MR78 Trimpots/Symptoms

[CLewis]

I had tried a search at the beginning of my troubleshooting and found nothing, so I am posting this in hopes of helping the next tech.

A customer's unit came to me with the following symptoms:
Very low and distorted audio output
Center of Channel Tuning (COT) meter pegged to the - side
Signal strength meter operating normally and indicating good sensitivity to the usual stations

Adjusting the bias pot (R603) would bring back the audio to some extent, but only through a VERY narrow range at the extremes.

Adjusting the COT meter pot (R610) would never put the meter in the center.

The trimpots were both defective. They are both supposed to be 500 ohms; R603 was almost 4k.
Replacement of both and re-cal brought audio and proper COT action back, but still left audio that was distorted at higher levels of modulation.

Signal tracing revealed symmetrical clipping of the final audio stages. Adjustment of Audio Level (R317) made no difference at all. Yep, yet another defective trimpot.

I ended up replacing all of the trimpots. Some were ok, but it just seemed prudent.

What a beautifully engineered tuner!

 

[clinic-audio]

A trim pot can't change of value without explanation . I suppose that a wrong spray has been used in some in this unit

I have refixed more than 20 and none had trim pot problems

 

[kevzep]

A trim pot can't change of value without explanation . I suppose that a wrong spray has been used in some in this unit

I have refixed more than 20 and none had trim pot problems

I agree, I haven't come across this issue either. Perhaps it was in a very humid environment near the ocean or something like that....
Discriminator wasn't off at all?

 

[jeffs79]

I agree, I haven't come across this issue either. Perhaps it was in a very humid environment near the ocean or something like that....
Discriminator wasn't off at all?

I've run into two open bias pots and another failed trim pot in 3 different MR-78's just in the past 3-4 years. Regardless of the reason for the failure, from my experience it's evident that they can fail.

 

[kevzep]

I've run into two open bias pots and another failed trim pot in 3 different MR-78's just in the past 3-4 years. Regardless of the reason for the failure, from my experience it's evident that they can fail.

well, there you go......
Yes they can fail and I guess as these units are ageing even more, then we can expect to see more of this, pays to keep an open mind right?

 

[CLewis]

The chassis was pretty nasty with dust, but no signs of excessive corrosion. There was also no evidence of cleaner spray residue. I was quite surprised to find even one of them defective - and none completely open, just a change in value

 

[twiiii]

Audio Classics has the original designer of the MR-77/78/80 available for repairs updates and calibration of the tuners mentioned. So that where I send my 78 and 80 when they need taken care of. He also has updates to make the tuners better than new. It ain't cheap. But they sound terrific.

 

[chazix]

Audio Classics has the original designer of the MR-77/78/80 available

!!!

(No doubt someone will correct me if I'm mistaken, but I believe Richard Modafferi passed away some years back.)

 

[twiiii]

Sorry for my error.

 

[kevzep]

!!!

(No doubt someone will correct me if I'm mistaken, but I believe Richard Modafferi passed away some years back.)

Are you sure, I was aware he was at AC, you can get the mods done to the MR78 etc etc......

 

[chazix]

Are you sure

Not so sure any more, honestly. I was going from my memory, which is pretty fallible. And since then I've tried surfing the web (which we all know is infallible), and not turned up anything.

D'oh!! I just figured out my error - I was confusing RM with the late James Bongiorno.

Sorry, all!