Impedance switch kit problem, outputs shorted to ground

[sbelyo]

Somehow I've messed up the impedance switch kit.  I decided to check for continuity after finishing the output wiring.  When I got continuity from both the left and right to ground I knew that couldn't be right.  I removed the output connections from the switches and checked each binding post and the headphone jack for continuity to ground and everything was fine so the problem is definitely with the switch boards.  There is continuity across the neg, pos, and ground pads when in unbalanced and balanced mode on one board.  On the other board when I switch to balanced there is only continuity between the neg and pos pads.  When in the unbalanced position there is continuity between all three pads.

The results are the same no matter what the other switches are set to

I checked for solder bridges and did not find any.  It appears I've got bad switches.  What else can I check, and what's the next step?

[Paul Birkeland]

When I got continuity from both the left and right to ground I knew that couldn't be right.

This actually is OK.  The secondary windings on the output transformers will be in the tenths of Ohms, which reads as a short.

What symptoms do you have? Are you missing a channel?

[sbelyo]

I see, I might be ok then.  That would explain why my meter was hesitant to emit a tone at first.

There is continuity across the neg, pos, and ground pads when in unbalanced and balanced mode on one board.  On the other board when I switch to balanced there is only continuity between the neg and pos pads.  When in the unbalanced position there is continuity between all three pads.

Is this part ok?  because one board reads a short between neg and ground in either position and the other board only reads a short when in unbalanced mode

To answer your question, I haven't finished the build all the way so there no real symptoms as of yet.  I only found this stuff out because I just wanted to make sure my output wiring was ok

[Paul Birkeland]

because one board reads a short between neg and ground in either position and the other board only reads a short when in unbalanced mode

This may be a real issue.  In balanced mode, the ground will be referenced with the pair of resistors on the board. Simply having the Neg and Ground connections switched could cause this problem.

To answer your question, I haven't finished the build all the way so there no real symptoms as of yet.  I only found this stuff out because I just wanted to make sure my output wiring was ok

It's very risky to build the amp with the switch kit at the same time.  If you run into any problems, that can't be diagnosed by resistance/voltages, removing the impedance switch kit will be the first step.

-PB

[sbelyo]

Understood...  I'll have another look at it tomorrow and may remove the switch kit

[sbelyo]

So here's an update...  I can reproduce the behavior on both sides.  There is NO continuity between neg and ground on the switch board with nothing connected to the output pads when set to bal and both impedance switches set to low.  There IS continuity between neg and ground in all the other switch combinations.

I'm inclined to think this is normal behavior.  Am I right?

[Paul Birkeland]

Yes, that is normal behavior.  You should see about the same resistance from each speaker post to ground in the balanced configuration, and continuity from both in the unbalanced configuration.

[sbelyo]

Awesome...  I'll reassemble in this case