Alctron BV200 - developed a mysterious hum

[DaxLiniere]

Hey gang,
I have a BV200 which sounds fantastic, but recently developed a hum. I have tried it with another PSU and cable, but the hum persists. I tried a different tube and re-capped the whole thing, but still no dice.
Anyone have a suggestion what next to check for? It hasn't seen any use for a while because of this problem and I dearly miss it for airy female vocals.

Thanks for any suggestions!
Dax.

 

[DaxLiniere]

Holy $hit, I've been a member here for 20 years! :O :O

 

[rock soderstrom]

Holy $hit, I've been a member here for 20 years! :O :O

Congratulations on the 20th anniversary!

Are all the body parts still in direct contact with ground? Have you changed the capsule? Maybe replace it with a small 60-70pF capacitor? Does it still hum then?

 

[DaxLiniere]

Thanks Rock
I can probe to test for ground continuity. Original capsule (sounds great). Ahh, the 'dummy capsule' idea is fantastic! I'll test and report back.

 

[DaxLiniere]

Hey everyone, I'm afraid the work hasn't yielded any progress. :/
I swapped the capsule for 2 strings of ceramic capacitors and the hum stayed the same. (Interesting, the caps are crazy-microphonic, but that's unrelated.)
I suppose that's good news, because it means the capsule is fine.

I also tested resistance in 15 different places around the mic PCB and body; everything is connected perfectly.

I noticed that the solder used is very strange. It has a higher melting point and always looks dull. In several key places (like where the main PCB meets the tube daughterboard), I have removed the solder with desolding braid and replaced with good ol' leaded solder.

I have, of course, tried several tubes, including a 12AT7 (lower gain than the default 12AX7) and the hum is always present. When the mic is powered up without a tube installed, output is silent. I re-capped the PSU a few years ago when I first noticed this problem. (Before I put this mic out of action.)

I've noticed that the mic is particularly susceptible to EMI when the outer shell is not installed. I discovered this while testing as I had it on top of my racks without it's outer shell installed and the hum was incredibly loud. Moving it around (rotating 90 deg, etc) reduces that *extra* hum, but the 'baseline hum' never changes.

The only 'new information' I found is that when I power up the mic, there are a few seconds of silence as the tube's heater gets up to temperature to start inducing the little electrons to make the jump, then there is a short bit of white-ish noise, then the hum fades up.

Any suggestions of what to try next? I can send a sample of the hum, including the start-up noise.

 

[rock soderstrom]

Hey everyone, I'm afraid the work hasn't yielded any progress. :/

Bummer!

I've noticed that the mic is particularly susceptible to EMI when the outer shell is not installed. I discovered this while testing as I had it on top of my racks without it's outer shell installed and the hum was incredibly loud. Moving it around (rotating 90 deg, etc) reduces that *extra* hum, but the 'baseline hum' never changes.

This is pretty normal, without the shell you have no Faradays cage around the high impedance nodes, the mic picks up any EMI field in the room --> hum orgy.

Since your original hum is not affected by this, it seems to have a different cause.
Is it a 50 or 100Hz hum?

You seem to have some kind of ground problem , have you tested the cable from the PSU to the mic? Do all pins have continuity? Do you have the possibility to exchange the cable and the PSU with a replacement to isolate the source of the problem? Is the mic really the problem?

The only 'new information' I found is that when I power up the mic, there are a few seconds of silence as the tube's heater gets up to temperature to start inducing the little electrons to make the jump, then there is a short bit of white-ish noise, then the hum fades up.

How loud is the hum? Slightly annoying in the background or really loud?

I think it would be advantageous to create a circuit diagram of the microphone and measure all voltages, such as B+, filament, anode and cathode voltages in operation. Maybe there is something conspicuous? Post it here.

Be sure to measure all ground points in the microphone again against the central ground in the PSU. Does everything really have continuity?

I can send a sample of the hum, including the start-up noise.

Yep, upload this and some good photos of the mic and PSU. Maybe someone will notice something that could help you...

 

[rock soderstrom]

I have tried it with another PSU and cable, but the hum persists

Okay, just read it again. The mic is the problem.

 

[rock soderstrom]

Also test with the continuity tester whether all pins of the 7pin XLR connector still have continuity to the corresponding points in the circuit of the mic.

 

[gyraf]

when you switch off the psu, is there hum present in the little while before mic shuts down?

if not, we're looking at PSU generated hum - perhaps some component is drawing too much current, upsetting regulation?