@Dneu2011
Yes, one pot will tend to control the DC-Offset the meter on R12 goes to one side of R12 and the other side of R12, this is your offset voltage. Try and keep the offset down, as you go up, so I crank the bias voltage up then use the other pot to bring down the offset, then up more on the other to bring up the bias, then trim down the offset, rinse and repeat.
Which pot controls the offset? You will see that one of the pot will change the offset a great deal, but make smaller changes on bias. The pot that does the bias will also change the offset, so you go to the other to bring the offset back down and continue. On mine, P1 is the dc offset and P2 is the bias.
Note: the direction you turn to increase bias will be the same. but you might be turning the offset pot one way or the other depending on the meter reading, you might see neg or positive on the meter. What matters is keeping it close to zero.
Also, I assume you mean 400mV not .004mV
Yes, one pot will tend to control the DC-Offset the meter on R12 goes to one side of R12 and the other side of R12, this is your offset voltage. Try and keep the offset down, as you go up, so I crank the bias voltage up then use the other pot to bring down the offset, then up more on the other to bring up the bias, then trim down the offset, rinse and repeat.
Which pot controls the offset? You will see that one of the pot will change the offset a great deal, but make smaller changes on bias. The pot that does the bias will also change the offset, so you go to the other to bring the offset back down and continue. On mine, P1 is the dc offset and P2 is the bias.
Note: the direction you turn to increase bias will be the same. but you might be turning the offset pot one way or the other depending on the meter reading, you might see neg or positive on the meter. What matters is keeping it close to zero.
Also, I assume you mean 400mV not .004mV
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Thank you for the help! I attempted to bias the BA3 again this morning but I’m having trouble seeing any DC Offset when monitoring with DMM across R12. I get 0v the whole time I’m adjusting the pots and again can’t get over .400v before I run out of trimmer.
I currently do not have an input or output connected to the board. Do I need to hook these up and add a dummy load? Could that be my problem?
I currently do not have an input or output connected to the board. Do I need to hook these up and add a dummy load? Could that be my problem?
No, ideally you short the input and it has been stated by 6L6 that it makes it easier to set bias with a load on the output 2-5k, but you can do it without either and it would not prevent you from seeing a dc offset. Why don't you post a close-up of your meter connections to rule out any meter connection errors. Also, get some good shots of the board.
In the picture below the black arrows show the connection point for DC offset and the blue for your bias voltage.
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Same here. Recent power supply had 2 missing parts. I emailed, but no response.....parts showed up just a few days later. I have ordered from them numerous times, no problems.
Russellc
Been awhile, but have 3 BA3 boards operating. seems like I remember a little confusion in offset measuring, isn't offset measured from at the cap and ground? Junction of R12 and C2, and other to ground?
Russellc
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Ya parts missing on one pcb and I still don't have the power switch ordered on 10/22 now 11/7 and no shipping. He said he shipped it, but what he shipped was another part in the order, so I pointed that out and never heard back. I don't know if it's coming or what. I got tired of waiting and put a toggle switch in the unit.
Thompsontechs, the resistor you labeled for offset is R13. Russellc mentioned using R12 and the ground of C3. I tried this yesterday and see about 8mv of offset. I’ll send a pic once I get home.
You are correct, I had it wrong, I was looking at R13. I will double check my own as well. I'm new too, so be skeptical of any help I may, "try" and give.
I get pretty much the same reading off both R13 and R12. R12 is very hard to get to because of the huge Clarity caps I have there. If I were to build another, I would make some test points for that one.
Not sure if on same page, one probe at the cap/resistor junction, (which is "before" the cap) the other probe to ground, right? This is for offset, not bias so you are shooting for 0 here, or as close as is manageable.
Russellc
"Place one voltmeter (Set to DC volts) across R12 - to observe DC offset"
Across R12 see picture. This is how I did it anyway.
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Don't use blue leds because noisy, just replace the fifth red led closest to rail per side with 2kOhm rheostats.
Bodge them there. They will give ample voltage range.
You may leave them in or measure what adjust resistance to rail goal each then put resistors in their place.
Its for a 45mA bias per side load. It will not move away from there, no more than few mA on line level loads.
With 18//18 Ohm 2W CC setting resistors you get +/- 200mA standing current limit on a Mez board's PSU section.
BTW Ultrabib is even better. But its not a selector + psu combo like the Mez board. Standalone shunt PSU sections.
Thanks for hint. I was not aware that blue leds are not as good as red ones.
I'm using 2SK117 in PSU with Idss ~5.5mA thus will need a bit more than 2k. I hope it's still ok.
What is the trade-off when using resistor to get extra voltage drop instead of leds? Noise, voltage stability?
Already signed up for next GB at Tea-Bag store
Yes use a higher value trimmer to meet your rail goal given your GR and not BL level Idss.
Trade off is higher impedance in the Vref which the K117 thermals can affect easier than the leds portion of final voltage. But not to a troublesome degree in this case. Leds are active hence stiffer. For noise not bad because the same Vref filter capacitor can filter a higher impedance more effectively.
Trade off is higher impedance in the Vref which the K117 thermals can affect easier than the leds portion of final voltage. But not to a troublesome degree in this case. Leds are active hence stiffer. For noise not bad because the same Vref filter capacitor can filter a higher impedance more effectively.
If Fairchild output devices are used it is recommended in the build guide to use 1k trimmers instead of 500R. I did that and had no problems with bias/offset. If P3 is adjusted after bias/offset it is important to adjust bias/offset again…..as it will drift. So maybe center P3 first (by measurement).
See post #45 where 6L6 corrects this instruction. You cannot measure offset "across a resistor"....it is DC on a circuits output, so it is measured at the output and ground.
See also post #23 by Zenmod as explaination. Always good to study entire thread for little details...I think where that instruction was aimed was that as bias is adjusted, offset will wonder, so its a back and forth procedure.
Russellc
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Maybe I should higher Idss JFETs (like in original BOM), as leds have steeper Vf/I curve in ~10mA area then in 5mA?
I didn't want to waste precious matched K170BL and used unmatched K117GR.