First time in a long time posting and it happens to be squeezing y'alls brains. I've just recently got a hold of some schematics and while I understand some of it, it is old, like 80's old. So old parts or some of the symbols I've never seen or can't find. I'll post here the parts I'm not understanding and wait for enlightenment (yes I'm dumb as a rock sometimes. Don't make me get my thermometer.) This is a schematic of a Randall RRM2-250 that Randall was very nice to give me free. (I'm definitely buying from them.) This is a sweet amplifier and was (and is) used extensively by Phil Collen of Def Leppard and other guitarists from the 80's.
Best quality I have. Now first question: What is this part?



Says GAIN 50k and that's it. Looks like a resistor symbol with an arrow pointed to it. What does this mean?

Been a long time since I've looked at any sort of circuit diagram, but if I had to guess, your diagram on the input side of the arrow (to the right) is equivalent to whatever resistor it's pointing to.

To be honest I'm not sure what exactly you want answered and for what purpose. I may be in over my head answering this for you (I certainly don't have an EE degree at any level), or an answer coming from that may be more than you want. Are you trying to rebuild this amp (you mentioned you were "buying from them" which is why this is even a question)? Are you just trying to fix something you have?

Actually, building from ground up. I know enough to put things together and not electrocute myself, but never learned schematics. What I'm wondering is could this be a potentiometer? There is one potentiometer each in the back for gain on the two channels, this must be one of them. But what part would I use?

Edit: Answered my own question . Thanks Mav for pointing me in a direction. It is a 50k potentiometer. The old way of drawing it was a zigzag like resistors, but the new way is a box with an arrow pointing to the center.

3
|
---
| |
| |<--2
| |
---
|
1

Pin 1 is usually ground, Pin 2 (one with arrow) is output, and pin 3 is input. Now next question: What is this?

That is the symbol for a Zener Diode, actually that is 2 zeners back to back (or front to front depending upon how you look at it).

Zener Diodes

You may find this useful

So I'm assuming 4V each or a total of 4V? Or am I completely wrong? And thanks for the link. This is the most detailed one I've found. I was using this one. Would this one work?

The voltage indicated in the drawing is 4V7 (silly way they do it so there is no decimal point) so it actually is 4.7 volts at that point for the zener. They are both the same so that no matter if the voltage (current flow) is positive or negative it will only pass when the reverse breakdown voltage is 4.7 volts. So according to the link you sent you would want the MM5Z4V7 one.
Unless of course what I think is a 7 is actually a Z then the 3V9 or 4V3 are your other two choices.

That's the thing I hate about scanned handwritten images. Hard to read in some areas because of penmanship and artifacts that could get in the way (Like their 1's and I's Look the same unless you know what they mean). So since we narrowed it down to three, what would result in picking one? Is this going to be a plug one in and see if we let the smoke out kinda deal or would it matter?

I'd go with the 4V7, anyway what's wrong with a bit of smoke, if you have worked in electronics and haven't blown anything up then you are doing it wrong.

Ain't that the truth.
I once saw on a forum a good quote. "Electronics must run on smoke 'cause once you let the smoke out, it no longer works." I'm about to watch the new Star Trek so I'll enter this in my notes, look at the think you gave, and get back to the Q and A. Still got a few left.

FragMe! wrote:I'd go with the 4V7, anyway what's wrong with a bit of smoke, if you have worked in electronics and haven't blown anything up then you are doing it wrong.

That's a Z, you can see the author's "7"s written differently:

Unless of course I am completely misunderstanding and you're referring to the 4.7k resistance shown over to the right of it, in which case feel free to ignore me.

Ya that is why I mentioned the other two parts jsut incase he was using the Z to indicated a zener. If he was and since they don't make a 4 volt zener I listed the other two choices.

Then I'll take the middle road and go with the 4V3.

What do they mean on this capacitor (C10)?


There are several like this with different variables. also, the circle with arrow has a value of -35 and the notes say "This symbol indicates a D.C. voltage from that point to the chassis ground. All voltages +/- 10%." There are several of those too. What the heck is that in layman? Man I wish I had the time to go back to school. I have 17 years until Faith is out of the house (Like that will happen now in days) so maybe when I feel like being the oldest and dumbest one in class (already felt that way at UTD in my core classes.) I really do appreciate the help guys. I love learning new things.

The top value is the size of the capacitor and the bottom value is working voltage (or rated voltage) so 4/40 means it is 4 mircrofards rated at 40 volt electrolytic capacitor. Important to make sure you observe the polarity on those as the blow up real good if you don't.
More reference
The other capacitors (anyone without polarity indicators) are just normal capacitors
As for the voltage measurement the are talking about it means put the positive lead (red) of the voltmeter on that point and the negative lead (black) on any of the V (stripped arrow looking thing) best place is bottom side of the 22000 microfarad capacitor on the power supply

Oh and in case you want to know what the zeners are doing, it is an input overload protection circuit. Not normally a problem if you are using unbalanced audio (the rca input) also known as consumer level, but the xlr and 1/4 phone (line level) have the possibility of exceeding 4 volts on the input so if it does exceed 4 volts it triggers the revere biasing of the zeners which shunts the signal to ground so to speak so it doesn't overload the circuitry of the amp or blow the cones out of the speakers. There are two of them back to back because audio as you may know is an alternating signal meaning it has both positive and negative volage swings.
Okay that is Professor FragMe!'s lesson for today.

So basically it is a test point, right? Also do all XLR connections have a standard pinout? The configuration shown on Neutrik's website shows the pinouts for the females as clockwise 2 1 3. The user manual that Randall sent me shows a pinout 3 1 2: They also used Neutrik chassis connectors so something changed in 20 years. According to the schematic matched with the manual, pins 3 and 1 are grounded. Should I go with the 3 1 ground or continue with the idea of the schematics on the new pinout making 2 1 grounded? Plus I'm planning on using Neutrik's combo 1/4" XLR connector for an either or choice.

Yes there is a standard for XLR connectors, they also have the little numbers stamped on them
1 = ground
2 = high
3 = low
So to do what he wants which is to make and unbalanced signal from a balanced then connecting pin 1 to pin 3 is correct.
His drawing is incorrect though it is

1 2 or 2 1 depending if is a male or female connector and which side you are looking at
3______3

edit:crap it didn't keep the 3's in the right place so I put in _

so instead of the schematics bridging 1 and 2, it should bridge 1 and 3?

yes





bet you were looking for a longer answer.

Well okay here is another link, near the bottom is good.
balanced to unbalanced wiring
The little resistor network they show at the bottom is actually what should be done to make the xlr input = the rca input. Now unless you are connecting professional equipment with line level outputs you really don't need the xlr inputs.

Main reason I was going to go with the hybrid. Anything that will plug into that slot would be guitar related so it would be a 1/4" jack, but I'm also planning to use it in a studio, so the XLR might be needed later on, but still linked to the guitar. So that would cut 3 inputs down to 2 with one still questionable. I really don't know what I'd be using the RCA for. Anything you can think of? Also, which is better, unbalanced or balanced, for the studio? If balanced, could I convert this schematic to balanced and how would i do it?

Edit: Would help if I actually read. Balanced is less noisy, so it is better for studio stuff. But still confused about the need for RCA and how to make this amp support balanced signals. This will be a studio amp plugged into a mixer connected to a PC.

Well the PC by it's very nature is unbalanced, the amp you are building is unbalanced, therefore all inputs to the amp should be unbalanced. They can however be balanced up to the point where they go into the amp. The RCAs you would use for something like the output of a CD or DVD player if you felt so inclined.

Balanced is better if you are going long distances or an eletrically noisy environment. One thing to note about balanced signals is that the ground should only be connected at one end, usually where it becomes an input. If you don't you can get what are known as ground loops and what these can cause is electrical hum.

By the way just in case you haven't guessed yet I do this for a living

Well, considering my use, don't see me needing the RCAs, so looks like I'll leave them out, but I'll leave a spot on the PCB for it if needed. Also unbalanced it is. I know about ground loops too well. Installed several stereos and that noise is very annoying. I can imagine playing through that. Are the boxes with arrows another test point? The schem says "This symbol indicates signal at 1 k.c.". what's a k.c.?

So now I have a new person to haunt if I electrocute myself (although can you haunt multiple places at once? I promised my wife I'd haunt her forever also, and my best friend, and the guy next door, and my ex-girlfriend...)