This was by far the most frustrating problem I ever had with an amp. While playing there was popping noises like breaking sticks and a "rainy" sound in the background with some modulation on it.
At first I assumed it´s the lamp that went bad. But these symptoms were atypical symptoms for a dyeing lamp. In most cases the lamp becomes microphonic. If you tap on it, you can hear the tap in the amp. This is called ``the pen test``. A microphonic lamp might interfere with the signal but only when the amp is played. Otherwise if dead, it wont amplify at all. In this particular amp the lamp was having the role of pre-amp stage, then rounting the signal further on to some solid state integrates. Anyway, just to rule it out, I bought a new lamp and changed it. As it was to be expected, the problem was still there. No change.
At first I assumed it´s the lamp that went bad. But these symptoms were atypical symptoms for a dyeing lamp. In most cases the lamp becomes microphonic. If you tap on it, you can hear the tap in the amp. This is called ``the pen test``. A microphonic lamp might interfere with the signal but only when the amp is played. Otherwise if dead, it wont amplify at all. In this particular amp the lamp was having the role of pre-amp stage, then rounting the signal further on to some solid state integrates. Anyway, just to rule it out, I bought a new lamp and changed it. As it was to be expected, the problem was still there. No change.
At this point, I decided it´s better to open it up and take a look inside.
everything seemed to be just fine, visually, except 2 germanium diodes who were transmitting the signal to the final stage amp, one for each speaker. They were smoked, and they when tested with mutimeter, they didnt responded right away always, and sometimes allowing the signal to flow backwards. That would of course not explain the modulation but I got it clear that first they broke, signal was flowing backwards so other components might had been damaged. I replaced them, just to see if it maked any difference, but as expected, nothing changed. The problem was the same.
Time to go eve further with my investigation so I verified ALL the resistors. They were all in good condition, working in parameters and without any signs of overheating.
Next on the list, CAPACITORS. Since I didnt had a measuring device for testing this ones, I decided to replace them all. I ordered high quality, audio capacitors and changed them all, replacing of course every one of them with one of the same specification. NO CHANGE!!!
Well, since it was popping noise and with modulation, I assumed it cant be the final stage amps, since it was very loud anyway. So i left those alone.
Whatever spirits were haunting my amp, it was happening somewhere in the Pre-Amp stage, and the entire "Shbang" was further amplified to obtaining a final crappy sound.
So I make a sumary:
- New Lamp
- New diodes (they remained unblown)
- New capacitors, ALL OF THEM
What was left where the Transformer, Bridge rectifier, potentiometers and some small integrated chips.
It is not the transformer since it´s working just fine (loud and crappy) at full power and it´s working in the first place.
It is not one the potentiometers, because I don't obtain any noise when I turned every one of them, and they all seem to do they're job just fine (volume, master, echo, re-verb, etc...)
It is not one of the integrated chips since all the functions are working just fine, when switching between channels (clean and distort) and the noise is just there all the time.
So whats left>???
What can give you modulation like a 50 Hz sine wave>???
That´s right. THE BRIDGE RECTIFIER!!!
Now this is a guess yet. But I´ll get it changed and I´ll continue the post, let you guys know what happened.
Time to go eve further with my investigation so I verified ALL the resistors. They were all in good condition, working in parameters and without any signs of overheating.
Next on the list, CAPACITORS. Since I didnt had a measuring device for testing this ones, I decided to replace them all. I ordered high quality, audio capacitors and changed them all, replacing of course every one of them with one of the same specification. NO CHANGE!!!
Well, since it was popping noise and with modulation, I assumed it cant be the final stage amps, since it was very loud anyway. So i left those alone.
Whatever spirits were haunting my amp, it was happening somewhere in the Pre-Amp stage, and the entire "Shbang" was further amplified to obtaining a final crappy sound.
So I make a sumary:
- New Lamp
- New diodes (they remained unblown)
- New capacitors, ALL OF THEM
What was left where the Transformer, Bridge rectifier, potentiometers and some small integrated chips.
It is not the transformer since it´s working just fine (loud and crappy) at full power and it´s working in the first place.
It is not one the potentiometers, because I don't obtain any noise when I turned every one of them, and they all seem to do they're job just fine (volume, master, echo, re-verb, etc...)
It is not one of the integrated chips since all the functions are working just fine, when switching between channels (clean and distort) and the noise is just there all the time.
So whats left>???
What can give you modulation like a 50 Hz sine wave>???
That´s right. THE BRIDGE RECTIFIER!!!
Now this is a guess yet. But I´ll get it changed and I´ll continue the post, let you guys know what happened.