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Thread: Fender Hot Rod Deluxe : instability when in More Drive mode

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    Fender Hot Rod Deluxe : instability when in More Drive mode

    Hi,

    A friend of mine gave me this Fender Hot Rod Deluxe to be repaired.

    Here's the schematic and layout :
    http://support.fender.com/schematics..._schematic.pdf

    The problem is : when in more drive mode, there are some oscillations happening.

    Here are the tests and reasonings I've done so far :

    - No oscillations happen in clean or drive mode, so I guess those are caused by the very high gain of the more drive mode (gain increased by bypassing both V2 cathode resistor)

    - I switched the preamp tubes, oscillations still happen with erevy tube I put into the amp

    - The oscillations are not affected by the drive gain pot or the master pot. The only control that affect the oscillations is the treble control. When this is at a very low value, the oscillations are reduced. I guess this is because less "high" frequencies come to V2 and excite the oscillating frequency.

    - I ran the amp without the power tubes to get rid of any "acoustic" coupling : no results. I even removed the phase splitting tube (12AX7) without any result.

    - I shorted R23 and R24 to get rid of the jfet switches, nothing changed.

    - When I touch the middle of ribbon cable of V2 (it connects G1-C1-A1 and G2-C2-A2 to the main board), I get some squealing. The wires in the middle of the ribbon cable connect A2 and C1. Seems to me that the circuit is instable and that the "link" made when I touch the ribbon cable is enouth to get the circuit into frank oscillations.

    I guess I could get rid of those oscillations reducing the gain of V2B by lowering the plate resistor, or adding a resistor in series with the cathode bypass capacitor (the one that are "activated" in "more drive" position). I could also put some low value capacitor (a few pF) in parallel with V2's plate resistor to lower the HF gain, but those mods would affect the amp's tone for sure.

    I also doubt that Fender let an oscilating amp go through their test procedures, so some part must have gone wrong !

    Here is a sound file illustrating the problem :

    https://drive.google.com/file/d/0B6K...ew?usp=sharing (please let me know if the link doesn't work)

    This has been recorded through the "preamp out", without the output tubes, so no sound at all on the speaker. There is no speaker simulator or LPF, so the distortion sounds harsh as expected

    At the beginning, the guitar's volume pot is turned all the way down. We can already hear some "phantom" oscillation behind the hiss. Then I played some notes where you can hear the oscillation ringing, especially during the sustain of the notes. Then I touched the V2 ribbon cable, and you can hear those frank squealings.

    Has anyone any idea of what could have gone wrong in this amp to cause such instability ?

    Thanx in advance.

    Best regards.

    Eric

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Perhaps there is an open somewhere in the path that connects C3 250pF cap to gnd when in drive. Candidates are relay section K1B pin 9, solder joints or C3 itself. Check for continuity to ground when in drive.

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    Supporting Member Jazz P Bass's Avatar
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    On the pcb, over at the input jacks, is a big electrolytic capacitor marked C36.
    Hook up a DMM to the two leads.
    Set your meter to read Volts/ac & measure the 'ripple' on the cap.
    10 to 1 you will find a high ripple reading.
    If so, that is your likely culprit.

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    Supporting Member Alex R's Avatar
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    yes the big filter caps would also be my prime suspect on these.

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    Absolutely the filter caps.I've repaired at least a dozen of these newer Fenders,Hot Rods,Devilles,Blues Jrs etc and some reissues,all had similar oscillations,all repaired with a re-cap.Those Illinois caps suck.Use good quality Spragues or F&T's.

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    Supporting Member Jazz P Bass's Avatar
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    If you have a scope (a DMM will suffice) & you get on the capacitor leads (C36) while set up for the issue at hand (more drive mode), when you strike a note, the note will be impressed on the positive lead of the cap.
    With a good cap, the signal on the lead will be very quickly dampened & then it will be gone.

    With a faulty cap (C36) the signal will continue to ride on that power supply node.

    This is why that particular capacitors function is refered to as a 'decoupling capacitor'.
    It is there to decouple any signal that is impressed on that power supply node by the action of the preamp tube plates, on that node.

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    Don't forget the joker g1's Avatar
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    If the typical filter cap issues don't get you anywhere, the actual placement of the ribbon cables can be pretty finicky. Being bent in non-stock positions can cause weird problems, and rear panel shielding/grounding can also.

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    That, and I've seen shielded cable bypassing the ribbon cable to V1 input grids pin 2 & 7.

    Try to use the quietest tube you have for v1.

    Also reflow the tube socket PC board connections and any bad looking ones on the main board like pots and jacks, dropping resistors etc.

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    Quote Originally Posted by g-one View Post
    rear panel shielding/grounding can also.
    !! This is an important factoid to remember with Fender amps. I've seen head-scratching oscillation issues that vanish when the rear panel is put back in place.

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    Especially with ribbon cables up against the rear panel.

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    Hi,
    Thanx a lot for your great answers !
    I checked the PSU caps (put another 22uF in // with the existing ones) and C3 and its associated relay, but everything looks OK, no improvement with the cap in //.
    As I have no earth connected on the living room where I temporarily have my bench, pulled a cable from the bathroom where I have an earth connection and... the oscillations disapeared !
    I guess my friend tested it without earth either...
    BTW, any idea of how to get rid of that behaviour when plugged without a proper earth connection ? I know this shouldn't happen, but I have even seen it in some "professional" clubs...
    Anyway, I'll change the PSU caps for better ones, just in order not have this amp on my bench too soon
    Thanx again for your contributions, every clue can turn into a solution the next time !
    Best regards.
    Eric

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    Last edited by ricothetroll; 12-15-2014 at 11:51 AM.

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    Supporting Member Jazz P Bass's Avatar
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    Does Post #9 help?

    The chassis should not be carrying any current, so this makes no sense to me.

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    Last edited by Jazz P Bass; 12-15-2014 at 03:10 PM.

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    Quote Originally Posted by Jazz P Bass View Post
    Does Post #9 help?

    The chassis should not be carrying any current, so this makes no sense to me.
    Well putting back the cover wasn't necessary to stop the oscillations....
    The audio ground does connect to the chassis, and thus to the earth when present. How this has an influence on the oscillations... I don't know

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    Senior Member audiopete's Avatar
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    Quote Originally Posted by stokes View Post
    Absolutely the filter caps.I've repaired at least a dozen of these newer Fenders,Hot Rods,Devilles,Blues Jrs etc and some reissues,all had similar oscillations,all repaired with a re-cap.Those Illinois caps suck.Use good quality Spragues or F&T's.
    Problem is, many time only those crap IC caps will fit on the board. Spragues won't fit and F&T probably won't either. Stupid Fender.....

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    Check both ends of the ribbon cable for broken conductors. Even though the wire is still soldered though a hole in the PC board, they can break right where the insulation starts. With the power off, just poke each wire at the ribbon cable and make sure it isn't broken before it goes into the insulation at both ends.

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    Quote Originally Posted by audiopete View Post
    Problem is, many time only those crap IC caps will fit on the board. Spragues won't fit and F&T probably won't either. Stupid Fender.....
    F&T's fit okay.Only problem is you have to cut the old caps out and leave the leads connected to the board and then twist the leads of the new cap to the leads on the board,as removing the board to get to the underside requires removing too many leads.Changing the bias caps on these amps requires removing the entire board,as these caps are not axial leads.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by ricothetroll View Post
    Well putting back the cover wasn't necessary to stop the oscillations....
    The audio ground does connect to the chassis, and thus to the earth when present. How this has an influence on the oscillations... I don't know
    So you tried it in the ungrounded outlet with the back cover on and it still had the oscillation?

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    I didn't try it again since those tests, as I've ordered F&T caps to replace those crappy IC, I left the amp open. But the first test I did was with the back cover, and there was oscillations.
    It's strange though, I played on a HRD during a few years and in a lot of places, and never had this oscillations problem. Now my HRD is gutted and the original circuit is replaced by a Vox AC15 circuit (only the original OT and the box left)

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    I've seen it too many amps from Fender.Even in amps that seemed to be okay after moving the ribbon cable,which by the way is a fix described in Fender tech notices,if the caps were changed the ribbon cables position didnt matter anymore.I really think Fender got a bad batch of those IC's.In about a 2 year period I saw about a dozen amps with the same problem.All of them were Hot Rods,Deluxes and Devilles,Blues Jr's and a coupke reissues.One that belonged to a friend seemed to be okay after repositioning the ribbon cable,we changed the caps anyway and the ribbon cable could be in any position and no problem.

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    I'll measure the IC caps after replacing them, to see how wrong those were.
    BTW I'll also replace the 5W/470R +/-16V voltage drop resistors by 10W offboard ones. She should then be fine for years, excepted with the 6L6GC...

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    Quote Originally Posted by ricothetroll View Post
    I'll measure the IC caps after replacing them, to see how wrong those were.
    BTW I'll also replace the 5W/470R +/-16V voltage drop resistors by 10W offboard ones. She should then be fine for years, excepted with the 6L6GC...
    Dont expect to see anything "wrong" when measuring the IC's.I'm not an engineer,so I cant explain why the caps fail to properly decouple the stage,but the failure occurs when high voltage is present,and in this case,when the extra gain of the drive channel is applied.I have checked some caps for leakage and at lower voltages the cap showed no leakage,but did show it when working voltage was applied.

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    Supporting Member Alex R's Avatar
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    actually the boards do come out ok like this. Have the amp upright on the bench and take off the rear panel. Free the pots and jacks from the control panel and take off the ground wire (green) that screws to the cab at the side. Cut the cable ties on the wires from the transformers etc, take out the board retaining screws, then carefully pull down the board till the pot shafts clear the lip of the panel then hinge the board towards you. You can work on it whilst it is down like that, the cable joints don't seem to crack, prob because there are so many of them. I've done it countless times with no problem.

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    Hi,
    I switched the PSU caps for F&T. BTW those fir the board perfectly as they are only slightly bigger in size as the IC.
    Now the oscillation problem is gone, with or without earthing the amp !
    You were right, those crap IC caps were the problem....
    Thank all of you guys for helping me with this one !
    Best regards
    Eric

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    Thank you Jazz P. You suggested this 4 years ago, but you saved me from wasting any more hours chasing my arse around the switching circuit.

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