RG100ES problems - new owner/new poster

[guyatron]

Hi, I'm new to the board here and found it while searching info about the amp.

I just got it a few days ago and first learned that it needs a footswitch to work right.
Am I correct though that jumper-ing the top two slots on the head's footswitch plug engages the red channel while jumpering the bottom two engages green (might have that reversed, but you get the idea)?

So that's what I did to test the thing out. Then I got this oscillation happening as you can see in these videos I made for the Ebay seller.

https://youtu.be/kj2teugcLP8

https://youtu.be/p-Ev1hTTzKs

I misspoke in the "red channel" video because the green channel did howl as well.

Today I opened it up, looked for anything obvious, couldn't find anything, cleaned the pots, and tested the spot on the board at the junction of the 1K and 1.5K resistor and it measured about -16.8 volts. I figured that was ok since the schem said +/- 10%. I probably should have tested other points but I just figured I was good to go. The schem though, the trim doesn't seem to sit in the same spot on my board as it does on the schem.Maybe I'm way off, but it seemed the trim on my board was located where the thermistor is drawn in the schematic.

here's the schem I'm referencing
http://el34world.com/charts/Schematics/files/randall/Randall%20RG100.PNG

So I plug it into an 8 ohm cab, leave the reverb disconnected, fire it up and it sounded good, aside from some background noise when not playing which i just assume is the hissy nature of fets. There was no more howling. I continue to jam along for maybe two or three minutes and then all of a sudden I lose a ton of volume. I powered it down and checked for continuity between pins on the power transistors and found that I did have a direct path from base to emitter on the outer pair of transistors (had done this previously and had no continuity between connections). 

So, other than dead 2n6254's, anything else you can think i should look at? I had the amp opened up while playing and there was no smoke and no funny smells. I didn't see any resistors go poof.


I'm not a total newb, but there's a ton I don't know. I've done a lot of work with tube amps and only some work/trouble shooting on SS gear. SS power amps are not anywhere near intuitive to me yet.

Thanks

[guyatron]

Ok, I'm like 1/3 idiot.
I jumped to the conclusion of dead 2n6254's because I have a GK400B, and when it's power transistors went, having continuity between the transistor's pins clued me in that they were fried. New power transistors and a couple other things in the GK's power amp fixed it right up.

I pulled the transistors from the Randall last night and tested them outside of the circuit, no continuity between B and E. I did find continuity between those spots in the circuit, but also found that yeah, my beeper will beep with the VERY low (< 1ohm) resistance between those spots. I put the transistors back in, fired it up, checked bias (what's with the 20V on the output jack? Would that occur with no load? I wasn't getting any significant voltage there) and played it.

The volume still kicked down after a little playing, but then came back up.

I have the reverb tank disconnected. I had the two reverb cables just tied t each other through a female to female connector and verb turned down.

It's noisy and i found turning the trim pot so that I get about -17volts where the schem says -16V made everything quieter ... not sure if I should do that though. The trim only allows be to go from about -16.5 to -17.3 or so ... all withing 10% of the schem's suggested -16.

Ideas of what I can do to quiet this bugger down?
The sticker says the amp was made in 1988. The supply caps don't look bulgy or anything. All the electro's inside look ok. Think the power supply caps should be replaced?

I'm going to disconnect the reverb leads to the board and try playing it while adjusting that trim and see what i get.

Any insight is greatly appreciated.

[phatt]

If you have 20VDC on the speaker output then the power amp section has a major issue.
The output should be well below 500mV,, as close to Zero as possible.
Don't connect a speaker until the Amp output is stable  otherwise you chance burning out the speaker coil which will cost a lot more than the power amp fix.
Power transistors may well be working but Q11, 12,13 may have issues.
Check those other voltages around the power amp and post the findings that will help the better minds here who may be able to give more detail to resolve the problem.


BTW,,,
NEVER!!!! connect the reverb In/Out plugs together as it may burn out the return circuit.

Oh and don't rely on beeper for testing low Ohms,, but of course you NOW know that

Phil.

[guyatron]

yep, did find that when connecting the reverb in/out leads together, it did not like having the reverb turned up!

I didn't measure 20V on the output, but the schem I linked above indicates that as a test point ... if I'm reading it right. 20V off the speaker out. No idea what that's about.

Here's where I'm at now. I completely removed the reverb leads from the PCB and popped out the reverb op amp. The amp is WAY quieter, no volume issues and sounds great ... plenty loud too.

I'll follow up with voltages though when I next get a chance.
I do have +/- 41 volts on the rails and about 40 off the collector of Q8

Thanks

[Enzo]

Read Note #5 on your schematic, lower right.  The 20v in a square box means at full output, or at least output under test conditions, you find 20v of SIGNAL there.

In fact, read all that list of notes, they are there for a reason.

[guyatron]

"indicates signal at 1 K.O."
or is it K.C?

So does that mean, on a scope, signal 1K, the signal hits 20V on the output?

[phatt]

Means an AC signal at 1,000 cycles per second or 1khZ.

BTW,, you said the signal dies away and comes back,, humm?  Jumper the EFX loop send return with a guitar lead and see if that helps.
Phil.

[guyatron]

here are my voltages with an 8 ohm load

1) 41.5v
2) -28.9v
3) -0.9v
4) -17.1v
5) 1.4v
6) 0.9v
7) -0.4v
42.7v
9) -42.3v

0.123mv on the output

Phatt: ahh, the "C" of K.C. means cycles. Got it.
The volume hasn't cut out since i disconnected the reverb cables from the board. I checked the reverb op amp socket and am getting +/- 12V on the proper  pins. I just popped a fresh 4558 in there for the hell of it.
Last time I played through it, I did jump the fx loop with an instrument cable. The jacks look clean and have good tension on the plugs.

I think my intermittent problem had something to do with the reverb, which is fine because I never use reverb anyway. I have no problems leaving it disconnected.

So how do those voltages look? They seem pretty darn close.

[guyatron]

also, here's that variation I was talking about.
There's a different network going on in mine between the collector of Q10 and R48

Pretty sure this is how it's laid out. Make sense?

[phatt]

With .1 VDC then a fair bet the circuit is ok.
Well if the reverb was causing volume drop out due to the miss connection then I'd say you have a working Amplifier. :dbtu:
Maybe time to stop now and enjoy the sound. 8|
Phil.

[guyatron]

yup, ran it through two 4x12's for a while yesterday after taking these voltages
love the drive of this thing!

Thanks

[vidmachine]

Volume drops were happening to me on the RG100ES XP when the rocker switch was failing.  Randall replaced the treble pull knob with a rocker.  I stuck a new mini toggle in there and the problem went away. 

I have a newer issue.

My RG100ES has recently started sounding loose in the bass end and sort of blah.  I put new filter caps in it and the humming at idle did drop by a noticeable amount.  But it just doesn't sound as good as it once did.  Someone suggested the output transistors?

Any ideas what could cause the low end to start sounding flabby?

Thanks, gang.

[DrGonz78]

Have you tried the amp out with a different cab or other speakers? Just a thought that you can try to eliminate the speakers as a possible culprit.

[vidmachine]

Thanks for the reply, Doc.

Yes I tried some other speaker setups.  I used a 1x12 Peavey cab, the 1x15 in my Randall bass combo, and also the 1X12 "special design" in my Fender Studio 85 with some wiring adapters.  I can't say they had the exact same issue because those speakers are drastically different.  None of these sounded "bad."  But something changed from what it used to sound like.   Normally I use a Tour cabinet loaded with 2 8 Ohm Peavey Scorpions wired down to 4 ohms. 

The problem appears to point to something in the Randall head wearing out or breaking down.  Don't get me wrong, it still sounds good but it used to sound awesome.

[DrGonz78]

To be honest I don't think it is the output transistors. They are not something that wears down, rather they just break. Things that wear down with age and grow dirt are your jacks, pots, solder joints, and the speakers just to mention a few. I mean transistors can get old and leaky too I guess but lets look another direction.

How do you play the amp (i.e. Style of music)? Do you use the effects loop? How long were you playing the amp and satisfied with the tone?