Hot Rod DeVille "More Drive" switching problem

[Labjr]

I installed a Fromel Supreme mod in a Hot Rod DeVille this week. Part of the mod was removing one J111 and associated circuitry around V2B. Upon testing the amp, I noticed the gain doesn't change on V2A when the "More Drive" button is activated, However, the LED turns red and everything is normal at TP36. Is the -15v and +16v supposed to also be present on the gate of the JFET or just at TP36?

I read voltages as follows

Activated vs Deactivated

0.0v ********* - 0.11v at the gate
.
+15.8v ******* -0.62v on the cathode end of the CR16.

Should I actually be able measure the resistance across the Source and Drain of the J111 while the amp is operating? I should see 100K while "More Drive" is activated? Because it measures about 20 ohms whether is activated or not?

Thanks

[Enzo]

A JFET is normally ON until a voltage at the gate turns it OFF. So unpowered or just sitting in your hand, a JFET will measure a low resistance Source to Drain.

TP36 is supposed to toggle -15 to +15. You report it toggles from +15 to -0.6. Go clear that up.

[Labjr]

Hi, Thanks for the quick reply. TP 36 is measuring what it's supposed to. There is a 100K resistor between TP36 and the cathode end of the diode where I measured.

[nevetslab]

Should I actually be able measure the resistance across the Source and Drain of the J111 while the amp is operating? I should see 100K while "More Drive" is activated? Because it measures about 20 ohms whether is activated or not?

Thanks

NO....your DMM is measuring the Drain-Source resistance, as it's turning it on with it's internal current source. 20 ohms is typical On Resistance for the J111 JFET. I'm not sure thus far why you're NOT seeing a gain change at V2A in More Drive mode. Assuming Q1 JFET is working, and you have the correct voltage at TP36, it would be shorting out the 100k resistor R23, which bypasses cathode resistor R17 with C8 1uF. You're just not increasing the gain at V2B that Q2 was achieving in More Drive mode.

[Enzo]

Or it won't turn off and he is stuck in MORE mode.

[Labjr]

I'm unsure if it's stuck in MORE DRIVE mode. I just see no gain change on the scope, at the plate of V2A. Maybe I will try to jump the resistor with a test lead to see if there's any change. Am I measuring the correct voltages at the Gate?

[Labjr]

What effect does the diode CR16 have on the +15 and -15 voltage from TP36?

[g1]

The diode means only the negative voltage will be allowed to the gate, which will turn the fet off, disabling 'more drive' as it relates to V2A.
You said the fet reads 20 ohms no matter which way the switch is, so that means V2A is stuck in 'more drive'.
Removing Q1 should make it stuck the other way (no 'more drive').
Did you save the old Q2 to try it in Q1 position? (assuming it's still good)

[Labjr]

Both FETs test the same. Like diodes between Gate to Source and Gate to Drain, and about 20 ohms between the Source and Drain.

I should probably order new FETs and CR16 because the PCB can't handle much soldering and de-soldering before the pads start peeling up.

[Labjr]

I snipped the anode side of diode CR16 so it's disconnected from the gate of the JFET and measured the voltage. Now I get -15V when "More Drive" is disengaged and close to zero when it's switched on. I guess the JFET is bad because it's loading down the control signal when it's connected to the gate, although it seems to test good?

[Enzo]

Did you not remove a JFET as part of your mod? WHy not install that one where the suspect one sits?

[Labjr]

Did you not remove a JFET as part of your mod? WHy not install that one where the suspect one sits?

Because, the other one tests the same. I don't want to keep changing parts. This PCB won't take the abuse. And every time I remove the board the ribbon cables flex risking breakage. Working on these amps causes a lot of collateral damage. Red Knob Twins are even worse.

[Enzo]

Tests the same? You mean with your ohm meter? ALL JFETs measure low resistance S-D when unpowered. The only test is to apply a gate voltage and see if it turns off.

[Labjr]

Received new JFETs today. Replaced Q1 and CR16. Problem solved. Thanks for the help.