...tearing my hair out about this. The amp won't switch properly into drive. With drive selected the relays chatter, the led comes on but often not brightly, and the amp falls silent, presumably because the signal remains grounded at a relay. Or sometimes there is no chattering but no switching either apart from the drive channel remaining silent.
I eventually replaced the relays and all the caps and all the opamps in the switching circuit, with no effect on the problem. R71 is 100K, and all the other resistors check out ok, and all the diodes beep nicely. The first thing I did was to fit two +-15v supply dropping resistors, wired off the board.
At TP30, where the switching is initiated, when the selector switch is in drive position, the voltage remains too high to initiate the rest of the process. However, with the opamp removed (I installed a socket), the TP30 voltages are as they should be. The opamp is ok, I swapped in plenty of good ones.
Here are the TP30 voltages:
Opamp U3 in:
clean 2.9vDC (fulfilling the schem requirement of >0.6vDC easily)
drive 0.6vDC or slightly more (not quite fulfilling the <0.6vDC required on the schem)
Opamp U3 out:
clean 2.9vDC to 3.3vDC (varies, I think according to how long the amp has been on for)
drive -1.7vDC (again slightly variable)
The "Opamp in" condition, ie hovering around the 0.6vDC level at TP30, means that none of the other drive conditions as marked on the schem in the test point boxes are fulfilled. TP31 either stays at about -15vDC or sometimes produces a small positive voltage, which makes the relays chatter, but not switch. With the opamp out the TP30 voltage goes healthily negative.
If I ground TP30 with drive engaged the relays switch fully and drive channel comes on loud and clear.
One more thing. A slight low-frequency whine is just audible on the clean channel, slightly and slowly rising in pitch. It is there with v1 and v2 removed, but is silenced when the PI tube is out.
What am I missing here? It's one of those jobs that sits by the bench and keeps getting looked at, nor solved, put back... it's been here months! The amp has had a lot of use and also been stored; previous repairers have failed to solve it so the owner is happy to be patient... I'd love to solve it!
Fender Blues Deluxe Service Manual.pdf
I eventually replaced the relays and all the caps and all the opamps in the switching circuit, with no effect on the problem. R71 is 100K, and all the other resistors check out ok, and all the diodes beep nicely. The first thing I did was to fit two +-15v supply dropping resistors, wired off the board.
At TP30, where the switching is initiated, when the selector switch is in drive position, the voltage remains too high to initiate the rest of the process. However, with the opamp removed (I installed a socket), the TP30 voltages are as they should be. The opamp is ok, I swapped in plenty of good ones.
Here are the TP30 voltages:
Opamp U3 in:
clean 2.9vDC (fulfilling the schem requirement of >0.6vDC easily)
drive 0.6vDC or slightly more (not quite fulfilling the <0.6vDC required on the schem)
Opamp U3 out:
clean 2.9vDC to 3.3vDC (varies, I think according to how long the amp has been on for)
drive -1.7vDC (again slightly variable)
The "Opamp in" condition, ie hovering around the 0.6vDC level at TP30, means that none of the other drive conditions as marked on the schem in the test point boxes are fulfilled. TP31 either stays at about -15vDC or sometimes produces a small positive voltage, which makes the relays chatter, but not switch. With the opamp out the TP30 voltage goes healthily negative.
If I ground TP30 with drive engaged the relays switch fully and drive channel comes on loud and clear.
One more thing. A slight low-frequency whine is just audible on the clean channel, slightly and slowly rising in pitch. It is there with v1 and v2 removed, but is silenced when the PI tube is out.
What am I missing here? It's one of those jobs that sits by the bench and keeps getting looked at, nor solved, put back... it's been here months! The amp has had a lot of use and also been stored; previous repairers have failed to solve it so the owner is happy to be patient... I'd love to solve it!
Fender Blues Deluxe Service Manual.pdf
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