DISCLAIMER #1: This post is technical in nature and there are no family photos of dogs or food. I'm posting it because there is very little information out there about this and I could have used it. Hopefully someone else will be able to use what I learned.
I picked up a used DSL40C from GC. It's an awesome all-tube amp that sounds great without any effects pedals. Except for the reverb. Or lack of reverb, depending on if your glass is half full or empty. With the reverb knob at 10, it was barely there. I know, I know... If you want Fender reverb, buy a Fender. I always thought the reverb was weak on my 6505, but this Marshall is so anemic, it makes the Peavey sound like a Fender. There is just enough there that you can tell a difference if it's on or not. I don't wanna drop $100 for an add-on reverb pedal, especially when reverb is one of the buttons on the amp footswitch.
Looking over the schematic, there's R43 that is in series on the output of the digital reverb circuit. R43 = 2.2 meg. That's a lot of Ohms packed into a single resistor. I found something online that talked about paralleling something to drop the effective resistance to 1 M for a 3 db boost. That still seemed weak, and I only had a 1 M resistor on hand. I thought of desoldering R43 to replace it with the 1 M, but that's a ton of work to pull all the knobs & connectors & .... So much easier to parallel the 1 M on top of the 2.2 M to get 680 k. That should work, right? The output is being driven by an op amp with a ±12 V supply; the lower resistance shouldn't tax it...
Put it all back together to try it out. It's kickass! Then a few minutes into it the amp went dead. Standby did nothing, but a full power cycle reset it. Played for 15 minutes and everything worked fine. Must have been my cheapo guitar cable. I put everything back together and went on with my Saturday.
Later in the day I played for about 5 minutes and then nothing. Power switch reset it and it was good for another few minutes. Crap!!! What did I do to my new-to-me amp?!? I really liked the new reverb sound available to me; I did not want to reverse my mod. But at the same time, an amp that cuts out every 5 minutes isn't very useful.
I pulled the schematic and did some Jimmy Neutron noodling on it while we did yard work. Pulled it all apart the next day and looked over the PCB for loose components. Everything looked new & shiny and secure. Then I wiggled the DFX board. It's connected to the main amp board by a crappy pin and socket connector that I've had issues with in the past on Arduinos. I pulled it apart and hit it with some De-oxit and cycled the pins in and out a dozen times. Played it for 30 minutes with no issues, so I put it back together. Played it for two hours straight after that with no dropouts. Problem solved!! My guess is that the supervisory microcontroller is talking to the DXF board and it takes the amp into mute mode if there are any problems.
If it holds up over the next month, I may pull it apart and replace the piggybacked setup with a single 680 k and do it right. Might replace the crappy pin & sockets with a 12-pin ribbon cable. And maybe set the bias.
DISCLAIMER #2 - I am not an amp technician. I am not an electrical engineer. I have a soldering iron, I can read schematics, and I understand Ohm's law. Do this at your own risk. Oh, and be careful opening up any tube amp - they run on really high voltage (~500 VDC) that can/will do nasty things to you if you don't know what you're doing.
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