Siemens V276 mic preamp - loud pops

[Ulrich]

I have several Siemens V276 mic preamps. They all work fine,
sonically. But some produce very loud pops when engaging the dB
selector switch. As some of those preamps come from different
sources, I assume that there is a common problem with this.

So far, I changed the selector in one unit (which is very expensive
and time-consuming to install) - with no improvement. Also, I changed
the big electrolytics - no improvement either. An audio engineer spend
quite some hours in trying to find the fault - but he did not succeed.

Does anybody have experienced the same fault, and did find a solution?

Thanks
Ulrich

 

[gyraf]

(All this is from V276A schematic..)

C20 (47uF) keeps DC away from S1 (F) - emitter bypass of T1

C4 (47uF) keeps DC away from S1 (D) - AC Feedback from emitter T3 to Emitter T1

If one of these bypass caps fails or leaks any DC, you will have changing DC with gain - ie loud pops.

Also, you should check for any DC inbalance on the input of the units - e.g. an external phantom supply with uneven feed resistors. Any DC on the input will be popping with gain switching.

last, check with an oscilloscope to se if the pops you hear ARE in fact just DC levels changing, ot if they are rooted in triggere'd HF oscillation. In that case yoy could try with some powersupply decoupling (100nF caps placed strategically from +supply to 0V)

Jakob E.

 

[andre tchmil]

I also have a V276 with fairly loud pops on certain gain settings ,
All lytic caps are replaced with quality ones.
I wonder if you can get this "old" pre dead silent ?
Anyone ?

 

[tubestation]

Hello!!!
I have 3  276 in my Studio.When I got them they all the same problem you described.After a elco recap the problem stayed.In my case the gainswitch was quiet dirty and oxicated.After excessive cleaning (not easy,had to unscrew the swtch) it got better,but not dead silent. maybee the coating of the switch
changed (no more plain contact between the metallips) and small condenser are created and discharge when switching the bridged switch?
Greatings

 

[Michael Tibes]

I've got a bunch of these and don't recall them popping too loud during switching. I obviously can't know why, but I do know that I always replace the lytics and for sure cleaned the switches. What revision do you have? I vaguely remember that there are at least 3 different versions, maybe you got an old one that needs some mods?

Michael

 

[andre tchmil]

Well , I don't say "loud" popping , but it's there.
Have some younger 276a and they are dead quiet.
Maybe the rotary switch needs some massage.