Problem with volume control - Valab

[richie60]

Hi, newbie here, just built the crack with the valab stepped attenuator, all went well, passed all the voltage and resistance checks.  Plugged it in to the system connected my phones and turned it up one notch from what I presume was the minimum position only to be blasted at maximum Volume!

As far as I can see, I've wired it up correctly.  Is there a way to fully test the volume control because I think it's faulty.

[kgoss]

What headphones are you testing with?  Ear buds can be very sensitive.

The other question is what happens when you turn the volume clockwise?  If it gets quieter the volume control is wired backwards.  Someone with the Valab will have to confirm wiring, but it looks right to me.

What value did you order (10K, 50K, 100K, etc)?

[richie60]

HD650.  At the minimum setting it's silent so would seem logical that it's wired correctly.  Didn't attempt to turn it clockwise because as soon as I got blasted with loud sound I immediately turned it back to the off position and shut the amp down.

I ordered the 100K attenuator.

[diynewbie]

There’s probably a better way, but I would think that you could do resistance measurements with the unit off.  First verify continuity for all of your grounds.  Then start at full counter clockwise position when viewed from the knob.

Resistance from in to ground should remain ~100k at each step.  Record resistance across in to out and out to ground.  I would expect out to ground resistance will increase at each step clockwise and in to out resistance will decrease with each step clockwise.  You should see the ratio of r2/(r1+r2) increase with each step where r1 is in to out resistance and r2 is out to ground resistance.

[richie60]

OK.   I only did 5 steps as I could see a pattern forming - input to ground on all 5 was 99.6, both left & right input to output was 99.6 and both left & right output to ground measured 0.

I take it that the volume control is faulty.  Never mind, I've just contacted the seller on eBay and notified them of the fault and hope I can return it for a working replacement...

[diynewbie]

Perhaps my suggestion was wrong and the attenuator needs to be isolated from the circuit.  An output to ground of 0 with an input to output of 96k would result in little to no sound and you would have not have been blasted on the first step.  It doesn’t make sense that the in to out would be 96k for different positions.

Maybe try checking the continuity of the wires and traces.  It looks to me like the their red wires are for the input and white wires are output. So you should see little to no resistance from your Right RCA In to the In solder spot and to the highest red wire by the resistors.  Similar results for the Your output wire to the output solder spot to the highest white wire.

EDIT
Actually the first few resistors from input to output may be the same.  The output to ground resistors will change and likely be low in value.  So, if your meter isn't auto ranging, you will have to lower the range to measure the resistance.  It looks like the outer resistors are output to ground and the inner resistors are input to output.  Try measuring across the each of the upper pair of resistors at position 2 to see what values to expect.

Or try what did with the attenuator in the next to last position - all the ways clockwise then counterclockwise one step.

[richie60]

OK, ignore my readings previously, was tired and the DMM wasn’t set up on the right settings...

I've attached the true readings below in a .PDF file.

Note that when I turned the switch to position 1, I got max volume in the right channel, this reads infinity on the multimeter so I think I can pretty much guess that the VALAB attenuator is faulty!

I would like to thank you for helping me figure out the fault.

Rich

[diynewbie]

I’m getting out of my depth here.  For the most part the numbers look okay except that:
1. input to output values should be in kilo ohms
2. left output to ground at position 17 should be 14.98k instead of 4.98k.

Position 0 doesn’t make sense.  I would have expected the reverse of position 23; i.e. 96k ohm for both R/L input to output and 0 for output to ground.

The only bad value appears to be R output to ground at your position 1 label.  This could be a bad resistor or a bad solder joint if you are seeing infinite resistance.  If you can’t get satisfaction from the seller, you could try reflowing the solder for that resistor or replacing it if it is bad.

Better still would be someone with more experience to comment.

[Doc B.]

If the functionality or hookup of the Valab attenuator is in question, the best resource for getting that sorted would be - you guessed it - Valab.

http://contact.ebay.com/ws/eBayISAPI.dll?FindAnswers&requested=vintage_audio_lab&_trksid=p2050430.m2531.l4583&rt=nc&iid=311796035728

[richie60]

Input to output values are in Kohms, just didn't mark them down as such, apart from that all readings are what they are.  Had no reply from the seller as of yet, don't expect one either so am probably going to ditch the Valab in favour of another pot.  Quality control seems to be a bit iffy on them.

[Doc B.]

Do you measure continuity between all of the black wires?

[richie60]

Do you mean all of the ground wires connected to the PCB the yes, there is continuity between them all.

[mcandmar]

I wonder if the PCB on top is wired backwards, i.e. the input and outputs are the wrong way around.

Also i don't like the tube cathode terminating at the attenuator, i would prefer it on the middle post of the terminal strip as per the manual.

[JamieMcC]

Are you able to reduce the volume via your Cracks source? If you can lower the volume via your pc or player software you would at least be able to then test to see if the attenuator as any effect at a reduced volume as its switched from step to step.

[richie60]

I did think of that, but I can't lower the volume at source as I don't use a pc.  My main source is a Pioneer N50 network player or turntable.