Bias pots

[tuffteef]

Lets talk bias pots boss

im considering moving the bias pot for the feedback on my space echo to the front for maximum face melting oscillation
but i was thinking if this would be a good idea or not unsoldering the trim pot, my understanding is that the bias for a feedback on delay kinda controls how many times it loops around and is capped at a certain limit depending on how its biased?

if im moving it to a pot and constantly adjusting the resistance of it can it cause problems in the circuit with other things

[culturejam]

im considering moving the bias pot for the feedback

For most *pedals* (low voltage, solid state) mucking around with bias is not something that is ever going to cause damage. But the Space Echo is a bit of a different beast. It's got tubes isn't exactly a pedal.

Let me gander at the schematic a bit and get back to you.

[tuffteef]

on a somewhat related subject
we were going over stuff like hysteresis and ac bias for tape machines
i somewhat formed an idea in my head using pedal bias pots to somehow work that stuff out too

i could be wrong though

[culturejam]

Sonofabitch. I just looked at the schematic(s) in the service manual. My god, what a complex beast.

I'm not even sure where the trimmer you're talking about is located. There are so many in there.

But, the important thing is that all of the main board is no higher than about 18v, so I feel confident that no harm would come from constantly adjusting any of those trimmers.

[tuffteef]

Sonofabitch. I just looked at the schematic(s) in the service manual. My god, what a complex beast.

I'm not even sure where the trimmer you're talking about is located. There are so many in there.

But, the important thing is that all of the main board is no higher than about 18v, so I feel confident that no harm would come from constantly adjusting any of those trimmers.

yeah ive looked at the schematic and its so confusing

so i attempted to just open it up and poke around and find them
theres three boards

two mounted on the bottom
and one long across the front where the pots are mounted onto the board and the trimmers are all labelled feedback so on so forth and soldered on the opposite side of everything else

[tuffteef]

oh crap i cant remember the value of the bias pot
i remember reading it somewhere i have the 150 so im assuming its closer to the 101

theres a 10kb 20kb and 50kb i think

any clues?

[culturejam]

You can pull the trimmer and measure the resistance across leads 3 and 1. That will give you the "stock" max resistance.

[tuffteef]

You can pull the trimmer and measure the resistance across leads 3 and 1. That will give you the "stock" max resistance.

or this
good work 99

thats if i can yank it out
desoldering is a furious business

[tuffteef]

on discharging caps

at what risk do i put myself at on electrocution with opening this up will it store a charge in the caps
most of em are like 25v caps other then those other areas?

just to bear on the safe side kids

[culturejam]

I can't imagine that 25v is going to be harmful, and caps of that rating usually don't hold a charge as long as the big boys. But you could always just unplug the thing and wait a couple hours. That would give ample time for the caps to fully discharge.

[McSpunckle]

Or rub a screwdriver across the caps. : D Sparks are pretty!

But, yeah, I looked up the schematic (of course, CJ here beat me), and it looks like nothing in the thing runs at that high of a voltage.

[mathias]

Or rub a screwdriver across the caps. : D Sparks are pretty!

But, yeah, I looked up the schematic (of course, CJ here beat me), and it looks like nothing in the thing runs at that high of a voltage.

Safer is to get a nice big resistor and alligator clips -- this is how I discharge tube amps. My resistor came from my uncle's appliance repair business, it's some crazy ceramic thing with a 250VDC (maybe AC?) rating. essentially it just slowly discharges the caps (relative to shorting them -- it only takes a second to discharge this way.) That said, I discharge each cap individually with this method, just to be sure, because you don't get that visual "spark" to be sure it discharged. Follow electrician's rule while attaching the leads: only one hand in the device, the other hand in your back pocket. That way, if you do get shocked, it won't go through your heart and kill you. That said, 25VDC isn't much, but it depends on the size of the cap, too. a few milliamps discharging through your heart will kill you. A 112V AC shock through your hand will hurt but you'll be fine. (I've been shocked a dozen times just across my hand, including a few recent due to not thinking to cover the terminals of the relay in my internet-coffee-pot.) It's all about where the electricity goes

[tuffteef]

Or rub a screwdriver across the caps. : D Sparks are pretty!

LOL

[tuffteef]

dragging this thread back up from the crypt

so ive taken out the intensity trimmer well what i think it is
its way different to the 101 and 201 weirdly

measured the trimmer and its coming up 91.2 k
measuring from pin 1 and 3

and im adjusting it all the way from one side to another and the resistance isnt changing on the meter
is it supposed to

would a 100k pot work? im guessing the resistance has swayed giving 90 ish intead?

[McSpunckle]

The outer lugs shouldn't change as you move the wiper (the thing that moves when you rotate it).

91K is within a 10% tolerance, so it might just not be super duper critical that it's 100K, but a 100K pot would totally work.