MX110 tuner popping sound

Google search hasn't revealed this exact problem, so here goes. My recently-acquired 1967 MX110Z has been "checked out," tube pins and controls cleaned, and aligned by a local electronics shop, and given a clean bill of health (not convinced all was done properly). Prior to that, I checked all tubes with my Knight tester, and all are serviceable. It's all original to include the power supply and all caps. Yes, still has the selenium rectifier, etc.
Upon turning on, Phono 1 and 2 work as advertised (haven't connected anything to aux yet), but the tuner sound is kind of thin and muffled, then after about 20 minutes of operation, it makes a mild pop through the speakers. Frequency response and especially stereo soundstage open up, and volume increases considerably. Listening to it right now, and approximately 40 minutes after the first pop, or one hour total, it's cycled twice (went weak for a number of seconds, then it became strong again) in the past couple of minutes.
Terry's backlogged for almost three months, so "send it to Terry" is not what I'm looking for right now, if it's an easy enough fix.
I'd appreciate any and all help.

did you try gently rocking tubes yet to see of the pins are connecting good ?

OctalSocket said:

Google search hasn't revealed this exact problem, so here goes. My recently-acquired 1967 MX110Z has been "checked out," tube pins and controls cleaned, and aligned by a local electronics shop, and given a clean bill of health

Click to expand...

Here are a few handy reference links:

http://www.geofex.com/ampdbug/popping.htm

http://www.geofex.com/ampdbug/ampdebug.htm

petehall347 said:

did you try gently rocking tubes yet to see of the pins are connecting good ?

Click to expand...

Further to Pete's suggestions, you need to first determine if the preamp is the problem or if the problem is the tuner. From the description of the problem, if the phono inputs work without hearing the problem, then the tuner is the likely source of the problem.

If this is the case, gently rock the row of tubes - one at a time, of course - that are in a row down the right hand side. There should be no scratchy sounds when you do this coming from the preamp's outputs.

Next, gently twist the last metal can on the right rear corner. Listen for scratchyness. Flick it with your finger to see if you hear any of those popping sounds. Move to the next metal can - they're I/F transformers - toward the front and do the same thing. If you get any of them where you can cause the problem at will, indicates a clear problem with the FM.

Lastly, do you hear any popping sounds when you switch the Input Selector or the Mode Selector? Popping sounds when these controls are switched indicates leaky caps - there's an MX-110 Service Bulletin on that, too.

Hope that gets you started on a diagnosis.

Cheers,

David

Yesterday, I was able to dedicate the time to troubleshooting my MX110 with your helpful suggestions. Within 30 minutes of power up again, it started exhibiting the same symptoms. Once that started, I gently twisted and tapped the IF transformers, and got it to pop a bit with each of the four transformers. None were loose, but only the minutest attempt at movement induced a pop. However, repeated jiggling and tapping could not get any of them to induce the popping. I rocked and tapped on all tubes (except the phono 12AX7s) and that had no effect. For the next 8 hours, it operated perfectly.
Another thing I noticed when it was acting up, along with reduced volume/soundstage, the MPX light was considerably dimmer. When it popped back to proper operation, the light brightened as well.
Since the malady is not continuous and occurs randomly, I haven't had the opportunity to see if it pops while the input selector is on FM, rather than MPX.
As for the input selector, no, it doesn't pop when changing sources.
It's been running for a little over an hour today, and it hasn't acted up, which is good news, but I'm curious as to the permanency of the "fix."

Quick check, how does it behave in mono mode vs. MPX mode, if it sounds OK in mono the problem is in the MPX section, if mono mode is also bad it is in the IF. To get the best reaction when troubleshooting the IF you will have to remove the bottom and check each can. To check a can I power up the tuner and using a pair of insulated needle nose pliers I give each pin on a can a wiggle and pull (gently), 9 out of 10 times the bad can will pop back to life or give a good indication of a problem. Remember you are playing with B+ and the IF may be de-tuned when you grab a pin but the problem should be noticeable.
If the tuner works fine in the mono mode the MPX probably is at fault, T6 (19K xfmr) or L5 (68K filter) are first to check, with C60 at the 19K phase coil next.
C60 is an easy check, unsolder one end of the capacitor, if the noise stops the cap is bad.
A quick hint when trouble shooting a 110, make sure the audio path is OK by using the AUX input or Tape Monitor input, then if the tuner is at fault check the Mono mode as the MPX is out of circuit in momo.
On the 'Z' Tape Monitor bypasses the 6D10, one stage less in the audio path.

Murphy struck again. Just after I submitted the previous post, it popped and became louder. Come to find out, it was in the "weaker" condition since I turned it on 90 minutes ago. I had a suspicion it was taking a higher knob position to deliver the same volume, but couldn't be positive. Shows how much of a small incremental change it's exhibiting, but when it goes from one to the other, it's unmistakable.
Wiggling and tapping the 5 transformers and all tubes is still having no effect. Ugh...

Dirty volume control or dirty Tape Monitor switch.....very common problems.

Just connected a CD player to the Aux this evening, and it sounds wonderful and strong, just as the phono does. While playing, I switched over to FM or MPX, the volume/sound quality was considerably diminished. I'll troubleshoot those IF cans in a few days per Terry's suggestions (greatly appreciated) and see what comes of that.