Weird Terminal 9 voltage on speedball + dead left channel [solved]

[FairDinkum]

Thanks Doc. Do I have to remove the transistors from the PCB for the resistance check? If not, can I measure them on the back of the PCB?

[Doc B.]

You should be able to measure them on the board.

[FairDinkum]

I've performed the resistance checks for both pairs of TIP-50 and 2N2222A (see picture). I've measure the TIP-50 using the pins on the top side, and the 2N2222A via the bottom of the PCB. The TIP50 readings are similar for the pair; but not the 2N2222A. In fact, on the "bad" side, one pair of the legs for 2N2222A is showing 17 ohms, which is unusual given that the resistance on the same pair of legs on the "good" side shows rapidly rising resistance to OL. I'm using a Fluke 87V.

Does this mean the 2N2222A on the "bad" side is broken, hence causing the resistor to break as well? If so, looks like we might found the problem.

[Doc B.]

yup.

[FairDinkum]

Hi Doc - thanks so much for helping me out! I have a lot of trouble sourcing that resistor here in Australia. It's an uncommon resistor and so far I haven't found any. I think I might be able to source the 2N2222A. In any case, would you or Eileen mind sending over some resistors and 2N2222A? This is to ensure product consistency. I don't mind paying for them. My order reference is #2521.

Thanks very much!

[Doc B.]

Yeah the resistor is kind of an umcommon value and needs to be accurate. Best bet is to contact Eileen directly at queen at bottlehead dot com and she can be sure she has all the info she needs to make sure you get the right parts. Bear in mind it's a three day weekend here (labor day) so the parts may not mail out for a couple of days.

[FairDinkum]

Thanks Doc I've emailed the Queen. Is there possibly a component failure up chain that could have caused the 2N2222A to blow? The LEDs or the TIP-50 on the same side maybe?

[FairDinkum]

I'd like to update this thread to say that I've received the replacement LEDs and resistors and I've replaced both the LEDs and the blown 31.6ohm resistor in the defective channel on the main PCB. Voltages on 7 and 9 are 103.3 and 106.5 volts (is this discrepancy normal?), and the other ones seem to check out ok. Hooked the amp up to a source and I heard sweet sweet music.

[Paul Birkeland]

Voltages on 7 and 9 are 103.3 and 106.5 volts (is this discrepancy normal?)

These voltages are dependent on both the 6080 itself, as well as the 12AU7 plate voltages (T1 and T5), some variation is certainly expected.

[FairDinkum]

Thanks Paul! This means that my issue is solved. Thank you everyone (including the Queen) for your help! I've had a lot of fun building my first electronic project.