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2 X QSC RMX2450 for £200?

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shagnasty View Drop Down
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    Posted: 20 March 2017 at 10:55pm
The RMX2450s I have open up are a bit of a bitch because the pot is on the main pcb (the EP2500 has a daughter board) TBH I would simply short the wiper to the signal which converts the pot to an on-off switch/Mute, no one in their right mind would use a pot on a an amp other than full open..

If you do crack them open 3in1 oil is the weapon of choice for clean, not contact cleaner as it cleans and lubes and stays put for more than 90 seconds...



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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2017 at 10:41pm
Originally posted by charlysays charlysays wrote:

 
Are they difficult to replace or hard to get hold of then? They work great now. I went mad with the IPA contact cleaner. Only one was a bit crackly and caused that channel to have 100MV dc offset. but all felt quite stiff. There's no resistance at all when turning them now, they feel pretty loose.
 
If the problem recurs I'll think about refurbing them, cheers for that.
 
How do you tell if the caps have gone bad? I guess they can dry up but still look fine and can bulge and leak. Can't see any bulging or leaks. 

The pots are actually meant to be quite stiff, they have a fairly viscous grease on the shaft which makes the operation smooth but with some resistance (so they're harder to adjust by accident etc).  If they're now really loose then it just means you've washed all the grease out with the contact cleaner.  I try to be very sparing with contact cleaner for that reason.  I've also found that just squirting it in doesn't really do a good job.

You can get hold of replacements quite easily but I was finding that the ones I was removing weren't worn out, just needed careful dismantling and cleaning.  I used to have some RMX's myself and looked after some installs that had them so it was useful to get at least one extra round of usage from them.  They probably fail quicker in an install in the corrosive environment you tend to get and because they never get moved.

Caps are a tricky one.  Bulging can be a sign, especially if it's at the bottom so you find they're no longer sitting flat and vertical.  Doming of the top of the cap can be a red herring if it's just the plastic insulation and not the metal can. If you can push it back down with your finger that's not a sign of wear-out.  The other sign might be an increase in mains hum on the channel or possibly less power/earlier clipping.  If they seem to be working fine then nothing to worry about.  Electrolytic caps don't like high temperatures so plenty of airflow in and around the rack (applies to all amplifiers) is always good.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Crashpc Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2017 at 8:44pm
While in direct comparison, these RMX amps felt weaker, the output from these is measured and verified. Have no idea what might that be then...
Anyway I'm running QSC amps too, and with one notch higher impedance speaker than its minimum, it behaves Very, very well.
Nikon and Canon people should not be married to each other. Why did you let this happen?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote charlysays Quote  Post ReplyReply Direct Link To This Post Posted: 16 March 2017 at 1:03pm
Originally posted by ceharden ceharden wrote:

You can 'refurb' the RMX pots.  I've posted this before but the problem with them isn't the carbon resistive track, it's the silver plated 'common' track in the middle which tarnishes.

With a bit of care you can actually dismantle the pots to gain access to the track, then with a bit of contact cleaner on a cotton bud, remove the tarnishing and reassemble.

The only other issue with the RMX's is that the electrolytic caps aren't the best and don't have a particularly long life.
 
Are they difficult to replace or hard to get hold of then? They work great now. I went mad with the IPA contact cleaner. Only one was a bit crackly and caused that channel to have 100MV dc offset. but all felt quite stiff. There's no resistance at all when turning them now, they feel pretty loose.
 
If the problem recurs I'll think about refurbing them, cheers for that.
 
How do you tell if the caps have gone bad? I guess they can dry up but still look fine and can bulge and leak. Can't see any bulging or leaks.
 
In the older of the two amps some of the coating/ paint on some of the resistors is chipped/ coming off. Maybe I'll just stick them on ebay or rig resale straight away. I really don't need them but it's nice having toroidal backups for my switchmode amps.
 
I usually use an SAE PQM 13 and 2 X Matrix UKP2100X. I've now got 2 X QSC RMX2450, 1 X Berry EP2500 and 1 X Crest CA9 in reserve.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 15 March 2017 at 11:27pm
You can 'refurb' the RMX pots.  I've posted this before but the problem with them isn't the carbon resistive track, it's the silver plated 'common' track in the middle which tarnishes.

With a bit of care you can actually dismantle the pots to gain access to the track, then with a bit of contact cleaner on a cotton bud, remove the tarnishing and reassemble.

The only other issue with the RMX's is that the electrolytic caps aren't the best and don't have a particularly long life.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote charlysays Quote  Post ReplyReply Direct Link To This Post Posted: 15 March 2017 at 12:52pm
Got em... amps are prettty clean and not beat up. He threw in some IEC cables and some nice neutric bridge cables too.
Did a full test today. Everything fine apart from a crackly pot on channel 2 on one amp.
That channel also had 100mv DC offset.
Took the lid off, blew the fluff out and blitzed the pots with contact cleaner. They don't feel stiff anymore.
However I must have got contact cleaner in the small ribon connector behind the pots and not got it out before switching on as when I turned it on both the signal lights were stuck on and I could see arcing coming from the connector when I turned the pots.
Blew the connector out with compressed air and it seems fine now.
DC offset is 22-24mv on both channels now, no crackling and plays music fine.

When I had it on with the wet ribbon connector I had no music going in or anything connected... all seems to be well so no harm done?

Internals look good, not dirty really. A bit of mysterious black hairy fluff wrapped around some of the legs of the transistors etc but all gone now. No bulging caps anywhere. Not looked at the other one as it was working perfectly but will clean it out and clean the pots making sure I don't switch it on with that ribbon connector full of contact cleaner lol


Edited by charlysays - 15 March 2017 at 12:56pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shagnasty Quote  Post ReplyReply Direct Link To This Post Posted: 15 March 2017 at 1:59am
Originally posted by RiddimKid RiddimKid wrote:

Always hated those amps.

They clip so early and have hardly any power, a behringer ep2500 or 4000 always played better

But

Qsc amps always sell for good money so at that price grab them quick, you will definitely double or triple your money 

learn to work an amp....


RMX are good budget amps with crap pots, you can't drive that, don't bother buying a K10 you won't know how that works either....


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Post Options Post Options   Thanks (0) Thanks(0)   Quote nickyburnell Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2017 at 5:55pm
Berry EP is a copy of RMX with smaller power supplyConfused
It's everything, not everythink!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RiddimKid Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2017 at 4:37pm
Always hated those amps.

They clip so early and have hardly any power, a behringer ep2500 or 4000 always played better

But

Qsc amps always sell for good money so at that price grab them quick, you will definitely double or triple your money 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nickyburnell Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2017 at 4:35pm
I've noticed an increase in all RMX on eBay
It's everything, not everythink!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote shagnasty Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2017 at 2:58pm
Assuming they work the thing that dies is the volume pots, if I have been near them they will be shorted out so they are on full all the time, if the pots crackle/drop out shorting the is a good bet.

Aprt from that for £100 a unit you could buy the 2 and sell one on ebay for £200 and end up with one for free...

:-)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote rosssss224 Quote  Post ReplyReply Direct Link To This Post Posted: 14 March 2017 at 1:59pm
they go for around £200 each and most of the RMX series seem to be going for about 30% more than when I bought a few 1450's and 1850's 2 years ago.

Edited by rosssss224 - 14 March 2017 at 1:59pm
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