Odd problem playing LPs

[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.

 

[Larpy]

Heater voltage on the PS board is fine.

But I caught a mistake I made reinstalling the board above the 7308: I reversed the wires to OC/OKA and OD/OKB.

Now that I've corrected that, I get these voltages:

IA  218  IB 151
OA 163  OB 144
OC 98    OD 140
OKA 99  OKB 139
OKC 2    OKD -.003

 

[Paul Birkeland]

If you pull the EF86s and 6922, does that 151V on IB pop up to 220?

 

[Larpy]

It goes up to 287v.

 

[Paul Birkeland]

My focus would be on the 12AU7 socket to get that regulator working.  What are the Kreg voltages?

 

[Larpy]

I put the 7308 back into the socket and measured pins 3 and 8, 147v and 258v respectively.

I then noticed a little smoke rising from the board above that tube and pulled the plug.  Nothing is obviously burned that I can see.

 

[Paul Birkeland]

You don't want to run the amp with just the 6922 tube installed.  You'd want to run the amp with just the 12AU7 installed in the D socket to check on power supply voltages. 

If you put a 7308 in the D socket instead of a 12AU7, that is going to cause some damage to the Eros for sure.

 

[Larpy]

No, I never took out the 12AU7.  I removed the 7308 and the EF86s, then put the 7308 back in.

Now I have all 4 tubes installed.  I think I found the cause of the smoke (a tiny solder whisker between legs of a transistor).  Fixed that and now I have:

IA 218  IB  280
OA 163 OB 230
OC 98  OD 53
OKA 99 OKB 59
OKC 2  OKD -.004

 

[Paul Birkeland]

Which transistor had the solder wiskers?  I would check to be sure you have the appropriate voltages at pins 4/5 of the EF86 on the sketchy channel (should be about 50 and 56V DC).  If that all looks good, then I'd swap in a new 2N2222 transistor on that side.

 

[Larpy]

Checked all the wires from the 7308 socket and all have continuity and measure correct resistance.  Checked all the wires coming from the board and they all have continuity.

Hard not to credit the demon for returning sevenfold.  At least today I got out of my bathrobe and got dressed before turning on the solder station.  I'll count that as a victory.

 

[Larpy]

Uh oh, I forget which one it was.  Pretty sure it was one on the left side.  I'll go check to see if I have any spare 2N2222s.