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Question about trim pots inside Walkabout head.

#4
BassIan said:
I've attached a picture of two trim pots I noticed inside my Walkabout head. I've circled them in red. I was simply wondering what their purpose was. If anybody can tell me about these, I'd really appreciate it.
Click to expand...

Ummmm!
Maybe bias adjustment for the pre tubes?
 
#6
Almost every good amp has trimpots like these. They are used to trim voltages after assembly and burn-in in order to achieve a specific voltage offset in the circuit, either 0 volts or some small positive or negative voltage. There are literally hundreds of possible circuits so I can't tell you more.

I'm too lazy to open up my W'bout but the ribbon cable near the trimpot on the left looks like it is going to the board with the FX loop and/or or DI jacks. The power supply regulator board is also back there, so it could be a bias pot for who knows what.

The one on the right is most certainly an internal balance trimpot for the MOSFET power amp, to achieve some static bias or offset.

Neither of those should be adjusted without the proper schematics, scope, and knowledge of what you're doing. The risk is to the W'bout, not the person doing it.

Hope this helps. Hope you're just curious and didn't tweak them already. :)

Nightbass
 
#7
Nightbass said:
Almost every good amp has trimpots like these. They are used to trim voltages after assembly and burn-in in order to achieve a specific voltage offset in the circuit, either 0 volts or some small positive or negative voltage. There are literally hundreds of possible circuits so I can't tell you more.

I'm too lazy to open up my W'bout but the ribbon cable near the trimpot on the left looks like it is going to the board with the FX loop and/or or DI jacks. The power supply regulator board is also back there, so it could be a bias pot for who knows what.

The one on the right is most certainly an internal balance trimpot for the MOSFET power amp, to achieve some static bias or offset.

Neither of those should be adjusted without the proper schematics, scope, and knowledge of what you're doing. The risk is to the W'bout, not the person doing it.

Hope this helps. Hope you're just curious and didn't tweak them already. :)

Nightbass
Click to expand...

Thanks, that's pretty helpful. Of course I don't tough anything 'till I know EXACTLY what it does.

Waiting to hear from Benjamin Strange too, he may be able to find out exactly.
 
#9
Nightbass said:
Hope this helps. Hope you're just curious and didn't tweak them already.
Click to expand...

:D

Yes, random messing with stuff like that is dangerous to equipment.

I used to work on wireless mic systems for a German mic company, and someone once sent in a handheld system for repair because it wasn't working. One of their "techs" had opened up the handheld unit, the transmitter, and saw the row of about four or five small RF transformers with adjustable ferrite cores. Normally you tune each stage for optimum linearity and bandwidth. He thought the cores were screws that somehow weren't tightened down. So he "tightened all the screws," but it still didn't work. I should've kept the note he enclosed when he sent it in. :hmm: :scowl: :help:
 
#10
BassIan said:
Did you find anything, Ben Strange?
Click to expand...

Well, I don't start work until August 5th, and I am out of town until then. I am supposed to meet up with one of my co-workers at Boogie this week; I will ask him and see if he knows. If not, you'll have to wait until next week for me to find out. Patience! :)
 
#13
Ok, got your answer. The pot on the right controls the threshold for the limiter. The early Walkabouts didn't have these, but they were getting some distortion at SERIOUSLY high volume levels that were less than pleasant. Thus, the limiter.

The ones on the right are the bias controls for the mosFETs. Don't touch those, dude.
 
#14
Benjamin Strange said:
Ok, got your answer. The pot on the right controls the threshold for the limiter. The early Walkabouts didn't have these, but they were getting some distortion at SERIOUSLY high volume levels that were less than pleasant. Thus, the limiter.

The ones on the right are the bias controls for the mosFETs. Don't touch those, dude.
Click to expand...

Did you mean one of those to say left?

This is really interesting. I really appreciate you finding this out for me.

Is the limiter pot something that would be worth playing with, or would I merely compromise the best setting?

I wouldn't want to play with the bias on the mosFETs, that'd be bad news. I assume that's in there to account for production tolerances and is permanently calibrated at the factory?
 

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