Restored MX-110z - one channel out on phono only

[jaaronb]

Hey folks - first and foremost, thanks to the wealth of information y’all have posted on here over the years, I’ve been enjoying the fire out of a really nice condition MX110z I picked up a few months ago.

Short story is, I’ve got a nice MX-110z I’ve been rebuilding a step at a time over the past few months. Looks great, sounds better. All serious listening up till now has been through AUX. FM works fine.

Finally got around to trying out the phonostages the other night with a Mac-obsessed buddy, we get no output from the left channel on STEREO (but man that one channel sounded glorious). Switch to mono, we get mono out of both speakers. Swith L+R to L, we get sound out of the left channel. But when there’s discreet signal to L channel, with no right blended in, I get nothin outta the left side.

More info and trying to answering questions in advance:

I’ve rebuilt a few other early Mac tube amps, and for this considerably more intricate project, I followed the AK drill: I scoured the rebuild threads and Hints n Kinks here, and benefited greatly from the generous advice of Dshoaf and Terry D and many many others who generously gave guidance and helped troubleshoot in the many MX-110 rebuild threads here on AK. All that info helped immensely.

So the rebuild:

Per Dshoaf’s advice, I used the preamp bone stock for a few weeks - rebuilding a section, listening for a few days or weeks, moving on to the next section. It sounded great with no issues stock and improved at each stage of the rebuild.

Here’s what I’ve done so far with days or weeks of listening between each step:

- Replace all three multisection caps with new production multisection cans.
- Replace rectifier bridge with full wave diode.
- Replace the below decks electrolytics in the power supply.
- Replace the two .1uf Sprague black beauties at the input selector.
- Replace the two .1 uf ‘lytics at the front bulkhead.
- Replace all ceramics and tropical fish in the signal path with identical values - used a combination of Miflex KPCU, Auricaps, Russian K75-Y10 hybrid oil and 2 each Audyn True Copper Max and Mundorf Silver Oil.
- Install thermistor after the fuse.
- Removed/cleaned front glass (got lucky here with sturdy printing on the glass).
- Removed foam/cleaned out and lubed the front rocker switches.
- Fader lubed all front pots (but forgot the input gain pots - these do all function with no noise though)
- Replaced the two 5 watt resistors after the rectifier bridge to lower HV supply. I used 2.2k there - HV supply is now ~10% stock.

To try to address this left channel out in phonostages only issue, we:

-swapped 12ax7 and 6u8 tubes and tried known good types of these tubes
-Turned the knobs, switched the switches repeatedly.
-Rolled the input gain knobs.
- tried a different turntable all together
- tried different phono rcas
- Tried phono 2

In all cases, the issue persisted with no change.

What I HAVEN’T done:
-Lube the input gain pots
-Try a new Compactron tube
- Replace the two real small value ceramics opposite the front side of the bulkhead (on the right, looking to the front of the unit with the bottom facing up. Not sure offhand, these may be in the loudness circuit(?)
- replace the two tiny value caps on the 12ax7s no one ever seems to bother with.

This is the first and only issue I’ve had with this unit - is sounded great from the beginning and better with each section I rebuilt. Never any pops, hisses or scratchiness. It’s always been dead quiet.

At the risk of facing Audio Karma ridicule, I’ll post pics of the unit and my work tomorrow.

I really appreciate any ideas or guidance about the source of this problem or figuring out what it might be, or ruling out what it isn’t.

My Lenco PTP/SME 3012 is about to be transformt from a table of parts to an actual turntable in the next few days, so I’m anxious to get this figured out!

Thanks in advance!

 

[mnmmt]

It sounds like you have done everything right so far. I would next suspect a wiring/continuity problem. If you have a scope you could trace the signal in from the rca jacks to see whereit disappears. If not, physically follow the wiring in from the rca jacks, testing or resoldering connections and testing for continuity along wire runs and across resistors. Also pay special attention to the connections to the socket of the 12ax7 for that channel. Perhaps a broken wire or slipped solder joint along the way. The input selector has a lot going on, so may be worth looking at extra closely. Good luck--I recently picked up a very heavily and poorly modded mx110z for my winter project and i am certain i will run into many problems like this.

 

[jaaronb]

I know you’re right about this being the most viable next step. I guess I could apply some Faderlube to the phono gain pots and see if that gets me anywhere.

I was hoping to hear it might be a bad Compactron tube or something, you know....easy!

 

[dcgillespie]

A couple of symptoms can help determine where to concentrate your efforts. As stated before, your general efforts should concentrate around V13, which is the left channel phono preamp tube. In doing that then:

1. If you advance the volume control up high to near maximum, do you hear the rush of the phono preamp? If so, then the problem is on the input side of the preamp -- that is, from the input jacks through the selector switch, and into the preamp itself. If you don't hear the rushing sound, then the problem is either the preamp itself, or the selector switch on the output side.

2. Is V13 in fact lighting up? If so, good. But if not, check the wiring to pins 4, 5, and 9 of the tube.

2. Do you have a DVM? If so, have you made any voltage measurements of V13 to know that it's receiving the necessary power to operate? I didn't see where you had made any measurements in the list of things you've done.

A new compactron tube is not the issue in this case, as all selected signals (both high and low level) pass through the compactron tube. If the rushing noise cannot be heard on Phono 1 or Phono 2, it is doubtful that the selector switch or the phono level controls is the problem. It's highly unprobable that the selector switch has a problem on both settings, or that both level controls went out at the same time. Without any further information, this really sounds like an issue associated with V13.

Dave

 

[jaaronb]

Wow Dave - thanks so much for this thorough explanation.

I’ll crack her open tomorrow, follow your steps and report back. All the tubes, including the preamp section, were lighting up as I recall. I’ll confirm that tomorrow and check voltages as well.

Given the otherwise perfect functionality of this unit, the fact that v13 was among the most difficult and intricate spots in the circuit to work on, and my inexperience, it seems likely I goofed it up around that section.

Thanks again. Stay tuned.

 

[Bob the Tech]

I took a look at that schematic for a few minutes, that input select switching seems quite intricate with the multiple dual sided decks. I had a thought that you might isolate it the input switching without even powering the unit by feeding in a decent signal at the phono input, pulling the phono tubes, and monitoring for the AC signal at the V13 input pin. All that input switching is passive so you can check this without power. You could even go further and drive a signal in at the V13 output pin and see if you see it further down the line by pulling the next tube in series, ...