Odd problem playing LPs

[Larpy]

I took John’s advice and asked a friend to bring over his Music Hall turntable (fitted with a Shure MX97xe cartridge).  While I waited for him, I rewired my Eros input jacks to bypass the Sowter SUTs. Once my friend arrived, we replaced my Linn LP12/Ekos/HanaML with his Music Hall/Shure and put on side 1 of Radiohead’s Kid A.

I heard the same distortion as I did before(!).

I have some speakers and an old Technics receiver (made back when they had internal MM phono stages) set up in another part of the house, and so we plugged the Music Hall turntable into it and the Radiohead LP we had just played sounded fine through it.  No distortion.

So the problem I’m having has to be with my Eros.

What would cause the Eros to distort in both channels on louder passages (usually bass-heavy) even while all of its voltages are spot on?  It obviously can’t be the Sowter SUTs, since they were not in the circuit.

Last night I did the chopstick test on the PS board above the PT and on nearby ground connections but didn’t hear anything through my headphones.

I'm feeling baffled and defeated.  Any help will be hugely appreciated.

[Paul Birkeland]

I would expect the voltages to deviate once the problem has occurred.  Are you 100% sure that the voltages are all Ok when this happens?

[Larpy]

Yesterday I had the Eros' chassis inverted while I played an LP.  I heard the distortion and took measurements then and there.

I don't remember if I raised the tonearm or not while I took the measurements.  I'll repeat the exercise and take measurements while the Eros is processing a signal and report back.

[Paul Birkeland]

It would also be worth a shot to roll in a different tube set, just to rule that out. 

[ccmccull]

Have you ruled out the audio chain after the Eros? What do you have the Eros plugged into?

To test this, when you have the distortion happening, you could unplug the Eros output from its subsequent amp input and plug it into the aux jack of your techniques receiver.

On the grounding question, I'd chopstick all the points along the ground bus near the inputs. Since it's a single ground bus for both channels, a bad solder joint there could affect both channels simultaneously.

[Larpy]

Changing all four tubes makes no difference.  I just tried it again to confirm.

Voltages are spot on measured while hearing the distortion:

IA        217
IB        216
OA        162
OB        166
OC        99
OD        99
OkA     100
OkB     100
OkC     1.7
OkD     1.7

The Eros is plugged into a preamp.  I tried plugging the Eros into different input jacks on the preamp but the distortion followed.  The digital devices plugged into the other input jacks sound fine.

[Larpy]

One more thing, the distortion is no longer intermittent but constant.  Good news, I guess, from a diagnostic perspective.

[Paul Birkeland]

I would poke around to see if the chopstick test can yield any results.  I suspect you have some kind of ground conductivity issue that isn't in the DC current path (so your voltages are OK) but is very much present with AC. 

[ssssly]

I had a similar intermittent, then constant, then intermittent distortion issue with my Eros a while back. After months of searching it ended up being badly oxidized tube pins and sockets. Cleaning the sockets with a torch tip cleaner, some steel wool on the tube pins and deoxit cleared it up.

I would reflow the joints for the ground bus while I was poking around in there as well. Gremlins can frequently take up residence in those crowded joints over time.

[Larpy]

Yes, I think you, Colin, and Paul B were all on target with the suggestion to probe the ground buss.  Doing so elicited some pops and I reflowed some solder joints.  Strangely, the distortion got worse rather than better.  Then i desoldered most of the connections on the ground buss and reassembled and resoldered them and the right channel sounded distortion-free but the left channel sounded like a 94 year old lifetime chain-smoker struggling for breath.  More probing with a chopstick revealed a questionable connection to the 7308's left channel plate.

I started reflowing solder on the board above the 7308 tube and, well, you know where that led.  Solder bridge along the legs of a transistor.  That led to removing the board to remove and reinstall said transistor.

Once I got it all back together, the Eros sounded like a broken distortion generator, like a Big Muff Pi guitar pedal with a near-dead battery in it.

I wearily came to the conclusion that my Eros was demonically possessed,  Scoff if you will, but the thing served proudly without a hitch for 3 years until suddenly all hell broke loose.  My every attempt to fix one problem only encouraged the gremlin to gleefully attack another part of the circuit.

Late yesterday afternoon, after fighting the demons for about 7 hours, still dressed in my bathrobe (I'm retired, so this happens more often than you might think), I remembered that I still had a few hours left on BH's 15% off sale.

Dear reader, I did it!  I ordered another Eros.  Nodding solemnly to the unhappy Eros before me, I congratulated the demon on his (its? their?) victory.

I'll start over, this time having my parish priest bless each component before I install it.


OK, seriously, yes, I did order another Eros.  I love building amps, but once they're built I hate working on them again.  All my pretty initial work starts looking ugly.  And removing and reworking a PCB is my idea of hell.

So I'm happy to do my little part to help Dan and Eileen gracefully transition to retirement.  I can't buy their company, but I can put a few more dollars into their bank account.  And I'll have a fun project for the winter.  That's a win-win.

And maybe some day I'll come back to the possessed Eros and see if I can free it from its demonic captivity but, honestly, I'm in no rush.

[Deluk]

I think I would have sent it back to BH for service. Perhaps we would all have found out where the demons were living.

[Paul Birkeland]

Yeah if I had it, I might break out the hot air gun and heat stuff up while it was distorting to see if the problematic node could be located.

[Larpy]

I have a friend whose Seduction stopped working, and now all he listens to is Qobuz.  I'll eventually rebuild the demonic Eros and give it to him as an inducement to start spinning vinyl again.  In the meantime I'll strip the demonic Eros for parts: I put some very nice caps and resistors into it.  They'll go into my new Eros once I wash them in holy water.

[Larpy]

UPDATE:  After a few nights' rest, I returned to my work bench and fired up the solder station.  I removed all the components along the ground bus and meticulously reinstalled them.  I removed the board above the 7308 tube and removed and reinstalled the transistor whose legs got bridged the other day.

Fired up the demonic beast and now I have some, I hope, tell-tale wonky voltages:
IA 218     IB  277
OA 216    0B 274
OC 201    OD 51
OkA 0      OkB -.5
OkC 8      Okd -.004

Both channels are unhappy but in different ways. 

[Paul Birkeland]

One channel isn't getting a proper regulated voltage from the board above the 12AU7 socket. This could be from a problematic solder joint at pins 4/5 there, a broken wire, etc.

That hypothesis seems to possibly be playing out in the forward section of the amp as well, as you have no current being drawn by the 6922. 

I would also be measuring voltages at the +6.3V pads on the board over the power transformer to be sure those aren't moving around on you too.