Fender Ultra Chorus Shuts Down within 3 Seconds at Power-On

Discussion in 'Amp Tech Center' started by Tenderfoot, Dec 21, 2017.

  1. Tenderfoot

    Tenderfoot Tele-Meister

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    Prior to the amp shutting down issue , there was a constant low level hum for several months along with loud pop's when turning the amp on and off. Given that the Power Supply Filter Capacitors are over 20 years old, I pulled them out of circuit and found that one would not hold a VDC test charge using a 9VDC Transistor Battery. One read 8.65VDC and the other one 0.004VDC. They are 4700 MFD @50VDC 105C.

    The fuse didn't blow (it's 4amp SB) and there is no indication of anywhere on the board of excessive current or overvoltage, i.e., no scorch/burn/component bulging. One interesting thing is that the Chorus LED remained ON with power switch in the OFF position but 120VAC power still being applied to the amp. Looking at a schematic the Chorus LED gets its feed from the power supply +/- 16VDC which is taken downstream from the subject filter capacitors. The other feed is +/- 40VDC to other components on the chassis (please note this is a Solid State Amp).

    I have ordered 2 replacements which I feel will correct the hum issue being experienced and has a good chance of fixing the shutdown issue as well. With one capacitor not holding a charge and both being well over their service life; I feel that on power-on the DC voltages (especially the +/- 40 VDC) is unstable resulting in the amp shutting down.

    Any input on the above would be most appreciated. If there are further troubleshooting steps that should be done, please pass them along. Of special interest would be hearing from anyone who has experienced the same issue and how it was resolved.

    Thanks!
     
  2. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Since you asked.....

    I like that amp, worth fixing and keeping IMO.

    Capacitor replacement (good ones, right specs, correct polarity, careful installed given that board) would be a respectable first order of business for this or any 20 year old amp with similar-sounding probs.

    If the cap job doesn't fix the prob, put it in the hands of an authorized Fender amp tech.

    *I know that amp somewhat and I guarantee it isn't going to be productive to address the probs with a "modder" skillset or mindset. Toyota-like...
     
  3. Tenderfoot

    Tenderfoot Tele-Meister

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    Thanks LudwigvonBirk. Since i posted I did further trouble shooting and verified that that the power supply diodes and protection devices (Current Surge Thermistor and Thermal Switch) for the 120 VAC input power to the transformer were good. I have a copy of the schematic and test equipment for signal tracing should that be necessary after the capacitors replacement. As an FYI I have never modded my amplifiers electronics with exception of tubes and of course biasing adjustment.

    Since its been a few years since I worked on "Black Boxes" in the Navy (was an aviation electronics technician) I felt it wise to reach out to others for any information that could help resolve the issue without taking more t/s steps than required. You mentioned "I know that amp somewhat..." and keeping mind that I know Ohms Laws, Test Equipment and how to read a Schematic; please advise me on any issue that you had and how to resolve them.

    Regards, Tenderfoot
     
  4. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Since you already have rigorous pro training and experience, I recommend a tube-amp-specific re-review of safety procedures (and an honest re-pledge to adhere to cap draining, ground and hot check safety- the usual). Then once your safety refresh is done, dive in! Walk the schematic from input to output and record all measurements.

    Good safety stuff here: https://robrobinette.com/Amp_Stuff.htm

    Also, haven't watched any but YouTube seems to have quite a few diagnosticy videos for your amp.

    Suit up!
     
    Last edited: Dec 24, 2017
  5. Tenderfoot

    Tenderfoot Tele-Meister

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    Good News! The cap job fixed the problem (and I am still alive). I did verify the diodes, caps, resistors, thermistor & thermal switches, etc. while waiting for the caps to arrive. YouTube videos were viewed as a refresher on safety and proper circuit board troubleshooting and soldering. LudwigvonBirk my anti-virus (Bitdefender Total Security) flagged the website that you provided so I didn't go there but thanks for offering it.
     
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  6. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Yay congrats!
     
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