Lab series L5 reverb on both channels?

[Zouto]

Hi,
I'd like to mod the reverb on my trusty Lab Series L5 to work on both channels.
Schematics and layout:
Lab Series L5/L7/L9/L11 Service Docs
Any ideas, help, suggestions much appreciated
Zouto

[loudthud]

There isn't an easy way to make the reverb effective on both channels. A109A is the mixing amplifier, the reverb return signal goes in there. The two channels are at very different signal levels going into that opamp and that difference is accounted for by the different input resistors to the summing node, R155 (6.8K) and R156 (39K). So another opamp will need to be added to sum Ch-1 and Ch-2 and boost the level of Ch-1 to match Ch-2. Then the signal can be connected to R220.

Are there any unused opamps on the circuit board? If there aren't you can just piggy back a 4558 on top of A109 to pickup the power supplys and sky-wire the three resistors. Disconnect the coax going to pin 4 of the preamp and connect that to the output of the new op-amp.

[loudthud]

Some comments are in order here. I've never seen the insides of one of these amps so I'm not sure how much clearance there is where you need to add the IC. If you can add it on the same side as the exsisting IC, that is prefered but it could go on the solder side of the PCB of there is more room there.

The idea is to attach the new IC on top of an exsisting one with pins 4 and 8 soldered to exsisting IC to pickup +15 and -15. A104 and A109 as well have pin 3 grounded so you can solder pin 3 of the new IC to pin 3 of the exsisting one. The rest of the new IC pins are bent out horizontally so resistors and/or wires can be attached and they won't short to the exsisting IC pins. Do this carefully, the IC pins sometimes break and you need a new IC. Use the smallest resistors you can find, 1/8 or 1/10 watt is best.

If you need to mount the new IC on the solder side of the PCB, it must attach upsidedown, dead bug style. Sometimes you can bend the IC leads to do this but some ICs have brittle leads that will break. Or you could use bare wire to make the short connections. You won't need to bend the other leads horizontally, just leave them sticking up.

I have revised the circuit slightly to use the un-used opamp to buffer the CH-1 signal and prevent any volume loss. Adjust the resistance of the 22K resistor so that CH-1 has the same amount of preceived reverb as CH-2. That way you won't have to re-adjust the reverb when changing channels.

If you are up for it, you can cut the etch between R135 and where the wire attaches and run a wire to that point so you won't need to run a new wire to the reverb board.

[Zouto]

loudthud: THANKS a lot for your detailed description+schemo.I think I'll use some perfboard + socket for the new IC wiring. At the moment I've got no time for modding, but I'll post the results as soon as possible. It's really important to keep the original sound quality, so I'm curious about the results and sound.
Good to have such a great forum,
Zouto

[Zouto]

Some comments are in order here. I've never seen the insides of one of these amps so I'm not sure how much clearance there is where you need to add the IC. If you can add it on the same side as the exsisting IC, that is prefered but it could go on the solder side of the PCB of there is more room there.

The idea is to attach the new IC on top of an exsisting one with pins 4 and 8 soldered to exsisting IC to pickup +15 and -15. A104 and A109 as well have pin 3 grounded so you can solder pin 3 of the new IC to pin 3 of the exsisting one. The rest of the new IC pins are bent out horizontally so resistors and/or wires can be attached and they won't short to the exsisting IC pins. Do this carefully, the IC pins sometimes break and you need a new IC. Use the smallest resistors you can find, 1/8 or 1/10 watt is best.

If you need to mount the new IC on the solder side of the PCB, it must attach upsidedown, dead bug style. Sometimes you can bend the IC leads to do this but some ICs have brittle leads that will break. Or you could use bare wire to make the short connections. You won't need to bend the other leads horizontally, just leave them sticking up.

I have revised the circuit slightly to use the un-used opamp to buffer the CH-1 signal and prevent any volume loss. Adjust the resistance of the 22K resistor so that CH-1 has the same amount of preceived reverb as CH-2. That way you won't have to re-adjust the reverb when changing channels.

If you are up for it, you can cut the etch between R135 and where the wire attaches and run a wire to that point so you won't need to run a new wire to the reverb board.

Should I take a 22K trimpot for the 22K resistor to adjust the reverb intensity of CH-1?

[loudthud]

Either use a 22K trimpot in series with a 10K resistor or a 50K trimpot. That way you can adjust the gain higher or lower. I just did the math in my head when I picked 100K and 22K values.

[Zouto]

I just finished the mod using the buffered version, results : a nice sounding reverb on channel 1, unfortunately no reverb on channel 2 anymore. I double checked all connections/schematic etc. but could not get it working. Any idea how to fix it?
Thanks
Zouto

[g1]

If you are getting reverb from Ch.1, then both halves of the added op amp are working. Your problem should be a connection issue between R156 and the 100K.

[Zouto]

Thanks, You're right, I just fixed a (barely visible) broken connection from 4558's pin2 to the 100K and now both channels have reverb. The only issue is some additional hum/noise on ch.1, sounding like an open input, depending on the volume level of ch.1. Since I used the "dead bug" mounting method, piggy back a 4558 on top of A109, the cables are really short, too short for shielding.There's no additional noise on ch.2.

[Zouto]

Culprits found: two 2.200UF/35Volts filter caps of the +/- 15 Volts supply. I replaced 'em with 63 V types, all noise is gone. I've also got to replace both of the 4.700UF/100Volts caps (with screw terminals, expensive stuff), but I must order it.

[J M Fahey]

You didn't need to go to a higher voltage in a part that is way overbuilt from the very beginning, but of course they were old and dry.
And yes, those 4700x100 may be dry too ... just don't replace them with 4700x160 or whatever
Congratulations on keeping alive a KILLER amp.
What speakers does it have?

[Zouto]

You didn't need to go to a higher voltage in a part that is way overbuilt from the very beginning, but of course they were old and dry.
And yes, those 4700x100 may be dry too ... just don't replace them with 4700x160 or whatever
Congratulations on keeping alive a KILLER amp.
What speakers does it have?

It has the standard speakers, nothing special, but great sounding. I think, the additional buffer was soundwise a great improvement . The amp sounds a lot "tubier" and warmer than before.

[loudthud]

The dissappearing attachments problem doesn't seem to be getting fixed, so I'm re-posting the schematics from above.

[Jazz P Bass]

Thanks for reloading the files, LT.
I was trying to follow along and, couldn't.
If that attachment was from 2011, there is not any 'fix'.
The files are gone.
Poof.
In the ether.
Bye, bye.
What a bummer.