Eros sounding distorted [resolved]

[Larpy]

Played a record this morning and it sounded terrible, like a dirty stylus.  Digital sounds fine, so I cleaned my stylus.  Still sounded bad.  So I pulled out my Eros and took measurements.  1A is 278 vdc, but otherwise everything measured within spec except for, get this, the tube heaters:  58 vdc coming from the big PC board above the transformer.

I swapped out the 12AU7 tube with another.  No difference.

How the heck can I be getting 58v on the heaters?

[Paul Birkeland]

If you have the Eros 2, the 6.3V pads are biased up about 51V above ground, so you'll see about 51V on one and about 57.3V on the other; there's nothing wrong with that.

If you have 278V on IA on the front PC board, that's a problem.  If you have 278V on IA on the back PC board, that's not a problem.

I would recommend posting all of the DC voltages on the front PC board here.

[Larpy]

All measurements from the "front" board--the one closest to the input jacks:

IA        278
IB        216
OA        170
OB        166
OC        96
OD        97
OkA     97
OkB     99
OkC          1.5
OkD     1.6

[Paul Birkeland]

One half of the regulator above the 12AU7 is not functioning properly.  This could be something as simple as the connection between D4 and D5 coming loose, or something more complicated.

What are the voltages on the board above the D socket? (IA, IB, OA, OB, KregA, KregB)

[Larpy]

1A  280
1B  280
OA  216
OB  278
KregA 7
KregB 24.5

[Paul Birkeland]

The 24.5V would have me examining the resistors that go to pins 2 and 7 on the D socket.  Although that 24V would tend to indicate that the regulator is drawing no current, in my experience when I have simply forgotten to install that part and powered up something with our active regulator, a weird high Kreg voltage and no regulator operation is what I will run into.

[Larpy]

This is puzzling.  I built my Eros 2 almost a year ago and all the voltages checked out during and after the build.  It's been working fine all this time.  I've not made any recent modifications.

I reflowed solder on the socket pins you mentioned, but no change.  Nothing looks amiss.  However, I did notice that the B side LEDs don't seem to be lighting up.

Could a transistor have gone bad?

[Paul Birkeland]

Which B side LEDs? On the regulator board above the D socket or somewhere else?

LEDs not lit up and the high Kreg voltage together would then tend to suggest that current isn't flowing through that half of the C4S.

I would not expect a random transistor to just stop working.  This is more likely to be a solder joint issue, a loose connection, or a broken wire.

[Larpy]

It's the LEDs on the B side of the PCB above the 12AU7 tube that aren't working.  I'll check more connections.

[Larpy]

Got the LEDs on the B side of the (12AU7) regulator board to light up again, but now the voltage on that side is too low:

IA  269
OA  215
Kreg A  7

IB   269
OB  111
Kreg B  .1

Does this suggest I look at any particular connections?

[Paul Birkeland]

What did you end up doing to create that change?

Change itself is good, but you're still not out of the woods, now you have the opposite problem it seems. 

[Larpy]

I reflowed solder connections on the regulator board and the wires from it going to the D tube socket pins and terminal strips.

[Paul Birkeland]

It's slightly possible that whatever wasn't connected damaged the TL431 on that side of the amp, and the 0.1V that you're seeing there is indicative of said damage.  You could try a replacement.

[Larpy]

Will do.  And then I'll report back.

[Larpy]

It's slightly possible that whatever wasn't connected damaged the TL431 on that side of the amp, and the 0.1V that you're seeing there is indicative of said damage.  You could try a replacement.

"Slightly possible"?  Nope.  Entirely possible.  Replacing the B-side TL431 got the voltages back in spec. 

Thank you, Paul, for your patience and good cheer.  They never fail.