Randall RG100 Classic info?

[TolerBear]

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.

[J M Fahey]

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.

[Roly]

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

I had also thought about converting it to the ES power supply

Why?

[TolerBear]

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.

[TolerBear]

I think the Randall RG 100 Classic predated the ES.

[TolerBear]

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.

[TolerBear]

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

[Roly]

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.

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 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.}

[TolerBear]

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.

[TolerBear]

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

[Roly]

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.

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?  "

[TolerBear]

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

[TolerBear]

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.

[J M Fahey]

No, not load enough to blow a fuse.

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

[TolerBear]

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