Strange problem on input stage of bass guitar amp. Need advice

Thread Starter

Jeroen Maesen

Joined Oct 6, 2018
6
Hello,

I have just accuierd a new bass guitar amp. It has 2 inputs (for an active and a passive bass guitar - one is louder than the other) de passive input does not work.

When measuring on the pins of the printboard with my scope, i noticed that on the pin of the input jack there is signal, but on the pin of the connector to the pre amp there is none. I reverted the setup and put signal on the cable coming from the pre amp and now I measure signal on the pin of that connector but no signal on the pin coming from the jack.

I resoldered both pins. But this did not help.

On the same route there is a 10pf cap going to gnd. Disconnected that one, but it did not have any effect.

Since there is just about 10mm of printboard in between both pins i highly doubt the printboard itself is defect.

The active input has the same setup, but does not show this behaviour and isnon the same printboard. Signal passes trough perfectly.

Does anybody have an idea what is going on? Or what I can test?

Br
Jeroen
 

Thread Starter

Jeroen Maesen

Joined Oct 6, 2018
6
Hye Eric!

Thanks for the link! It did give me a better insight in active vs passive. I was not aware of the advantage an active bass has vs a passive one. I'll definitly consider one next time I search for a new bass :)

However, this article does not answer my question! My problem is on the input stage of my amp.. and since I have a passive Bass guitar and only the active input works, I am not able of getting all the power out of the amp! (believe me, I need it next to our drummer ;)

So my question remains, what am I over-looking testing the input.

Here is a little schematic of the problem:
upload_2018-11-19_10-23-44.png

Br
Jeroen
 

ericgibbs

Joined Jan 29, 2010
11,073
hi J,
As you may have guessed I am not a musician.o_O
I have been searching the web looking for possible electronic cause of the problem

Is this related, ie: missing power supply.?
Clip:
Simply put, guitars or basses that have “active” electronics use a power source to power an onboard preamp. This power source is typically a 9-volt battery that is housed within the instrument.
 

ericgibbs

Joined Jan 29, 2010
11,073
hi J,
I am sure a fellow member will have seen a similar problem and hopefully have a quick fix.
Pity I cannot look over your shoulder.:)
E
 

Alec_t

Joined Sep 17, 2013
11,726
Did you have a plug inserted in the passive jack when making your connector pin measurement? If not, the passive input is shorted to ground.
 

Thread Starter

Jeroen Maesen

Joined Oct 6, 2018
6
Second thought, It might be a good idea to remove the jack plug and inspect it properly. I did inspect it, but what if it is faulty? (a moving component after all...)

Let us presume:
The input connector is faulty, when entering the male jack, the tip is not lifted from the ground properly.

Could it be that close to the jack's tip I measure signal, but at a next point in the circuit there is none?
I have built a few audio amps before and have not yet encountered this. Always presumed that ground = ground = no signal.

Do you think it could trigger this behaviour?
 

recklessrog

Joined May 23, 2013
985
From your drawing I can only assume that there is an open circuit between the two points you indicate. it could be as simple as a dry joint or crack in the print. Solder a short piece of wire between the two points and see if it works. The 100p capacitors are there to prevent R.F signal pick up at the input, a few times I've been playing and suddenly had the local taxi cab calls come through loud and clear on some older high gain amps.
 
Last edited:

Thread Starter

Jeroen Maesen

Joined Oct 6, 2018
6
Thank you all for your input!

Finally i have had the time (2 kids and a full time job) to finish the project, and found the solution!

It was in fact a cracked print!

Could not believe it, because the print was so big. Anyway the issue is solved.

Thanks again for your help!
 
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