Fender Blues Jr Noise

[Mark Black]

Just a report in case it can help someone (and a nod to using a scope - WITH proper loading...)

I was presented with a Fender Blues Jr that reportedly had a distortion-on-decay issue. Well - I eventually agreed that it did. Man - that symptom was subtle (for my 50+ Yr-old-rock-guitar-player ears anyway). After I played it for about 15 minutes I DID FINALLY start to notice a quick "BRRZZZZ" (like that?) at a VERY low level following the attack & during the decay of a note, most noticeable at higher (guitar) frequencies like G @ 12th fret and disappearing almost as quickly as it appeared.

I tried the leasyl stuff at first: Ran it into an external cab with same results, tapped/poked/prodded tubes, PCB & all manner of other components, used a guitar and/or a signal generator for input.

Eventually (since I was listening anyway) I put the scope probe on the output while it was still hooked to my test speaker with a 1KHz square wave input and immediately noticed a HF oscillation on the positive side of the waveform at lower levels. I then switched to a resistive dummy load and the artifact was gone! There was lesson/reminder #1 for me: a dummy load is NOT THE SAME as a speaker load in many ways. OK hope I remember...

Lesson #2 was that I probably would NEVER have homed in on the problem without a scope, AND a real speaker load. Dammit - when am I going to get out of school and start earning a real living...?

A little prodding/probing/moving stuff around with a chopstick revealed the most reactive components to be the wiring in the vicinity of the OT primaries. Clipping the (very neat but too numerous) cable ties and moving the wires around got rid of the basic problem, but there was STILL a hint of oscillation visible on the scope.

OK to wrap up: I desoldered the ribbon-cable conductors which carried the power tube plate connections from the main PCB at the tube PCB and clipped the conductors at the tube PCB to eliminate any signal path. I then clipped off the QD terminals from the OT P-P primary wires and soldered them directly to the tube socket PCB pins at pin-7 of each of the EL84 power tubes. Result - problem solved!

Hope this helps someone sometime...

Mark

[wizard333]

Interesting! Thanks for the heads up!

[Rick Erickson]

Hey Mark – I was just reading your post about the Blues Jr. and chuckling about the Blues Jr. that I just worked on. I had made a comment to Morris about how dumb I thought it was to run the plate connections down that ribbon cable. I didn’t notice any oscillation from that amp but will log this for future reference. Talk to you later.

RE

[Enzo]

Two days ago I repaired two Blues Juniors for a dealer. New store stock, each had a broken control to replace. That meant pulling the circuit board down for solder access. One of them hummed badly - in the way an oscillating amp hums. Sure enough the scope shows the power oscillation going on.

I then wondered why the OT wire should be running right across the ribbon cable with the grid leads, touching. Dress the OT wire up and over the other one's push-on and dress the ribbon down and away, no more oscillation.

[Mark Black]

Aah - so it would appear the one I saw was not an isolated case. I kind of figured so and that's what prompted me to post.

I actually did the lead dress first exactly as you described if I understand correctly and it did eliminate the vast majority of the oscillation. I was watching the scope as I moved the wiring and the oscillation, which was stretched across the entire positive portion of the square wave signal, would shrink in amplitude and length back towards the leading edge as things got better. At the end of my wire manipulation phase my best result still left a tiny wiggle - about one cycle actually - of the oscillation frequency just following the positive leading edge. That's when I made the decision to actually alter the wiring and see if it would get rid of that last little bit, which thankfully it did.

I say thankfully because I am actually quite reluctant to alter factory wiring or component layout unless I feel I have pretty good cause. I figure that some much-better-paid-than-me engineer would have designed things to avoid exactly the kind of problem we're discussing here and that I must be missing something.

In this case it did just look wrong for those wires to be in such close proximity. I'm also not so sure what a good idea it was for the factory to cable-tie the OT primary and secondary wiring together tightly even if only for a couple of inches so I left those ties off on reassembly.

When all was done I ended up with the ribbon cables dressed as stock (minus the plate connection) and the OT primary wires dressed as far away from them as possible and running along the chassis straight to the tube PCB.

[Rick Erickson]

I figure that some much-better-paid-than-me engineer would have designed things to avoid exactly the kind of problem we're discussing here and that I must be missing something.

You should know better than that by now.