Hay all, I know that these are falling out of popularity, but they’re are definitely uses for vacuum tube meters as many of you know. Like the ultra high input impedance. I like having as modern and accurate equipment as I could possibly afford, looking at some 6-1/2 digit Keithleys, Agilents, and Tektronix bench meters, but if I come across a very good priced vacuum meter that is in good condition when I attend local electronic swap meets I will grab them. Last year I grabbed an HP 410B which I fully restored, replacing all the capacitors, tubes, and installing one percent resistors in place of the carbon comp. After the alignment, this thing is dead on accurate, I see why this meter has the hype behind it.
Onto my questions for help. This year I picked up an HP 412A. Like the 410 it got a full recap, 1% resistors, cleaning of controls, and I picked out all brand new tubes. The two regulators measured good, looked in my stash and found NOS JAN 12AX7, 6AU8A and 6X4, they all tested very strong and were installed.
Just some pulmonary testing measuring some DC voltages on the different input levels showed me that it was actually already very close to being aligned. I’d be very happy with leaving. It is where it is with measuring DC, but if I were to just nudge the Neal, it would be perfect, and I’m talking maybe half a needle width.
The first thing to adjust is the hum balance, and I can’t even get that, getting stuck on the first thing lol.
It says to attach a scope probe to the amplifier output on the back of the meter.
Have the meter set to volts and it says to adjust the hum control for the minimum 10 Hz signal on the scope, at least that’s what I think it is saying. I have the scope settings pretty darn low, 2mV/div or 5 mV/div. With where the meter was I wasn’t seen anywhere near 10 Hz. it was fluctuating quite a bit and the several kHz. I was able to trigger on the signal, but it was moving around a bit.
I started adjustment of the hum balance and it started adjusting the waveform I was seeing. I got the frequency closer, but it was jumping around from as low as 7 Hz up to just about one kHz. it was incredibly difficult to get it to a point where I thought it was lowest.
Am I doing this incorrectly? I did notice that when adjusting the hum balance that it greatly affected the zero point of the meter. So after making an adjustment on the balance, I would have to go to the small screw right below the minor to zero it. Is that movement from the zero point noise? Meaning would it be best to adjust for the lowest point on that meter, and that would be the lowest hum point? I’m guessing that lol.
Would love some pointers if anybody has them, thank you. Bring this beauty from yesteryear back to life and let it feel useful again.
Thank you,
Dan
Onto my questions for help. This year I picked up an HP 412A. Like the 410 it got a full recap, 1% resistors, cleaning of controls, and I picked out all brand new tubes. The two regulators measured good, looked in my stash and found NOS JAN 12AX7, 6AU8A and 6X4, they all tested very strong and were installed.
Just some pulmonary testing measuring some DC voltages on the different input levels showed me that it was actually already very close to being aligned. I’d be very happy with leaving. It is where it is with measuring DC, but if I were to just nudge the Neal, it would be perfect, and I’m talking maybe half a needle width.
The first thing to adjust is the hum balance, and I can’t even get that, getting stuck on the first thing lol.
It says to attach a scope probe to the amplifier output on the back of the meter.
Have the meter set to volts and it says to adjust the hum control for the minimum 10 Hz signal on the scope, at least that’s what I think it is saying. I have the scope settings pretty darn low, 2mV/div or 5 mV/div. With where the meter was I wasn’t seen anywhere near 10 Hz. it was fluctuating quite a bit and the several kHz. I was able to trigger on the signal, but it was moving around a bit.
I started adjustment of the hum balance and it started adjusting the waveform I was seeing. I got the frequency closer, but it was jumping around from as low as 7 Hz up to just about one kHz. it was incredibly difficult to get it to a point where I thought it was lowest.
Am I doing this incorrectly? I did notice that when adjusting the hum balance that it greatly affected the zero point of the meter. So after making an adjustment on the balance, I would have to go to the small screw right below the minor to zero it. Is that movement from the zero point noise? Meaning would it be best to adjust for the lowest point on that meter, and that would be the lowest hum point? I’m guessing that lol.
Would love some pointers if anybody has them, thank you. Bring this beauty from yesteryear back to life and let it feel useful again.
Thank you,
Dan