MR74 squealing noise

[Max77]

I have an MR74 that was worked on by Modafferi in '08 and when using a cheapie wire dipole I get overall pretty good reception here in Chicago. I picked up a pair of rabbit ear antennas with a tuning knob and mostly in the 88-92 band I get a very loud squealing when tuning stations. Sometimes I will fine tune a station that has a good signal and a slight touch of the tuning knob on the MR74 will send the tuner putting out a high pitch squeal. Sometimes if I wait a few seconds it will finally stop squealing and tune. Other times it doesn't and I have to continue to fine tune, jumping between a strong signal and the squealing. This happens on one other station in the 94MHz band, but is not at all an issue anywhere else on the dial. Using the tuning knob on the antenna can switch the antenna from perfect reception to squealing as well.

When I'm using my wire dipole, I get the squealing on 1 or 2 stations but nothing major. Using narrow selectivity seems to fix this to some degree, but if I tune a station using narrow and switch to wide, that may induce the squealing noise. Also this seems to be a much bigger problem at night as opposed to the day.

Any ideas on what may be wrong here? I can't imagine it to be an alignment issue since it was just done recently and above 95Mhz there are absolutely no issues and aside from the squealing noise while tuning, everything is where it should be on the dial. Perhaps some RF issue since the problem seems increased by the better antenna? I also thought it might be because the 88-92 band is very crowded here. Thanks!

 

[jlovda]

One of the stations in the 88-92 section of the band may be broaadcasting a subcarrier signal.

 

[dshoaf]

Here's what you can try to start with:

- Rock the tuning dial across the low end of the band a bunch of times. Yes, I know that 'bunch' isn't really a good term but do this back and forth 10 to 20 times. Then see if you still have the problem or if it moves elsewhere.

I'm suspecting you have some oxidation/grunge in the tuning cap. It can be cleaned but requires the removal of the cover, which in turn, requires removal of the bottom cover and getting into the chassis with a nut driver. Then you'll need to know _exactly_ where to use contact cleaner on the tuning cap.

I would always suspect mechanical problems before the electronic ones. The fact that you have changed the antenna may or may not be problematic. Is it powered? I know that those cheap wall-wart power supplies can cause all sorts of birdies up and down the dial.

Cheers,

David

 

[Max77]

Both antennas I've used are unpowered. I also unplugged all switching power supplies nearby to see if they were causing any noise. Last night I did go up and down the 88-92 band quite a bit trying to figure things out and that seemed to clear the issue up briefly. The fact that the tuning was so touchy did seem mechanical, kind of the same way a dirty pot is touchy. However, the fact that if I let it sit on the squeal for a few seconds it eventually tuned in lead me to believe it may be another issue. There is quite a bunch of dust built up behind the glass since '08, so it might be worth taking it apart soon to do a cleaning. Thanks for the input and I'll try that again tonight!

Edit: I do know that the previous owner only listened to 2 stations on this tuner, then only 1 and finally 0 after the last one went off the air. Perhaps dust has built up on the tuning cap as a result of the dial not being moved the last 3+ years. Though I'm not sure why this would be worse at night than day. Strange issue.

 

[dewickt]

David is right on the button, the original lubricant on the grounding fingers to the rotor of the tuning capacitor has hardened and the contact is bad causing the RF amplifier to oscillate randomly. This can be cured buy use, end to end tuning, or more reliably by cleaning and re-lubricating the fingers. This is very common in older tuners of all makes, once cured you will need to do a front end alignment to get you tracking and sensitivity back to where it belongs.

 

[Max77]

Terry, thanks for the confirmation. After reading David's post I did go back and work the wheel considerably between the affected band and that seemed to cure the issue. Must be from lack of use by the previous owner. Thanks guys! I've been having a lot of fun with this tuner and getting some amazing reception.

 

[Max77]

Thought I'd follow up here in case anyone comes across this in a search, or cares to listen I took the MR74 apart tonight only to realize the tuning cap is easily accessible via 1 screw on the top. There didn't seem to be any dried up lube, but there were some tiny brown bits of what looked like solder resin from the tuning cap on some of the fins and a couple pieces stuck between the fins. I didn't have canned air or anything to clean it so I left it as is for now. I will probably at the least spray some canned air in there and hope that helps solve the issue, which is rare now that I've been using the tuner a lot.

 

[dshoaf]

Here's a link to a Stickey over in the Tuner forum discussing basic cleaning of tuners. It goes on for a while but I'd suggest you go to post #41.

Also, here's a pic from that thread with arrows where you'd want to use just a bit of contact cleaner: http://www.audiokarma.org/forums/attachment.php?attachmentid=245161&d=1292440449

The MR-74's variable cap will look somewhat different but they all pretty much work the same. The key for you is to find those points where the moving fin shaft touches some brass fingers that make the contact. That's your sweet spot for cleaning.

Cheers,

David

 

[Max77]

David, thanks for the link, I was poking around that thread earlier but didn't get to that pic.

I also noticed a lot of light bleeding through the front face of the tuner, after removing the glass it turns out that is because all of the foam has been removed. So I've got a foam replacement project scheduled for this weekend. I did manage to give the glass a nice cleaning to remove all the built up dust. After the foam replacement I should have one very nice looking tuner!