Randall RG100 ES solid state questions

[Randall]

Amp comes in, it works, sorta, albeit with very low volume for a 120 watt amp. I pull the RCA 6254 outputs and all measure open case to emitter. I wonder how this could even make any sound under these conditions? The drivers TIP31C and TIP32C seem OK in circuit. The .18 ohm 10 watt resistor going from speaker ground to ground looks dodgy with a corner piece of it broken off, and it fell away at both ends when I put fingers on it to examine it, so obviously someone has been in here with less than perfect soldering skills.

So my questions are, replace the four outputs with originals if I can find them, or if I can't, sub them out with NTE? How is it that this amp can make sound in this condition? I will also replace that .18 ohm resistor. Is there anything else I should consider?

randall_rg100es+manual.pdf

[Jazz P Bass]

That link is 'dead'.

[Leo_Gnardo]

So my questions are, replace the four outputs with originals if I can find them, or if I can't, sub them out with NTE? How is it that this amp can make sound in this condition? I will also replace that .18 ohm resistor. Is there anything else I should consider?

Randall fixes a Randall - it had to happen someday! Unusual failure mode for the output transistors. It was the driver transistors supplying audio to the speaker, probably just a couple watts at best. You may have a heck of a time finding a 0.18 ohm 10 watt resistor. Though battered I'd leave that one in there unless you do locate an exact replacement. For the output tran's, double check the right ones were in the right places so you don't repeat someone else's error, then swap in some fresh ones. I'll bet there's a cheap MJ transistor that will save you a bucket of cash vs. NTE replacements, like a 15003 or something like that. Ya know who's gonna know? Enzo! Don't order anything 'til you hear from him.

[The Dude]

http://music-electronics-forum.com/a...ll-rg100es.jpg

[The Dude]

As Leo suggested, the 15003 should work just fine. I happen to stock MJ15024 and have them readily available. That's what I would use if it were here. They are sort of the transistor version of the 1N4007 in that they work in most instances.

[Randall]

Thanks Dude, I don't know why I can't get that link to work.

Leo, Mouser has wire a Vishay wound .18/10w for a couple of bucks, so no sweat there. I can find the original RCA outputs, but they ain't cheap at around $12/ea plus shipping. I also see "Motorola" branded 6254's from China, but I don't want to go there. And I don't understand what you mean about putting them in the proper position? What mistake am I trying to avoid?

And I had my emitters and bases mixed up, they are all open E to C.

[Enzo]

They are TO3 transistors? SO wouldn't you WANT it to check open from emitter to collector? E to C shorts will blow fuses.

To review, meter set to diode test, red probe on the base, black probe on emitter pin, and then black probe on the case. This is NPN, so that should indicate like a diode in each case. The case is the collector.

Do they say "RCA6254" on them, or do they say RCA 2N6254" on them. In other words are you being casual with the part number?

The series 0.18 ohm resistor may look crummy, but does it have continuity? It is either going to be open or OK pretty much. And if it is electrically intact, then it is not responsible for low output.

Why oh why oh why would we ever replace the outputs with NTEs? The only time I'd even consider it would be if a show is going on in an hour and they were the only thing I could get. In fact, even then I'd probably tear some good ones out of some other amp in the shop. Original types are great in most cases, but if you are going to sub something, then just select a reasonable real part, instead of paying several times more for something merely close from NTE. I suspect those NTE subs are no closer than the MJ15003 we mentioned.

Low output is RARELY due to output transistors. If they all fail open you get no sound. If they short in any of the three combinations of pins, you blow fuses. If one on top opens and one on the bottom, it sstill makes just as loud, just can't drive as low an impedance.

Did you isolate the problem to the power amp? Did you cross the FX loop with a cord? Did you plug a signal into the FX return to test the power amp alone? Does the preamp out from the FX send sound OK through a different amp and speaker? Does your speaker cone move freely? Does your speaker sound OK driven by a different amp?

In my mind, the very first thing I check when i get a low volume complaint, is the FX return jack cutout contact.

[nosaj]

As Leo suggested, the 15003 should work just fine. I happen to stock MJ15024 and have them readily available. That's what I would use if it were here. They are sort of the transistor version of the 1N4007 in that they work in most instances.

At the chance of derailing are there other transistors that are like swiss army knives for a wide range of other transistors?

Thanks,
nosaj

[Enzo]

That is the joy of transistors, you can sub a TON of different types into this.

[Leo_Gnardo]

And I don't understand what you mean about putting them in the proper position? What mistake am I trying to avoid?

At first I thought you had some NPN and other PNP output xtors. If that was the case and a previous repair person had swapped one for the other, you could expect a major fail. But that wasn't the case so nothing to worry about.

At the chance of derailing are there other transistors that are like swiss army knives for a wide range of other transistors?

Seems that's pretty much the case with MJ-series outputs, also some of the TIP series. MJ15003, 15004, 15015, 15016, 15024 & 15025 would make a good sub set for many TO3 transistors. I'm sure there's a similar set of TIP for plastic cased output transistors both singles & darlington types.

[The Dude]

Speaking in generalizations: The 1N4007 is pretty much the same as all other 1N400* rectifier diodes, except it has the higher voltage rating. So, there's really no reason to stock the others, IMO. The same sort of thing works in the TO3 MJ series,...... for the most part.

[Randall]

Boy, do I feel stupid. I don't know what I was thinking. Nothing I guess. I can't believe I forgot how to test a transistor. What a maroon.

The outputs are fine. And yes, I also misrepresented them, as they are actually RCA 2N6254. I put them back in with some fresh heat sink compound. The FX loop jacks are not the problem, it was the first thing I tried. I have found that it puts out a clean 72 watts at 8 ohms feeding a sine wave into the FX return jack. It's not even remotely close to that going thru the preamp. I also now notice some of the tone controls do not work when going in thru the input jack, so that's a clue. I hit that tomorrow with a fresh head.

Thanks for indulging my dumbassedness.

[Enzo]

Hey, I forget what I had for lunch.

I think one thing to take away from this is the common idea that power is loudness. When an amp works but has low output, it usually is not about the power stage. If a 100 watt amp makes 10 watts, that is not a 90% reduction in loudness, it makes it half as loud.

You have two channels in that preamp. Are both of them affected? If so, look for what they have in common. Comes to my mind first: Q4 and the tone stack.

But also, turn up the reverb, does a spring crash come out loud? Does playing into the input make reverb? Or is reverb diminished along with everything else?

[Randall]

Both channels are effected. Reverb spring crash is not loud. Playing into the input makes a little bit of reverb, about on par with everything else lacking volume. Mid control does nothing scoped at the treble middle lug, where bass does much. I'm going after the mid cap next. Treble control makes a slight change on the scope, but I can't hear it, same for presence pot.

[Enzo]

Play music through it, rather than guitar. Turn bass and treble controls to zero. Now listen while turning mids up and down. That will give you an idea if it is doing anything or not.