S.E.X. 2.1 voltage discrepancies in heater circuit [solved]

[physicsmajor]

I've got a S.E.X. 2.1 built with the impedance and C4S upgrades. All resistances check out. The amp turns on quietly, all the LEDs light, the tubes glow, and nearly all of the voltages check out.

However, the heater circuit does not!

On the DC scale, A7/B7 show about +0.15V, and A8/B8 show are around -0.15V. The tubes are glowing quite happily, though.

I've triple checked my work in the heater supply around back, near the input jacks. The Schottky diodes are all connected and oriented properly, as is the tall cap, the 0.1 Ohm resistor, and the black jumper wire. There are no shorts in the Schottky diode leads or any other leads in the heater supply.

On a whim I decided to try to the AC scale, and both A7/B7 and A8/B8 actually have about 35V running through them - it's just alternating current rather than DC.

The heater circuit is connected correctly; this is also observed (both DC and AC voltages) where it connects on C1 and C2. I stopped here and haven't tried listening to anything with it yet.

Ideas? I'd be happy to provide pictures or any additional readings/tests tomorrow.

Edit: Without a picture, Paul speculated I might be missing the connection between power transformer 10 and 3L. I was. Fixing this resulted in A7/B7 at -3.7 V and A8/B8 measuring +2.4 V

[vetmed]

I've learned that when faced with readings that make little sense to put a new battery in the meter. Got a new inductance meter the other day and got weird readings. Checked the battery and it measured only 7 volts. Put a new 9 volt battery in and no more strange readings Also consider which scale you are using and whether or not AC or DC are correctly selected.

Regards
   Robert Lees

[physicsmajor]

Always a good point, but this Fluke 87 III recently had its battery replaced. It's also measuring correctly on the DC and AC volt scales for a couple of point measurements I have available (9V battery, AA battery, wall mains).

I'd love to blame this on the meter, but seeing as at least one of the odd measurements is between these tested values (+/-1.5V and +/-9V) I'm inclined to believe the meter here.

[Paul Birkeland]

On the DC scale, A7/B7 show about +0.15V, and A8/B8 show are around -0.15V. The tubes are glowing quite happily, though.

Well, the nice thing for you is that having 0.3V across A7/8 and B7/8 won't allow the tubes to glow and the amp just won't work, so we can say with some certainty that the measurement is what is at fault here.

You can try setting your meter to DC and just measuring across A7/A8 and B7/B8 to see if you get a more consistent reading there.  It also occurred to me that if you didn't have the black wire from power transformer terminal 10 to 3L, or it wasn't soldered on one (or both) end, then you'd get some nonsense readings like these, but your amp would still work.

-PB

[physicsmajor]

You are borderline psychic, Paul!

Indeed, I was somehow missing the connection from power transformer 10 to 3L. Fixing that now, and expect this will resolve the problem.

Edit: That seems to have resolved the measurements. A7/B7 now at -3.7 V, A8/B8 at +2.4 V.

[Paul Birkeland]

Enjoy!