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Author Topic: RG100ES Biasing the POWER transistors???  (Read 3397 times)

BMWFREQ

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RG100ES Biasing the POWER transistors???
« on: March 23, 2011, 04:35:46 PM »

I just replaced all four of the power transistors in my RG100ES, and would like to make sure they are biased to the correct voltage.  I know there is a trimmer pot on the PC board that is used for this purpose, but I am not sure where the voltage is taken.  I believe they should be biased to -.6volts, just not sure where to probe the board when setting this voltage.

If anyone has done this and can help me out, it would be greatly appreciated.

BMW FREQ
« Last Edit: March 24, 2011, 12:38:52 AM by BMWFREQ »
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J M Fahey

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Re: RG100ES Biasing the POWER transistors???
« Reply #1 on: March 23, 2011, 08:18:33 PM »

It's not exactly as on tube amps, and transistors are 1000000 times more consistent than tubes, probably it won't even be needed, but to take that idea out of your mind, measure voltage across cathode resistors ... he he, I mean those 0.47 ohm emitter resistors (or were they 0.33 ohm?).
Anyway, any voltage between 5 mV and 20mV across them is fine.
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BMWFREQ

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Re: RG100ES Biasing the POWER transistors???
« Reply #2 on: March 24, 2011, 12:15:01 AM »

Thanks for the reply.  You sound like you know your way around an RG100ES.  Would you know what would happen if the value of these resistors were to be changed?

There are two .6 ohm and two .27 ohm 5 watt resistors on the board.  Are two of these for the clean channel and the other two for the gain channel?  The reason I ask is, I have been looking for a way to add more gain to the red channel, and clean-up the green channel.  I know I know...you're gonna say "The red channel already has monsterous gain, and the clean channel is already crisp and clean - why would you want to change them?".  And to answer that question before you ask it...it's just my nature; I like to tweak.

If you would happen to know how to occomplish this, your knowledge would be greatly appreciated.

Thanks in advance-

Jay
« Last Edit: March 24, 2011, 12:39:35 AM by BMWFREQ »
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J M Fahey

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Re: RG100ES Biasing the POWER transistors???
« Reply #3 on: March 24, 2011, 01:06:54 AM »

Quote
There are two .6 ohm and two .72 ohm 5 watt resistors on the board
Read those ".72" the right way and they will be the correct  .27  ;).
That's the power amp side, which you must leave well alone.
Randalls are finely tuned, excellent design, they really squeezed all juice possible, no untapped gain there; I suggest you build a clean boost ahead for a scary sound.
The MXR MicroAmp is very good and layout is available; if you don't find it, it's just a Distortion+ without the clipping diodes and the output volume control; the board is the same with those parts omitted.
Or you can use a Tube Screamer, more as a gain booster than for its (light) distortion which will *not* be noticed after being brutalized by Randall's own..
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JHow

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Re: RG100ES Biasing the POWER transistors???
« Reply #4 on: March 29, 2011, 05:20:34 PM »

It's all personal preference... but I like my JRC4558-based BYOC "British Blues overdrive" in the effect send-receive of my rg80 - I'm not going for hardcore metal, but it leaves a lot of room for rude sounds in the red channel.  Think of Zep live Killing Floor.  If you want extra crazy, you can turn on the diode-clipping.  The green channel I like with the reverb up high and no boost.  Think Earl Hooker  on dust my broom. 

The modification I am most interested in for the randall is the trick to set it up to switch the channels via relays.  I have read that people have done this to cut some of the noise introduced by the long footswitch cable.
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BMWFREQ

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Re: RG100ES Biasing the POWER transistors???
« Reply #5 on: April 01, 2011, 04:13:54 PM »

So, does changing the Op Amp really change the sound of the RG100ES?  I thought the Op Amp chip was used to amplify the reverb and effects loop?  If it DOES change the sound of the amp, how does it do that? If I look at the schematic, I do not see how it has an effect on the sound of the amp...
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JHow

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Re: RG100ES Biasing the POWER transistors???
« Reply #6 on: April 01, 2011, 04:54:41 PM »

The op amp on the pc board is for the reverb send and return circuit.  It happens to be 4558 on the schematic, but in my post I was referring to a pedal that I particularly like, which concidentally also uses the same chip.  Regarding, changing the op amp, I have done that.  When I got my amp it had a socket for the chip already installed.  I don't know if Randall did that or someone else, but a previous owner or experimenter had mis-installed the op amp upside down.  When I got the amp the owner (he said it was his brother's amp) told me the reverb didn't work, and really it was just the chip inserted wrong, I flipped it over and it worked.  Anyhow, the chip in my amp (the upside down one) when I got it was a 5332.  I also tried a 4558, which worked fine.  It may be the 5332 was a little quiter, less noisy, but I wondered if it wouldn't pull a little more current, so I left the 4558 in there.  I didn't notice a big difference in the reverb, to be honest.
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vidmachine

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Re: RG100ES Biasing the POWER transistors???
« Reply #7 on: March 16, 2017, 11:36:30 PM »

what are the symptoms of a bad transistor?
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J M Fahey

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Re: RG100ES Biasing the POWER transistors???
« Reply #8 on: March 17, 2017, 06:46:39 PM »

what are the symptoms of a bad transistor?
It doesnīt work that way.
You tell us what is the actual problem with your amp or describe its symptoms, then we proceed.
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vidmachine

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Re: RG100ES Biasing the POWER transistors???
« Reply #9 on: March 29, 2017, 08:36:46 PM »

what are the symptoms of a bad transistor?
It doesnīt work that way.
You tell us what is the actual problem with your amp or describe its symptoms, then we proceed.


I was double checking the bias based on RG and PRR's post here http://www.diystompboxes.com/smfforum/index.php?topic=31737.msg899678#msg899678

It has me concerned. 

When I measure my R55 & R56 .6 ohm resistors across the terminals I get a reading that quickly decreases to zero mV.  One of the .27's is in the 48 mV range and the other is about half of that. They do not decrease.  I have 43 Volts going into the set closest to the molded plugs. Oddly I get a chassis voltage reading of 43 on either leg of both of these. Maybe I am doing this wrong. This was with no signal attached and both channels on with the volume cracked open a little.  The other set of .6/.27s shows about 250 mV on either side to ground. 
« Last Edit: March 30, 2017, 11:37:59 AM by vidmachine »
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g1

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Re: RG100ES Biasing the POWER transistors???
« Reply #10 on: March 29, 2017, 10:53:11 PM »

Measure with DC millivolt range.  One probe at each end of resistor.
Post your results for each and specify whether .6 or .27 resistor.
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vidmachine

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Re: RG100ES Biasing the POWER transistors???
« Reply #11 on: March 30, 2017, 12:16:43 AM »

Measure with DC millivolt range.  One probe at each end of resistor.
Post your results for each and specify whether .6 or .27 resistor.

I put the controls all at noon, no signal plugged into the input, reverb off with footswitch, and both channels on with fs-5 footswitch

Looking at the picture I posted starting with the top left .6 Ohm
.6 = 3.3 mV then runs to zero
.27 = 106 mV holds steady

next bank next to the molded connectors
.6 = 6 mV then runs to zero
.27 = 83 mV holds steady.

I have adjusted the trim anywhere from .550 to .650 Volts.  And there is a sweet spot that sounds pretty decent, but I can feel the back heating up a bit more than yesterday.  My initial reason for the investigation started when I noticed the Bass was sounding loose and flabby.  I changed the filter caps to 6800 uF - 50V cans.  Some of the ever-present hum cleaned up, but the bass was sounding off.  It was suggested on the thread https://www.ssguitar.com/index.php?topic=4201.msg33508;topicseen#msg33508 to change a few capacitors that I haven't had a chance to swap. 



« Last Edit: March 30, 2017, 11:39:32 AM by vidmachine »
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J M Fahey

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Re: RG100ES Biasing the POWER transistors???
« Reply #12 on: March 30, 2017, 05:26:12 PM »

Measure with DC millivolt range.  One probe at each end of resistor.
Post your results for each and specify whether .6 or .27 resistor.

I put the controls all at noon, no signal plugged into the input, reverb off with footswitch, and both channels on with fs-5 footswitch
Put all controls to 0 , we donīt want any signal at the output, not even self generated hiss or buzz.

Quote
Looking at the picture I posted starting with the top left .6 Ohm
.6 = 3.3 mV then runs to zero
.27 = 106 mV holds steady
Weird, according to schematic you should have 30 to 50mV across the 0.6 ohm ones, and *zero* across the 0.27 ohm ones.
Maybe somebody pulled them and replaced the wrong way ?  :o

Quote
next bank next to the molded connectors
.6 = 6 mV then runs to zero
.27 = 83 mV holds steady.
Same here, we expect 40/50mV across the 0.6 ohm ones and zero across the 0.27 ohm ones.
In any case we do NOT want more than 50mV across *any* of them.

Quote
I have adjusted the trim anywhere from .550 to .650 Volts.

550/650mV *where*????
Quote
And there is a sweet spot that sounds pretty decent, but I can feel the back heating up a bit more than yesterday.

You must NOT repeat NOT adjust bias "by ear", not even by bias voltage, only by measuring transistor current which is voltage drop/emitter resistor ; in this case 50mV across 0.27 or 0.6 ohm resistors means around 185 or 80 mA through each transistor, whih is already a lot and should never be surpassed.
In fact I would be happy with half that.

Quote
My initial reason for the investigation started when I noticed the Bass was sounding loose and flabby.

Maybe true but that is absolutely unrelated to bias, so do not mess with it.
And in any case you are supplying values opposite to what is expected.
please reread as suggested above.
EDIT: measure bias with NO speaker or load connected.
Now that you are there, also measure DC offset, from Spk out to ground.
We expect less than 100mV DC there.
And recheck that the 0.6 ohm and the 0.27 ohm resistors are in their proper places.
Even if parts are not labelled on the PCB:
* R55 0.6 ohms must be connected to emitter of Q13 which is the one which has its base conected to Q11 emitter and 100 ohm R53 . Follow wiring to confirm this.
* R57 0.27 ohm is conected to Q15 emitter whose base is connected to R55 and Q13 emitter. Confirm.
* R56 0.6 ohms must be connected to emitter of Q14 which is the one which has its base conected to Q12 collector and 100 ohm R54 .
* R60 0.27 ohm is conected to Q16 emitter whose base is connected to R56 and Q14 emitter.

Quote
I changed the filter caps to 6800 uF - 50V cans.  Some of the ever-present hum cleaned up, but the bass was sounding off.

If hum lowered then new caps work way better than old ones.
If anything, Bass should improve.
If you still donīt like it, at least now you are certain that itīs not a supply problem any more.

Quote
It was suggested on the thread https://www.ssguitar.com/index.php?topic=4201.msg33508;topicseen#msg33508 to change a few capacitors that I haven't had a chance to swap.
*Maybe*, but your sound as shown in your mp3 is killer as is.
If anything, it might be more of a "taste"  problem than a "tech problem"  one,and itīs difficult to argue on taste, I wonīt even try.

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vidmachine

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Re: RG100ES Biasing the POWER transistors???
« Reply #13 on: March 31, 2017, 12:41:50 PM »

Quote
Looking at the picture I posted starting with the top left .6 Ohm
.6 = 3.3 mV then runs to zero
.27 = 106 mV holds steady

Weird, according to schematic you should have 30 to 50mV across the 0.6 ohm ones, and *zero* across the 0.27 ohm ones.
Maybe somebody pulled them and replaced the wrong way ?  :o

I'll be dipped in dookie.  Every picture I can find has these two resistors opposite of the way they are in mine.  I will trace the circuit to be sure.  I can't imagine that this would have passed QC at Randall but it appears these are originals.  Maybe a trainee or a hungover tech was assembling it?  Gee whiz. 

I can tell the amp has had a repair before I bought it. A power transistor appears to have been replaced before I bought the amp used.  I can tell because there is evidence that there was an overheat. (bluing on the metal straps inside the chassis and the part looks like a later model with different silk screening on the top.)  Maybe that is why the owner traded it in.  "Hey man, this amp doesn't like Lynch or Dime.... blah, blah"

It sounded great for the 19 years I had it but something is off for sure. 

Thanks for the details on the voltage readings.  I am so grateful for your help.

The jpg file at the other thread looks like mine and has the two op amps and TL604C switcher.  Compared to my amp, the resistor's position is different.  (from https://www.ssguitar.com/index.php?topic=2847.0)

ADDITIONAL INFO: 
Yeah the resistors are definitely in the wrong place.  I have 46 mV on the .27 labeled R55 In my attachment.  There is 243 mV on the speaker outs.  Is this reading due to the manufacturing defect/botched mod/repair?  Would there be permanent damage?   Can we verify that I identified the correct parts with the markings on the image attached?  Also a photo of the QC tag for shame.  :trouble

The XP series is supposed to not lose as much volume when you engage the sustain boost.  Would that have anything to do with swapping the resistors around?
« Last Edit: April 01, 2017, 03:18:26 PM by vidmachine »
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g1

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Re: RG100ES Biasing the POWER transistors???
« Reply #14 on: April 01, 2017, 03:14:36 PM »

The yellow wire coming out of that connector should read short to one of those nearest resistor legs.  It should be a .6 ohm
If not then they are all reversed and your labeling is correct.
« Last Edit: April 01, 2017, 03:17:16 PM by g1 »
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