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Fender Hot Rod Deluxe Drive/More Drive switching problem

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  • Fender Hot Rod Deluxe Drive/More Drive switching problem

    I recently purchased a Fender Hot Rod Deluxe. I really love the tones I can get from the clean channel and have no problems there, it's only when I go to the Drive channel that things get odd. For some reason the amp switches to the "More Drive" channel on it's own and back again. It happens repeatedly while on the Drive channel. I'm not using a footswitch, nor do I use the effects loop. I didn't have this problem when I was 'test driving' the amp. I checked the bias, it's all good there. Also the seller said that he had never changed the 6L6s (have JJ on the way). I have some electronic training but not extensive.

    Umm... Please Help!?!

  • #2
    I was told that switching out 2 resistors (R78 and R79) may fix this problem. They are 5w 470ohm resistors. It was suggested that switching them to a 330ohm will fix the problem. Any thoughts?

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    • #3
      IIRC, a bad solder joint on the LED will do this (it's part of the switching circuit; not just an indicator). The circuit board will need to come out, and that's not exactly easy. If you've never worked inside a tube amp with potentially lethal voltage, it might be best to take it to a tech and sort it out with your seller. BTW, you'll need to at least check the bias when the new output tubes are installed.

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      • #4
        Right on. I'll check that out. I went and picked up a couple resistors to switch out also. All I would really need to do with the LED would be retouch the solder correct? And also I would need to drain the filter caps... I think. Maybe I should take it to a tech. hah.

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        • #5
          I wouldn't recommend lowering the value of those resistors, I'd just make certain that they are solidly soldered in place. Lowering the value will cause the zeners to have to work even harder than they already are.

          Also as Dave pointed out, the led is an integral part of the switching circuit and if it breaks loose, will cause an intermittent like the one you describe.

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          • #6
            My HRD is an earlier 90s model. Fender has since been using 330 ohm rather than the 470s on R78 and 79. But everything else in the new schematic is the same. I'm still planning on checking for solder problems in the chain.

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            • #7
              Originally posted by ehouck View Post
              My HRD is an earlier 90s model. Fender has since been using 330 ohm rather than the 470s on R78 and 79. But everything else in the new schematic is the same. I'm still planning on checking for solder problems in the chain.
              I will have to check my service discs, but my recollection is just the opposite. Early ones had the 330 ohm resistors and all have been changed to 470 ohms, except for Devilles that had a different power transformer and a lower incoming supply voltage.

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              • #8
                Hi guys

                I am a new member but as a repair tech in the UK I can say that in my experience this problem has allways been the 5Watt dropping resistors. Basically because they run so hot they transfer this heat to the circuit board which causes problems with the solder joints on them and also the zenner diodes. Whenever I get a Blues or Hot Rod series amp in for repair and or overhaul I always remove the resistors sqeeze some silicon onto the board and sit the resistors on that and resolder. This does three things. 1) Keeps resistor off the board. 2) Forms a barrier between resitors and board hence board stays cool. 3) Prevents any chance of the resistors vibrating and breaking their joints. I did this to my own Blues Deluxe in 2000 and have never had the switching circuit problem. Hope this helps.

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                • #9
                  Thanks Kenda47. Also what is your opinion about switching the 5w to 330 ohm? And my little jewel light started having problems. It gets really dim then all the sudden it lights up and the amp jumps in volume (clean channel, I only use the clean for now cause I can't stand the switch from drive to more drive). Thanks.

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                  • #10
                    The Tech note I have refers to the Blues Deluxe, and talks about changing the 5W resistors, and recommends a change FROM 330 ohm to 470 ohm, and specifies a "footed" resistor to keep the heat off the board. I believe the 470 ohm resistors have always been standard on the HRDlx, at the least my copy of "Revision A" circuit diagram shows 470 ohm resistors and I cant recall ever seeing 330 ohm in a HRDlx. Even so, I have seen many HRDlx that showed signs of heat stress on the PCB in the area of these resistors, and it is always one of the things I check whether the amp is having switching problems or not.

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                    • #11
                      Riddle me this Billy R. We can take those cement block resistors and elevate them off the board and insulate them but if they're getting that hot it's bound to translate down the leads and desolder itself. Next time I have a De-whatever here I'll aim my infrared thermometer at it and see what I get off those reeeeee-sistors. How about hanging them out on inch long leads or maybe mounting them to the chassis as a heat sink?

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                      • #12
                        Billy R, the 5Ws that are on it are 470 ohm resistors. The newer HRD III amps are using the 330 ohm in place of the 470s. The majority of the people I talk to say that switching is the best option. And putting some silicone under them to raise them up a bit. Unfortunately it's no longer just a switching problem. The longer I have the amp on and playing I start to loose the reverb, it goes in and out with a nasty sound... And the clean channel jumps in volume when the jewel light dims and then gets brighter again.

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                        • #13
                          Here is a MEF link with a thermal image of the zener resistors.
                          271F.
                          Link: http://www.google.com/imgres?imgurl=...ed=0CBQQ9QEwAA
                          They do run toasty!
                          As to the indicator lamp dimming & the volume loss: the lamp is supplied by the heater windings.
                          You may have a bad tube that is pulling down the heater voltage or bad solder joints in the heater circuit.

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                          • #14
                            I had a Deville with a pilot light that was doing exactly as was described. The solution to THAT problem was to remove the bulb and clean up the base and the contact in the socket with an eraser on the end of a pencil.

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                            • #15
                              Yeah it may be the lamp itself.
                              I have went as far as adding a small blob of solder on the lamp end.
                              My concern is that the OP seems to see a volume drop at the same time that the lamp flickers.
                              It may be coincidental or it may point to other issues.
                              My take on these HR amps that have multiple symptoms is to resolder the whole board.
                              You have to take care that you do not cause more issues, like with the interboard wires.
                              Granted , the ones that show up need work, but that does not mean all of them are bad.

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