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

[brilliantblue]

Hello, and thanks for reading. In a 1999 issue of a Fender Hot Rod Deluxe, which I'm trying to restore to stock for a friend, I found that the "More Drive" feature appeared to be stuck on despite the color of the LED (properly toggling from yellow to red).

It appears the diode CR16 is letting a negative control voltage of -16v to the gates of the Q1 & Q2 J111 "switch" jfets, presumably to open them, as from what I can find reading online they are "shut" or essentially shorted in the non-energized state. When "More Drive" is selected, the voltage drops to 0v.

However, measuring resistance to ground at R23 and R24, it never reads 100k in either Drive nor More Drive mode, only about 30 ohms, the resistance between Drain and Source on the jfets.

I was curious to know if the jfets were "toast", and desoldered them. Currently they are out of the amp. I see the 100k at R23 & R24 now. Testing the jfets with a simple multimeter, I see about 30 ohms between the S & D, and one reads about 2meg between either and the gate, and the other reads about 600k between either and the gate.

One question is - is it safe to run the amplifier for an extended period without the jfets installed? I turned it on and verified that there is less gain now in the drive channel - only Drive is available at the moment. I'm planning to replace the jfets, but the friend is going to be in town over the weekend and hasn't heard the amp with just "Drive" available, always instead suffering the "More Drive" and opting to only use the Normal channel as a result.

I'm attempting to upload a schematic of the preamp - R23 and R24 are the 100K's next to the JFets that are linked at the gates. Also C44 appears to be functioning correctly.

Thanks for any and all help! I'm sure to check back tomorrow evening but my daytime access is limited.

Jeff

[Enzo]

So the JFET appears not to work, stick another one in and find out.


It won;t harm anything any more than forgetting to turn on a microphone will hurt the PA.

[52 Bill]

The two jfets Q1 and Q2 are there to act like a gain control for the two tubes stages V2A and V2B only when the more drive switch is on.

The cathode bypass caps C8 and C9 are limited or turned off by the 100K series resistors R23 and R24. When the jfets are in their normal state, their low resistance is paralleled with R23 and R24 which allows the bypass caps to increase the gain. When the jfets are activated, their resistance increases, effectively turning off the bypass caps.

If the jfets do not change resistance when activated, they should be replaced. Leaving them out of the amp will basically be the same as if they were in high resistance mode.

You didn't mention it, but besides the lights switching I will assume that the relays are working so that the master volume and drive volume controls turn on when switched.

[brilliantblue]

Thank you both for the replies, and for previous information you've shared in other posts - I'm a long time lurker.

I obtained two new jfets (j231 - which appear to be effective replacements). This cured my problem.

Yes, Bill, Drive/Master were switching with Volume during channel selection

Now I have a new problem! I'm hesitant to make a new thread, but may do so if this doesn't stir up some insight.

The circuit board also had the notorious 16 +/- dropper resistor blackening, and the 330 ohm resistors were tight to the board, not helping things. I determined to help avert future problems there by lifting the resistors, but found that a previous tech had apparently used some of their lead-length to jump parts of the trace that had been lost. I opted to use wire, and the specific points where I used them was from R78 (330ohm) to R91 (10k), and from R79 (330ohm) to the neg lead of C42.

Now, unfortunately, I have oscillation in the Drive/More Drive Channels. Do any of you see where these jumps would cause it?

The tone is low midrange, near a G#, probably second octave on a guitar.

Pulling v1 does not stop it, pulling v2 does.

The note of the oscillation drops several cents when turning the treble knob to one side.

No oscillation in the clean channel.

I've found a couple of other threads elsewhere online recommending reflowing solder joints and testing power caps with a known good power cap - I tested all power caps and reflowed all joints in the preamp section. I found one thread with this exact problem but the last post was "take it to a good service person".

I recognize that something may have been loosened during the numerous times moving the circuitboard out from "home" position and flipping it for soldering/testing etc, but I've not been able to find the culprit. I reflowed all solders on the tube connection multi-lead cables as well.

To be clear - I had the first problem fixed - clean, drive, and more drive were all working. I decided to do this one last thing to hopefully add to the longevity of the amp <facepalm>

Thanks again for any help.

[Jazz P Bass]

Most probably, C36 decoupling cap is bad.
If you have a spare high voltage cap, hook it up in parallel & see if the problem goes away.
If that is the cure, replace the capacitor. (22uf/500V)

[brilliantblue]

Most probably, C36 decoupling cap is bad.
If you have a spare high voltage cap, hook it up in parallel & see if the problem goes away.
If that is the cure, replace the capacitor. (22uf/500V)

I tried that already, as well as all other large filter caps, to no change - except the tone dropped as the new cap was charging, then returned to steady low mid note.

[52 Bill]

I tried that already, as well as all other large filter caps, to no change - except the tone dropped as the new cap was charging, then returned to steady low mid note.

Live cap attachment? Brave man.

First thing is that the 330 ohm resistors should have been changed out with 470 ohm values. Fender recommends footed resistors glued in with silicon.

Wire jumpers to fix the circuit board should not cause the oscillation to happen. Not being a big believer in coincidence, double check the work you did when you replaced the jfets. If you read the resistance across R23 and R24 with the more drive on and off, what readings do you get?

I think that the fact that the tone dropped when you paralleled one of the caps may be a hint. It may be more than one cap that is bad. Did you try reading the ac ripple voltage across the caps?

I see that kind of oscillation occasionally with these amps, but in most cases the frequency is higher and the cause is a bad filter cap.

[brilliantblue]

I just deleted my last post, because lo-and-behold, those new jfets failed..... causing the oscillation, and therefore both of my threads are actually related <facepalm>

(there really needs to be a face-palm smiley)

I also discovered along the way that some aspect of the switching circuit is apparently grounding (by design?) to the faceplate - it won't switch channels without being firmly inserted into the faceplate. This means that I didn't get a truly accurate reading of the original jfet's resistance and functionality.

Thanks for helping me chase down this thing - and if you have knowledge as to whether or not this circuit board by design needs to be in contact with the faceplate, or not, please let me know!

Now I will try the old jfet's again, for they may not read the same resistance g to d or s, but they do have strong resistance! The new ones shorted!

(I swear it worked fine for a few minutes)

[Enzo]

And any chance your oscillation was actually one of the relays buzzing? I have encountered that.

[brilliantblue]

I'm not 100% clear on the oscillation cause, Enzo. The jfets that a local tech had given me as substitutes were j231, and the limited data I could conjure on them indicated that they should work - n channel switching, same pin-out, though I may have missed something in their ratings that would point to this failure. They worked for awhile but then the gate shorted to the other elements, and the oscillation resulted. Since the original jfets also don't function properly, I am aware that there might be an issue in the 16v supply. However, I do not have new J111's to try at this time.

I'm also going to take the dropping resistors back to the stock 470ohm per Bill's suggestion and per the explanation in this article: Playing it Cool with a Hot Rod Deluxe | 2013-07-11 | Premier Guitar.

This is, of course, dependent on the owner - he has never heard the "drive" channel as that channel was previously stuck on the "more drive" feature - he may opt to take the amp back in its currently-tonally-improved state (he's coming through town this weekend on other business, and lives 2 hours away). When it came to me, the amp also had a cathode bypass cap of 47uf wired to the V1b section, boosting gain and disabling the Bright function - I took it out and wired the section to stock. The owner wanted to be able to get a cleaner sound with more headroom, and its definitely got that now.

[g1]

Are you sure about the Fet's being bad? Some of them measure low when not powered and need a voltage to turn "off".
Don't remember if this is the case with the J111's.
You mentioned the gates being shorted to other terminals, so I think you are correct about them being bad, but just wanted to mention that low resistance readings can be proper with some Fet's.

[brilliantblue]

I'm not sure if the originals are truly bad. You are right that they require voltage to turn "off", and the source voltage is 16.4 with them pulled out, but drops to around 5v with them in circuit - it makes me wonder if the lower resistance one (600k compared to 1.8meg on the "high" one) is leaking - or possible the higher one isn't "opening"?

I replaced one of the filter caps in the low voltage supply due to it seeming overly "wiggly", when trying to determine if I knocked something loose causing the oscillation previously mentioned. I plan to replace the rest and restore the droppers to 470 ohm. I'll put in an order for that and get a few J111's at the same time. These caps do not appear to have been replaced when the B+ caps were.

Are you sure about the Fet's being bad? Some of them measure low when not powered and need a voltage to turn "off".
Don't remember if this is the case with the J111's.
You mentioned the gates being shorted to other terminals, so I think you are correct about them being bad, but just wanted to mention that low resistance readings can be proper with some Fet's.

[52 Bill]

Most jfets will test just like bipolar transistors using the gate as the base connection. The major difference will be that there is always a low reading from drain to source. If your jfets are testing oddly, then they probably are bad, especially if they cause the control voltage to drop to 5 volts.

I don't know why the jfets would cause an oscillation. The low voltage filter caps don't seem to fail very often as they aren't under much stress.

[brilliantblue]

Most jfets will test just like bipolar transistors using the gate as the base connection. The major difference will be that there is always a low reading from drain to source. If your jfets are testing oddly, then they probably are bad, especially if they cause the control voltage to drop to 5 volts.

I don't know why the jfets would cause an oscillation. The low voltage filter caps don't seem to fail very often as they aren't under much stress.

Hey Bill,

The only thing I could figure was that maybe, since these are switching a 100k resistor in and out at the cathodes of V2, possibly some feedback was being induced there.

To be clear, the oscillation was there (jfets in) regardless of volume settings - I could hear it even with Drive and Master turned all the way down. It did jump in volume when I turned those controls up, but did not require either control to be up for starting.

Well, I'll get some supplies and return with at least a summary when complete if not a play by play.

Thanks again!

[brilliantblue]

Do any of you have a tip on where to find the 5w 470 ohm footed resistors? I've checked Mouser, Digikey, and Allied but haven't been able to get a result that looks right. I guess "footed" makes me think of the ones I see in tower computer power supplies, with square base with one "toe" that goes through the circuit board hole. Thanks for any help!