need help with diagnosing a problem with my amp head Randall RG100 Classic

[reldvs]

ok first off, hello everyone

i have a Randall RG100 classic amp head that seems to have a mind of its own

it will cut out on sound and if you crank it up max u can barely hear it on either channel.

what i have done so far. resolder every pot on board and also used pot cleaner on all pots. after i put it back together it played amazing for all of 5 mins. when i hit the channel change button from dirt back to clean the most annoying high pitch squeal started (i also resolder the channel change button as well while doing the pots) and again if i maxed the vol i could hear a signal faintly along with the steady squeal. this is not the first time it has done this either.

out of pure frustration i banged hard on my strings and bingo it popped back into sound and so far has been sounding just fine. since i have already resoldered and cleaned the pots, what could be the issues.

i fear it is a failing component but i guess i need the gurus here to tell me for myself. and if possible what component may be going bye bye. since it seems to come and go and tonight that whole banging on the strings fixed it, i feel clueless.

i dont think it is something real simple. the squeal was present even with no cord plugged into the head. changing the vol or gain does not change squeal at all, it stays constant when it does this and no knobs will change the squeal sound at all (just banging hard on the strings lol)

please help me, i am in a band and there is nothing more embarrassing than watching ur bands mates make fun of you for ur bad equipment and i personally think the RG100 is a good head even for the classic. but when it does this it makes me want to chuck it against a wall (not that i would)

thanks in advance
david

[Enzo]

Can you link us to a schematic?


Does this amp have an FX loop?   If so, plug a spare cord from FX send to FX return.  ANy help?


Same thing if it has a preamp out jack next to a power amp in jack.  Plug a spare cord from preamp to power amp there.  Any help?

[reldvs]

thanks for the reply.

i have been looking for a very long time for a schematic so if anyone can help out with this, id be ever so grateful.seems like everyone loves the rg100 that came after this one cuz u can find it lots of places but the rg100 Classic is very hard to find. best i could do is take pictures of the board with my camera unless someone know where i can come across a schematic, sorry  :-(

i will try that as soon as i can.  the only problem is that the amp only has this issue intermittently so i will have to wait for it to mess up again but the track record shows it won't be long before it happens again.

when it does i will try ur suggestion. what is it you r trying to see? do u think my fx loop may be bad?  i am not using it currently and if this fixes it(fingers crossed) can i just leave it that way or will i need to fix it before something worse happens?

  also i dont think i have a pre amp out jack  or a power amp in jack but i will look in the morning.

thanks again so much for ur help. i am curious to see if that help any.

[Roly]

i dont think it is something real simple.

David, I've been fixing amps for 50-odd years and it's almost always something braindead simple, but that doesn't mean that it's easy to locate.

A squeal frequently means that the signal is going through a poor contact and that the contact is getting feedback from the speakers, much like a microphone.

We want you to try taking a known good lead and plugging it into "Effects Send" and "Effects Return" because the internal bridging contact is often a source of trouble, and by plugging a bypass lead into the Effects loop it takes this contact out of circuit (and frequently "cures" the problem).  In this case it is either a matter of cleaning the switch contact, or replacing the switchjack with a new one.

In the event that jumpering the Fx loop doesn't make any change, try feeding a signal in to the main amp alone from something like an MP3 player, into the Effects Return socket.  If the amp works okay like that (and yes, give it a bit of moderate thumping) then we start looking back towards the preamp/input.

You can also try picking up the signal at the Effects Send and taking it to another amp to see if that is clean and trouble-free.

Attached is a PDF of the circuit.

i am in a band and there is nothing more embarrassing...

Yeah, that's not good, but there actually is worse; Henry Rollins tells a story about coming out on stage as his band plays an intro in front of 100,000 people in Brazil, and promptly knocks himself out with the microphone - stone cold, centre stage, first number.  A chick here managed to do something similar when, blinded by the lights, she walked out, then right off the edge of the stage into the orchestra pit, and unlike Rollins wound up in hospital with broken bones.   

Do the tests, report results.  :tu:

[QReuCk]

The fact that no knob tweakage was altering the sound indicates for 99% sure that the preamp is not at fault.
So that leaves us with anything after the master volume, starting with the effect send connector, which turns off the internal bypass when something is plugged in. Hence the jumper test.
The mp3 test would be a good idea also, because it would indicate if the problem lies in anything after the return effect plug (but as the amp now works fine, it is difficult to test that until the next occurence of your problem).

Was this amp stored and not played for a long period of time?
Because even if I may not be able to explain exactly why, but I suspect dry dust to be a electric isolation material for weak current, whereas it might be burnt and turned conductive (more or less permanently) by a stronger signal, which is consistant whith what I observed on amps that where not played for a long time and with your symptoms.

So what are your options now?
Play this amp often, and when you rehearse with your band, always bring with you a spare patch cord to shortcut the effect loop if this happens again. Anyway some carefull visual inspection might be a good idea but I fear the faulty contact is tiny enough for not being noticed this way. Maybe ohm testing every connexion past the master volume knob would help, but I'm not even sure you'll find anything and that can prove an harassment to do so.

[reldvs]

i dont think it is something real simple.

David, I've been fixing amps for 50-odd years and it's almost always something braindead simple, but that doesn't mean that it's easy to locate.

A squeal frequently means that the signal is going through a poor contact and that the contact is getting feedback from the speakers, much like a microphone.

We want you to try taking a known good lead and plugging it into "Effects Send" and "Effects Return" because the internal bridging contact is often a source of trouble, and by plugging a bypass lead into the Effects loop it takes this contact out of circuit (and frequently "cures" the problem).  In this case it is either a matter of cleaning the switch contact, or replacing the switchjack with a new one.

In the event that jumpering the Fx loop doesn't make any change, try feeding a signal in to the main amp alone from something like an MP3 player, into the Effects Return socket.  If the amp works okay like that (and yes, give it a bit of moderate thumping) then we start looking back towards the preamp/input.

You can also try picking up the signal at the Effects Send and taking it to another amp to see if that is clean and trouble-free.

Attached is a PDF of the circuit.

i am in a band and there is nothing more embarrassing...

Yeah, that's not good, but there actually is worse; Henry Rollins tells a story about coming out on stage as his band plays an intro in front of 100,000 people in Brazil, and promptly knocks himself out with the microphone - stone cold, centre stage, first number.  A chick here managed to do something similar when, blinded by the lights, she walked out, then right off the edge of the stage into the orchestra pit, and unlike Rollins wound up in hospital with broken bones.   

Do the tests, report results.  :tu:

thanks for all the information you are giving me, i really appreciate it. i also appreciate the attempt to get me the schematic but this is not my amp head.  i have Randall RG100 Classic...classic is on the amp head. what i have learned is this is an amp that was made before the usmusiccorp buyout by the original Randall.  what u sent me is not what my amp head looks like so the schematic hunt continues. but thanks for trying. 

attached is a pic of the way mine looks

your advice is still sound and i will try to use it as soon as it messes up again.

thanks again

[reldvs]

Was this amp stored and not played for a long period of time?
Because even if I may not be able to explain exactly why, but I suspect dry dust to be a electric isolation material for weak current, whereas it might be burnt and turned conductive (more or less permanently) by a stronger signal, which is consistant whith what I observed on amps that where not played for a long time and with your symptoms.

yes the amp was stored for a long time while i was playing bass in different bands. then i got a gig as a guitarist and as soon as i am trying to use it. it started with having all kinds of issues with playing fine then go out for a cig and return and no sound unless u max out channels. then i thought it might be a guitar but i have now eliminated that from the equation.i took it home with my head in shame lol that is when i took out the circuit board and re-soldered all the pots from the masters to the gains on down to the eq and channel selector. and then gave the pots a cleaning with pot cleaner. hoping it solved this, i reassembled everything and at first, it was rocking but then as i attempted to change to the clean channel ...well u know the rest.

i will try the suggestions with the fx loop and report back just as soon as it does it again. and i am sure it prolly will lol

but yes it was stored for around 3 or 4 years before i used it again. maybe using it will help?and patching the fx loop will help too. i really hope so. NOW that would be an easy fix  :dbtu: even replacing the jack connectors would be easy enuff.  but hunting down a tiny contact that only messes up when it feels like it. you can pull out ur hair over  :grr  i will try the patch fx loop idea and report back

in the mean time....still looking for the ever elusive schematic for my specific amp head. see pic above for front control layout.

thanks for ur help and advice

[Roly]

Well it's a fair bet that your problem is a bit of oxide on the bypass contact on the Effects Return (sometimes there is another on the Fx Send as well).  If these are open type sockets you should be able to locate the contact without much trouble, and if so, take a strip of clean writing paper, open the contacts and insert the paper, let the contacts close on the paper then withdraw it from between the contacts.  Do that a few times and the contact area will be scrubbed as clean as you'll get it.

Unless you happen to make regular use of the Fx loop these bypass contacts don't get "worked" and a bit of oxide build up cause just the sort of intermittent operation you have been experiencing.

[reldvs]

thank u soooo much. i will try that and i was also gonna resolder the jacks on the pcb as well.  if that fixes it, i will be so grateful and have one less headache to worry about. it makes sense too becuz i hardly ever use the fx loop if at all so a build up is very possible

thanks again for all ur help

i will report back after i try this and let u know my results

[reldvs]

just wanted to touch base and let you know that i did not forget to report back, ive just been soooo busy i have not had a chance to get to trying the things that have been suggested

but i will though lol, sometime soon.   :dbtu:

thanks