This where a variac comes in handy. Slowly up voltage until bias begins to show. Fiddle with pots to zero offset without affecting bias too much. Work your way up until at full house wall voltage.
If pots turned wrong way, turn bias down, more voltage, turn down, etc until at full wall voltage. Then fiddle with offset while watching bias.
Russellc
If pots turned wrong way, turn bias down, more voltage, turn down, etc until at full wall voltage. Then fiddle with offset while watching bias.
Russellc
All true.
You can get totally down the right track just with the bulb tester - start biasing, start nulling the offset, add more bias, null the offset, etc... until you get a bit of a faint glow on the tester.
DON't try to set full bias with the tester in the loop, as you'll have it set all the way to 11, since the tester will be severely limiting the voltage, and when you remove the tester and replace with a normal mains lead it will be set for maximum skookum, and all the mad pixies will run out in the ensuing heat and release of smoke.
You can get totally down the right track just with the bulb tester - start biasing, start nulling the offset, add more bias, null the offset, etc... until you get a bit of a faint glow on the tester.
DON't try to set full bias with the tester in the loop, as you'll have it set all the way to 11, since the tester will be severely limiting the voltage, and when you remove the tester and replace with a normal mains lead it will be set for maximum skookum, and all the mad pixies will run out in the ensuing heat and release of smoke.
perhaps I should turn down Sphongle a tad then!
Never heard of them before now, listening to this - it's fantastic!!!!!!!
YouTube
Okay ... humor me now and understand you are speaking with an over-glorified plumber who almost always uses a meter to simply verify continuity in a switch. I have two DMM with me at the moment, an old Mastercraft and a newer one with auto-ranging. When I read across R5 with the auto-ranging unit it takes a while to get there but eventually reads 569.9 ohms. Adjusting the pot does nothing. So I assume I am doing the measuring incorrectly.
This IS with the amp powered down to begin with correct?
Yes, with amp powered down.
Hmmm....make sure batteries in the meters are ok. Measure some
known resistors to test. Concentrate for now on R5. Measure
resistance across it. (10K ohm scale is enough). With the pot P1
going all the way from one end to the other, what resistance
do you see across R5? It would be weird if you see no change at
all.
Also, try another meter and see if it makes a difference.
This morning I put a Fluke 324 DMM on to R6 and waited about 5 minutes for it to stabilize at 581 ohms. I came back at lunch to pick something up and did it again just to record what was going on better: When 1st powering up the DMM it started at 60.something and moved fairly steadily up to 450 where it jumped to 4 digits at about 3200. No decimal points. From there it fell back down to 581. This took 7 minutes. It fell another point and then I turned the pot 10 full turns CW where I waited for another 4ish minutes and it settled in at 551. Then another 10 turns CW and 4 minutes and down to 452.
I'll leave it at that for now, have to get back to work but I thought CCW brought the values down, not CW. These are Bourn 3296W-201-ND.
I'll leave it at that for now, have to get back to work but I thought CCW brought the values down, not CW. These are Bourn 3296W-201-ND.
Dennis did you sneak into Trismos' house or something, trying to complete the F5 like one of Santa's elves or are you crazy eagle eyed? I think I can make out a 201 for P3 but can't tell what the other pots are from the pictures.
Super impressive either way and impressively intuitive if you didn't fly into Yellowknife.
That is P3 actually Dennis - sorry about that, my mistake that I forgot they were different. As they are mounted and with clips on R6, it was easiest to grab a pic of P3.
Back to my measurements - why does it take so long for the reading to settle out? And why is CW bringing the value down? Or am I simply not reading it right?
Well now. I didn't bother touching the left channel yet, I merely unplugged the V+ wire and left the other two connected. After hooking up my DMMs and double checking everything I bit the bullet and turned the power on - I was rewarded with a nice blue LED on the Right channel board, and then I got some smoke out of the Left channel. What did I do? It appeared to come from the transistor at Q5. Thoughts?
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