Sick S.E.X.

[Audiocaudex]

Hi all:

I am a long-time lurker here on the forums and a big lover of Bottlehead products including my SEX 2.0 amp with C4S which is currently sick. I haven’t used the amp in a year or so (and it was working fine then), but when I fired it up recently, I only heard (distorted) sound from the left channel and nothing from the right channel.

Looking at the LEDs on the C4S boards, the left channel’s LEDs are glowing about ½ as bright as those on the right channel.

Since this unit had been working fine (and tested initially at the correct voltages) both before and after installation of the C4S, I went through and re-measured voltages at all terminals and came up with some pretty surprising results. I’m hoping that the collective knowledge here on the forum might be able to help me understand why some of the readings are so off from where they should be. I’m trying to understand whether the abnormalities measured at the terminals of the power transformer might be the cause of some (if not all?) of the abnormalities further on in the circuit? Given that some things are so far off, I’m including voltage measurements on all terminals. I have checked and re-checked all measurements, and have also checked the batteries on my Velleman DVM 850BL meter. Out of range values are flagged with a * below.

Terminals:   (Right / Left)
1,21:              192 / 190
2, 22:            192 / 190
4, 24:      192 / 190
5, 25:       0 / 0
6, 26:      408 / 404
9, 29:      -17.8 / -18.13
10, 30:      0 / 0
11, 31:      389 / 385
12, 32:      -17.04 / -18.11
14, 34:      -15.4 / -16.45
15, 35:      0 / 0
16, 36:      310 / 306
17, 37:      0 / 0
18, 38:      0 / 0
19*, 39:           2.26 / 0
20, 40:      0 / 0

A1:      -15.38 / -16.42
A2*:      370 / 363
A3:      0 / 0
A4*:      7.35 / 0
A5*:      7.8 / 78   (7.8 VDC not a typo!)
A6:      1.72 / 2.26
A7*:      0.1 / 0.1
A8*:      5.09 / 5

C1:      0
C2:      5.09
C4:      0
C5:      5.25

VAC measured on all transformer terminals
T1:      122
T2:      0
T4*:      6
T5*:      7
T6*:      435
T7*:      426
T9*:      431
T10*:      421

Plate Chokes
1,8      384
4      391
5      373

I’d appreciate any and all help that I might be able to get! Given how strange some of my readings are, I don’t feel too capable of proceeding with troubleshooting without the advice of some more experienced builders. I have built a Foreplay 1 as well as a scratch-built single-ended 2A3 amp in the past, so I have some experience, but not enough to approach this problem!

[Paul Joppa]

A8, B8, C2, and C5 are the heater supply and should be close to 6.3 volts DC. If your measurements are accurate (no dead batteries in the meter, etc) then the heater supply is at fault. I would suspect a bad solder joint on one or more of the Schottky diodes - if it's operating as a half-wave with a missing diode, that would drop the voltage.

A4 and A5 are the grid and plate respectively of the driver section. The fact that they are so close in voltage suggests the possibility that they are shorted together. A6 is the cathode, and the slightly low voltage indicates a bit less than the C4S current is flowing there, which would make sense if som of that current goes from the plate/grid through the 220 ohm grid stopper and volume control to ground. If you can't find anything and re-heating the solder joints does not help, swap the tubes to see if a tube has developed an internal short.

It's a start ...  :^)