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Randall RG50-112 Distortion Channel intermittent then quit working

Started by LBeast, November 18, 2023, 05:55:32 PM

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LBeast

I am new to the forum and am looking for some help regarding repairing my favorite Randall RG50-112 combo amp. Over the years the distortion channel has intermittently stopped working and then would return to working, if I left it alone, for a few days. However, now it seems the problem has become more acute, and the distortion channel is no longer working. Both the normal channel and the distorted channel lights LEDs do not work when the amp is acting up like this. I opened up the amp to see if I could see any solder joints which may have been broken, and I found a cement (sand?) resistor that appeared to be disintegrating as there was powder below it on the PCB board. It is near where the distortion channel knob for master volume is located and thought that may be the problem. I am not an amp tech, but I can replace a component here and there if I know what to replace and how to diagnose it. However, I am a complete amateur/novice at electrical repairs. Any help would be greatly appreciated.

Tassieviking

Be very very careful in that amp, there is 444 volts DC going to the valves and the capacitors can still have voltage in them even when the amp is unplugged.

I have one version of the schematic only for the RG50.
Is there a reference number for that resistor on the PCB ?
You cannot view this attachment.
There are no stupid questions.
There are only stupid mistakes.

g1

Is this the later tube version (TC) or is it solid-state?
I have a feeling you may be here because it is solid-state.  :)

Attached is the SS version schematic

LBeast

Thanks for responding and supplying those schematics. This is a SS version RG50-112. Since I am a novice, schematics are very much Greek to me. However, I can try to take a pic of the board tomorrow and highlight the cement resister which appeared to be shedding material; that way if you already know your way around the circuit, you can possibly pin point it on the schematic for me so I can possibly learn how to read schematics somewhat. I thank you both in advance for valued assistance.


LBeast

Rg50-2

Having difficulty uploading pics. I suppose I have to host the pics somewhere else and add the link in. I guess it is painfully obvious I haven't been on a forum in years. 😜 Hopefully you see the links I posted.

saturated

Hi LBeast
go to preview post and try to do it from there
(I didnt figure this out somebody else did possibly G1)

good luck with your amp and welcome to the forum
I ask stupid questions
and make stupid mistakes

criticism, critique, derision, flaming, verbal abuse welcome

phatt

Hi LB, and welcome,  8)
If the Channel led indicators are not working then the issue may well be in the switching circuit, (RL1 at top of drawing)
The relay contacts maybe faulty or power not going to B (the 1k5 1W resistor)..
 Should be over 50VDC at B. Then at the relay should be 12VDC controlled by the 12V Zener.
Oh and your sand box resistors are fine, that is just the filler crumbling nothing to worry about.
HTH, Phil.

LBeast

Hi Phil,

Thanks for the tips. I think you are referring to the RG50TC schematic, the RG50-112 that I have is SS and was built in 1990. It has a 1/4" jack for the footswitch instead of the old style 6 prong one my RG100ES has. Not sure what changed between the SS to the tube version.

g1

No, he is talking about the SS version.
Look how the footswitch jack connects to a point marked 'X'.
That X goes to the relay he mentioned.

LBeast

Hi G1,

Ah okay. Thank you for correcting me. I just couldn't find RL 1 on the SS schematic, because I was looking at the wrong schematic, though I did find it on the TC schematic. My fault. Showing my noobies. I'll take a look again at the proper RG50-112 schematic. Thanks for zeroing in on it for me above.

Tassieviking

If I have the facts right, the following happens:
1.Distortion channel stops working but amplifier still works but only on the normal channel.
2. When this happens both the normal and distortion LED's go out.

When you want to use the distortion channel you have to pull the Treble knob out, or use the foot switch.
The relay operates and switches the amp to the distortion channel, the relay determines the channel that is used.
power is lost to both the LED's and the relay when fault appears, pointing to the 1k5 1w resistor to me.

I would suspect that the power to the relay goes out, possibly a bad solder joint or a faulty 1k5 1W resistor that goes to the relay from the +58v rail.

It is strange that you are worried about that power resistor that is also 1k5 that is dropping all that powder, there are only 2x 1k5 resistors on that schematic that I can see and they should be 1 watt.

In a previous post the same component was questioned.
https://www.ssguitar.com/index.php?topic=4596.msg36810#msg36810

I think you should try to trace where the 1k5 resistor is that feeds the relay, and then replace it. also re-flow all the solder joints in that part of the circuit.

If you take the PCB out and take some photos of the bottom tracks and the top of the PCB we might be able to help you trace it better.
Inspect the solder joints with a jewelers loupe or strong magnifying glass and you might see the problem.
I have 3 jewelers loupes I use for that job, 10x, 20x, 30x magnification I got from E-bay for dirt cheap. Best buy I ever did.

Edit: I think the red and green LED might be swapped around in the schematic as well.
Cheers
Mick
There are no stupid questions.
There are only stupid mistakes.

g1

Quote from: Tassieviking on November 24, 2023, 01:53:47 AMIt is strange that you are worried about that power resistor that is also 1k5 that is dropping all that powder, there are only 2x 1k5 resistors on that schematic that I can see and they should be 1 watt.
I may have missed it, but I don't think he mentioned a value for the cement resistor?
There are several 7 watt resistors in the circuit and I assumed he was speaking of one of those.

LBeast

Quote from: Tassieviking on November 24, 2023, 01:53:47 AMIf I have the facts right, the following happens:
1.Distortion channel stops working but amplifier still works but only on the normal channel.
2. When this happens both the normal and distortion LED's go out.

When you want to use the distortion channel you have to pull the Treble knob out, or use the foot switch.
The relay operates and switches the amp to the distortion channel, the relay determines the channel that is used.
power is lost to both the LED's and the relay when fault appears, pointing to the 1k5 1w resistor to me.

I would suspect that the power to the relay goes out, possibly a bad solder joint or a faulty 1k5 1W resistor that goes to the relay from the +58v rail.

It is strange that you are worried about that power resistor that is also 1k5 that is dropping all that powder, there are only 2x 1k5 resistors on that schematic that I can see and they should be 1 watt.

In a previous post the same component was questioned.
https://www.ssguitar.com/index.php?topic=4596.msg36810#msg36810

I think you should try to trace where the 1k5 resistor is that feeds the relay, and then replace it. also re-flow all the solder joints in that part of the circuit.

If you take the PCB out and take some photos of the bottom tracks and the top of the PCB we might be able to help you trace it better.
Inspect the solder joints with a jewelers loupe or strong magnifying glass and you might see the problem.
I have 3 jewelers loupes I use for that job, 10x, 20x, 30x magnification I got from E-bay for dirt cheap. Best buy I ever did.

Edit: I think the red and green LED might be swapped around in the schematic as well.
Cheers
Mick


Tassieviking,

That explanation is kind of it in a nutshell. I will pull the PCB out and look at the hidden top side of the board to see if there are any bad cold solder joints; crossing my fingers that re-soldering any possible bad solder joints does the trick. If not I'll try to go further to trace the bad component.

The sand resistor that dropped powder is a RGA 1.5k ohm 5% CW5 which stands on one end located toward the front of the amp near the master volume control. Again thanks for the help.

LBeast