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Randall RG100 G3 combo and its many problems!

Started by Dinky, February 18, 2015, 01:09:48 PM

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Dinky

Greetings!

I hope someone can give me some info and help regarding my Randall RG100 G3!

I got this amp in a trade and well it pretty much went nuts.
First to save time and confusion i will  describe what is not causing problems and i promise you these things are working fine.

The power amp: Ive had the power amp at a local who fixes electronics and he builds mosfet car amps. He tested the power amp and did a modification to it to keep the temps low. All the components are 100%

The valve in the power section. Yes its a hybrid amp with a valve in the power section. Ive tested it with brand new valves. Ive tested the pre amp by bypassing the power amp and that pretty much eliminated the valve hey, pre amps behaves the same when bypassing the power amp with the valve or even removing the valve to make sure its got nothing to do with the behavior of the pre amp.

The speakers: They are 100%, ive used a few different ones including a 4x12 cab.

The power supply: Working 100% the tech tested it thoroughly!

What my problem is:
Ok here what the amp does and sounds like.
The amps sounds like a farting mess on the distortion channels. It gets worse as you turn up the volume on the gain channels (not the master)
It will sound like fart mixed with crackling.
The clean channel sounds fine.
After playing the amp in this state, whether im on the OD or clean channel, it will eventually die!

The amp will sometimes work again after a few hours or a few days!

If any of these symptoms sound familiar to anyone on this forum please please inform me as to what you think it can be!

The amp is currently without speakers and disassembled.
I would really like to get it back to working condition again.

ps.its got waaaaay too much low end as well.

Many thanks for reading all of this!

Thanks in advance!

J M Fahey

#1
Maybe somebody read too much Internet and applied a  couple Mods to the poor amp.

In such cases best by far is to get it back to Factory specs.

Your farting pooping way too much bass description strongly hints at some bedroom rocker trying to get wall shaking bass from his bedside 6" (or was it 4" ? ) speaker.

High gain amps, starting with early Marshalls eons ago, liberally used strong bass cut in at least a couple places to make distortion acceptably clear.
Distortion pedals too.

Dinky

#2
Quote from: J M Fahey on February 18, 2015, 07:03:15 PM
Maybe somebody read too much Internet and applied a  couple Mods to the poor amp.

In such cases best by far is to get it back to Factory specs.

Your farting pooping way too much bass description strongly hints at some bedroom rocker trying to get wall shaking bass from his bedside 6" (or was it 4" ? ) speaker.

High gain amps, starting with early Marshalls eons ago, liberally used strong bass cut in at least a couple places to make distortion acceptably clear.
Distortion pedals too.

Hey man, the amp uses two 12" speakers.
Ive heard people refer to that sound as blocking distortion.
It was not modded! Ive had the tech go over it wit the diagrams! not modded!

Someone on here must have proper knowledge about amps and what components usually causes a amp to behave like that.

I play with gain, not on a clean channel and i do not use a distortion pedal.

Ive played guitar for years now and i do know how a amp should sound.

I would just appreciate any usefull information.

Many thanks!

I have attached diagrams for anyone thats intrested in having a look.

Some basic specs if google doesnt work in your country.

The amp is a hybrid like ive stated.
It uses two TWELVE inch speakers.
Its closed back
It is 100watts

I am not a bedroom rocker cranking my amps as ive got bad tinnitus and i cant bear loud music/turning up my amp as it causes me extreme pain in my ears.

DrGonz78

Have you tried plugging a guitar cable from send to return jack? If the signal is cutting out like that it could be as simple as one of those jacks being dirty and failing to carry signal. Google "the dreaded switching jack". Spray contact cleaner in the send & return jacks and work a cable plug in and out a few times. Let it dry and test it. That might be the culprit to your signal failure.

There are many symptoms that are the same but caused by different faults. It is not so simple as just tell me what it is and fix it. If you have a tech that worked on the power amp then what prevents him from finishing the rest of the amp?

With that said it is important to first figure out why your signal is cutting out. Then to address the bass driven flatulence problem with the amp. Also an audio sample of the amp can help us understand what you are hearing. Best of luck.
"A person who never made a mistake never tried anything new." -Albert Einstein

Dinky

Quote from: DrGonz78 on February 19, 2015, 05:55:01 AM
Have you tried plugging a guitar cable from send to return jack? If the signal is cutting out like that it could be as simple as one of those jacks being dirty and failing to carry signal. Google "the dreaded switching jack". Spray contact cleaner in the send & return jacks and work a cable plug in and out a few times. Let it dry and test it. That might be the culprit to your signal failure.

There are many symptoms that are the same but caused by different faults. It is not so simple as just tell me what it is and fix it. If you have a tech that worked on the power amp then what prevents him from finishing the rest of the amp?

With that said it is important to first figure out why your signal is cutting out. Then to address the bass driven flatulence problem with the amp. Also an audio sample of the amp can help us understand what you are hearing. Best of luck.

hello.

Yes ive done that!

The problem with the tech I took it too is that does not play guitar and he does not know what a distorted guitar amp should sound like, its is also very difficult to work with him, it took him 3 months to fix the power amp as he is extremely busy, he stays very very far from me as well. There arent many amp techs in my country.

Ive been troubleshooting for more than a year and reading up on amps etc as well as gotten input from other musicians thats why im so stuburn about what i know is not causing the problems/or atleast what i know works, I see it as having a faulty computer, when you trouble shoot you start to remove parts and change them untill you find the culprit, in my case the culprit is the pre amp.

Many may not agree with me and thats fine.

Something that might be of use/help to someone that wants to help is that when the amp dies and i play a chord you can see the 2 LEDs on the pre amp, i assume they are there for clipping, light up.

thanks yes understand that the bass problem should be addresses last, its not that terrible but its got more bass on 0 than my marshall has on 10 (using a dial thats marked from 0 to 10)


Roly

Quote from: DinkyIve heard people refer to that sound as blocking distortion.

Which isn't a lot of help because what you describe is nothing like "blocking distortion".  Blocking distortion (a.k.a. choking) is caused when a valve output stage is overdriven and grid rectification adds to the stage bias causing it to be over biased and reducing power.  I differ with Aiken about what it sounds like, not "farting" or "cutting in and out" but rather like you are playing through an over active compressor that pulls the level back excessively then lets it fade up again.  You back off and it recovers.  It doesn't wait for half an hour to cut out all together (that's more likely a dirty  Fx loop socket).

This is somewhat unlikely here because you say the problem is confined to the OverDrive channel.  This is in the preamp and entirely solid-state.  You say you suffer from Tinnitus (as do I) and play at low level which makes blocking distortion even more remote.

As Doc says, the first test to do here is insert a known good lead between Fx Send and Fx Return and see if that clears the fault.  I also agree that the cutting out, and being bass heavy, are unlikely to stem from the same cause.

{Yes the clipping LED's lighting when the amp had dies is a significant observation.  Since you have tried the lead between Fx Send and Return, have you tried contact cleaner on the controls?}

When it cuts out do you still have signal at FX Send?

Are the IC's socketed or soldered in?


Quote from: DinkySomeone on here must have proper knowledge about amps and what components usually causes a amp to behave like that.

You are addressing more than two hundred years of professional bench experience servicing, designing and building guitar amps.

Occasionally it is that simple but normally it is rather more complex than that, and sometimes it can be diabolically difficult to work out what is wrong.

In a "typical" guitar amp there might be 100 components (give or take -50% +300%).  Any of these can fail in at least two different ways, and some in a lot more ways.  Sometimes a failure in one component may cause a cascade failure of several other components.  Then there are non-component failures such as corrosion disconnecting things that should be connected, and surface contamination connecting things that shouldn't.

Multiply all these together and you could have perhaps a million, or at least many thousands of, different ways an amp could fail. And depending on the type of amp and particular circuit the symptoms of any given failure may be quite different.

Sure there are faults that are more common and straight forward than others, but many of those "others" can have even the most experienced tech running around in circles screaming and ripping their hair out in frustration.

What I (and others here) do only looks easy coz we have been doing it for many years, 50 in my case, but not much less with several others.

Quote from: Dinkywhen you trouble shoot you start to remove parts and change them untill you find the culprit

No actually.  This is called scattergun or blunderbuss servicing and it doesn't work - it's why you have been working on this problem for a year without making progress.  As professional techs we have to produce results in a few days or weeks, so we use methodical diagnosis, locate the fault and fix it.

We collectively guide a lot of people through repairs, some seriously difficult, and almost all the time get a going amp at the end, but every fault is its own journey of discovery, and the only way to do it properly is diagnose, narrow it down and corner the problem by careful tests, measurement, and the application of strict logic.

Every amp arrives on this forum as a totally unknown quantity (which we can't even see), so we have to take a methodical approach starting with the simple and obvious first.


If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

#6
QuoteI see it as having a faulty computer, when you trouble shoot you start to remove parts and change them untill you find the culprit,
That applies ONLY to computers, and that because you can buy generic, functionally the same boards , with same connectors, by the MILLIONS and for low price, much lower at least than full computer price.
And you just pull , say, a Network card, plug in another in the same slot, you have thousands different models to choose, and for only a few dollars, and it will work, guaranteed.

At most you must configure a couple software parameters, you don't have to use a multimeter, soldering iron, pliers, anything.

Now just tell me where is the shop where I can buy, over the counter, a plug-in Guitar preamp which fits any and all Fender/Marshall/Laney/Crate/Peavey/ ........ /Yamaha/etc. etc. etc. and for,say, 20 or 30 bucks and I will definitely become a faithful customer there.

Same with Reverb sections, tremolo, channel switching, power amps, power supplies, etc.

So far, the closest available is , say, buying a Crown full replacement board (sold only to Authorized Techs, by the way) for U$380, to repair an amp which costs new some U$800 but can be found used for about U$400.

I guess I'll warm the soldering iron, fire up the scope, and do some troubleshooting instead  ::)

magnus0re

Way too much low end
and maybe the dying (depends how it dies)
point my brain towards the preamp

Methinks maybe the low end is caused by a failed short-circuited cap. (because, IMO caps fail more often than resistors)
so maybe check C1 and C2.
after that go through the gain structure and see if you can find a point where the gain is WAY off for the bass.

Since you hear blocking distortion on the distortion channel, the filtering that removes the bass is before the clipping diodes. And because they are marked as polarized, they might be electrolytics, so check C18 and C22

Also it would be easy to check the switching voltages with a multimeter.

This is unfortunately only a guess at a diagnosis for the farty sound, not the dying.

more information about the momentus mortis would be nice. Does it still light up? Have you probed parts of the preamp to see where the signal stops ? Is the switching FETs getting the correct DC voltage?

Loudthud

What's up with the circuitry around IC2B on page 2 of the power amp schematic? Looks like a graphic EQ except there are no pots, just fixed resistors setting the gain on all the bands. Then in the feedback network around the power amp, R30 and C26 looks like a resonance type bass boost. Seems like the factory is setting the EQ more than the player.

Roly

Quote from: Loudthud on March 06, 2015, 08:48:26 PM
What's up with the circuitry around IC2B on page 2 of the power amp schematic? Looks like a graphic EQ except there are no pots, just fixed resistors setting the gain on all the bands. Then in the feedback network around the power amp, R30 and C26 looks like a resonance type bass boost. Seems like the factory is setting the EQ more than the player.


... well there's ya problem right there ...

(what were they thinking?)

{e&oe}
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

#10
Oh yes they are cheating  :trouble

Problem is they missed the train and are now frantically running along the tracks, trying to catch it.

They lost THE Randall signature sound just because they have no clue, and are trying to get it back the wrong way.

Original Randalls used the most extreme mixed feedback available, in fact so extreme that it was not "mixed" at all, but pure current feedback, meaning they had as high output impedance as possible, meaning they had "natural" EQ following a speaker impedance curve, period.


They *roughly* mimic it with:


See that it rougly matches:  bass boost; a minimum around 400Hz; a rising response above.

The difference is that bass boost area is much wider, 400Hz wide dip is not so deep, high boost is weaker than before.

Probably the original response was too brutal for sissy Japanese Engineers' ears, who listen to Classical Music at home with 5W , 2A3 triode single ended power amps ;)

Or, more probable, original Randall response is gut wrenching and ear piercing if driving 4x12"closed back cabinets, but not so much in 2x12"combos ... or smaller ones.

So they artificially created a wide "one size fits all" bass boost, and tamed the treble bite ... washing down the original sound.

They reverted back to conventional NFB long ago, and Musicians hanging to their old RG100 heads must have complained.

So they stuck a graphic EQ between preamp and power amp and adjusted it by ear.

I also see a deep notch around 220/250Hz; they probably had some ugly resonance on the test cabinet or speaker and they killed it ... problem is that it works with that particular unit, not necessarily with others.

A clunky solution at best.

MESA does a similar thing with their MOJO modules: an encapsulated fixed set 5 band transistor graphic equalizer.

Roly

It's 6dB up in the bass and 12dB down mid band, so that's an effective 18dB bass boost before you do anything else.   :o


Well, after all that, my inclination would be to lift the output cap from the multi-band gyrators, and hop its input over to the output and see what the vanilla unadorned amp sounds like; i.e. would it be better to just lose this stage?
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

#12
Agree that a simple bypass switch, choosing either filter input or output would be a useful mod.

After all, boosting bass beyond what the speaker can reproduce, results only in flapping and farting.

Personally I would insert a graphic EQ (adjustable of course)  in the effec ts loop and tune by ear or, for the brave, mod the PCB to add 5 tiny screwdriver adjustable trimmer pots similar to:



after all, they would be adjusted once, maybe slightly tweaked after a couple rehearsals with the band to see how it sounds with other instruments and at a realistic level and then left there forever.