Another Blues Jr. Crackling Problem

[Perkinsman]

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??

[Enzo]

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?

[nevetslab]

Also, those pots used in the Blues Jr are poor quality. I've had noise issues with them as well. They could use a dose of contact lubrication & exercising.

[Enzo]

And the connections on the ends of the ribbon cables can start to break. Lots of possibilities. We need to troubleshoot.

[The Dude]

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.

[Enzo]

Or use a signal tracer and listen for the noise.

[Perkinsman]

You 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.

This RevA schematic, cream board 2003. Tubes are all new. OnlyMaster & to some extent treble affect noise volume. Pulled v1, still noise & put back in. Pulled V2, noise dissapeared. Switched 1& 2, still noise. C10 is not a filter cap on this model, it’s a Mylar. I’m still learning to read schematics so is it correct to say that V1a,V1b & V2b share the B+ from C28? The voltage is 242 & there are no bulges & it’s not leaking. I didn’t realize that resistors can be noisy even if in spec, how would you know? There is one, R47 that has turned the board a light shade of brown but it measured exactly 2.2k

[The Dude]

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?

[Enzo]

Yes, C28 for all three.

resistors can be noisy even if in spec, how would you know?

BY troubleshooting. I use a 0.047/630v cap, because I have a ton of them and they are handy, but value not critical. I can check a 100k plate resistor by clipping my cap across it. Sometimes just applying heat or cold to the resistor will change the noise as a clue. I can't short a tube plate to ground, but I can connect my cap from that plate to ground. It acts like I am grounding it for AC. Signal, noise or otherwise is AC by its nature.

[Perkinsman]

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've been trying to load the schematic but I can't figure it out using my phone, sorry, so I'm posting the link below. I do have a scope and the probe but not sure how to use it to troubleshoot yet. So would C28 be suspect even though the voltage is in spec? Also, I don't understand how to determine when a resistor can be noisy when it measures in spec. What tests can i do to a resistor besides using the DMM ohm meter?



https://el34world.com/charts/Schemat...nior_rev_a.pdf

[The Dude]

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.

[Enzo]

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.

[Perkinsman]

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.

[mac dillard]

Silver mica caps have a history of being bad.

[nosaj]

Silver mica caps have a history of being bad.

Best description of what happens I've ever seen from https://www.antiqueradios.com/forums...ic.php?t=61153

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.