Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hey there. I'm new to this so I'm going to try to be brief in my description of my problem. I have looked high and low, near and far for a schematic for the Randall RG100 Classic. I'm a novice at discerning the layout of an amplifier. I know how to read a schematic and I know how to navigate through it enough to know what parts are used. My only source for any kind of schematic for this amp is that of an RG100ES. There's differences but I can't tell what all the differences are as I haven't went through it with a fine toothed comb. I know the output transistors are difference but they seem to run off the same dc voltage. Beyond that I have no clue what's the same/different. I need the right schematic. Can anybody help me? I had also thought about converting it to the ES power supply if that would work. Any thoughts on that, too? Thanks in advance for any help.

Never saw the Classic, which I guess must be a reedition, but by definition it should be practically the same as the .... uh ..... "classic"... ES.
Difference in power transistors is probably because they use what's available and popular today, instead of 30 years ago, but does not change the sound at all, so don't waste time and money worrying on that.

Same with power supply.

um ... so brief you didn't actually say what your problem is with this amp. (???)

Quote from: TolerBearI had also thought about converting it to the ES power supply

Why?

It was made when the original owner of Randall Amps made them. It's blowing fuses which tells me there's a short in it. One of the resistors was slightly discolored in the power supply and I'm not sure if I replaced it with the right one. I measured it with a multimeter and it read 300 ohms but I think it was weak and slightly toasted. So it's not gonna read right. At first the amp would come on and then blow the fuse, now it blew it once and it don't come on at all. I think the original problem was somebody attached a couple of diodes onto the clipping diodes and it was causing a short. I removed them then I replaced the output transistors, regulator, rectifiers, and the filter capacitors. And that resistor. But Idk if I replaced the resistor with the right one. The reason I thought about replacing it with the ES power supply is the schematic is available. I just don't know if the specs are the same.

I think the Randall RG 100 Classic predated the ES.

It also uses a different footswitch. The Randall RG100 ES uses a rectangular plug for the footswitch the classic uses 2 mono 1/4 inch plugs.

I have pictures of the Classic but Idk how to attach them on here.

Quote from: TolerBearI think the original problem was somebody attached a couple of diodes onto the clipping diodes and it was causing a short. I removed them then I replaced the output transistors, regulator, rectifiers, and the filter capacitors. And that resistor.

Now you are deep in what we call the "scattergun" approach, and the fact that you have half rebuilt the amp and still have the fault illustrates why we deprecate this method - it doesn't work.

What we do is diagnose the fault, track down what it really is by voltage and resistance measurements, then replace the offending component(s).  In the long run it is quicker, cheaper, more effective, and runs much less risk of introducing new faults to complicate matters.

Wirewound ceramic power resistors (generally square) can be assumed to be about the right value (even if well fried), or open circuit, they don't go "weak" i.e. off value like other resistor types can.

The clipper diodes are in the preamp and it's vanishingly unlikely that they would cause fuse blowing, however I'll make a guess that you are referring to the diodes in the output stage which are not clipper but output stage bias diodes, and messing about with these, particularly adding more in series, could easily result in the output stage turning hard on and blowing fuses (if not transistors).

So first we need to clear that up, clippers in the preamp, or bias diodes (generally two or three in series) in the power amp?

Second, disconnect te speaker to avoid damage and do not reconnect until the amp is healthy again.

Third, if you have fuse blowing then a limiting lamp (http://www.ssguitar.com/index.php?topic=2093.0) is essential to allow servicing with some voltage but with the current limited to safe levels.

Once you have a limiting lamp in series with the mains supply we can move on to actual fault diagnosis.

Amps tend to be fairly generic, and as JMF said the circuit we have is likely to be close enough to give a pretty good guide.


{click "attachments and other options" under your post to expose "Attach: browse".  An overview pic of the internals would be very helpful.}

Awesome. I was wondering how I could do a controlled supply voltage to test with. I don't have the bench equipment to do that so this is really a good way of doing it. I'll have to post pictures of the amp's innards. I did have a couple problems on one of the rectifier circuit board traces. I thought it was desoldered and loose from the circuit board and it wasn't and the trace lifted off the board.  (double sided circuit board)  :-(    I fixed it the best I could with solder lead clippings. I know this throws a bad part in it but as far as I know, it's connected as it should be. Pics later. I'm also sorry I'm not on here more. I get distracted by life. Lol.

The clipping diodes I was talking about was on the preamp board. I'll show where they were as best I can.

Quote from: TolerBearI fixed it the best I could with solder lead clippings. I know this throws a bad part in it but as far as I know, it's connected as it should be.

I hope you mean "resistor lead clippings" and not solder; it's great for joints but isn't that great as a wire substitute.  This sort of kluge is so common that we don't talk about it much, but it's quite common to have to repair PCB's with odd bits of wire (e.g. broken traces around wrenched PCB mounting sockets).  As long as it has been done "in a workman like manner" there shouldn't be any downside, in fact in some cases it can be better than the original.

I use resistor and cap lead offcuts for stuff all the time.  Just knocking up a multi-LED replacement for my rear numberplate light using LED's recovered from a torch (flashlight) that have very short leads, so I'm using wire offcuts off the bench to renew and extend them.  Wire is wire.

Yeah, life is what gets in the way while you are trying to get stuff done.

"Johann put out the garbage, Johann put out the cat - how's a guy supposed to write a Toccata?  ::) "

Yeah. I think it was the left over clippings off of the capacitors I put on there. Maybe the rectifiers. Lol.

The more I think about it, the more I'm wondering if it blew the fuse because of the possible wrong resistor value since its one of the first things the power sees after the bridge rectifier and the filter capacitors.

No, not load enough to blow a fuse.

Think blown/shorted power transistors/drivers/psu rectifiers/big filter caps in that order

Oh Lord. I just replaced all of those. Lol.

I'll be putting those pictures on here maybe tomorrow. If not tomorrow I'll get them up Monday. I have a gig this weekend.

http://s18.photobucket.com/user/Jskrapper/media/P1120876.jpg.html

http://i18.photobucket.com/albums/b112/Jskrapper/P1120881.jpg

Quote from: TolerBear on August 21, 2014, 07:52:26 PM
Oh Lord. I just replaced all of those. Lol.

Then we´ll have to start troubleshooting instead of shotgunning ;)

As of your pictures, you show the amp exterior which *looks*  like an old ES and just one small unidentified PCB which shows it is NOT a classic Randall.
(By the way, there´s an old saying which goes: "if it says Classic or Vintage on it ... then it´s not :( )
That PCB is pupulated with SNT parts and .... horror of horrors .... two 80 or 100 pin square ICs, so there´s some heavy digital stuff brewing in there :(

Not useful for the problem at hand.

We need clean and readable pictures of the power transformer area, power supply and power amp at least.

No. This amp is totally analogue. Lol. It's got the older components in it. Barrel resistors, 8 pin op amps, etc.  :-)

 So this picture is not from the Randall?
http://s18.photobucket.com/user/Jskrapper/media/P1120828.jpg.html

Ok, we need inside pictures ;)

Quote from: g1 on August 24, 2014, 11:12:29 AM
So this picture is not from the Randall?
http://s18.photobucket.com/user/Jskrapper/media/P1120828.jpg.html

Yeah I saw that pic too at first and was not sure if it was from that amp or not. Looks like a helicopter... :lmao:

Yeah. That's from the helicopter. Lol.

And how does it sound ?   :lmao:

Good news. I got it to turn on. It changes channels too but still no sound and that resistor I was talking about is getting way too hot and it's smoking a little, making me think it's the wrong resistor value. It acts as if to me it's got too much current flowing through it making it overheat. The other problem was I didn't realize it but one of the rectifiers had a globe of solder bridging two of the leads together.   ::)   I'm gonna try to put pictures up now of the different parts of the amp.

Did you replace the Zener diode?
What voltage do you measure on each end of the resistor, relative to ground?

I haven't measured it yet. The amp is at work. I took it there because we have benches and better lighting than I have here at my apartment. I'll measure it tomorrow. The resistor in this picture is the original which I have misplaced. :-(

I haven't replaced the zener diode. Do you think I ought to? Is there a good way to check the one I have?

I couldn't tell what the original resistor's value was either. I'm color blind to an extent. Pastels and close colors get me every time.

I see Orange in the resistor colors but I can't tell the others. Lol.

Did you see the other pictures I put on here. I accidentally created a new topic and there's a couple under that one.

I'm asking because in the SS Marshall thread a few days ago exactly the same happened.

It was a 220 ohms resistor, which normally had some 26V on one end (one main rail) and 15V on the other.

Normal dissipation was (26-15)^2/220=0.55W , well within its 1 W rating, but with the shorted diode it was 26^2/220=3W .
Of course it toasted, and I suspect something similar here.

That's why instead of guessing or replacing parts at random, I always INSIST on "measuring, the shortest path to success"

Gee, I think I should patent that phrase .... only that Enzo beat me to that long ago ;)  .

Lol. I'll try that. I guess you measure it just like a regular diode?

It did have a 1w resistor in it too. I hope I can read it. I'm gonna have to research it to see about zener diodes.

Ok I checked the zener. It checks out fine. Do you have any idea what the color bands are on the original resistor in this picture down below? I can't tell but I think it's Orange, but the other 2 I'm not sure of. The tolerance band looks silver to me.

Another try at a pic.

Quote from: TolerBear on August 26, 2014, 04:17:24 PM
Another try at a pic.

https://plus.google.com/103513281531958273078/posts/1Y2kt7kGn57

Yay! I think it worked!  :tu: 8| :duh. Lol.

http://spaceryerson.com/pics/RG80-Schematic-1.jpg

This is pretty close to mine but it's lacking information. 

Here´s an earlier version.
Basically the same as the one you posted, but somewhat simpler.
As: no +24V regulator IC, just a simple resistor+zener, one less gain stage .
But should be close enough.

Thanks. I'm about at my wits end on this thing. Lol. Maybe this is close.

Did you see the pictures I posted a link to? I couldn't figure out how to post them all on here so I posted the link.

  Yes, the link from reply #37 is working and there are pics there.  Click on the main pic and it sends you to the rest.

Are any of them showing the original discolored resistor you are trying to figure out the value of?

Yeah I put it in an earlier post. Here it is. 20140606_124514[1].jpg

Well it didn't work like I thought it would.  :-(

It's an actual picture though. I can't seem too get the picture thing to work.

Quote from: TolerBear on August 25, 2014, 03:37:05 PM

This is the original resistor?  What value did you replace it with?  From the other picture it looks like you installed a 300 ohm?
  I guess it's possible it was a 3.3K ohm, or maybe there is a fault causing the overheating.
  Because of the cap near it that does not have a pair, I'm guessing this is the 24V supply, not the + or - 12V.
  Can you read the numbers off D4?  Looks like 1N47 something, need 2 more digits.

Edit:  I don't know what I did wrong with that quote, just click where it says "quote from..." and it will take you to the pic.

Agree, I also guess it's the resistor in series with the 24V regulator, to lower somewhat its heat dissipation .

Just thinking aloud:

> Now anyway, why would it heat so much?
> Unless the regulator is shorted?
>That Fet based preamp by itself uses very little current, a few mA.
>In fact the original ES 24V supply was made out of a resistor and a ¿1W? Zener, quite light duty.

Food for thought.

I believe that zener is a 1N4752. And yes it was the original 1w  resistor. I replaced it with a 3w 300 ohm resistor. I'm slightly colorblind. I couldn't tell what the 2nd band was.  :-(  That's why I've been concerned about that resistor. The power supply to me looks like it splits in front of the filter capacitors. Then one side I guess runs the output the other half runs everything else. But if that were true it would have a negative side because of the ground reference. That's just speculation on my part though. According to my research that zener diode is a 33 volt one. There's 2 7832 regulators and 1 7831. I was thinking about what could cause that resistor to heat up so the only thing I could figure was it was the wrong value. I'm pretty confident now it's the wrong resistor value. I wonder if it could've messed my outputs up or something. They don't seem to be they didn't smoke or anything. According to the specs they're pretty hardy.

Something's not right here (going by the circuit JMF posted).

The 1N4752 is a 33V 1W Zener (mistakenly shown as "3.3V" at digchip) - where were you thinking of placing this in the circuit, coz I suggest that you don't.  I think it should be a 1N4749 24V 1W zener with a 1K 1W resistor in series.

Get a lens and see if you can read the number on the zener (you may have to lift one end if the number is underneath).

Quote from: TolerBearThere's 2 7832 regulators and 1 7831

That doesn't look right either; three-pin 78xx regulators don't go up to 32V (7832) and they aren't spaced by one volt (7831), and more generally you would use three-pin regulators or zeners but normally not both.    ???

Quote from: TolerBear on August 28, 2014, 01:24:04 PM
There's 2 7832 regulators and 1 7831. .

I think you mean two TIP32 and a TIP31.  Then the 4th device on those individual heatsinks can be seen in the pics as a 7824 regulator.
  So perhaps it is a 33V zener limiting the input voltage to the 7824.

Edit:  looking again at the schematic tolerbear linked: http://spaceryerson.com/pics/RG80-Schematic-1.jpg  it appears to be correct, just missing values.

  So R1 should have 45V at one end and 33V at the other, for a voltage drop of 12V.
With a 330 ohm for R1, it would be running about 1/2W.  So the heat there is probably normal if you have 33V at the zener.
Time to get back to proper troubleshooting of the actual fault.

What is the voltage at the junction of R1 and the zener?

Yeah yeah. That's it. That's exactly right. I screwed up listing the part numbers. Lol. I'll check the voltage and get back with you.

Sorry for the mix up. Lol.

The only thing is, it's cooking with the 3 watt resistor I replaced it with. So my guess is that the wattage is above 3 watts as it stands.

  It's hard to believe how hot some of these things run "normally".  But you said it was smoking, which it should not.
  The voltage at the zener will tell you.  If you have 33V there, then the heat is the normal amount.  If there is a fault, you will not have 33V at the zener, but some lower voltage.  (this is assuming the other end of R1 has it's proper approx. 45V)

{ okay, sorry guyz, it may be helpful if I actually work from the right circuit.  Thanks g1.  }


+45V supply --> R1 --> 33V 1W zener --> 7824 reg, 24V out.

Given: 45-33=12 volt drop across R1.

Max zener current;
33Vz 1W
P=E*I
1/33=0.03030303A = 30mA max

30mA @ 12V
R = E/I
12/0.03=400 ohms min

It emerges that with R1 = 300 ohms everything is fine as long as the fixed load of the preamp is connected, BUT;

- if the preamp gets disconnected (open) the zener is slightly over-powered.

- if something shorts in the preamp the current is limited by the 1A rating of the 7824, but also by R1 - which may smoke as a result.

R1 is 300 ohms, right?

And is getting hot?


What is the actual voltage across the 33V zener?

What is the voltage on the 24V rail?

Is the 7824 getting warm/hot?

Are any of the op-amps on the board getting warm/hot?


{There will be a test next period.  Neatest correct entry wins.}

I hope y'all had a nice weekend. I didn't think to check the voltage on the 24 volt rail I can do that tomorrow. I checked the voltage on the resistor and it says 40 volts dc.

I haven't had the amp on long enough to check the op amps to see if they're getting hot. I've been concerned about that resistor smoking. Lol. Btw, I changed that resistor to a 3.2 k ohms 3 watt but it did the same thing but more so I think. So I'm gonna change it back to the 300 ohm resistor. I was following the advice of one of the older techs at work.

Dang you said the voltage on the zener. I forgot to measure that. I'll do that tomorrow too.

Quote from: TolerBear on September 02, 2014, 02:32:51 PM
Dang you said the voltage on the zener. I forgot to measure that. I'll do that tomorrow too.

{#$%& man, the suspense is killing me}


I predict...
(http://img.photobucket.com/albums/v236/multimoog/Character.jpg)
...that both the 33V and 24V lines will be low.

Lol. I'm sorry!    :-D

So I measured it today and I found that the voltage on the zener is 1.6 volts dc. The other way it shows 0 volts. I got busy and I wasn't able to do any other measurements. I'm hoping tomorrow to do more measurements. I don't understand this thing. I'll talk to y'all tomorrow. I'm soooo grateful for y'all's help. I haven't said that yet but I am.

  Measure the resistance across D4 (regulator input) and C18 (regulator output).

Quote from: TolerBearSo I measured it today and I found that the voltage on the zener is 1.6 volts dc.

Powered up we mean; if it's +1.6V one way it should measure -1.6V with the probes reversed.  Measurements on an unpowered zener are almost meaningless (unless it's shorted).

This should be 33V.

Pull connection P2 between Power Amp PCB and Front Panel PCB, and see if 33V input and 24V output supplies are restored.
Post results.

Personally my money is still on a shorting op-amp on the board, and these can be tricky to locate.

On the circuit <RG80-Schematic-1.jpg> (upthread) the 24V goes to the bit in the middle where it becomes VC1 and VC2.

If you had a short on VC2 then R26 would be letting you know, so it has to be on VC1 rail.  A few things run off this, FET preamps mainly that are incapable of drawing significant current even if they wanted to.

So, the reverb driver and recovery is U1, a 4558, which is right on the VC1 rail.

There are a couple of caps, but that's my hit pick of the week.


I can bearly tolerate this drip feed...  :P
(http://www.leavingworkbehind.com/wp-content/uploads/2012/01/drip-feed.jpg)

Lol. I'll check it asap. I have a dentist appointment this evening so it may not be until tomorrow when I get to it. Drip.....Drip.....Drip :duh

And I powered it up when I measured it.

attached is the elusive schematic for this version, full page view on page 5 of pdf

Thanks  :dbtu:

Me too.   :dbtu:

I do hope TolerBear hasn't gone and done something rash.   ???

Sorry y'all.  My phone died and I had to get another one.  This is awesome!  I'll check it ASAP.  I had a 300 ohm resistor and it looks like it calls for a 330 ohm.  Is that enough to make it act like it is or should I be looking elsewhere still?  Btw it sucked being away that long.  Lol.  May God bless each one of you.

330 ohm is the common value, close enough, anyway can tweek later if needed.

Hey. I gave up on the Randall for a while. I replaced the regulators and output transistors and it  kinda worked for a bit in that it had a hum going through it for a bit but it never passed a signal. I'm gonna start working on it again soon I just got aggravated with it. Also I had a couple circuit tracks that lifted and I think some of the through the board hole taps came out. Ugh! xP