What's the best way to replace a signal strength meter on an MR78?

[eedork]

My recently acquired MR78 had a dead signal strength meter. I have sourced a new one, but am wondering how best to get at it. It is extremely tight behind the front panel .. any tips on how best to do this? It should only take a few minutes once I figure out how to get in there.

Thanks!
-Matt

 

[chuckworkb]

Maybe reach out to Ryan at Audio Classics for hints.

 

[jlovda]

I replaced one in my MR-77 about three yrs ago. I am making the assumption here that the front assemblies of the two models are very similar. It is tedious. Both the outer and inner glass must be removed. After removing the glass, you must unscrew the black steel back plate and move it forward. The moving dial pointer must be rotated upward and lifted slightly so the black plate can pass under it when moving outward. I remember there is a resistor that is soldered across the terminals and resistor hanging in the air that comes from inside the chassis and connects to one terminal. I unsoldered the wire going to one terminal and clipped the resistor off the other terminal. I am very hazy here but I believe I used two new resistors so as to have longer wires to work with. The parallel resistor was attached while the meter was out. The meter is held in position by pressure with a "clamp like" bracket on each side. The little window centers it in position. In my situation the meter worked fine but the back piece with the printing discolored. I had to use an MR78 meter as a replacement because the 77 meter was no long available. Because the little front window was a different size I had to disassemble the two meters and use the back piece from the 78 meter and put it on the original 77 meter. Sorry I can't be of more help. Maybe this thread can provide some help.
http://audiokarma.org/forums/index.php?threads/tuner-indicators-repair-mr74.699403/#post-9369039

 

[eedork]

Thanks John. It is definitely going to be tedious. I was hoping to find a way to do it without having to remove the entire front panel assembly, but I don't think that's going to happen. I'll take my time and open it up. I need to put new glass on anyway.

-Matt

 

[c_dk]

Have you tested the meter coil and found it open?

 

[eedork]

Yes, I believe the meter is open. It has a 4.7k resistor across the terminals which is what my DMM reads when I measure it.

-Matt

 

[jlovda]

Did you check with Mac Parts to see if the meter was still available? The part is 124-019. Their number is (800) 538-6576. Let me know what you find.

 

[eedork]

Hey John - yes, the meter is still available and one arrived a few days ago. I've got everything I need, I'm just dreading the disassembly. It should only take a few minutes to actually replace the meter once it is in a position where I can get my soldering iron on it.

-Matt

 

[c_dk]

What voltage do you have at tp1? Note the alighnment instructions vs. the service notes.

 

[eedork]

Hi c_dk - I'll have to check again tonight after work, but I recall seeing voltage on TP1 and it was varying as I tuned in to strong vs. weak stations. This was a few days ago, so I'll have to double check that.

-Matt

 

[clinic-audio]

Hello eedork

My recently acquired MR78 had a dead signal strength meter

Sorry but how can you say that your signal strength is dead ??

 

[eedork]

Hi Patrice,

I acquired this MR78 knowing that the signal strength meter was not working. Exactly why it stopped working is a mystery. However, it appears that the meter itself is actually open. I came to this conclusion because the meter has a 4.7k resistor across its terminals which is exactly what I measure with my DMM.

I have a new meter from McIntosh parts. As a simple test, I used clip leads to connect the new meter to the old one. The new meter seems to work as expected while the old meter is still dead.

At this point I'm fairly certain the old meter is open. I suppose it could have some debris in the movement, but either way, I need to remove it from the chassis.

-Matt

 

[eedork]

The meter has been replaced and it's working perfectly now. Thanks jlovda - I basically followed your instructions to get the job done. It was a little fiddly, but not too bad.

Now I need to order a new front glass and replace the handful of remaining electrolytic and ROE caps (the tantalums on the MPX have already been done). This is a great sounding tuner!

-Matt