Hi
I am working on a friend's amplifier, Messa Boogie (Solo Head). He said the amp sounds dirty. What I noticed is that, there is severe crackling, popping and weird noises from the amp even without any signal present. It is present on all 3 channels. It gets worst if I turn up the volume or gain on each channel. I have isolated and checked all the electrolytic and orange caps, they are all within range. I did not check the tiny (picofarad) caps. I tried replacing all the 12AX7 tubes , one at a time with a good working preamp tube with no difference. Cleaned all the pots, switches and sockets with contact spray. I totally have no idea what is causing this problem. Can someone please guide me on what can cause such problems and what to check for. Schematics attached.
Thanks
I am working on a friend's amplifier, Messa Boogie (Solo Head). He said the amp sounds dirty. What I noticed is that, there is severe crackling, popping and weird noises from the amp even without any signal present. It is present on all 3 channels. It gets worst if I turn up the volume or gain on each channel. I have isolated and checked all the electrolytic and orange caps, they are all within range. I did not check the tiny (picofarad) caps. I tried replacing all the 12AX7 tubes , one at a time with a good working preamp tube with no difference. Cleaned all the pots, switches and sockets with contact spray. I totally have no idea what is causing this problem. Can someone please guide me on what can cause such problems and what to check for. Schematics attached.
Thanks
Random replacing parts (shotgunning) is the long uncertain path .
You may pull all parts to be tested outside, replace all components by new ones, and yet not find the problem.
You must scope the amp at all points in the chain, from input jack to speaker terminals, to find where that noise appears.
Waveform or screen image will also hint at what kind of trouble you have.
Reduce suspect components from maddening 200-300 to 7-10 suspects.
And it may not be s bad component at all but cracked solder, corroded wire, poor contacts, you-name-it.
So scope it and post screen images showing noise.
You may pull all parts to be tested outside, replace all components by new ones, and yet not find the problem.
You must scope the amp at all points in the chain, from input jack to speaker terminals, to find where that noise appears.
Waveform or screen image will also hint at what kind of trouble you have.
Reduce suspect components from maddening 200-300 to 7-10 suspects.
And it may not be s bad component at all but cracked solder, corroded wire, poor contacts, you-name-it.
So scope it and post screen images showing noise.
Is there any difference if you switch between the two different rectifier options?
jeff
I have tried switching from tube to solid state rectifier but no difference. I took the signal out from effects "send" to another amp and it is crackling, inserted an audio signal into "return", the signal was clean but when I turn up the gain or volume of that channel then the crackling comes in, So I guessed that the problem is in the preamp. Following the block diagram on page 1 of the schematic, the signal from input goes into V1a (grid pin2) then out of V1a (plate pin1) then into V2a (grid pin2) then out of V2a (plate pin1) and back to V1b(grid pin7) and out of V1b(plate pin6). I do not have a scope, so as an alternate I used my signsl tracer. Amp set to channel 1 (clean), probed pin 2 of V1, no crackling, then pin1 of V1, no crackling. From pin1 of V1 to pin2(in) of V2, no crackling. Now when I probed pin1(plate out) of V2a there is some slight crackling noise and the crackling noise want up about 3 to 4 times louder when I changed the switch from "clean" to "pushed" on channel 1. What I see the noise came in at V2a ( out) and was present also at V1b (in). I have tried replacing V1 and V2 with a good working tube but no difference. What could be the problem. Please assist.
Thanks
.
Thanks
.
[I]Carlosraj[/I]
I have tried switching from tube to solid state rectifier but no difference. I took the signal out from effects "send" to another amp and it is crackling, inserted an audio signal into "return", the signal was clean but when I turn up the gain or volume of that channel then the crackling comes in,
You are guessing.
So I guessed that the problem is in the preamp.
Don't guess.
Measure/troubleshoot/Scope instead.
I do not have a scope, so as an alternate I used my signsl tracer.
Better than nothing, but you can't see waveforms, peaks, buzz, etc. which is very useful.
Amp set to channel 1 (clean), probed pin 2 of V1, no crackling, then pin1 of V1, no crackling. From pin1 of V1 to pin2(in) of V2, no crackling. Now when I probed pin1(plate out) of V2a there is some slight crackling noise and the crackling noise want up about 3 to 4 times louder when I changed the switch from "clean" to "pushed" on channel 1.
What I see the noise came in at V2a ( out) and was present also at V1b (in). I have tried replacing V1 and V2 with a good working tube but no difference.
You do not see yet.
What could be the problem. Please assist.
Should we guess too?.😊Best assistance is to suggest you do it the right way.
You seem to be doing this kind of work often, so get a scope.
I paid $21 for one, plus $6 slow (surface) shipping.
Regular Mail or Courier is faster.Very useful, only limitation is it handles 50V peak "as is"
Solved by building a simple 20:1n attenuator: 2 resistors (1M + 47k) plus a .01uF 1200V capacitor to remove DC.
Nowadays there is no excuse not to have a scope.
Sorry Forum turned most of the text into bold.