Hey Doc,
If a speaker is connected to the "troubled" amp while you are trying to trace the noise, you will continue to hear that noise through the speaker. This could interfere with your ability to hear the stethoscope signal through the dedicated listening amp.
With a dummy load, there is no audible output from the troubled amp. You could plug a set of headphones into the stethoscope (as opposed to a dedicated listening amp) and keep the speaker connected to the troubled amp to provide the required load. But....it could be difficult to hear the stethoscope signal with the speaker blaring away in close proximity. Especially when you are probing the low signal amplitude sections of the troubled amp. Those circuit sections will just barely drive the headphones enough to hear it.
A 0.1uf capactitor in the stethoscope will present the "best" overall representation of the frequency band present in typical audio amplification circuits at the listening amp's (or..headphone's) input.
I am with Cornelius on the 0.022 uf selection! I am certain that you will hear the high freq. hiss. Not sure about the lower freq. hum.
Dave O.
Help! I tried to recap my amp!
Moderators: pompeiisneaks, Colossal
Re: Help! I tried to recap my amp!
Ok, I ordered the parts I need to build those tools. But in the mean time I tried a bit more. And either I am getting nearer to the source of the problem, or I messed up bigtime!
I changed the plate resistors, the cathode resistors, replaced the 30uF cathode capacitor, replaced two 100k coupling resistors between the volume pot and the tone pot. I think this is where the signal is fed to the effect unit. - Again please correct me if I'm wrong
None of this helped the noise or the hum, but now I have more problems. The reverb and tremolo are no longer working, and when playing a guitar through it the open B string is noticably louder than the rest, and it also distorts! What the heck is going on? I made shure that the new resistors were the correct value, that they were soldered to the same places where the old ones were, and I checked with multimeter after installation to ensure good connections. Where did I go wrong? The new problems started after I installed the new resistors between the volume pot right lug and terminal strip.
DocZ
I changed the plate resistors, the cathode resistors, replaced the 30uF cathode capacitor, replaced two 100k coupling resistors between the volume pot and the tone pot. I think this is where the signal is fed to the effect unit. - Again please correct me if I'm wrong
None of this helped the noise or the hum, but now I have more problems. The reverb and tremolo are no longer working, and when playing a guitar through it the open B string is noticably louder than the rest, and it also distorts! What the heck is going on? I made shure that the new resistors were the correct value, that they were soldered to the same places where the old ones were, and I checked with multimeter after installation to ensure good connections. Where did I go wrong? The new problems started after I installed the new resistors between the volume pot right lug and terminal strip.
DocZ
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Re: Help! I tried to recap my amp!
Not sure.
I was looking at the first photos again.
That is a very unorganised point to point construction.
So many caps (Rubycon!) !
It's a mess in there.
Do you know how to trace the schematic out as far as the signal flow in that?
That is what I would do.
Start at the input jack and match each part with the schematic and try to follow the signal path.
I'll bet there are some wrong connections there or shorted components.
It looks like somebody dumped a bucket of parts into that chassis!
No offense, I know you didn't do it, just try to trace it out and verify each stage.
I was looking at the first photos again.
That is a very unorganised point to point construction.
So many caps (Rubycon!) !
It's a mess in there.
Do you know how to trace the schematic out as far as the signal flow in that?
That is what I would do.
Start at the input jack and match each part with the schematic and try to follow the signal path.
I'll bet there are some wrong connections there or shorted components.
It looks like somebody dumped a bucket of parts into that chassis!
No offense, I know you didn't do it, just try to trace it out and verify each stage.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Help! I tried to recap my amp!
Hey Doc,
Double check the orientation of the newly installed cathode bypass capacitors. The nice shiny blue one (33uf/25V) looks like it is installed backwards. It is a little hard to see in the picture to be positive on that though.
Electrolytic capacitors have a definite polarity. The "+" side goes to the cathode socket pin and the "-" side goes to ground.
Good luck,
Dave O.
Double check the orientation of the newly installed cathode bypass capacitors. The nice shiny blue one (33uf/25V) looks like it is installed backwards. It is a little hard to see in the picture to be positive on that though.
Electrolytic capacitors have a definite polarity. The "+" side goes to the cathode socket pin and the "-" side goes to ground.
Good luck,
Dave O.
Re: Help! I tried to recap my amp!
No offense taken, I didn't build the thing
It has taken time, but I now can trace the parts inside and find them on the schematic.
Those small blue caps had a horizontal black stripe on the bottom, I assumed that was the negative side? This is the side I connected to the ground terminal.
DocZ
It has taken time, but I now can trace the parts inside and find them on the schematic.
Those small blue caps had a horizontal black stripe on the bottom, I assumed that was the negative side? This is the side I connected to the ground terminal.
DocZ
Re: Help! I tried to recap my amp!
Take a look at where the legs comes out on that cap. The leg that comes out from someting like black rubber, would be the positive, and the leg that seems fasten to the 'chassis' of the cap, are negative.
It seems to me that the cap are the wrong way? (the negative soldered someplace on the tube socket?)
It seems to me that the cap are the wrong way? (the negative soldered someplace on the tube socket?)
Re: Help! I tried to recap my amp!
The "bucket of parts" was funny. 
When replacing items only do 1 at a time and test out the amp again. If a problem is found you know where to start looking.
The signal flow through your amp is from the input jack to V1a, up through the plate, through the .01uF coupling cap where it has a tap for the reverb.
The reverb has a 100k attenuation resistor which allows it to see only a certain amount of the signal, most of the signal then continues on to the tone pot, coming out of the center lug. There it is joined by the return loop from the reverb. This is how they setup the delay (signal from the reverb is late getting there because it had further to travel). Out the center lug of the volume pot on into V1b.
There is a bypass setup around the tone pot which contains 2-100k resistors, a .05uf cap and a 10k resistor to ground. The 500pF is the treble, I assume the .05uF is the bass and the bypass combination of the resistors and the .005uF controls the mids. (i've not seen this setup used before)
V1b is where the tremolo comes in. That circuit sets up an oscillation which is fed into the cathode of V1b. This makes that tube oscillate giving the tremolo effect to the passing signal by varying how the tube sees ground through the cathode resistor and bypass cap. (something like that anyway). It doesn't actually see the signal, but it controls how the tube reacts which effects the passing signal. (words?)
When replacing items only do 1 at a time and test out the amp again. If a problem is found you know where to start looking.
The signal flow through your amp is from the input jack to V1a, up through the plate, through the .01uF coupling cap where it has a tap for the reverb.
The reverb has a 100k attenuation resistor which allows it to see only a certain amount of the signal, most of the signal then continues on to the tone pot, coming out of the center lug. There it is joined by the return loop from the reverb. This is how they setup the delay (signal from the reverb is late getting there because it had further to travel). Out the center lug of the volume pot on into V1b.
There is a bypass setup around the tone pot which contains 2-100k resistors, a .05uf cap and a 10k resistor to ground. The 500pF is the treble, I assume the .05uF is the bass and the bypass combination of the resistors and the .005uF controls the mids. (i've not seen this setup used before)
V1b is where the tremolo comes in. That circuit sets up an oscillation which is fed into the cathode of V1b. This makes that tube oscillate giving the tremolo effect to the passing signal by varying how the tube sees ground through the cathode resistor and bypass cap. (something like that anyway). It doesn't actually see the signal, but it controls how the tube reacts which effects the passing signal. (words?)
Last edited by MBD115 on Sun Jan 10, 2010 1:13 pm, edited 1 time in total.
Re: Help! I tried to recap my amp!
Yes that is what I figured out, and I only replaced one part at a time, then powered up the amp to see what happened. After I replaced the first 100k it all seemed fine, after the second one the new problems started happening, but I can't find anything wrong there.. I'll inspect more closely, maybe something is shorting or something.
DocZ
DocZ
Re: Help! I tried to recap my amp!
In your post you said the reverb is not working now. The signal from the reverb comes back to one side of the volume pot and mixes in with the passing main signal. I would look there for a bad solder joint(?) since you were changing parts in that area.
If the tremolo isn't working, and again if you were replacing items there, then you need to check the cathode components at V1b for a bad joint or wrong value/etc. The tremolo isn't signal but it effects the signal passing through the tube at the cathode of V1b.
Also you say that none of this helped the hum or noise. I still believe that is because of a poor ground somewhere. That can be found by tracing every ground point on the schematic and inspecting/metering/cleaning/etc each one of them. Have you did that yet?
If the tremolo isn't working, and again if you were replacing items there, then you need to check the cathode components at V1b for a bad joint or wrong value/etc. The tremolo isn't signal but it effects the signal passing through the tube at the cathode of V1b.
Also you say that none of this helped the hum or noise. I still believe that is because of a poor ground somewhere. That can be found by tracing every ground point on the schematic and inspecting/metering/cleaning/etc each one of them. Have you did that yet?
Re: Help! I tried to recap my amp!
The best I can tell by the photos here is what it is.
The return loop from the reverb comes off the left lug of the reverb pot - through the 100k resistor - to the center lug of the tone control. Through the white wire going to the left lug of the volume pot. There a 100k goes to a terminal strip lug. At the strip it joins with the second 100k and the .05 cap (which goes to the end lug on the strip where it joins to the 10k resistor to ground (center lug of the strip), the second 100k goes to the red 500pf cap which goes back to the left lug of the tone control. At that junction on the terminal strip (500pF/100k) there is a white wire coming up from underneath. That is the signal from V1a which should be coming from the V1a coupling cap.
Boy what a mess
The tap going to the reverb looks like its connect to the v1a coupling cap at the terminal strip (second lug from the left) through a resistor to another terminal strip (right lug) to a yellow wire that goes to V2a (reverb).
The return loop from the reverb comes off the left lug of the reverb pot - through the 100k resistor - to the center lug of the tone control. Through the white wire going to the left lug of the volume pot. There a 100k goes to a terminal strip lug. At the strip it joins with the second 100k and the .05 cap (which goes to the end lug on the strip where it joins to the 10k resistor to ground (center lug of the strip), the second 100k goes to the red 500pf cap which goes back to the left lug of the tone control. At that junction on the terminal strip (500pF/100k) there is a white wire coming up from underneath. That is the signal from V1a which should be coming from the V1a coupling cap.
Boy what a mess
The tap going to the reverb looks like its connect to the v1a coupling cap at the terminal strip (second lug from the left) through a resistor to another terminal strip (right lug) to a yellow wire that goes to V2a (reverb).
Last edited by MBD115 on Sun Jan 10, 2010 2:05 pm, edited 1 time in total.
Re: Help! I tried to recap my amp!
I did check every ground point and component connected to ground with my multimeter, I didn't clean anything though. I'm not sure what to clean with and what to clean up
DocZ
DocZ
Re: Help! I tried to recap my amp!
IMO only. Some of those terminal strips are insulated but some have at least one lug that is grounded. Some are also grounded for all the lugs I suppose. This ground is established by the mounting lug and bolt that goes through the chassis. If there is any corrosion between the chassis and the terminal strip mounting lug then this could be a problem. Given how that amp is so cluttered around those grounding terminal strips that will be a difficult job.
I don't know.
Good luck.
I don't know.
Good luck.
Re: Help! I tried to recap my amp!
Ok I found that the .005uF cap on pin 6 of V2 was pushed down so far that the metal on the front touched pin 7 of V2 when I fixed that the distortion went a way and the reverb is working again. I also switched the new 33uF cap the correct way this gave me better tone and suddenly the amp was louder. Actually a little louder than it was before I started this thing.
Noise and hum still present though. But the noise takes a while longer to present itself.
Tremolo is still not working, I am coninuing to inspect around to see if something is shorting - just a quick thought maybe something got broken when I had the cap the wrong way or the pins on v2 were connected?
Well the tremolo seem to work for 1 second after I switch it on, then it stops.
DocZ
Noise and hum still present though. But the noise takes a while longer to present itself.
Tremolo is still not working, I am coninuing to inspect around to see if something is shorting - just a quick thought maybe something got broken when I had the cap the wrong way or the pins on v2 were connected?
Well the tremolo seem to work for 1 second after I switch it on, then it stops.
DocZ
Re: Help! I tried to recap my amp!
In that last picture showing the new blue cap, are you sure you have that connected right?
That is most likely the bypass cap, should be on pin 3 or 8 depending which have of the tube is used there.
The middle pin on the tube is just a shield which is rarely used.
Also you have the positive end of the cap going to ground.
On most axial leaded E caps, the + end has a indented ring around it.
Also as mentioned it will most likely have a rubber stopper on the + end and the - end will be the metal of the can.
On that cap you have the indentation, the black ring around the - end.
If you install these backwards with any kind of high voltage on them, they can blow up!
That is most likely the bypass cap, should be on pin 3 or 8 depending which have of the tube is used there.
The middle pin on the tube is just a shield which is rarely used.
Also you have the positive end of the cap going to ground.
On most axial leaded E caps, the + end has a indented ring around it.
Also as mentioned it will most likely have a rubber stopper on the + end and the - end will be the metal of the can.
On that cap you have the indentation, the black ring around the - end.
If you install these backwards with any kind of high voltage on them, they can blow up!
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Help! I tried to recap my amp!
I noticed from the previous tips that it was put in backwards, the center pin of the tube is used as a convenient connection point. It is connected together with a resistor to pin 3.
DocZ
DocZ