Odd problem playing LPs

[Paul Birkeland]

No current really flows through that resistor.  If it burned, that could indicate a severe short in the 12AU7 (which would be very rare) or some kind of contaminant shorting pin 6 to pin 7.  Does the resistor still read the correct value? 

[Larpy]

It does.

[Paul Birkeland]

Well at least you have that going for you! How are the Kreg voltages on the power supply SR board?

[Larpy]

I'll check tomorrow.  I need to call it a day.  But thank you so much for helping me.

Round two goes to the Demon.

I'm down, but I'm not out yet.

[Larpy]

OK, a new day, a good night's sleep, and a sunny demeanor have me back at my workbench.

Two things to report:

KregB on the D socket board measures 5.6v (B side output is 218v).
KregA on the D socket board measures 24v (A side output is 294v).

At this point, I have only the 12AU7 installed.

Second item:
I checked for continuity among the legs of all 6 transistors on the C socket board (above the 6922/7308) and there's no continuity except for the 2N2222 on the left channel side of the board.  That means this one needs to be replaced, correct?

[Paul Birkeland]

KregA on the D socket board measures 24v (A side output is 294v).

One of the resistors from grid to ground (pins 2/7) on that 12AU7 isn't doing its job.

I checked for continuity among the legs of all 6 transistors on the C socket board (above the 6922/7308) and there's no continuity except for the 2N2222 on the left channel side of the board.  That means this one needs to be replaced, correct?

Yes, that is definitely causing one of your issues.

[Larpy]

Some success!  I fixed the regulated B+ problem.  I now have 218v coming to both sides of the C socket board.  Kreg on the D socket board is 5.9v on both sides.

The problem turned out to be the 431 regulator.  The only reason I figured this out is because as I was reflowing the solder on the kreg wires between the socket and the board I had a feeling of déja vu.  I checked the Eros forum archives and, sure enough, almost 2 years ago I had the exact same issue playing LPs through my Eros ("sounds like a dirty stylus," I moped) and you eventually helped me diagnose the problem as the 431 regulator.  Then it was the B side; this time it was the A side. I still had extra 431s from fixing the problem then.  Since the legs of the "bad" regulator didn't seem to be shorted, I guess the problem was a questionable soldering job that took a few years to become a problem.

Once I get some more 2N2222s delivered next week and replace the shorted one on the C board, maybe I'll have a happy Eros again.

Thank you again for your help and amazing patience.

To be continued. . . .

[Larpy]

Replacements transistors arrived this morning, and I replaced all 5 on the "bad" side of the C socket board: 2 PN250s, 2 MJE350s, and 1 2N2222.  I also reflowed the solder on all the joints on that side of the boards.  And I replaced all the wires going to that side of the board.

Still no luck.  Here are my voltages, unchanged from before I changed out the resistors:

1A 217   IB 216
OA 163   OB 213
OC 97     OD 51
OKA 99   OKB 57
OKC 2.0  OKD -.003

I've checked all the connections on the 6922 and EF86 tube pins, and they all seem fine.

I'm at a loss.

[Paul Birkeland]

Replacing everything in one shot can make more problems than it solves since board rework is rather challenging. 

Do you still measure a short around the 2N2222?  It could be that there's something amiss on the board or socket that's shorting out the cathode.  Maybe try removing the wire going into OkD and measure the resistance between the earth post on the chassis and that loose wire that leaves pins 3/8 on the EF86 socket to see if there's a short there.

[Larpy]

Excellent advice, Paul (as usual).  I did have a short between that side of the board's cathode circuit and ground, and the problem turned out to be the tube itself.  Its cathode (pin 3) was shorted to pins 2 and 7 (the tube's internal shield?).  I checked the other tube (on the "good" side of the Eros), and there was no continuity between pin 3 and pins 2 and 7.  Whether the tube died from suicide or murder (by my blundering hand) I don't know, but it was previously a healthy tube.  Somehow I managed to take out a 431 regulator and a 2N2222 as well.

So now I have all the DC voltages back to where they should be. 

But!  I still have the distorted sound I initially heard that prompted my first post.  I was hoping it was the 431 regulator, but apparently not.  Apparently, all the troubleshooting I've been doing over the past week was just tracking down the problems I created myself in trying to diagnose the distortion sound.

I hear it in the center of the soundstage and most noticeably on louder passages.

Paul, you suggested earlier that it's a problem with the preamp's AC, since I heard the distortion even while measuring the correct DC voltages.  The DC is happy but somewhere the AC is not.  A dodgy connection to ground perhaps?  Would anything else cause these symptoms?

[Paul Birkeland]

  Apparently, all the troubleshooting I've been doing over the past week was just tracking down the problems I created myself in trying to diagnose the distortion sound.

This sounds very familiar 

Paul, you suggested earlier that it's a problem with the preamp's AC, since I heard the distortion even while measuring the correct DC voltages.  The DC is happy but somewhere the AC is not.  A dodgy connection to ground perhaps?  Would anything else cause these symptoms?

Yes, I suspect there's something that's not in the DC current flow path that's poorly connected and causing AC issues.  If you have a phone with a headphone jack on it, you can play music at one volume setting above mute into your Eros and poke around while it's playing to see if you can change the behavior of the issue.

[Larpy]

Another head-scratching update:  I could not reproduce the distortion using my phone to feed a low level music signal through the Eros.  The EQ was horrible, of course, but the sound was clear.

One thing I noticed is that one of my Sowter 1990s has half as much secondary resistance as the other (860Ω vs 1.4KΩ).  I find that alarming but I didn't notice the channels sounding different through headphones when I listened to just the left or the right channel.  So I don't think the distortion is coming from one of the Sowters.  (But I wonder if I should contact Sowter and ask about the impedance mismatch.)

In fact, I don't think the distortion is coming from the Eros.  When I play records, the distortion is now constant, but I didn't hear the distortion in the music played through the Eros from my phone. 

I found an old Koetsu cartridge in my box of used cartridges that still might have some life left, so I installed it on the Linn LP12 and the distortion was quite noticeable, so that rules out the new Hana ML I bought.  I replaced the tonearm cable from my Linn tonearm, just to test that possibility (I have two stock Linn tonearm cables, and they can be fiddly and need to be installed just so).  No difference.

Could the distortion be coming from my LP12's tonearm?  It's a Linn Ekos and the only thing about it I don't like is that it has a very small headshell area and that makes fitting cartridges difficult.  The Hana wanted to be mounted back further in the headshell than other carts, and I ended up very slightly bending (or so I thought) two of the four cartridge posts on the Ekos--you know, the gold plated pins that emerge from the tonearm where you install cartridge leads.  If I disturbed the wiring connection inside the Ekos, could that explain the distortion I'm hearing?

[Paul Birkeland]

I would absolutely try a different TT. 

The Sowter DCR should be about dead nuts on between sides, so I'd also look to be sure all of that wiring is correct and that the same loading resistors are present on both channels. 

It does sound like you're narrowing things down a bit though!

[2wo]

How about cranking up the tracking Force to a reasonable limit and giving that a test... John

[Larpy]

I went up to 2.5 grams on the Hana but heard no difference.  Since I heard the same distortion through the Koetsu, I now think the problem is not the cartridge.  I'm leaning toward the wiring inside the tonearm.  I have a talent for destroying tonearm wiring--done it at least twice over the years, and this might well be the third time--so I'm going to take this opportunity to get the Ekos rewired.  Even if it turns out not to be the source of the distortion I'm hearing, the guy who is going to rewire it will use a single run of Cardas wires from cartridge leads to RCA jacks: no DIN plug or headshell plug/pins, which will make fitting cartridges onto the headshell a gazillion times easier.  Fitting most cartridges onto an Ekos is like trying to fit into the pants you wore in high school.

I'm going to contact Sowter about the SUT impedance mismatch.  I desoldered the low-measuring one's output wires and even out of circuit they measure half what the other one does.

Eventually, I'll get all this figured out and, when I do, I'll post a follow-up.  I suspect it's going to be awhile though.