Author Topic: Strange Volume Issue  (Read 1515 times)

dnelson03

Strange Volume Issue
« on: August 19, 2014, 01:01:02 PM »
I am repairing a pedal (JHS Banana Boost) for a friend and have run into a problem I can't figure out.  The symptom was a volume pot that was scratchy and the signal would cut in and out when turned, so I wrongfully assumed the pot was bad and replaced with another 10kA pot.

The problem remained, so using an audio probe, I traced the path all the way to the output jack tip turning the volume pot and the circuit functions perfectly.  If I unplug the amp from my audio probe and plug it into the output jack, the volume starts cracking and cutting out again.  It does this when the volume pot is turned down or up; when the pot is around the middle, the signal comes through fine.  I do not understand what would cause this.  I even went so far as to replace the output jack and ensuring valid ground between input and output jack sleeves.  Everything "appears" to be fine ground-wise. 

Additionally, there is a trim pot to bias the 2N5457 JFET, but I wouldn't think that to be an issue as the circuit appears to perform just fine with the audio probe.

I have searched and can't find an issue like this, so I am hoping someone can help me out.  Thanks in advance.

JerS

Re: Strange Volume Issue
« Reply #1 on: August 19, 2014, 05:50:37 PM »
It sounds like an intermittent/loose ground issue to me. Is the output jack grounded through the enclosure? If it is, it may possibly not have good contact. A quick fix is to run a ground lead from the ground tab of the input jack to the ground tab of the output jack. Let us know what you find out.

dnelson03

Re: Strange Volume Issue
« Reply #2 on: August 19, 2014, 06:55:17 PM »
Thanks for the reply.  The output jack is grounded through the enclosure and it also has a ground connection from the board to the sleeve of the jack.  Continuity tests good, but I did just try an alligator clip from the input jack sleeve to the output jack sleeve and it made no difference.

I did find that when the signal cuts out by turning the volume pot, strumming hard will allow the signal to come through at the correct level, but as the signal strength fades, it cuts out again.  I am new to the whole biasing thing, but from what I read, this would be a sign of incorrect bias, right?. 

What I don't understand is that this issue is happening on the volume pot which is at the tail end of the circuit and should have nothing to do with how the bias is set.  Also, I get a perfectly clean signal where the volume increases and decreases just fine with my audio probe on the output jack and volume pot pin 2 itself.

ashcat_lt

Re: Strange Volume Issue
« Reply #3 on: August 19, 2014, 07:06:39 PM »
Sounds to me like you must have a DC voltage leaking through to the output.  Whatever coupling cap is in there is either faulty or shorted.  The cap on the audio probe solves the issue.

dnelson03

Re: Strange Volume Issue
« Reply #4 on: August 19, 2014, 09:28:39 PM »
I had wondered if that was the case; it definitely seems plausible.  I have already replaced the output cap and that wasn't it.  I will have to order a replacement input cap and see if that does the trick.  Thanks for your help.

dnelson03

Re: Strange Volume Issue
« Reply #5 on: August 19, 2014, 11:26:13 PM »
I had something close enough to substitute the input cap with, so I replaced it.  Unfortunately, that didn't change anything either.  ???

JerS

Re: Strange Volume Issue
« Reply #6 on: August 20, 2014, 08:00:49 PM »
Llike dnelson03 mentioned, if DC is leaking through the coupling cap - you would need to replace the output cap instead of the input. Also, not being 100% familiar with this circuit, I am wondering if the volume controls the bias (like on a rangemaster) or if it is just a 100KA pot on the output. If it is like a rangemaster, it is normal to get some crackling when turning the knob - although not so much that the signal cuts out. Can you take some clear pics of the board so that we can see the traces and the components?

dnelson03

Re: Strange Volume Issue
« Reply #7 on: August 20, 2014, 10:47:14 PM »
I really appreciate the help.  I did already replace the output cap.  It was a 4.7uF electrolytic.

The circuit has a 2N5457 and BS170 and allows you to blend between them.

I temporarily put the pictures up here:
https://plus.google.com/photos/108631747654862844614/albums/6049822708041588769?authkey=CPHt9LLbsZSPBQ

On the solder-side picture, output is on the left and the BS170 is under the 10kA volume pot.