This BJ came to me with the crackling/sizzling that I've heard before but with an unusual twist. The noise moves with the master volume and once the master is engaged, the treble knob will also increase & decrease the noise. I repaired a loose trace on V5 plate pin with no improvement. All preamp tubes socket solders were good. All plate resistors were tested in spec. C1 spec'd 47/16v but was actually a 22/16 so that was replaced. C4 has a bad trace also & was repaired. I check both the master & treble pot's & found their solders were good. Treble pot tested at 197k, spec'd as 250k and master at 30k, spec'd as 50k but was smooth. What did I miss here??
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Another Blues Jr. Crackling Problem
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What version do you have? Look printed on the main board either right by the input jack or over near the Master and Middle control. There will be a year and sometimes also a revision level.
Keep your eye on the prize. You are chasing down a noise, not a tone problem. SO I urge you not to be changing out cathode bypass caps because they don't agree with your schematic. In the later versions, the cathode cap in the input stage is 47uf, but in the original series, the first few revisions are 22uf there. But that cap value was not making crackle noises. It could possibly, though doubtful, alter the bottom end tone a little bit, but not make crackles.
Pots are notorious for being "off value". It doesn't matter. A 250k resistor being 200k will not make crackles.
Any control that affects the noise in ANY way means the noise comes from BEFORE that pot.
You can measure a 100k resistor and have it measure exactly 100k, but that tells you nothing about whether it is noisy.
What you missed is tracking down the source of the noise. The master volume control affects the noise. OK, two stages of the amp are in V1 and the third in V2.. Have you tried a different 12AX7 there? Does the volume control, not the master, affect the noise? Does the FAT switch affect the noise?Education is what you're left with after you have forgotten what you have learned.
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You could simply remove V1 and see if the noise persists. If it doesn't, you know the problem is in that gain stage. If the noise is still there, try removing V2 and see if that gets rid of it. Both of those tubes share a power supply node. You could have a bad filter cap (C10) on that node, which is not at all uncommon on this era Fender amp. If you have a scope, scope for noise on that PS node. You can also use a scope to follow signal through the preamp to see where the noise is coming from.Last edited by The Dude; 05-09-2020, 12:03 AM."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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Originally posted by The Dude View PostYou could simply remove V1 and see if the noise persists. If it does, you know the problem is in that gain stage. If the noise is still there, try removing V2 and see if that gets rid of it. Both of those tubes share a power supply node. You could have a bad filter cap (C10) on that node, which is not at all uncommon on this era Fender amp. If you have a scope, scope for noise on that PS node. You can also use a scope to follow signal through the preamp to see where the noise is coming from.
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Oh sorry. I was looking at a different revision schematic. Yes, C28 would be the filter for that supply node. FWIW: It's a good idea to post a schematic or schematic link when starting a thread for this very reason. It keeps us all on the same page. Do you have a scope?"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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Yes, C28 for all three.
resistors can be noisy even if in spec, how would you know?Education is what you're left with after you have forgotten what you have learned.
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Originally posted by The Dude View PostOh sorry. I was looking at a different revision schematic. Yes, C28 would be the filter for that supply node. FWIW: It's a good idea to post a schematic or schematic link when starting a thread for this very reason. It keeps us all on the same page. Do you have a scope?
https://el34world.com/charts/Schemat...nior_rev_a.pdfAttached Files
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If you have a scope, that's the easiest way to find the noise. It might be a good time to learn how to use that thing. There are plenty of tutorials on the web. A Google search will land you plenty of them. I'd start by scoping that power supply node and see if you detect noise on it."I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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Try this, scope the output. You hear the noise in the speaker, so you know the noise signal is there. It may not be a large signal. But you will see what it looks like. Probably just little wiggles and jumps. If the sound is coming from the preamp, the signal there will be smaller, but it will look the same. So you know what you are looking for.Education is what you're left with after you have forgotten what you have learned.
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My scope isn’t working correctly & may need repair. Until then, we know the noise dissapeared when I remove V2. If I’m reading the schematic correctly, is it correct to say that R15 & C5 are the only 2 components feeding V2b grid from the V1a plate, making them highly suspect? Are C6, C7 & R11 also part of that circuit? I pulled C5 & the noise stopped but no sound either. I see that it’s in the signal path. I noticed that 250pf/1kV is called for but they used only a 250pf/500v. The cap measures 550pf, twice the value. It’s a silver mica dogbone type. Not sure I have one of those.Last edited by Perkinsman; 05-09-2020, 07:54 PM.
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Originally posted by mac dillard View PostSilver mica caps have a history of being bad.
nosaj
The so-called "silver mica disease" is actually an electroplating process which occurs most commonly in capacitors with a high imposed potential. A minute current starts to flow through surface contamination, dirt, etc. This current carries silver ions, just as would occur in a silver electroplating process. Eventually enough ions migrate to form an electrical connection, which is a short circuit.
However, this conductor has a miniscule cross-section, and instantly opens like a fuse as soon as fault current begins to flow. Then you're back to leakage current and plating action until the gap is bridged and the cycle repeats. This closing and opening of the silver bridge can happen quite rapidly, and cause noise when it occurs.
This phenomenon can occur in any silvered mica capacitor, although it is much more likely in open ones such as those found molded into the bottom of IF transformers. In these the bridge occurs between the capacitor on the primary side and the one on the secondary, which are physically adjacent. The primary cap has B+ on both electrodes, while the one on the secondary has ground potential (approximately) on both sides.
The only application in which a high DC potential appears across an individual capacitor is in bypassing, and silver mica caps are not normally used for this purpose. However, the same problem can occur with lower potentials, and failures have been seen in micas used in tuned circuits. I don't know if the failure mode is the same as I described above or different.
We used to consider silver mica caps to be eternal. That's no longer true. It's just taken longer for their failures to show up. So now they are suspect, and probably in another 20 years we'll be replacing them routinely just like we do with paper caps now.
Silver mica caps are still being made by CDE and Elmenco IIRC. Stocked by Mouser www.mouser.com and other online vendors.soldering stuff that's broken, breaking stuff that works, Yeah!
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