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Randall RG100 Classic info?

Started by TolerBear, August 04, 2014, 08:24:55 PM

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TolerBear

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

TolerBear

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

g1

#47
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.

J M Fahey

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.

TolerBear

#49
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.

Roly

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.    ???
If you say theory and practice don't agree you haven't applied enough theory.

g1

#51
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.

g1

#52
  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?

TolerBear

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.

TolerBear


TolerBear

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.

g1

  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)

Roly

{ 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.}
If you say theory and practice don't agree you haven't applied enough theory.

TolerBear

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.

TolerBear

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.