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

[Grainger49]

I see a a small dark spot.  But to be sure throw a meter across it.  The manual will give you the resistance.

 

[FairDinkum]

I see a a small dark spot.  But to be sure throw a meter across it.  The manual will give you the resistance.

Oh my gosh the resistance is way off! It's not measuring 31.6 ohms at all! I'll replace the resistor tomorrow. Nice pick up!

 

[FairDinkum]

Alright just to clarify: I should shop for a 31.6 ohm 1/8 watt film resistor. Does it matter what tolerance it is or the manufacturer?

 

[Grainger49]

First lift one leg of the resistor to make the resistance check.  It is probably in parallel with something that will mess up the resistance you read.

Only one lead needs to be lifted, that takes it out of the circuit.

 

[FairDinkum]

First lift one leg of the resistor to make the resistance check.  It is probably in parallel with something that will mess up the resistance you read.

Only one lead needs to be lifted, that takes it out of the circuit.

I did exactly that and it's showing 320 kilo ohms when it should be 31.6 ohms.

Now I'll just have to go find a replacement and test it out! Thanks guys!

 

[Grainger49]

First verify the color code.  If it is correct according to the manual you need a replacement.  Bottlehead will send it or you can source it locally, maybe faster.

 

[Paul Birkeland]

I would measure the other 31.6 Ohm resistor to be sure your meter will measure it properly. 

 

[FairDinkum]

Thanks Paul. The other resistor is measuring 31.6 ohms on the dot. I hope that this is the main problem. As I recall, initially only one of the LEDs didn't light up, and I proceeded to reflow the center leg of the TIP-50, leading to sparks under the PCB which must have blown the resistor. Hopefully both LEDs will light up with the replacement of this resistor.

 

[Grainger49]

You are not reflowing with the Crack turned on, are you?  That is not a good idea.

 

[Doc B]

Generally a resistor goes like that because it drew way too much current (during that spark, the source of which we don't know). So you have treated the symptom but not necessarily the cause of the symptom. I would follow PBs suggestion and check the resistance between each pair of pins on the transistors. Measure the resistance between each possible pairing of the three transistor leads on the 2N2222A and also check the TIP50 the same way. If you see a very low (under 100 ohms) reading across any pair the transistor is blown.

 

[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.