- #1
Planobilly
- 440
- 105
Hi,
The issue: The "more drive" circuit is not working on this Fender Hot Rod Deluxe amp.
There is a push button switch that selects the drive channel which is working and the yellow LED comes on. There is another push button call "more drive" which increases the gain to the drive channel by around 13db through the use of two JFETs (J111)
Three amp techs have looked at the amp without results. Link to schematic http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Hot-Rod-Deluxe-Schematic.pdf
I have replaced many of the components in this circuit.
Test point readings:
Per schematic TP 31 +1.37 +1.07 +1.07 (pin 5 of the op amp)
Actual TP 31 +1.36 +1.09 +1.09
Per schematic TP 32 +.548 +9.76 (junction of CR23 and CR28)
Actual TP 32 +.498 +9.78Per schematic TP 33 +16 -13.5 -13.9 (pin 7 of the op amp)
Actual TP 33 +16.37 -10.17 -10.17
Per schematic TP34 -13.0 -5.46 -4.96 (pin 3 of the op amp)
Actual TP34 -11.88 -12.11 - 12.11Per schematic TP 35 -.54 -10.0 ( junction CR24 and CR26)
Actual TP 35 -.51 -9.5
Per schematic TP 36 -16 -16 +16 ( pin 1 of the op amp)
Actual Tp 36 -16 -16 -16
Link to the 4558 op amp pin out/data sheet http://www.ti.com/lit/ds/symlink/rc4558.pdf
I have verified all resistors and zener diodes and changed any that were out of value. I replaced the op amp which was a 4558 on the board from the MFG but shows a 4560 on the schematic. The only difference in the two op amps is the slew rate. I replaced the two (Q1 and Q2) JFETs and Q3 and Q4 transistors. The push button switches are working correctly.
It is obvious that the voltages at TP33, pin 7 of the op amp are not correct. TP34 is not correct, and the voltage at TP36 does not change to positive. I replaced the op amp twice.
I do not fully understand what controls the voltages at TP33, and TP34, ( pins 7 and 3 of the op amp) and why they are out of value. I ASSUME the reason TP36 is not changing to positive when the "more drive" is engaged is due to the out of value conditions at TP33 and TP34. The red LED associated with "more drive" is also not coming on.
I can provide any other measurements that may help.
This has been a perplexing issue that no one to date has been able to figure out. I am sure we are missing something.
Anyone have any ideas of what to do next based on the faults described ?
Thanks,
Billy
The issue: The "more drive" circuit is not working on this Fender Hot Rod Deluxe amp.
There is a push button switch that selects the drive channel which is working and the yellow LED comes on. There is another push button call "more drive" which increases the gain to the drive channel by around 13db through the use of two JFETs (J111)
Three amp techs have looked at the amp without results. Link to schematic http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Hot-Rod-Deluxe-Schematic.pdf
I have replaced many of the components in this circuit.
Test point readings:
Per schematic TP 31 +1.37 +1.07 +1.07 (pin 5 of the op amp)
Actual TP 31 +1.36 +1.09 +1.09
Per schematic TP 32 +.548 +9.76 (junction of CR23 and CR28)
Actual TP 32 +.498 +9.78Per schematic TP 33 +16 -13.5 -13.9 (pin 7 of the op amp)
Actual TP 33 +16.37 -10.17 -10.17
Per schematic TP34 -13.0 -5.46 -4.96 (pin 3 of the op amp)
Actual TP34 -11.88 -12.11 - 12.11Per schematic TP 35 -.54 -10.0 ( junction CR24 and CR26)
Actual TP 35 -.51 -9.5
Per schematic TP 36 -16 -16 +16 ( pin 1 of the op amp)
Actual Tp 36 -16 -16 -16
Link to the 4558 op amp pin out/data sheet http://www.ti.com/lit/ds/symlink/rc4558.pdf
I have verified all resistors and zener diodes and changed any that were out of value. I replaced the op amp which was a 4558 on the board from the MFG but shows a 4560 on the schematic. The only difference in the two op amps is the slew rate. I replaced the two (Q1 and Q2) JFETs and Q3 and Q4 transistors. The push button switches are working correctly.
It is obvious that the voltages at TP33, pin 7 of the op amp are not correct. TP34 is not correct, and the voltage at TP36 does not change to positive. I replaced the op amp twice.
I do not fully understand what controls the voltages at TP33, and TP34, ( pins 7 and 3 of the op amp) and why they are out of value. I ASSUME the reason TP36 is not changing to positive when the "more drive" is engaged is due to the out of value conditions at TP33 and TP34. The red LED associated with "more drive" is also not coming on.
I can provide any other measurements that may help.
This has been a perplexing issue that no one to date has been able to figure out. I am sure we are missing something.
Anyone have any ideas of what to do next based on the faults described ?
Thanks,
Billy