A8 LED dead. No right channel.

[mithrandir39]

After nine months, I have no sound in the right channel of my headphones through the crack.  I checked the components, and it seems the A8 led has died.  Would this cause the right channel to fail?

Thx. Russell

[Paul Birkeland]

Hello there Russell,

The LED not being lit is almost always an indicator of a different issue, so I would plan to leave it as-is for now.

If you measure the voltage at terminal 1, it is probably the same as at terminal 2?

Can you see both halves of the 12AU7 glowing?

A lot of the time this issue is caused by the wire that connects to both A4 and A5 not connecting to A4, which doesn't allow the right half of the 12AU7 to operate.

-PB

[mithrandir39]

both sides are glowing, yes.  I'll have to check the voltage in the morning.  and A4 and A5 are connected.

[mithrandir39]

I did some more voltage measurements.  It seems that post A8 is at around 12 V whereas is should be at 1.5.  Again, this is a new issue.  I didn't have thisl problem in October when I got the Crack,  I can't find another culprit through all of my checks.
if an LED dies, can that kill one channel?

[Paul Birkeland]

I did some more voltage measurements.  It seems that post A8 is at around 12 V whereas is should be at 1.5.

The wire feeding A7 isn't making a proper connection.  I would also triple check the solder joints on the volume pot, as a lifted ground on the left channel would also cause this issue.

-PB

[mithrandir39]

The wire feeding A7 isn't making a proper connection.  I would also triple check the solder joints on the volume pot, as a lifted ground on the left channel would also cause this issue.

-PB

resoldered A7 and the volume pot is fine.  No difference.  The amp has been working perfectly since it was built and nothing has changed until now, except for the dead LED.  Literally working one night, dead the next morning.

Pls answer my question...Can a dead LED on A8 cause the right channel to drop?

[Doc B.]

Yes a dead LED could cause a channel to drop. But an unlit LED does not always mean that LED is dead. There may be a bad connection elsewhere that is keeping the current flowing through the LED, If the heaters are not heating or voltage is not making it to the plate of the tube the LED would not light even though perfectly functional. One needs to be careful to consider all of the possible scenarios that might create the condition, and then the best bet is usually to try the easiest solution first. That is usually not to pull the LED and replace it, but rather to make sure everything associated with it is properly soldered. You can test the LED by measuring the resistace across its two leads. With the test leads oriented one way you will see a relativley low resistance like maybe 1500 ohms. If you swap the test leads the other way you will see very high or infinite resistance. In some cases the LED will also light when you try the tester on it, but not always.

[Paul Birkeland]

Pls answer my question...Can a dead LED on A8 cause the right channel to drop?

It can, but having 12V at A8 is an indicator that A7 isn't seeing a solid reference to ground (this particular issue has happened before).

This would happen if the wire from A7 to the volume pot was loose, or if a ground connection on the volume pot was loose. 

-PB