RM20BP Boost Button not working

[ronin4hire]

Hello everyone, I'm new to the forum. I just purchased RM20BP new with an XTC module, after being a long time Marshall user.
The amp sounds and works great, except as soon as I push the "boost" button all I get is static like hum. No sound from the guitar, just the hum. adjusting boost level and boost gain knob doesn't seem to do anything. Toggle the boost button off, and the amp's fine again.
I thought it maybe the preamp tubes, so I did the "tap" test, but I didn't see or hear anything weird. and I can see faint glow on all of them too. Any ideas? I don't mind paying for new set of preamp tubes, but I don't want to spend 60 bucks and find out that's not the problem.
Any help would be appreciated.
By the way, the distortion on this amp sounds great even without overdrive pedals. My Marshall DSL never sounded very good without some help.

 

[ronin4hire]

I've managed to get some sound out of the amp with the boost button on, by cranking the boost level. However, the "wave" like static is still there. pretty unplayable with the button on.

 

[m0jo]

Doesn't the boost switch in an extra tube?

I'd try replacing the preamp tubes in the amp (not the module, those are fine).

That tube would also not react to the tap-test since it is out of the signal path when the boost is off.

 

[ronin4hire]

there are 3 preamp tubes- I tried tapping them with the boost button engaged, but nothing. The presence and density controls work fine though. aren't those part of the preamp? Maybe the tubes aren't plugged in right?(if that's even possible)

 

[ronin4hire]

After some research: V1 is input, V2 is tone stack and PI is phase inverter, right? I guess the boost button is boosting the tonestack.(Presence and Density I guess)
Setting the amp to standby for 5+ min and warming up the amp first seems to have fixed the problem- Somehow it's working now. I wasn't aware preamp tubes needs to be broken in- or maybe my amp is just possessed.

 

[JKD]

It sounds like an intermittent connection...which could be a cold solder joint or just a 'loose' preamp tube or dirty socket connection.

If it's the latter, that's easy to fix by unplugging/plugging the tubes a couple of times.

Could also be a loose tube socket...you can re-tension these but I've never tried it.

 

[ronin4hire]

I took preamp tubes out and swapped their positions. Seems to be working so I'm just going to leave it alone until it gives me truble again.
Thanks JKD.
Is it just me or does the boost add thickness to the sound? My strat sounds bassy(but in a good way)