Hot Rod Deluxe "More Drive" stuck on - Jfet question

[Enzo]

Footed? When someone says footed cement resistor I think the rectangular ones we all know, but they have a little ridge near each end, so if you lie the thing flat on the board, it sits up off the board a millimeter or two because of those ridges or feet.

But I don;t find that necessary.

The resistors get hot. MOunting them a half inch up in the air means the wire leads are there to act as heat sinks. Not a lot of heat sinking, but it helps. if you don;t like them up on their own leads, squirt a bead of silicone under each to support it.

On the solder side of the board, the original connection was the common solder pad with a hole in it. To make it more sturdy, I remove the solder mask - that green coating - from the copper from the pads back along the traces a good half inch or more. Nice clean bare copper. Now when I mount the resistor, I do leave it up in the air on top, but the remaining lead wire, instead of trimming it off, I fold it down flush along the bared traces, then run a bead of solder the length of that. That half inch long bit of solderwork is a lot more sturdy than a simple pad, plus the extra lead wire and solder there ALSO acts as more heat sink.

[52 Bill]

What he said^^^.

The only place that I have gotten them is from Fender direct and that was back when we were replacing them a lot under warranty.

[brilliantblue]

Thanks for the help guys. I'll be back when I have more to report. I used to email with Rob Mercure about this sort of thing, but he passed away last year from liver problems. He was always a great source of information and "color". I'm glad to have found some helpful people here.

[Enzo]

He used to post here, I was not aware of his passing. That is sad news to me.

[brilliantblue]

I'm sorry to have dropped the sad news on you, Enzo, except that I've run across threads in the past with both of you contributing, and thought you might have known him.

I got several aspects of the low voltage supply replaced today, including the jfets, swapped 330 ohm resistors out and replaced with the 470 ohmers, new 1n5062 diodes and I replaced the low voltage power supply caps.

Now I've got three distinct "settings" - Normal, Drive, and More Drive - but More Drive has unholy oscillation! With both Drive and Master volumes down I can hear it at a low level, and raising the master the slightest makes it dramatically louder. The tone knobs slightly "detune" the tone - an Eb - but don't affect it much more than that. I'm going to reflow my cap joints.

Many threads refer to C36 as a culprit, but would that be the case if the Drive selection was fine? Its just the More Drive that is malfunctioning.

I'd appreciate any insight. Thanks again for the help!

Jeff

[52 Bill]

Off the top of my head, the only difference between Drive and More Drive is the gain increase caused by adding the cathode bypass caps to both stages of V2. C36 could still be a problem, but I thought that you tried paralleling a new cap across it earlier on. Recheck this if you didn't. The circuit might be okay until the extra gain is switched on.

Have you tried a different tube at V2? Check the ribbon wires that connect the tube board with the main pc board, check the socket soldering at V2, check the soldering of the pots and jacks paying close attention to the grounds, etc.

Edit: Looking at the schematic, relay K1B does two things; one it disables the bright switch and two, it adds a 250pf (C3) bleeder cap to V2A's input grid, reducing the treble response of the circuit. Check to see that the relay is working and that the contact for the bleeder cap is making good contact.

[brilliantblue]

I did try paralleling in a 20uf 500v cap at each filter cap, including c36. No change.
I swapped V1 & V2 - No change.

I'll try moving the ribbon cable and check the soldering at V2 and related controls. I did refresh the joints on all filter caps, but I'll go ahead and swap c36 out to see if there is a change.

I've got an oscilloscope and may be able to track down the source with it, but I'm not a master of the oscilloscope so sometimes I doubt my ability to use it correctly.


Have you tried a different tube at V2? Check the ribbon wires that connect the tube board with the main pc board, check the socket soldering at V2, check the soldering of the pots and jacks paying close attention to the grounds, etc.

"Looking at the schematic, relay K1B does two things; one it disables the bright switch and two, it adds a 250pf (C3) bleeder cap to V2A's input grid, reducing the treble response of the circuit. Check to see that the relay is working and that the contact for the bleeder cap is making good contact."

I'll check this - but this tone is solid midrange, not a high pitched squeal that I'd expect. However, I've still got some newbie-dust on my shoulders from sporadic experience and studying - you may have pointed out the culprit. Thanks for your eyes and consideration.

I'll report back hopefully tonight or tomorrow afternoon.

Jeff