erwin_ve wrote: ↑Mon Aug 21, 2023 3:35 pmIf your FET is defective your B+ voltages will be slightly lower.
Unless it's open, in which case they would be slightly higher.
I would measure the voltages at the top and bottom of the 10k drain resistor to see if it is biased correctly, drawing about 1 mA.
Correct, the 423v is too low. I had this with my 2nd gen hybrid. Defective FET; lower voltage. B+ should be around 440v with his secondary taps.
But the possibilty something else is off remains.
Thanks guys, I will start with the FET board and see what's going on. I suspect I have two issues that need sorting. Hopefully will have some time tonight to work on this.
martin manning wrote: ↑Mon Aug 21, 2023 3:53 pm
Unless it's open, in which case they would be slightly higher.
I would measure the voltages at the top and bottom of the 10k drain resistor to see if it is biased correctly, drawing about 1 mA.
Correct, the 423v is too low. I had this with my 2nd gen hybrid. Defective FET; lower voltage. B+ should be around 440v with his secondary taps.
But the possibilty something else is off remains.
Thanks guys, I will start with the FET board and see what's going on. I suspect I have two issues that need sorting. Hopefully will have some time tonight to work on this.
-Dan
Vdd is just about 9V and Vd was about 0.7V. I had set this up to be biased for "low Vd". I am going to pull the FET, replace it, and see.
I swapped the FET for an NTE 452 and rebiased it with my bench 20 volt power supply on the bread board. Everything was good there Vdd at 19.9V and Vd at 7 with a 1.2K source resistor. Weird it back into the amp and same issue as before.
martin manning wrote: ↑Wed Aug 23, 2023 12:33 pm
Can you disconnect and ground the input to the FET board? You should then be able to verify the voltages and operating point of the FET.
9.29 Vdd and 1.17 Vd with input wire disconnected and input of FET board grounded. B+ came up to about 430V like this. When I was bread boarding the FET should I have taken my supply voltage down to 10V? Not sure how linear these curves are.
dbharris wrote: ↑Wed Aug 23, 2023 2:55 pm9.29 Vdd and 1.17 Vd with input wire disconnected and input of FET board grounded. B+ came up to about 430V like this. When I was bread boarding the FET should I have taken my supply voltage down to 10V? Not sure how linear these curves are.
Yes, if you bias on a breadboard you need to set the Vdd where you want (or expect) it to be in the amp. The FET current is non-linear at low bias settings, plus the FET current changes the B+ voltage, plus the the FET supply voltage divider has high output impedance. All three make this tricky. If you bias it on a breadboard assuming Vdd = 10V, then you would have to adjust the lower resistor in the voltage divider on the FET board to recover that 10V Vdd in the amp. If you want to keep the FET voltage divider resistor values where they are (as in the original), I would adjust the source resistor value in situ, using the amp for the power supply.
dbharris wrote: ↑Wed Aug 23, 2023 2:55 pm9.29 Vdd and 1.17 Vd with input wire disconnected and input of FET board grounded. B+ came up to about 430V like this. When I was bread boarding the FET should I have taken my supply voltage down to 10V? Not sure how linear these curves are.
Yes, if you bias on a breadboard you need to set the Vdd where you want (or expect) it to be in the amp. The FET current is non-linear at low bias settings, plus the FET current changes the B+ voltage, plus the the FET supply voltage divider has high output impedance. All three make this tricky. If you bias it on a breadboard assuming Vdd = 10V, then you would have to adjust the lower resistor in the voltage divider on the FET board to recover that 10V Vdd in the amp. If you want to keep the FET voltage divider resistor values where they are (as in the original), I would adjust the source resistor value in situ, using the amp for the power supply.
I was able to rebias while connected to the amp power supply. Lesson learned, I will bias that way in the future. But the problem persisted. I think I have narrowed down the location though. I got out the signal generator and scope to trace the circuit.
The entrance to the volume trimmer on the FET board would change amplitude as the trimmer is turned. The wiper would pass no signal (effectively). I also noticed that depending on where the wiper was set the bias point for the FET would change quite a bit.
I disconnected the FET output wire from the normal jack. If I connect the probe to the output wire end of the wiper, then everything acts as it should. So the problem must be with the normal jack wiring, right? I cannot for the life of me figure out what though. Everything is wired matching the layout, I reflowed joints, I tested for continuity with and without jacks. All seems as it should but something is attenuating the FET signal. I even cleaned the jacks with deoxit.
Do I need to try a new normal and/or FET jack at this point? Can't believe one little transistor is getting the best of me lol.
So with the FET board input and outputs disconnected from the input jacks, leaving just the B+ and ground, FET bias and gain look good? If so, the problem is with the input jacks for sure. Could be a wiring error or poor contact in one or more of the switching contacts.
martin manning wrote: ↑Thu Aug 24, 2023 12:02 pm
So with the FET board input and outputs disconnected from the input jacks, leaving just the B+ and ground, FET bias and gain look good? If so, the problem is with the input jacks for sure. Could be a wiring error or poor contact in one or more of the switching contacts.
That's correct. I used switchcraft jacks 14B for the FET and 12A for the normal. Am I correct that those match the layout in the files section?
dbharris wrote: ↑Thu Aug 24, 2023 3:55 am
The entrance to the volume trimmer on the FET board would change amplitude as the trimmer is turned. The wiper would pass no signal (effectively). I also noticed that depending on where the wiper was set the bias point for the FET would change quite a bit.
A low resistance or short to ground could cause this behaviour.
With the FET output disconnected at the input jack you should have about 1MegaOhm between the FET inputs point on the Input jack where the wire was disconnected and chassis ground with nothing plugged into the input jack.
Edit: If the two wires on the RING terminals on the FET 14b jack are reversed it could cause this.
Won't bore you with all of the details and steps, but the 14B jack is faulty. I completely rewired both jacks, got it to work for a bit, then went intermittent, then out. I do have some more on hand, but decided to order the HAD style ones instead. I found some fender branded for about $9 a jack.
On the overdrive issue, I traced the signal out. After the OD1 coupling cap all is well, but on the other side of the 100K resistor (mine is actually 110K) signal drops to almost nothing. Checked and reflowed solder joints on both sides of the shielded cable run, also confirmed the cable was not shorted to ground. These old clarostat pots were in rough shape from eBay. They did not want to turn easily when I bought them but they measure fine from lugs 1 to 3. I cleaned them a while ago with deoxit. Did that again yesterday and no change. Ordered some 100K CTS pots so that I can either swap it and fix the issue or eliminate the pot as being the cause.
Built the foot pedal. Got the new FET input jack but haven't had time to wire up yet. Also got a package from amplified parts, but it wasn't my package unfortunately. So, still waiting on the 100K pots.
PXL_20230828_140726875.jpg
PXL_20230828_140734659.jpg
PXL_20230828_143457288.jpg
PXL_20230828_231345375.jpg
PXL_20230828_143443708.jpg
-Dan
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The FET is working perfectly now, thanks everyone for help and suggestions. The new jack did the trick.
The overdrive is still an issue. I replaced the drive and volume pots with new CTS parts. Same issue. I replaced the shielded wire run from 100K (mine is 110K) resistor after the coupling cap of V2a to the drive pot input. Now I do get signal flowing to that lug. But as I turn the knob up the signal decreases in a linear fashion.
No signal flows from lug 2 to V2b grid. I am going to try replacing that run unless anyone else has suggestions? If that doesn't work I will post some updated pictures.
