Failed Voltage Check on New Mainline [resolved]

[Doc B.]

It could be valuable to the community if a measurement of the resistance from either end pin of the trimpot to the center pin was measured before and after that kind of exercise of the trim pot. The result would imply that there was some change to the contact inside the pot, or it might imply that the conductivity of the trimpot's solder connections was improved mechanically by working on the pot.

[Toobuzz]

That is definitely a better suggestion than “turn it twenty times and see what happens”. My goal is to eventually be able to help someone on this forum. I hope I’m not creating too much noise in the meantime.

[Paul Birkeland]

That is a good reminder. Those little blue pots are multiturn pots, and you can turn them endlessly in one direction (they just cam out when you reach the end of the rotation, and you'll hear a tiny click when that happens).

You also have to turn them in the opposite direction that you would intuitively imagine that you'd need to.

[ScottAstroNut]

I just tested the trim pot. It has no trouble going from 0 ohms to 2K ohms.

[ScottAstroNut]

I am working here with a friend who is a retired EE who worked in the audio industry. He thinks he has narrowed it down to the 2N2222 transistor on the B side of the board. We pulled the transistor from the board to test it, and it measured open circuit on the base-emitter junction. Bad transistor?

[Doc B.]

Would probably measure open with probes one way, and some resistance maybe around 500ohms-2K with the probes reversed. A failed one typically shows a short rather than an open. Typically, but not always.

[ScottAstroNut]

The base-emitter junction measures open circuit both ways. The base-collector junction operates as it should, and just as you say. Shouldn't the base-emitter junction work the same way as the base-collector junction?

[ScottAstroNut]

In any case, at this point I think I will put a new 2N2222 back in the regulator board, so I need another. Should I order one from you, or would you prefer that I simply get one from Digi-Key or Mouser?

[Doc B.]

Yup you will probably see either open or a resistance around 500ohms  - 2K depending upon which way the probes are oriented with any of the three possible combinations of b to e, b to c, c to e. You can order one through us by emailing [email protected].

[ScottAstroNut]

Thanks, Doc! Will do.

Also, sorry for sidestepping the usual troubleshooting process a bit. My friend had removed the transistor before I could stop him to test it. Not that I'm overly upset that he did so, but normally I wouldn't have taken that step until instructed by you to do so.

A new transistor will help rule that part out as the problem, but...

Thanks for your help. Always appreciated.

[ScottAstroNut]

The new transistor arrived today. A two-minute install of the new part, then a quick test. Immediately I noticed all eight LED's on the C4S boards light up, than a thorough re-check of the voltages showed that all the values were now within specified guidelines. Yippee!

The new Mainline is now installed in my system and making sweet music! Looks like it was just a defective transistor after all, just as my EE friend suspected.

So far I am really enjoying the Mainline. It sounds awesome!

Thanks, everybody, for the help in getting this kit up and running. This forum makes all the difference!

[Paul Birkeland]

Thanks for returning to let us know it worked out!

-PB