Fender Stage Lead problem

[acorkos]

this amp had numerous problems...i've sorted all of them out except that the drive channel doesn't have as much gain as it should (compared to other stage and studio lead amps i've played). i can't find the schematic (100 watt stage lead...not stage lead II). the studio lead (50 watt version) schematic was available on this forum...the design is similar, if not identical, but some of the component values are different. anyway, the components in the vicinity of the volume/gain/master controls all seem to check out...but something is still amiss. i though the diodes on IC5, or the IC itself might be the problem, but they all meter ok. any ideas where the lack of gain is coming from? here's the studio lead schematic again...i'd appreciate someone posting the stage lead schematic if they have it. thanks!

[Enzo]

The only difference I can see between the Stage Lead and the Studio Lead is the power supply to the power amp and a couple resistor values there. The preamps appear to be identical and use the same board.

Isolate the problem. APply a test signal and trace it through the channel. The op amp stages are conventional. I donl;t know what the CMOS IC is at IC4, you tell me. But whatever it is, is there signal coming in and out at reasonable levels? If you changed it, it is possible the brands matters in this application.

And the TL604 up at IC 6 is always suspect, but it is just a switch IC, so when in the channel selected, does the signal look the same at pin 3 as it does at pin 7?

[acorkos]

The only difference I can see between the Stage Lead and the Studio Lead is the power supply to the power amp and a couple resistor values there. The preamps appear to be identical and use the same board.

Isolate the problem. APply a test signal and trace it through the channel. The op amp stages are conventional. I donl;t know what the CMOS IC is at IC4, you tell me. But whatever it is, is there signal coming in and out at reasonable levels? If you changed it, it is possible the brands matters in this application.

And the TL604 up at IC 6 is always suspect, but it is just a switch IC, so when in the channel selected, does the signal look the same at pin 3 as it does at pin 7?

ic4 is a 14-pin rca 145...i didn't change it

Q1 is missing on this amp...on the schematic, it connects to pin2 of IC6

the pots on this amp have different values than the schematic....does that matter?
the studio lead drive channel volume/gain/master pots are 50KA, 10KRA, 50KL
this stage lead has the some controls with values of 20KA, 20KL, 20KL

i'll trace the signal and report back

[acorkos]

APply a test signal and trace it through the channel. The op amp stages are conventional. I donl;t know what the CMOS IC is at IC4, you tell me. But whatever it is, is there signal coming in and out at reasonable levels? If you changed it, it is possible the brands matters in this application.

And the TL604 up at IC 6 is always suspect, but it is just a switch IC, so when in the channel selected, does the signal look the same at pin 3 as it does at pin 7?

IC 4 has hotter signal level at pin 5 than at pin 6
IC6 - signal is same at pin 3 + 7

is the linear pot in this amp a problem where the schemtaic calls for reverse audio taper?

[Enzo]

Q1 is just a mute, you can live without it.

IC4 seems to be amplifying, so it is OK. And IC6 works.

The Studio Lead and Stage Lead have the same pots in the schematic, no differences.

Pot taper should have no effect on the gain of an amp stage, only the knob setting.

The volume controls are 50k in the drawing, and they are feedback resistors around rthe op amp. Lowering that to 20k would reduce the stage gain wouldn;t it? Try a 50k.

20k in place of the 10k gain control should add some gain.

The master shouldn;t matter.

[acorkos]

the pots i think are stock...all the pots in the amp have the same D-shaft with micro-clicks all the way around the pot's travel. for whatever reason, i'm figuring they used what they had at the factory and this is what they came up with. i'd like to see more gain from this channel, but i'm starting to wonder if this is how it was supposed to sound given the parts on board.

[Enzo]

The preamp schematics are the same. If there was a revisiion during the production run, it is not reflected in the schematic.