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RM20 Combo Boost Not Working

 
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yourguitarhero
RM100


Joined: 30 Oct 2009
Posts: 110
Location: Edinburgh

PostPosted: Wed Jan 11, 2017 3:24 pm    Post subject: RM20 Combo Boost Not Working Reply with quote

Got a wee problem with my RM20 combo

If I turn on the boost function (either with the front panel switch or via a footswitch) it does the opposite of boost: the sound loses 90% of its volume and some gain.
Without the boost switch in, the amps plays as you'd expect. However press the boost switch and it goes very quiet. Turning the boost level knob does increase the (overall very low) volume of the amp, turning the boost gain does absolutely nothing.

I've tried replacing all 3 pre-amp valves in the amp (not the pre-amp module) but this hasn't made any difference.

Where would I look next?
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Spencer4hire
RM100


Joined: 03 Mar 2006
Posts: 184
Location: Camas Washington

PostPosted: Wed Jan 11, 2017 6:22 pm    Post subject: Reply with quote

I am not being an ass but you have to turn up both the gain and the boost volume controls to match the amp with the gain boost off. There are 2 different knobs for the gain boost. 1 for volume and 1 for gain level. If that does not fix your issue I am sorry I do not know what is wrong.
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CrunchBunch
RM50


Joined: 28 Oct 2015
Posts: 92

PostPosted: Wed Jan 11, 2017 9:59 pm    Post subject: Reply with quote

You may well have a bad valve in V1 or dirty pins/socket. Try another 12AX7 in there. V1 is the tube closest to the input socket.
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yourguitarhero
RM100


Joined: 30 Oct 2009
Posts: 110
Location: Edinburgh

PostPosted: Wed Jan 11, 2017 10:01 pm    Post subject: Reply with quote

Even with the boost gain and level controls on full the amplifier loses gain and volume when it is engaged. The level control has some effect but you can get nowhere near the volume of the amp when it is disengaged. And still, the boost gain has no effect on the sound.
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CrunchBunch
RM50


Joined: 28 Oct 2015
Posts: 92

PostPosted: Thu Jan 12, 2017 7:10 am    Post subject: Reply with quote

Did you try replacing V1?
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tschrama
RM100


Joined: 04 Jun 2014
Posts: 190
Location: Netherlands

PostPosted: Thu Jan 12, 2017 10:05 am    Post subject: Reply with quote

the boost function is SS.. V1 is not switched in or out. My best guess with the limmited info given is that the swichting ellement is faulty... might be H1F11s
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CrunchBunch
RM50


Joined: 28 Oct 2015
Posts: 92

PostPosted: Thu Jan 12, 2017 7:21 pm    Post subject: Reply with quote

tschrama wrote:
the boost function is SS.. V1 is not switched in or out...


On what do you base this? See circuit diagram below of RM20 input stage showing how the second half of V1 is switched into the circuit when the boost function is engaged. The boost gain pot (not shown) acts as half of a voltage divider on V1:

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tschrama
RM100


Joined: 04 Jun 2014
Posts: 190
Location: Netherlands

PostPosted: Fri Jan 13, 2017 1:28 am    Post subject: Reply with quote

Well I was clearly miss informed. From memory I read it in the manual once when I got my RM20 combo. I might have mixed up with a Friendman amp?

Thanks for the correction!

Would be great if you could share the RM20 schemo with me!

Thx!
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Building amplifiers in my small atelier called 's-Gravenmade | Follow me www.sgravenmade.com | instagram @sgravenmade

Good deals with: C2ply, BenMesaBoog, YouguitarHero, nitrous12, kylendm.
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