Buzzing Fender HRD

Discussion in 'Amp Tech Center' started by bluezmax, Mar 20, 2019.

  1. bluezmax

    bluezmax TDPRI Member

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    Hi everybody,
    I'm trying to fix this HRD for a friend.
    He keeps it in his van in the lovely Miami weather.
    He went to play it at a show and it blew the fuse as soon as he took it off from stand-by.
    I threw in a new fuse and turned it on. Did fine when I turned on the power but made a very loud buzzing sound and popped the fuse even though I had the volume and the master at 0. There was a slight electrical burning smell when it popped but that may have been the fuse itself.
    Put in new power tubes and a new fuse, tried again and heard the same buzzing soon as it came off stand-by. I turned it off before it blew the fuse.
    I'm thinking it's one of the electrolytic caps in the power supply circuit but I'm not sure that would blow the fuse. After doing some reading online, I'm suspecting it might be the OT shorting out... I tried to test for a short and got continuity between all the leads coming out of the OT to the speaker jack. I'm not 100% sure that's normal.
    I've worked on some HRDs before replacing the usual suspect resistors, the 2 wire wound resistors, cracked ribbon cables, bad jacks and bad solders. I checked all those and they seem fine. The only thing I would replace is Thermistor 1 as it looks like a very small piece is missing by one of the legs but It must be working or the rest of the board wouldn't be getting power.
    This one's got me puzzled... One small detail that might help is that the buzzing/crackling sound (it's almost like the sound of High Voltage electrical arcing) gradually trails off when I put it back on stand-by or power it off...
    So what do you guys think ? Should I order a few caps and try those or am I definitely looking at a new OT ?

    Thanks !
    Bluezmax
     
  2. clintj

    clintj Friend of Leo's

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    Open it up and have a look. I'll bet dollars to donuts you have a filter cap that's failed or about to fail.

    Sent from my Pixel 2 using Tapatalk
     
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  3. bluezmax

    bluezmax TDPRI Member

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    It's sitting on the workbench opened right now...
    I've been looking at those caps under every angle but they look fine from the outside.
     
  4. schmee

    schmee Poster Extraordinaire

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    I'll bet you will be able to see something burned. I'm thinking Filter Cap or other Elytic or worst case the OT is gone. But IIRC there are a couple resistors in those that burn and go sometimes. Here are some notes that apply to various versions of those Fender amps, so may not fit yours 100%:

    "The two large ceramic resistors in the power supply get very hot and try to desolder two of the rectifier diodes, causing all manner of voltage fluctuations, thus making all sorts of weird happenings. Easy fix (if you're competent), resolder the diodes, cut the resistor leads, add a bit to them so that they sit higher and not dissipating their heat on to the diodes. Remove the rear cover and take a peek. You can't miss them, and see if they are looking a bit 'brown'.

    Or: It's the 5 watt, 470 ohm power resistors (R85 & R86) that overheat and cause switching problems in the 16 volt line. The very early models had lower value resistors (350 ohms I think) but Fender changed to 470 ohms later on in the original manufacturing run. If yours are the lower value, upgrade them to 470 ohms. They often take out the two zener diodes right next to them and can cause track damage too.

    The large resistors are rated at either 3 or 5 watt, I can't remember which. The small 'cylindrical' resistors are typically 1/4 to 2 watt rated so will NOT be suitable.

    The schematic asks for 5 watt. Replacing them with smaller wattage resistors can cause damage, they need to withstand the amount of energy they are faced with, in terms of current and voltage (P=V*I).

    Had that problem couple weeks ago. As usual with these amps, the solder joint was broken to pin 9 filament of V1. It would fluctuate voltage just enough to have a volume fade. While you're in ther just go ahead and change out ALL of the plate resistors, especially the PI. If they haven't failed yet, they will.

    Cutting out: You almost certainly have one or more poor solder joints, or broken trace(s), in the area of the 5-watt resistors, and zener diodes, in the +/- 16 VDC supplies. The resistors are R85 and R86, (R78 and R79 in later versions), and the zeners are CR22 and CR23 (D13 and D14 in later versions). The 16 volt supplies are for the channel switching, and other functions of the amp. It is quite a job to remove the main circuit board to inspect the joints and reflow solder or repair broken trace."
     
    Last edited: Mar 20, 2019
  5. clintj

    clintj Friend of Leo's

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    Oh, how old is this amp? I've seen a few cases where Hot Rods lost filter caps at or before the ten year point. There was a period where poorly made IC caps intersected with Fender specifying too low a voltage rating that caused premature failures.

    Looks have nothing to do with it. By the time damage like electrolyte crust or bulging is visible, they're way past needing replacement. What's on the inside matters more. Do you have a decent multimeter you can test them with for capacitance, or even better an ESR meter?

    Sent from my Pixel 2 using Tapatalk
     
  6. bluezmax

    bluezmax TDPRI Member

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    Nothing looks burnt...
    I've checked the ceramic resistors and they are fine. Checked the grid resistors too and they seemed fine except for R80 and R81 giving me a reading of 50 ohm instead of 100 ohm each but that may be because they are wired together.
    I've fixed a couple of HRDs that had the problems you mentioned and of course, they were the first things I checked.
    Except for R80 and R81 readings, everything else looks perfect. I still believe it's one of the caps. I would have tried another one already but all the caps I had around were rated at 450V so I ordered some F&T already.

    We'll see if that was it when I get them...
     
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  7. bluezmax

    bluezmax TDPRI Member

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    Replaced the caps and that wasn't it... Same symptoms.
    I'm replacing a couple of diodes that look like they got hot. Let's see what happens !
     
  8. BigDaddy23

    BigDaddy23 Tele-Meister

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    You may have an intermittent short to ground somewhere that draws excess current and hence the hum leading to the fuse blowing. Check all of the power/B+ connections throughout the circuit
     
  9. Andy B

    Andy B Tele-Afflicted Gold Supporter

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    Check the resistors in the PI circuit.
     
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