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Post subject: Blues DeVille Shrieking & Scary noises
Posted: Fri Feb 08, 2013 3:47 pm
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I handle service on CenterStaging, LLC's backline gear, and have a well-seasoned Blues DeVille 4-10 combo amp in the shop being particularily nasty.

Basic symptom is (in the Overdrive Mode) it will shriek (hi freq pitch) loudly on any chords played heavily (controls mid-position), then emit electronic-like feedback to scare the dickens out of you.

I've had the main PCB out several times, have already replaced the 3 ribbon cables, as those appeared to be the main culprit. I've had the tube PCB out, repaired all the fractured solder joints (many found); pulled the preamp tube sockets off & re-tensioned the pin contacts to restore contact force; replaced the film caps in the preamp section (C2, C6, C7, C9); the ceramic caps in the preamp section (C4, C5), cathode bypass electrolytics (C1, C8); repaired numerous solder joint fractures on the main PCB, as well as repaired some circuit traces no longer connected to the solder pads; replaced V1 preamp tube (selected thru several for lowest micro-phonics) & fashioned a vibration damper from some packing foam, fitting it snugly around the 3 preamp tubes.

With the rear panel removed (has the adhesive-backed aluminum shield), I thought I finally had the problem licked, until I put the rear panel back on....then I got the same shrieking & very loud barking sounds over the duration of my test signal. I use a burst Pink Noise signal for tone & transient testing, not unlike playing full chords on a guitar to try and get an amp to 'come unglued' when you're looking for that sort of thing....1 sec duration, 1.5 sec rep rate with a GenRad 1382 Pink NOise Gen & GenRad 1396 Tone Burst Gen.

The problem doesn't occur when you inject the signal (level adjusted) into the Power Amp stage. It also doesn't occur if you use a external 8 ohm speaker (replacing the 4-10's, same jack), nor does it occur using an 8-ohm dummy load. All looks fine on the scope (feed from my Amber 3501a Analyzer). Only happens with the 4-10's connected, AND, happens as soon as the shielded rear panel is attached. So, there's still something resonanting on the PCB's. Haven't found it using a wooden probe (the ceramic caps C4 & C5 were culprits, now replaced).

Power supply potentials are all nominal. V1 ribbon (now short discrete wires) very sensitive, as before, still micro-phonic. Dressed the preamp tube wiring away from rear panel, dressed & bundled the output xfmr wiring neatly.


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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Fri Feb 08, 2013 4:54 pm
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Location: Natural Bridge, Virginia
So it only does this with the 4x10s and the back cover on.

Patient: "Doctor, it hurts when I do this."

Doctor: "Well don't do that!"

:lol: :lol: :lol: :lol:

Sorry. Couldn't resist. :D

It sounds like you've addressed all the typical problems that this amp series sees, but given that the problem only occurs as stated above, it sounds to me like a tube is still microphonic and is breaking into heavy oscillations with the sound of the internal speakers beating on it.

You said you replaced V1. What about V2? That is the drive tube and tonestack gain make up tube and has a lot of gain. You seem to have eliminated the PI and power tubes. I would look closely at V1 and V2 and the PCB and mountings immediately around them. As far as making a vibration damper out of packing foam, try these:

http://www.eurotubes.com/eurotubes-Euro ... -Rings.htm

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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Fri Feb 08, 2013 6:02 pm
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I've replaced both V2 & V3 with no success, so returned both to their home locations. When you mechanically jar the tubes, looking for the tell-tale signs of micro-phonic excitation, only V1 is a little sensitive, being at the head of the gain string. I get a whole lot more micro-phonics when I tap on the grid wires to V1A & V1B than I do on the tube itself. I had also gone thru several iterations of replacing the 6-cond ribbon, with 22AWG Teflon stranded wire, various lead-dress on those, including wrapping the discrete wires around a short piece of cable jacketing to hold the spacing and damp the wires, before returning to the original ribbon cable, only shorter.

I'm still not convinced the wiring between the motherboard and the tube board isn't the problem. The rear panel ground plane is close to it. I had also carved away a relief in the tube PCB (with a Foredom hand grinder/carbide bit) to gain some additional clearance from that ground plane.

It's an odd one, as far as daily dirt problems go. Usually it's fractured solder joints, which I spot on a regular basis (wearing surgical loupes & fiber optic headlight to see that stuff). I had to finally put this aside to get back to making progress, and try again when I ain't made to feel stupid by some cheaply-made Fender product. Horrible PCB's, maybe 1/2 oz copper, foil adhesive ain't worth diddly.

BTY, what is Q1 in the system doing? FET Sw (J111 device) sitting across the two back-to-back 15V zeners CR2 & CR3?? I haven't analyzed the drive ckt to that part yet.

Thanks.


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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Fri Feb 08, 2013 6:16 pm
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Muting circuit, activated briefly when switching channels. :idea:

This might sound odd, but have you looked closely at the Pate resistors for V2 and the PI? In fact, check all Plate resistors. :idea:

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Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Fri Feb 08, 2013 6:41 pm
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shimmilou has some good suggestions.

As far as vibration damping, I had some vibrations in my BDRI caused by the ribbon cables touching the back panel. I redressed those and used clear silicon where the cablea touches the circuit board. I also spaced the back panel away from the amp a little using a single washer between the back panel and chassis at each screw point.

You said the problem occurs with the stock speakers and the back panel on, but not with an external cab. Does the problem occur with the stock speakers and the back panel off?

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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Thu Feb 14, 2013 11:52 am
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I had it nearly free of problem with the back panel removed, as well as it installed but using external cab. Raising the acoustic level high enough (SPL level not measured, but VERY loud), I was able to get the amp to 'bark' with the 4-10's driven. With the panel attached, it happens at a much lower acoustic level. As I had stated previously, the cable between the motherboard and the tube PCB now has a relief carved into the tube PCB, with the cable dressed away from the rear panel, no longer in contact...as are the other two groupings of wires (replaced the other two ribbon cables with short discrete wires, not taught).

Just got back from being sidelined by the flu, but my current thinking is to again replace the V1 tube wiring between the two boards with some small dia Mogami flexible wires, appropriately dressed. With the amp powered up, the 2nd & 3rd wire groups are not vibration sensitive, while both grid wires of the V1 are. I had thought of adding spacers between the panel & mounting cleats, but will leave that until I've exhausted the vibration sensitivity issues first. Never ran into this issue on any of the other similar Fender amps in this class (Hot Rod DeVilles, DeLuxes, Blues Deluxe, etc).

I didn't find any vibration sensitivity of any of the components in the V1 & V2 stages, apart from the two ceramic caps I had replaced, using a wooden probing stick in attempt to induce the problem (no signal applied). I had also thought of the plate resistors, as well as cathode resistors & grid bias resistors. I rarely go and just change parts, but on the 4th time back onto the motherboard, I did recap the signal path as stated, even though there wasn't any outward static evidence of DC leakage on any of the coupling caps. I have had that occur and cause similar behavior (other products), but this time out, it made no difference.

When I get back to the amp, I'll report the findings. Many thanks for the suggestions. I looked at the tube dampers suggested, and what I have in place is similar to the 'stock' double-tube damper Fender supplies on their current production models. During my screening of preamp tubes for V1, the tubes' sensitivity to the high acoustic level was the main criteria, and just using my fingers holding the tube snuggly revealed that being one of the contributing factors to the problem. The applied damper I installed does a similar job as holding it (not as well).


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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Thu Apr 25, 2013 12:05 pm
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After letting the amp sit in the corner a couple months and getting involved in more exotic efforts (hydrophone preamp designs for towed arrays used in cetecean research), I finally got the amp back up on the bench yesterday, and reviewed my service notes from February.

I replaced (again) the input ribbon cable that I had slightly shortened the previous time with some flexible Mogami twisted pairs (grid & cathode wires for V1A, V1B) & the plate wires discretly, same Mogami 26AWG wire. Made no difference (not surprised, but put that to rest for good). No longer microphonic, at any rate, as it was before.

Went thru a reluctant exercise in replacing plate & cathode resistors on the preamp stages, which of course made no difference. Eliminated that. While I had the power amp stage dead-patched, and the preamp connected to my Amber 3501a analyzer, which feeds my headphone system in parallel with that to my scope, I again had the source connected (burst pink noise, filtered) and resumed proding areas on the amp.....V1 tube, V2 tube, tube PCB, motherboard, preamp stage components, and power supply caps. I suddenly got a major tonal & level change when I thumped on C46...the power supply cap from which the preamp tubes are fed. Gripping the body of the cap, I was able to get changes while flexing the part....solder fracture!

I had previously gone thru the main board searching for any & all fractures, and repaired all I found....not realizing I missed one. After powering down, discharging the supplies & dropping the PCB down from the control panel, I again had my surgical loupes & fiber optic headlight on, and found not only this one, but a number of fractures that I missed in February! Egg on my face. Repaired all I found, and, just to be sure, de-soldered & re-soldered all the supply caps in the process. Then, after cleaning, put it all back together again.

Problem gone, now solid as a rock! Solder fractures. I spend most of my time finding and repairing these than anything else. Surprised I had missed it, as it was the first thing I searched for.


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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Thu Apr 25, 2013 12:11 pm
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Excellent. Glad things finally resolved themselves. :D

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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Thu Apr 25, 2013 12:42 pm
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nevetslab, you've done this before, for sure. Nice "flowchart" method. Glad you got the puppy fixed.

Time to enjoy it!

:D


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Post subject: Re: Blues DeVille Shrieking & Scary noises
Posted: Thu Apr 25, 2013 1:21 pm
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While I'm relatively new to the Fender Forum (and others), I keep a daily journal & keep copious notes (often with photos if uncharted territory) on my service efforts. Over time, it's provided a good history on the company's rental inventory (CenterStaging, LLC) along with all my other clients, with solutions achieved in the past. There's always something a piece of gear can teach you, so I tend to be a student in the process. Some gear (such as this lower cost amp with poor quality PCB's) will envoke colorful language from me, when their response to careful surgery results in PCB traces & pads lifting. The end results, though, have always been successful, and don't come back with the same problems.

Happy to have contributed to the forum.

Steve


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