voltage measurement problems after output upgrade BeePre 2

[Toobuzz]

Plug something like this into a computer or old phone (the 1/8” trs side) and run RCA cables from this adapter to you BeePre2 input.

ANDTOBO 3.5 mm TRS to Dual RCA F (2-Pack), 3.5mm Stereo Male to Dual RCA Female Audio Breakout Adapter https://a.co/d/2NiSSZ8

Please unplug all outputs from the BeePre. Max out the volume on both the computer/phone and BeePre2, play this, and take the measurements that PB has indicated.

https://youtu.be/TULW_-zRjgo

 

[denteom]

I got 0 ACV on all the pins that you told me to check using the red as test lead and the black one clipped on number 4 terminal. That's weird, what could be the problem? I plugged the pre amp yesterday to my speakers with switching of the 6CM7 tubes location.  Still same problem very soft volume and sounding like mono (not stereo) despite everything on the pre amp is maxed out.

 

[Paul Birkeland]

Could you post a picture of your meter or the make and model?  If it is one that only has a 200V and 700V scale for AC, then you may need a different meter. 

Another test you can try is playing the tone through the cable with the volume on your electronic device maxed out, then just use your meter across the AC plugs to check the AC voltage there.

 

[denteom]

This is antique multi-meter from dad, still working fine.  How low AC volt do I have to turn it down to?  BTW it is now switch to DC cause I rechecked again the DCV so far they are still within the parameters. I did placed it on 1000, 250, 50 ACV (I think I tried 10 also). Below the photo is the TRS adapter with the RCA.  I plugged it to my phone, then tried it on the headphone out of the Macbook air as well.

 

[Paul Birkeland]

Yeah that analog meter is really good for certain things, but not for chasing signals that start out at less than 1V.

If you're in the US, I would grab one of these:
https://www.harborfreight.com/electrical/electrician-s-tools/multimeters-testers/11-function-digital-multimeter-with-audible-continuity-61593.html

On the 2V AC scale, you can resolve these signal voltages pretty easily.

 

[denteom]

Ok I'll try it again at 10. If still can't get a signal, I have a friend who has an oscilloscope.  Not sure if he has a less than 2VAC.  What should I expect from the voltage test?

 

[Paul Birkeland]

If you measure the AC voltage on the RCA plugs before plugging them into the BeePre, that would be your starting signal (probably around 0.3V-0.5V).  That same voltage should appear on pin 3 of each 4 pin socket (you can leave the preamp off for that test).

With the preamp running and the volume controls turned all the way up, you would expect somewhere around 1.5V or so on the center post of each 9 pin socket, and you would expect to see nearly that same voltage at each RCA output jack.

 

[denteom]

Finally found the problem. It was the 220 ohm 1/4 watt resistor connected to C7 and the center pin of socket C.  I forgot to trim off the excess wire after soldering it to C7.
My last problem is the fine volume control 0 db. it looses contact on the left channel, I have to tap it, press on it or wiggle it.  The other fine channels are ok.  I've sprayed contact cleaner to the switch and switch it several times but the performance is kinda erratic.  Not sure if this problem is also causing the ever so slight softer sound on the left channel. How do I resolve that problem?

 

[Paul Birkeland]

The fine switch doesn't really work that way.  The input/output are connected by wire, not the switch, so what you're more than likely dealing with is one of the empty terminals on that switch making contact with the metal hardware on the switch itself.

 

[denteom]

So how do I resolve that problem on the 0 db? Switching from -1.5 to 0 db, yes there is sometimes a slight loss in sound in between.  Which does not happen when I switch between the other fine channel switch, transitioning on the other fine channels is ok. It's only between the -1.5 to 0.  The -1.5 db and lower all work fine.

 

[Paul Birkeland]

Is the channel dropping or not getting louder?

A channel dropping means you lose the signal completely.

 

[denteom]

Yes I lose the signal completely on the left channel. But I regain it by pressing on the knob, at times tapping or wiggling it helps.  Also it really sounds a bit softer on the left.  If I switch it to -1.5 db and lower db knobs it is balanced.

 

[Paul Birkeland]

This means that the 0dB position is in close contact with metal on the switch body and needs to be spaced out.  This does not mean that the switch is not making contact on the 0dB fine position.

 

[denteom]

Do I need to try to bend with pliers, sandpaper it to have better contact or add solder?

 

[Paul Birkeland]

Maybe you could post some photos of that switch in your build.

 

[denteom]

hope these photos help.

 

[Paul Birkeland]

With the fine switch not on -0dB, can you check the DC resistance between the chassis and each empty lug on the fine switch?

 

[denteom]

Where will I put the Black lead? Is the black lead the chassis? Or do I clip the black lead to the chassis?
Does that mean checking the lug without the a resistor attached to it?

 

[Paul Birkeland]

One probe on one lug of the fine switch not connected to anything, one probe to the chassis with your meter set on DC resistance.

 

[denteom]

Switched at -4.5 db fine knob
At -1.5 db 85 k Ohm
At -3.0 db 36 k Ohm
(not sure of the decimal point. get confused by the analog meter)