Voltage and other issues post-speedball

[cottonmouth]

Greetings,

I recently completed the speedball upgrade to a perfectly functioning crack. After the first board was installed there was a significant improvement in sound but an intermittent hum in the headphones from time to time. I thought it sounded like dirty power so I continued on to install the next board. (all checks were within range after first board). After installing the second board, I noticed during checks that some voltages were high (readings below). I decided to give a listen and noticed a more pronounced hum in the left channel and no sound from the right. Any ideas?

B+ 195
OA 123
OB 136

[Paul Birkeland]

What are the OA and OB voltages on the front board? 

The voltages you posted on the big C4S tend to suggest a working amplifier.  A dropped channel is most likely a result of an issue with your underlying build.  It may be that you need to go back to the stock resistors to continue debugging the kit.

-PB

[cottonmouth]

Thank you, PB. I'll start troubleshooting the base build. Thankfully, I clipped the resistors so I'll need to figure that out. Any idea what's contributing to the hum? It follows the volume knob. Below are the numbers for the front board.

Front Board:
OA: 84
IA: 198
B-A-B: 0
IB: 198
OB: 90

[Paul Birkeland]

I suspect that when you get a pair of 22K and 10K resistors reinstalled that you'll find that your issues are still present.  This is more than likely a problem with a solder joint or two that aren't 100%.

[cottonmouth]

So, I have re-done all the solder points on the base system...no change. An odd thing occurs when I turn the unit on. I can hear music in the right channel, then it fades to the left after a few seconds and stays there. I'll have to track down resistors before I can reassemble to stock.

[Paul Birkeland]

That would have me suspecting that a black wire is loose in your amp.  Still, this is easier to debug without the Speedball present. It is extremely common for new builders to have a loose black wire at the headphone jack, but I would suspect a little more than that at work here.

[cottonmouth]

I was able to re-engineer the clipped resistors back into place successfully. I went backward in the steps and removed the 2nd (large) speedball board. At this step, I tested with the small board still integrated and found that audio was restored and working on both channels. Again, the sound is quite improved from the stock config.

Apparently, there's an issue with the large board. I have re-soldered it to no avail. I'm wondering if I cooked something in the initial assembly. I will include a pic for reference. I did flip the resistor in R2A since the pic to match the build guide.

The hum: There is a noticeable hum now with the stage 1 speedball. I wonder if it's related to the greater apparent gain? I'm running at 40% vs 65% at the same volume as stock with stage 1 installed. With music going, it's only noticeable on very quiet recordings. I also get an intermittent and loud shhhhh sound that comes and goes for a few seconds every several minutes.

[cottonmouth]

Looks like it just needed some run time. The hum has substantially decreased after a day of being on.

[Paul Birkeland]

I can't see any of the solder joints on the big PC board.  Having a look at those could be helpful.

[cottonmouth]

Please let me know if you need any other shots. Thank You!

[Paul Birkeland]

If your iron has a temperature adjustment, I would turn it all the way up.

[cottonmouth]

This was soldered at 700 deg. I will bump it up and reflow these joints. Hopefully this will do it.

[cottonmouth]

PB, I have done a few experiments. All of the connections were re-flowed at 800 deg. No change. I then tested the TIP-50 transistors and I suspect that one is fouled up. I did swap them on the board prior to the test and the problem persisted. I expected it to move to the other channel if it’s that transistor but it didn’t. (Maybe something with one of the smaller transistors too?) Below are some measurements.

Q2B:
base -/emitter +, 0
base +/emitter -, 0
Emitter & collector, .03MΩ both ways

Q2A:
base -/emitter +, 0
base +/emitter -, 1.48 MΩ

[Paul Birkeland]

If you desoldered and swapped the TIP50 transistors, it's entirely possible that the board didn't survive that process.  That's pretty difficult to do properly and without some strong desoldering tools, the heat required can certainly cause some damage.

[cottonmouth]

I feel confident that something deeper is at fault and that it is confined to the 2nd board. The symptoms are identical prior to the re-soldering so I can't directly link the two. Looks like I'm in for a new 2nd board with components at least at this point.