Randall rg75d humm

[Jazz P Bass]

It looks to me as if Q10 is the center tap of the power transformer secondary.
If R13 is cracked you will need to replace it.

[cmanningjr]

Thanks I have a resistor ordered. No luck with Radio Shack.

QC10 connects to PS2...I just looked

What's the difference between 2% and 5%??

[Jazz P Bass]

Resistors are made in batches & then 'selected' for the tolerance of the actual value.
A 2% resistor will be within 2% of the specified value.
A 5% will be within 5%.
R13 is a 4700 ohm/ 1 watt resistor.
The tolerance is not specified on the schematic.
The way the resistor is being used in the circuit, either a 2% or a 5% will be fine.
The stated 4700 ohm value is not all that critical.

[cmanningjr]

4700ohm = 4k7 correct??

Reason I'm asking is I got the resistors in I ordered and thr colors don't look the same..

[52 Bill]

4700ohm = 4k7 correct??

Reason I'm asking is I got the resistors in I ordered and thr colors don't look the same..

Yes, placing the k between the 4 and 7 replaces the decimal point in 4.7k or 4700 ohms. What are the colors on the part? How many stripes are there? When there are 5 stripes instead of 4 there is an additional significant figure in the value.

[cmanningjr]

Old



New

Yellow, Purple, Red, Red...The old one looks to have 5 stripes..

[Enzo]

The colors are not the same because the old part is burnt up. Like a colorful cardboard box from some product will no longer have the same colors if you burn it.

[DrGonz78]

I think it really is a four band resistor. I can kinda see how it may look like a fifth band there, but pretty sure that it is just burn marks. Also, if it was a 4.7k resistor w/ 5 bands it would be 2% tolerance and that is the difference in standard terms. Actually the resistors you have are 2% tolerance and then would be a perfect match to the 5 band standard 4.7k. Nothing to worry about as those resistors you have are good as replacements.

[cmanningjr]

Well, I replaced the resistor, plugged it in... Same thing. R13 started smoking.. I turned it off before it burnt..Apparently I have crossed something up when I replaced the caps..

Help!!

[52 Bill]

R13 is in the high voltage power supply for the tube. If it cracked, did it burn? If it broke from overheating, then something is drawing a lot of current through it. Or did it break from some sort of physical damage?

I'd start by replacing the resistor. If you pull the plug at QC14 that will isolate the power supply. If it powers up and doesn't overheat R13 then the problem is in tube part of the circuit. If it does overheat then C13 is suspect.

I posted the above earlier. The caps you replaced earlier have nothing to do with this resistor. Double check to see that you didn't leave a solder bridge on the board and that any connectors that you pulled have been replaced correctly. Refer to the schematic to be sure.

You can try and take a resistance reading from the output side of R13 to ground. It should be fairly high. If it's low or zero follow along the supply line and see if you can find anything that might be causing it. Keep moving along the power supply line and try to isolate the section that is shorted to ground until you find the problem.

[Enzo]

If a resistor is burning, it is not the resistors fault. If yo put a new one in without fixing the underlying problem, it will burn up too. As you found. COnsider the problem. The problem right now is the resistor burns up. Whatever else might be wrong with the amp, this needs to be corrected. So...isolate the problem.

R13 is the tube power supply, as someone mentioned. SO just pull the tube. Does that make a difference? I bet it does not, but if it stops the resistor fire, then it was a bad tube. If this only started after you swapped out caps, then I suspect either the cap or the work. Caps like that always have the polarity marked, usually a stripe down one side with a bunch of negative signs. That is the side closest to the negative terminal on the cap. The circuit board probably has a polarity mark like a + sign printed next to the cap. Other boards will have a round dot on one edge of the cap outline to indicate where the stripe sits. SOmetimes the cap circle will have one side shaded in. Look at the other caps on the board and the symbols printed on the board as a guide.

If you installed C13 in the power supply backwards, that would do it, or if yo umade a solder bridge in there.

QC14 on the power supply connects to QC5 on the power amp. If you managed to plug one of those in backwards, it MIGHT cause trouble, IN fact, unplug that connetor, now the tube power supply is isolated. If the resistor still burnsw, your problem is right there. If it stops, thenthe problem is on the other board.

A friend of mine installed a bunch of new caps in something once. Did a nice job, good solder work. He forgot to trim off the exscess wires from one of the caps, so the extra inch and a half of wire was left hanging. With the board installed in the chassis, those wire leads grounded out the power supply.

[DrGonz78]

It was mentioned earlier... Light bulb limiter... I hope you plan on using this device to troubleshoot, because it might help out in situations like these. Simple to build and can reduce damage to amps circuits etc.

[DrGonz78]

Let me clarify why I think you should use a dim bulb tester here... If you are at all unsure of anything on this amp blowing up, heating up, etc... Then you would be very wise to limit the current after doing any parts replacement. Not positive that the bulb tester would prevent blowing of this component, but if there is a short somewhere it might have saved it. I am not saying that the bulb tester will help you fix this amp, but rather it will give you a green light to plug it in to mains power source. Learning this way is always memorable though and the next time you simply replace a blown component you will know what to do. Keep at it this thing you are bound to fix it soon.

[cmanningjr]

Thanks for all the advice. I'll definately take all I can get.

I know it's a mistake on my part. Just not sure What I did. I have an idea but stupidity on my part by not taking pictures left me in this mess.




This picture is after I replaced the caps.I 'think' I put the black wire in the lower right hand corner that runs to the board on the front of the amp on the wrong pin of the cap I replaced.

[52 Bill]

I 'think' I put the black wire in the lower right hand corner that runs to the board on the front of the amp on the wrong pin of the cap I replaced.

Do you mean that this black wire just solders to the underside of the board? What does it connect to on the front panel board?

There is no easy fix here, you will have to methodically retrace your steps and check everything that you did against the schematic drawing.

You know that the resistor that is burning is in the high voltage power supply for the tube, so start there. Identify the connector that carries the high voltage from the power supply board to the tube. Pull the connector at one end and measure the resistance to ground at both ends of the wire. Is either side a low or zero reading?

Hopefully there wasn't a lot of damage done when whatever went wrong, went wrong.