jcm 900 channel switching problem

pogohead · Not So Newbie Joined: 29 Jun 2006 Posts: 6

Hi all. This is my first post so hello to everyone.

I've got a jcm900 dual reverb 4500. I had some trouble with the sound fading in and out, replaced all the valves (not sure if I really needed to replace all of them) and the problem was solved. Since then, the head has been transported about and now I've got a similar problem, but only to one channel. The clean channel (channel A) seems to pick up gain from the channel B gain control. It switches between the two randomly (clean/distorted/clean) and sounds scratchy. Channel B never does anything like this, it's just A.

Also, whenever I use the channel footswitch it cuts the reverb out completely. The reverb works fine without the pedal but the second I plug in the footswitch the reverb disappears.

PS. I kept all the valves from the last change. Is there anyway of finding out if any of them are usable?

Thanks guys Smile

Edit: I've just tried putting all the old valves back in to see if it made a difference. It didn't, but the old fading in and out problem seems to have gone Mad

Any ideas?

pizza8guy888 · Little Hamster Joined: 17 Jun 2005 Posts: 77 Location: NJ

The only 2 things I can think of is a cold solder joint somewhere.....so when the amp is turned on and is heated up, the cold solder joint possibly randomly makes a connection whenever it decides causing the fading in and out.

My second thought is possibly The footswitch. If you are using a marshall footswitch and it is always connected to the amp, try disconnecting it and playing to see if the fading is still there without the footswitch connected. marshall footswitches arent the greatest. They use cheap buttons that are faulty and occasionaly fade between channels and switch randomly sometimes. I know this because i have experienced it first hand with my one Marshall combo. It would fade halfway between channels, and it only occured a few times. I read up about it online and it is a common problem among people using the Marshall footswitches. I purchased a Roland footswitch and have had no problem since.

Best of luck with it, I hope you find the problem easily.

pogohead · Not So Newbie Joined: 29 Jun 2006 Posts: 6

Had a go without the pedal, its still there Sad

How would I find a cold solder joint? Is it an easy fault to find?

Thanks Very Happy

pizza8guy888 · Little Hamster Joined: 17 Jun 2005 Posts: 77 Location: NJ

About the cold solder joint. I dont have experience with them first hand so I dont wanna give you faulty information. If you are feeling daring maybe open your head up and just check around visually for anmy noticable solder joints that do not look good. A cold i guess can be annoying to find, if all else fales then maybe take it to a tech or if you have a electrician friend or anyone like that. I know taking it to a tech stinks because you have to cough up a lot of dough. goodluck with it, sorry I couldnt be moreo fhelp about finding a cold solder joint. maybe these links can help a little bit.

http://home.cogeco.ca/~rpaisley4/xSolder.html

One more tiny test that could rule out certain sections of tour gutiar amp electronics is this........Hook up your guitar to the amps FX loop like this. If your amp is labeled the same as my Marshall combo then hook your guitar into the "FX Return" jack. Turn on your amp and play. Dont be alarmed that no matter what channcel you are on (clean, gain) you will only be hearing clean. Also it will be playing at relatively low volumes only no matter what your volume knobs on the front of the amp are set to. The reason for this is that you are hooked up to your JCM900's preamp only, so what you are hearing from your speaker is coming from the preamp only.
If you still experience the cutting out while your guitar is hooked up like this, then you can have a very high suspicion that the problem is in your preamp. However if you do NOT experience any more cutting out problems when hooked up like this, then you can assume that your preamp circuit is in good shape and the problem is elsewhere.

I hope I didnt explain that poorly and confused you. If yo have any questions about hooking your gutiar up like I described, leave me an e-mail or anything.

pizza8guy888 · Little Hamster Joined: 17 Jun 2005 Posts: 77 Location: NJ

Ok well I just re0reasd your original post about the problem and relized I am just babbling, rushing mess. Most likely your preamp is fine, it is definately something with a colde solder joing or a jump in your clean channel circuit. Like I mentioned before I have never experience that sort of complication first hand so I dont want to give bad information. Just visually check the circuit and look for any broken joints or pieces of free metal laying on the board (possibly causing the circuit to jump) but always keep in mind i would NOT physicaly touch anything inside the amp unless you know what you are doing because tube amps are very high voltage and even unplugged hold lethal charges in the capacitors.....so dont be scared of at least taking a peek inside just take caution and dont touch anything or wear a watch and jewelry.

pogohead · Not So Newbie Joined: 29 Jun 2006 Posts: 6

Thanks mate, I've just gone and bought a ground fault switch so I'm a bit safer Very Happy

pogohead · Not So Newbie Joined: 29 Jun 2006 Posts: 6

Just checked the preamp like you said, all good (nice tone too) but I found something new.

I had the lead in the input and tried the clean channel again (since it worked so nicely in the return socket). Still crackly, farty, scratchy etc. I then took the lead out of the input but instead of shutting off like all amps do when you take the lead out it continued to make the crackly noise. The input jack and clean gain (the one thats making noise) are side by side so do you think it could be the input at all?

pizza8guy888 · Little Hamster Joined: 17 Jun 2005 Posts: 77 Location: NJ

Ahhh ok im only a tad confused about what you were saying about your input jack but definately it could be an input jack.......I dont though since you said you only expereince the cracking when on the clean channel. Maybe I read it wrong. Maybe gently mess wit the input jack when you have your cable hooked into it. Lightly wiggle ot or spin it. Here is a future side note for future guitar amp or palying......to protect my amps input jack when I am all hooked up.....(this is hard to explain and imagine if oyu dont know wut i am describing already) but take your cable and loop it through the handle on top of your amp/head it is loosely up there so if anyone gets snagged on ur line or trips it will not put stress ont he input jack but instead just pull the loop of wire out of the amps carrying handle. Let me know what happens when you gently mess with your input jack
dont forget to try it on both clean and gain channels

pogohead · Not So Newbie Joined: 29 Jun 2006 Posts: 6

Ok, sounds fine with distorted channel but crackly with the clean channel (same noise as pot).

I've opened the head but I'm struggling to see any problems. I hate this amp! Crying or Very sad

pizza8guy888 · Little Hamster Joined: 17 Jun 2005 Posts: 77 Location: NJ

Yea Id say you can rule out your input jack then since it does not misbehave on the distortion channel. Im not sure if maybe it could be one of your pots on the clean channel (i dont know if that would cause your problem) but i guess it is a possability. Try this to test your knobs,e on the clean channel try turning all the knobs down and messing with each knob one by one. like turning each one (with a little bit of volume on so you can notice any fading or cracking) to see if any particular pot starts cutting out or crackling. Because maybe you'll be lucky and it is just a pot messed up which wouldnt be terribly bad. I totally feel for ya on your situation with this amp. I apologize i cannot 100% help the problem with you. I havent given up yet though Ill still be here with moral support and random ideas to try out. We need to get you rockin out again on your amp...haha

pogohead · Not So Newbie Joined: 29 Jun 2006 Posts: 6

Here's a bit of good news:

I had a look in my amp, still struggling to find a problem so I plugged it in to try testing things. Then my little finger touched a transformer and I nearly got slapped in the face by my electrocuted hand! Rolling Eyes

I then tried WD40 on the crackly pot. I tried this before without success but when I tested the amp EVERYTHING WORKED!!!!

Maybe I need to be shocked more often hehe.

Still can't get reverb when I used the footswitch but atleast the clean channel is on. Might need to replace the pot soon though, it sounds a bit crappy.

Thanks for your help though Smile

pizza8guy888 · Little Hamster Joined: 17 Jun 2005 Posts: 77 Location: NJ

ahh glad to hear you got it workin, yea It must be the pot. I dont know if WD-40 is the greatest thing to use on electric equipment, I know if you go to an eletronics store or I dont know if you have a radio shack near you but you can get special cleaner to spray into stuff like switches and pots etc. Also glad to h ear you did not get seriously hurt by the shock. I to once zapped myself from an amp my arm was numb and my tongue got numb....haha.