Newbie needs help in figuring out what is wrong...desperate!

[tengcs]

Hi everyone,

I am a newbie and I tried to build the Stereomour 2A3. This is what I get when doing the voltage checks with valves installed and power on:
The resistance checks out without power. With power, another story:

A1, A4: 2.5V YES
C1, C4 2.5V YES
HV+ 396V looks ok
Terminal Strips:
1  410V
2  190 V
3  0
4  0
5  0
6  0. This is clearly wrong. Manual says should be 60V. Also when I check the PCB, only 2 diodes light up.
7  0
8  0
9  0
10  0
11  60
12  0
13  0
14  0. This is clearly wrong. Manual says 165-220
15  367
16  0 This is clearly wrong. It should read 60V
17  0. Wrong, shd read 380V
18  0
19  390
20  60

Four pin socket
A1  0. Wrong. Should be 60V
A2  410
A3  0
A4 0. Wrong. Shd be 60V
C1  60
C2  360
C3 0
C4 60.
Do the C readings mean this channel is ok? When I look at the amp, one side does not glow.
Active Load PCB
1A  390
1B  410
0A  190
0B  0. This is wrong. Shd be 165-220
K reg both sides 2.5 & 0. 0 is wrong
-reg both sides  0

Can someone let me know how to trouble shoot this?

[Paul Birkeland]

The first thing to do is to get the A1/A4 voltage up! 

Seeing that A2 is 410, and not ~360, you are not drawing any current through the tube.

Check the connection of the cathode resistors.  If I remember, it is a 4.7K and a 1.6K resistor.  One end of these resistors (or both) is not properly connected, or the wire leaving these connections to the hum pot is not properly connected. (do also note that the 3 connections on the hum pot MUST NOT touch the body of the hum pot or the chassis plate) 

Diagnosing this may fix some of your other voltage issues, so it seems like a logical place to start.

-PB

[tengcs]

Hi and thanks for the help. I have fixed the power supply to the valve. The terminal voltages 1 to 20 check out (except I think the manual has a typo and it is 5 that shd be 60V and not 6.)

The next discrepancy is this:

Active Load PCB

0B is 0, when it should be between 165 and 220
K reg on one side is 0.9V when it should be 2.5.

All assistance is deeply appreciated! Thanks in advance.

[Paul Birkeland]

Terminal 5 is listed at 0V in the manual, and it should be 0V!

0B being 0V means that you have a short somewhere, I'd look at the 9 pin sockets - pins 1 and 6 are the plate, are they touching something they shouldn't be?  There are also wires leaving those pins going to terminals 2 and 14, which may also have an issue.

Kreg at 0.9V is most likely a response to the 0V on OB, that voltage needs to come up for the Kreg voltage to stabilize at 2.5V.

-PB

[tengcs]

Hi Caucasian Backplate,

I took a look to visually ascertain if wires were touching. I did not find any. I desoldered the active load PCB for better access. In testing (with the active load PCB totally disconnected):

2 & 14. infinity
2 & B1 = 0
14 & B6 = 0
B1 & B6 = infinity

Then I tried Chassis vs:
B1 = infinity
B2 = 97kohm
B3, B4, B5, B6 = infinity
B7 = 97kohm
B8 = infinity
B9 = 0

I also checked B1 against B2, B3....B9 etc....all read infinity. What can I try next? I do appreciate the support.

Chieh

[tengcs]

Hi Caucasian Backplate,

One more thing when I inspected the active load PCB, the wire that connects "-reg" to "bA" (L shaped) is loose.

Thanks again

Chieh 

[Grainger49]

Chieh,

I'm not following too closely but the jumpers on the boards should not be loose.  Touch up the solder on the end that is loose and this could make a difference.

[Paul Birkeland]

Yeah, a loose black wire on the active load board could create issues similar to what you're experiencing, I'd remeasure the voltages and let us know.

[tengcs]

Hi guys and thanks again,

I have tightened up the wire, and in probing the active load board, I found that the 237 ohm resistor on the "B" side of the board reads infinity ohms, whereas the "A" side reads 236 ohms. The 300k resistors both read 298k ohms.

Could this be what is wrong? I am trying to check the active load board before resoldering the board back.

Thanks again.

[Paul Birkeland]

That is a potential issue, infinite ohms there would be no current through the C4S, though a failure like this is rare.

[Grainger49]

To be clear, the 237 ohm reading is bad, the other ones are within specification. 

[tengcs]

Thanks again guys. I have made some progress.

This is the latest. I bought a 240 ohm resistor (It is slightly larger than the 237 ohm from Bottlehead) and reassembled the amp: I get music from one channel ("A" side), which I am enjoying, amazing amp. These are the readings (I will only put the out of spec ones):
Terminal:
9 .................reads 2.5V shd be zero
14 ...............300V shd be 165-220
Active load PCB
0B reads ...300, shd be 165-220.

K reg and -reg are ok now, within spec.

Is my problem caused by my substitution of the 240ohm resistor instead of the Bottlehead 237 ohm (which was bust)? Can I get a replacement from Bottlehead?

Thanks again.

[Paul Birkeland]

Do you have terminal 9 wired to B8?  Do you have kreg wired to B7?  Something is very amiss here with the wiring on the 9 pin socket.

[tengcs]

Hi Caucasian Backplate.

I noticed something in the Manual. On pg 45, it shows B9 to terminal 8 This is repeated on page 50. But on pg 61, B9 is now connected to 10.....help! 

My connections are:
B9 .. 8
B8 .. K reg
B7 .. 9
B6 ..OB & 14
B5, B4 to transformer
B3 ...K reg
B2 .. 7
B1 ..OA & 2

Is this what is wrong? Thanks again.

Chieh

[Paul Birkeland]

What's wrong is that you have DC voltage at terminal 9, which should have 0V, and which is grounded through the 100K pot.  How does that 220 Ohm resistor measure?  (Is it still ~220 Ohms?)