ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Mesa Studio Preamp problem

10/10/2005 9:14 AM
Tom	Mesa Studio Preamp problem Hi all. I have problem with switching channels in my studio pre.When switched to lead channel there was only low distortion. First i have found, that when switched,the Z1 voltage drops from 9,1V to 7,3V.. The second thing i found is that someone add 470uF capacitor between D5 and R502.. i don´t know why but when i put it away the drop was much bigger the Z1 voltage was about 5V. I put it back and thought about it.. Maybe the 3300uF capacitor was discharging to fast in lead mode..I try to replace R352 (270ohm) with 1k2 and that was it! It run well for two days.. and now it is worse more then ever. the zeners seems to be good, rezistors are good, it must be in the ldrs maybe they are getting old. I measured only 1,2V behind Ldrs LEDs and earth.. when switched to lead mode the LDR rezistance was 300k,900k,150k,2M3.. so i put them all away and put in PC817 optoisolators,i seems to work fine in lead mode,but when switched to clean there is big distortion/buzzing sound adding to clean sound.. it drives me mad.. the original LDRs are: VACTEC 21L884 8922 ASSM.IN RP i can´t find them anywhere..please help! Where could be the fault? What could be the best substitute, if the LDRs are realy bad? I found some vactec on www.aikenamps.com but they are a bit expensive especially when shipped to europe i suppose.. and second i don´t know the parametres it should have.. thanks for your tips! regards Tom

10/10/2005 9:14 AM

Tom

Mesa Studio Preamp problem

Hi all. I have problem with switching channels in my studio pre.When switched to lead channel there was only low distortion. First i have found, that when switched,the Z1 voltage drops from 9,1V to 7,3V.. The second thing i found is that someone add 470uF capacitor between D5 and R502.. i don´t know why but when i put it away the drop was much bigger the Z1 voltage was about 5V. I put it back and thought about it.. Maybe the 3300uF capacitor was discharging to fast in lead mode..I try to replace R352 (270ohm) with 1k2 and that was it!

It run well for two days.. and now it is worse more then ever. the zeners seems to be good, rezistors are good, it must be in the ldrs maybe they are getting old. I measured only 1,2V behind Ldrs LEDs and earth.. when switched to lead mode the LDR rezistance was 300k,900k,150k,2M3.. so i put them all away and put in PC817 optoisolators,i seems to work fine in lead mode,but when switched to clean there is big distortion/buzzing sound adding to clean sound.. it drives me mad..
the original LDRs are:
VACTEC
21L884
8922
ASSM.IN RP

i can´t find them anywhere..please help!

Where could be the fault?
What could be the best substitute, if the LDRs are realy bad?
I found some vactec on www.aikenamps.com but they are a bit expensive especially when shipped to europe i suppose.. and second i don´t know the parametres it should have..
thanks for your tips!
regards Tom

Replies:

KB You can get the Vactec optoisolator... -- 10/20/2005 8:04 PM