Sun T – Buzzy Output

[Thomas a Meacham]

Hi everyone, I just finished assembly of the Sun T and when I hit the strings, it is a fizzy, buzzy signal that decays quickly. The bias controls work however will not change the Buzzy sound. I have tried replacing the transistors and there s no change in the output sound.The bright switch does work as well as the rest of the controls.

 

Any idea where I went wrong?  Thanks in advance

 

Tom

 

[Billy]

Did you bias the transistor drain pins to between 4.5 to 6v and where did you get your transistors from

[Thomas a Meacham]

I have the Bias set to 4.95 on Q1, Q2, and Q3     I just got them from Barry last week.

 

[Barry]

This would be an excellent opportunity to use that Audio Probe (you mentioned having) to find out where things are going wrong in the audio path and report back your findings.

[Thomas a Meacham]

Thanks, I will report back tomorrow

 

 

[Thomas a Meacham]

Hi everyone, I have been trying to determine where the problem is with the audio probe and have not found the problem. I have replaced resistors and metal film capacitors and nothing fixed yet. Interesting find was I removed Q2 and there was no affect to the sound. If I pulled oany of the remaining three I loose sound all together. I am hoping this helps to troubleshoot.

 

On a side note, is there anyone on the forum who does repairs?

Thanks again

 

Tom

[Barry]

The audio probe should be revealing in that you follow the audio path using the schematic and listen to what is happening as you go from the beginning of the circuit to the end of the circuit. Along the way you will hear something that does not sound correct like a drop in volume or bad sound. When that happens you will know where to look for things like bad solder joints, solder bridges, bad component value etc.. Note that not every single point along the way is going to produce a sound since not every place is part of the audio path.

That said I highly suggest re-flowing the entire PCB as well as checking off-board wiring to make sure it is secure.

85% of the time issues are related to bad solder joints and sometimes solder bridges. Re-flowing will address that. Just re-heat and have some solder handy to use where ever needed. You do not want to create blobs but you do want to make sure the solder flows into the pad. An easy place to get a bridge especially if you are using a large solder tip is when pads are close together like transistor pads.

When I zoom in on your potentiometer wiring I see many suspect places since it looks like you are trying to solder the wire from that same side of the board. Ideally you should push the wire through till the PVC hits the board and solder from the other side. You are burning PVC and some of those wires may be hanging by a thread. Particularly Lug 1 of both the Volume and the Gain pots

 

Also I see a spot just above the Brite Gain potentiometer pads that may be a bridge. Hard to tell,

Many times over the years despite building up thousands of PCB’s I have corrected an issue by doing a re-flow and making sure my off board wiring wasn’t just hanging by a thread.

[Author]

Posts