Fender Hod Rod Deluxe

[Jazz P Bass]

I wonder what made Fender change the gate drive diode to a 10M resistor.

Hot_Rod_Deluxe.pdf

HotRod_DeluxeIII.pdf

[Enzo]

perhaps we could be more specific than "won;t go into more drive." What exactly is the problem? The two JFETs are switching gain caps into bypass of the two cathode resistors. When the JFET is ON, then the bottom of the cap is grounded, so it is IN the circuit, and thus increasing gain. In two places. If you remove the JFETs, then NOTHING can ground the caps, so no increase in gain can happen.

I see a difference between won;t switch into more gain mode, and switches into the mode but doesn;t change the sound. The net result might be the same to the player, but those are two differnt problems.

Jazz explained it, the JFETs short across the 100k resistors to ENABLE the gain caps. With the JFETs out, shorting the 100k resistors with a wire will do the same thing the JFET does, so you can test each one to see if you hear the gain going up or down as you test.

If your JFETs were stuck on 23 ohms D-S, then perhaps the problem with not going into more gain was that it was stuck ther already, and the regular gain was actually already on more. if you get the red and yellow lights - never green - and it switches from clean to drive, at least the relays work. Well one at least. There are two parallel relay coils, so one could open and the other still work.

[bsco]

Just to be clear on how this circuit functions.

In the Clean & Drive modes, there is a negative voltage on the gates of the jfets.
That turns the Jfets Off. (basically an open circuit D to S)
Being off, the 100K resistors are in circuit.

In the More Drive mode, a positive voltage is applied to the gates.
That will leave the D to S essentially shorted, which bypasses the 100K resistors.

So to test the Jfets, I would measure across the 100K resistors to see if they measure 100K in Drive and shorted in More Drive.

The fact that you are saying the circuit will not go into More Drive mode is confusing.
First post you measured 23 ohms from D to S.
That IS the More Drive condition.
23 ohms will short out the 100K resistors.

Yes I know...It's very confusing to me.....

[bsco]

perhaps we could be more specific than "won;t go into more drive." What exactly is the problem? The two JFETs are switching gain caps into bypass of the two cathode resistors. When the JFET is ON, then the bottom of the cap is grounded, so it is IN the circuit, and thus increasing gain. In two places. If you remove the JFETs, then NOTHING can ground the caps, so no increase in gain can happen.

I see a difference between won;t switch into more gain mode, and switches into the mode but doesn;t change the sound. The net result might be the same to the player, but those are two differnt problems.

Jazz explained it, the JFETs short across the 100k resistors to ENABLE the gain caps. With the JFETs out, shorting the 100k resistors with a wire will do the same thing the JFET does, so you can test each one to see if you hear the gain going up or down as you test.

If your JFETs were stuck on 23 ohms D-S, then perhaps the problem with not going into more gain was that it was stuck ther already, and the regular gain was actually already on more. if you get the red and yellow lights - never green - and it switches from clean to drive, at least the relays work. Well one at least. There are two parallel relay coils, so one could open and the other still work.

Thanks for the reply, Enzo....actually, after reading your post I think you are right..The amp is always in More Drive Mode as soon as you switch to the drive channel..But the indicator indicates that is in the Drive Mode....Yellow.....and therefore when switching to MOre Drive.....(red), then there is no change in the sound......Switching between Clean and Drive works but in our case it sounds like I am switching from Clean to More Drive as far as the sound is concerned.....I will check the relays tomorrow and let you know what I find....
Cheers

[Enzo]

They probably both work, but can't hurt to check.

[bsco]

They probably both work, but can't hurt to check.

Yes sir...they both work...I took them out and checked them on the bench and checked the contact resistance when activated.....SO I installed them and I jumped out those 100K resistors....with both of them shorted, it goes into more drive...when only one of them is shorted out it goes into the regular drive mode and when switching to the clean channel. the drive channels are silent....so...the fets are not switching to turn them off....they are both shorted from source and drain.....therefore the more drive function is always on.in order for the regular drive to function, only one of the fets need to be turned on and the other one off...correct me if I am wrong here......
Cheers
P.S. I see that your elections are getting interesting...Donald now wears a bullet proof vest so they say....

[Jazz P Bass]

Both Jfets are activated/ deactivated at the same time.

I don't see how it could be any other way, as the gates are connected together.

[bsco]

Both Jfets are activated/ deactivated at the same time.

I don't see how it could be any other way, as the gates are connected together.

After looking at the schematic I saw that....I should have came back and did an edit on my post.....sorry.........They both turn on and off together...I do have it fixed.....After I bench tested those relays, I check out the traces and everything there seemed fine....so I installed two new JFETS, which by the way tested exactly the same as the originals, and it worked....The channels are switching fine.....There is a negative 7 something volts on the gates and when the More Drive is activated, there is about a positive half volt on the gates....and the amp goes into the More Drive mode........Ran it for a bit, completed re-assembly and all is good.....
Cheers

[bsco]

Hahahaaa. Wouldn't you know it...I got another one....well almost...This one, a Deville has a dead drive channel......and the good part!!! it is a bad V2......That helps to make up for the first one....I did check the JFET's....curious as to the switching voltage.....same thing....actually, that was a very good lesson...