Help me with 90's Blues Deville 410 Effects Loop

Discussion in 'Glowing Bottle Tube Amp Forum' started by fraggy0117, Jul 10, 2020.

  1. fraggy0117

    fraggy0117 NEW MEMBER!

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    I've got a 1990's Fender Blues Deville 410 that's been in the family for a while. As part of a quest to make the volume a little more manageable, I've been wanting to put a little volume "pedal" in the effects loop. However, that's where I started running into problems.

    It's been a bit of a journey but I think I've addressed some issues. I'll try to put everything in a succinct format:

    • When I first got the amp from my dad some months ago, I was unable to get any sound when patching a volume pedal from the preamp-out into the power-in jacks
    • Soon after, the amp started having issues with it switching to the gain channel on it's own and no sound coming out. Eventually, this increased in frequency until it basically sat in the no-volume-gain-channel state whenever turned on.
    • I'm not sure if it's helpful, but I'd typically get it to work by flipping the standby switch back and forth a bunch, but eventually, that didn't even help.
    • I did a bunch of research and figured out the reverb, channel switching, and Effects loop were all connected by a set of sandblock resistors that have a history of going bad from heat. My plan was to replace these.
    • Before I tackled the amp, I practiced working on PCB by making some guitar pedals. Not too important but the amp sat for a long time
    • Recently I got the amp on the bench, checked the caps, and opened it up. The two resistors I thought were going to be bad looked immaculate and the solder looked good.
    • Just in case I reflowed those connections (looked a little dull) and noticed that the only "bad" looking soldering was for a resistor and a cap near the reverb ICs (C18 & R35). It looked chunky and away from the board, so I solder sucked it away and resoldered them.
    • I turned the amp on and it slowly, but increasingly, created a loud farting sound. I turned it on and off a few times but it just got worse.
    • The next day I noticed the second Preamp tube wiggled a lot more than the other, so I pushed it back up
    • The amp turned on fine. 'Success!' I thought. Unfortunately the FX loop still didn't work.
    At this point I did some testing to get as much info as I could:
    • Plugging my guitar directly into the Poweramp-in produces no sound. I was thinking this meant a faulty jack, but:
    • Bridging the effects loop with one cable makes a very quiet distorted sound when playing. Not popping but like a distorted signal. However, it is so quiet that you have to turn the volume up all the way to hear anything.
    • Turning on the drive channel with the volume all the way up attenuates the signal even more - basically unhearable
    • Patching the preamp-out into the input of another amp creates sound like normal.
    • The presence knob seems to have [roughly] its normal effect when bridging the effects loop
    • Not sure if it's related, but the treble knob (normal playing, not using effects loop) seems to greatly change the volume of the amp in a way the other parts of the tonestack don't. Turning treble up turns the volume down. Now, this may have been true before but I don't remember it working that way
    I'm not sure where to go from here, and I'm hoping you all can at least point me in the correct direction.

    Thanks!
     
  2. MuddyWolf

    MuddyWolf Tele-Meister

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    It seems pretty far from your problems but I read there are relays that switch channels that fail in that amp.
     
  3. Dunedin2019

    Dunedin2019 TDPRI Member

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    These amps have +-16VDC supply rails (for channel + FX switching) using large dropper resistors which typically run hot, burn up the PCB and cause all sorts of intermittent problems until they fail.

    https://irationaudio.files.wordpress.com/2015/07/fender-hot-rod-deville-schematic-1.jpg

    See R78 and R79 330R 5W resistors in A4 area

    Fix is to replace the resistors (mount them away from the PCB) and repair any track damage. http://www.unbrokenstring.com/wp-content/uploads/2017/02/FenderHotRodDeluxe/12HRD-new-parts.jpg

    Zeners CR13 CR14 also may scorch the PCB.

    That's where I'd start
     
  4. fraggy0117

    fraggy0117 NEW MEMBER!

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    Dunedin, R78 and R79 were my original thoughts, too. But those two resistors look immaculate and I don't see any sign of trace damage. My multimeter reads them in-spec, as well.

    I just checked CR13 and CR14, but seem to be functioning as normal and I don't see any sign of trace damage there, either. Do you have any idea for next steps?

    MuddyWolf, I'll look into the relays. I'll have to figure out how to test them. Thank you!
     
  5. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    I would think C7 (treble 250pF cap in the tone stack) is faulty for the treble knob to act this way. Probably not related to the other issues.
    Does the amp have full volume with nothing plugged into the preamp-out, power-in jacks?

    FYI
    There are several op amps in the Blues Deville. Fender has them run close to their max voltage rating. They get their power through the resistors R78 and R79. This is a problem area for the BD as well as the HRDlx and others. The resistors drop the 34volts, then the diodes further drop the voltage to provide ~16volts to run the op amps. These resistors and diodes get very hot, sometimes hot enough to make components and/or the solder joints fail. On a lot of these amps that is the extent of the failure. Change out the resistors and/or diodes and it is good to go.

    The failure mode can be more extensive. When the power is not dropped sufficiently the op amps fail when the max voltage rating is exceeded causing reverb problems, switching problems and there is an op amp to buffer the signal when the power in jack is used.

    Luckily the op amps are very common and do not cost a lot. Unfortunately, I usually replace them with op amps with a higher max voltage rating to reduce the possibility of it happening again. Mo money. Ha.
     
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