Dynamic range switching acting weird

[jannne]

The dynamic range switching on my brand new 2266 started acting a bit weird last week. If I'm on the low dynamic range and turn the amp off it will boot up on the high dynamic range the next time it's turned on. And high dynamic range will reset to low dynamic range on the next boot (which is normal). It started doing this two days after I received the amp and it happens on every boot. Also, sometimes when using the dynamic range button or the footswitch the dynamic range changes back when the button is released.

I'm not going to take the amp to a tech since this does not affect the sound or playability and the amp doesn't have any other issues. I have a feeling that the problem lies somewhere between IC2 and IC3 so I opened the amp yesterday and measured their voltages etc. IC3 seems to work fine. IC2a sends a clock pulse on every boot which causes the IC2b output to change state. But I don't think that IC2b should remember it's previous state after power off so could the problem be in the IC2b set & reset pins or a bad connection on R74/C61? Or maybe I'll just replace the IC2 to see if it was a faulty chip.

[lowenzz]

I can't assist you any, sounds like you know your way around PCBs much better than I. I am jealous though. I would love for my amp to boot up in High Range. Hell, I wish it was just a small toggle switch for the high/low range.

[SteveD]

You will need to track the signals through the switching circuit to see if there are any discrepancies.

Verify that IC2 is being set on power up. IC2a creates the mute window when the Dynamic Range is changed so there is no switching noise through the amp. It's time period is governed by the R75/C62 time constant. IC2b simply works as a bi stable which toggles it's state every time it receives a trigger pulse at it's clock input, it's time constant is governed by R76/C63 which is obviously shorter than the mute window's. Once it is powered down though it should not retain it's state and the circuit is set any way at power on by R74/C61. IC3 is simply Schmitt triggers to clean up the clock input signals and convert the CR time constants to fast trigger pulses that IC2 will react to more readily.

The circuit should always default to Low Dynamic Range on power up unless you switch it on immediately after switching it off in which case it could go either way.

[jannne]

Oh crap... The problem might have been standing right in front of the amp :oops: Seems that it can take up to about 15 seconds after power off for the IC2 voltage to drop low enough to reset it while the leds fade in just two seconds...

The reason I suspected the switching circuit in the first place was because one day I got no sound from the amp after a few minutes break in playing. I got a sound when I plugged the guitar in the fx return jack so I thought the relay1 could be muting the V3 input. I can't remember which tubes I had in there since I've been swapping them in and out, but I noticed yesterday that the stock V1 tube heater was broken....