overnight and see where its at in the morning.
I'll also tickle up the IF alignment in the morning too.
It does sound better than it ever has, but I think I can get it a bit better, I reckon I can get the THD a bit lower.
So here we are, right where I thought I would never be with this thing, its been in bits in my Lab for about 12 months.
Anyway, progress may have been made so we will see.
Fingers crossed, and if it does seem that this problem is fixed, I'll jump on that thread and update it.
Interesting to note, right at the beginning, Terry suggested the 5V may have been noisy or dirty causing the circuit to play up.
He was very close, looks like a dirty Local oscillator signal.....
Ferrari Side Strake Heaven!!
My car is the one in the middle.....
Now that the engine is back in the car (well, has been since last year), I have had time to put the MR80 back on the bench.
New Zealand is locked down hard due to Covid, so today two weeks in, it was a day dedicated to the MR80.
To re-cap this problem.
The display will randomly scroll very fast, it has a fair amount of instability. The actual tuner will stay on the station its tuned to.
Here's what I did today.
I checked all the power supplies focussing on the +/- 5VDC which runs the Frequency Counter assembly.
I could not find any issues here. I even ran the Frequency Counter Assembly off my very clean bench supply to fully eliminate a supply issue.
I then checked the 200khz Oscillator, then followed the frequency dividers, base calculators and so forth, I just could not find any issues here.
As I started to run through the rest of the testing procedure for the Frequency Counter assembly. I could not find anything wrong, yet there it was, the display unstable, and then randomly scrolling.
How the frequency counter driver works is it receives the Carrier Frequency + 10.7mhz (Intermediate Frequency). This is from the Oscillator + Buffer with carrier wave. It then goes through a 3 stage buffer and then into some dividers and multipliers and other mysterious cmos chips and then drives the display to read the right number.
Part of the testing procedure is to unplug the RCA plug which comes from the Oscillator in the Front end circuit and short the plug out. This then makes the display read a nominal number they call the test pre-load number.
Now, as soon as I disconnected that plug, the display stopped scrolling and being unstable.
It read the 89.3 as it was supposed to, and solid as a rock.
The next few processes came up fine as well.
Then I read that to test the display you could inject a Carrier Wave into the input of the Frequency counter board and it should read your carrier wave "XXmhz" - 10.7mhz.
So I tried this, and at first it wasn't working, because I didn't really know how much level it was going to take to drive the circuit.
This is where it gets interesting.
As I increased level it got to a point where it started influencing the display, but it wasn't accurate, and the display was unstable, and I thought for a minute it might have been the triple line receiver chip they use as a buffer, but I thought, what the heck, I started increasing the level, and it got to a point of threshold where all of a sudden the display snapped into the number it was supposed to be reading.
I was injecting 100.7mhz to get a reading of 90mhz on the display, and it was reading 90.0 mhz solid. I had my spectrum analyser on the input of the Frequency counter driver circuit, and I took this and verified the three gain stages in the triple line receiver and this was all stable and working correctly.
I then swept through the entire range the frequency readout can do, and it was all working nicely.
I thought, right, it's not the Frequency Counter circuit. We need to possibly work back into the IF stage and check this oscillator output.
As soon as I plugged the oscillator back in, the display went crazy again. I wanted to look at this on the scope, but its not really possible to do this as the scope loads the circuit and pulls the signal right down, however I could see it on my spectrum analyser, but it was't able to show me how "noisy" the signal was, it only really shows there is a signal present.
I wasn't able to ascertain if the signal was clean or not, so I decided to replace the oscillator transistor. As luck would have it, I work on a lot of tuners and I had a suitable replacement, it had more HFE, but I thought that shouldn't be an issue, which is wasn't, its just an oscillator.
So I pulled the original transistor and put it on the tester and it came up quite a bit lower in HFE than it is spec'd.
Anyway I fitted the replacement, and WOO HOO, IMMEDIATELY it started working properly!!
I then decided to do a full alignment on the MR80, so that took about 4 hours, there is a LOT to aligning a MR80, during the entire time, not one glitch or anything dodgy happening with the display.
I have been listening to it for over an hour and a half, its still fine. The Auto scan works, the main tuning works, but I have some issue with preset 2 and 4, but I am not concerned with that right now, I will diagnose that tomorrow, but I think I might need some of Patrice's gold springs.
But the presets that work and hold the frequencies now, and they never used to.
So I am going to leave the MR80 running overnight and see where its at in the morning.
I'll also tickle up the IF alignment in the morning too.
It does sound better than it ever has, but I think I can get it a bit better, I reckon I can get the THD a bit lower.
So here we are, right where I thought I would never be with this thing, its been in bits in my Lab for about 12 months.
Anyway, progress may have been made so we will see.
Fingers crossed, and if it does seem that this problem is fixed, I'll jump on that thread and update it.
Interesting to note, right at the beginning, Terry suggested the 5V may have been noisy or dirty causing the circuit to play up.
He was very close, looks like a dirty Local oscillator signal.....
Ferrari Side Strake Heaven!!
My car is the one in the middle.....
kevzep
Its all about the Music
Thanks John, it was a very interesting journey, you gotta love the discrete digital circuits!!That's really excellent Kev, the fault finding and the car!
I'm really hoping this is going to help a few guys here as there were a few members including Gregory here who seem to have the same issue.
The MR80 still working like a champ, solid, the most stable since I've owned it.....
kevzep
Its all about the Music
Yes this is correct.
Part of the fault with my MR80 was slight frequency drift, and it makes sense that the failing LO could contribute to this.
It would randomly go 100-200khz off frequency, you'd retune it and a few hours later it would do it again, and sometimes it would end up on a nearby station. Then, a while later it would start the fast scrolling on the display.
So in order for the tuner to work correctly, the LO needs to be rock solid as you would expect. With this tuner though, old as they are, its easy to head straight for the digital circuits as they are discrete and complicated which is both good, and bad.
After 5hrs and several power cycles and some preset fault diagnoses, the MR80 is still working like a champ.
kevzep
Its all about the Music
You'll get the supplement out and look for "Section 3".
Have a look at the schematic and you will be able to find Q205. On the right hand side on the edge you will see "15" which is the oscillator output.
You can follow this back to the oscillator circuit.
I also checked R226 and R227 were in spec, they are 820Ω and 82Ω respectively.
Here you go. 430 in stock, except I just bought 50!! They are at the end of their life cycle!!
https://nz.mouser.com/ProductDetail...=sGAEpiMZZMutXGli8Ay4kLoxswOeVTYq42hQxm5WN/E=
Whilst you're waiting, power it up, wait until it starts faulting. Then unplug the input (the grey RCA plug) from the frequency counter assembly.
When you do this, the display should then read 89.3. This is the test pre-load number.
When you plug the cable back in, it should start going crazy again if the oscillator is the problem.
Let me know how you go.
Gregory
Soundco Kid
This is the results I got in the US:
https://www.mouser.com/ProductDetai...=/ha2pyFaduhp0ImzzPRatiIlyEUlQQH/YUeOE4f5Das=
https://www.mouser.com/onsemi/RF-Bipolar-Transistors/KSC1674-Series/_/N-1z0zlrjZax1s5Z1yxk8hp
Where to order from? NZ?
Thanks
https://www.mouser.com/ProductDetai...=/ha2pyFaduhp0ImzzPRatiIlyEUlQQH/YUeOE4f5Das=
https://www.mouser.com/onsemi/RF-Bipolar-Transistors/KSC1674-Series/_/N-1z0zlrjZax1s5Z1yxk8hp
Where to order from? NZ?
Thanks
kevzep
Its all about the Music
Whenever I access Mouser, I am directed to the NZ site, I cant access the US site. The parts are all accessed from the same warehouse, so if I can see 430 in stock then there will be 430 in stock on the US site too.
You are looking for KSC1674YBU.
jbailey930
AK Member
Great news on the MR80! @Gregory has been looking for a fix for a long time. This will make him happy......
Also, beautiful F-car. Do you maintain that as well as your Mc gear?
Gregory
Soundco Kid
Which means: it's been sitting in the rack for about 10 years with a cover on top.
The Antenna switch on the MR80 top panel wore out from swapping Omni vs Yagi, and Omni vs Yagi array, which are relative antenna gain or signal strength comparisons. That was part of the Chicago and Baltimore FM Log experiments. Fortunately I ordered a NEW 4-ganged switch a long time ago and have it in stock.
It would be difficult to put an FM antenna with rotator on the roof of this three story house, and would probably be against HOA rules, never asked them. Lots of stations here in the MD/VA/DC area. The MR80 is good for classical music listening. https://weta.org/fm
And of course DX'ing.
-Greg
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kevzep
Its all about the Music
Thanks, and yes I do all the work on the Ferrari. I was actually an automotive mechanic in a previous life and still enjoy working on well engineered cars. I had the engine out to do the cam-belt service, as well as a bunch of other "while you're in there" things too..... Yes the engine comes out to do the belts, can't access them in the car. Its very easy to take the engine out though not nearly as big a drama as you might think.
I'm very excited to be able to finally work out what this issue is with the MR80 as mine has been sitting around for about 8 years, and from time to time I'd have a fiddle with it and think I'd fixed it only to have it fail on my soon after.
But its been rock solid since I fixed the issue, a decent alignment has put a spring in its step, the MR80 is an insanely good sounding tuner!!
I highly recommend aligning the MR80, there are so many parameters involved which makes it beneficial to stay on top if tis alignment.
kevzep
Its all about the Music
Replacing the transistor Q205 wont affect the alignment as long as the MR80 wasn't aligned with the LO malfunctioning.
It will make its tuning more stable.
Nice post showing us the area of concern.
What you can do whilst waiting is take the top lid off, power it up, wait for it to start faulting with the display scrolling.
Then, unplug that grey (gray) RCA plug which goes into the Frequency Display section.
When you do this, the display should read "89.3". This is its test pre-load number.
This will prove the oscillator is the issue.
** only do this if the display is in fact stable enough otherwise wait until the LO transistor is replaced**
Also check the frequency span, all you need is a small screwdriver, a DMM and a steady hand.
Using the main tuning control, take it all the way down, it should stop at 87.4mhz. The voltage should be reading 4.9VDC +/- 0.05VDC on Pin 18A Section 3.
Then take it all the way up to the top, where it should read 108.1mhz. The voltage reading on Pin 18A Section 3 should be 26.0VDC +/- 0.25VDC.
If the span is not correct, follow the alignment procedure B: Main Tuning knob Upper and Lower band limits. When you achieve the correct span, those voltages have to be correct, if they are not, it gets a little more complicated, you'll need to adjust the upper Vari-cap and lower Inductor in the LO.
You can do this without a RF gen, all you need is the DMM for this part, it will not upset any other part of the alignment.
You can also re-seat all those chips in that section if you haven't already. There are a couple of E-caps in there too, three to be exact. You might have already done them I think I remember reading about that.
Anyway let me know how this works out.
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Fast996
Mod that Vintage Gear
Hi,
I've been following this thread as I'm ready to send my MR80 in for a restoration. I purchased from a gentleman that was using it in his 2nd system and he was selling it as he was downsizing. I have not used it.
I would guess that a full recap would be in order and replacing the Q205 transistor. Anything else that you would recommend...other than the standard protocol? It is a late model CK374*
I've been following this thread as I'm ready to send my MR80 in for a restoration. I purchased from a gentleman that was using it in his 2nd system and he was selling it as he was downsizing. I have not used it.
I would guess that a full recap would be in order and replacing the Q205 transistor. Anything else that you would recommend...other than the standard protocol? It is a late model CK374*