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.
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 ?
Randall_rg50tc.pdf
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
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.
Rg50 (https://acrobat.adobe.com/id/urn:aaid:sc:VA6C2:e98d2ee1-ddab-4b56-ba29-88f4ac3a7be0)
Rg50-2 (https://acrobat.adobe.com/id/urn:aaid:sc:VA6C2:43160994-ca27-47ff-9487-c222942f10a6)
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.
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
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.
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.
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.
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.
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
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.
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.
Here are some pics of my RG50-112 guts for errbody.
The picture was in post #4, I had to open it in a new window and then download it before I could see it.
It is the resistor standing up near the front panel, it is circled in blue.
I think it is 1k5 5 watt, it is marked "1k5 ohms 5% CW5"
He has re-posted the picture in post #8
I think the small square blue thing behind it is the relay for the channel switching.
There is a chance that with the resistor standing up like that the solder could develop cracks and cause a bad contact just from the vibrations from the speaker also.
The link I posted in post #8 shows a much smaller component so I wonder if that big resistor is not original, it makes no sense to mount a large resistor standing up like that. Too much weight relying on the solder to support it, it's bound to suffer from mechanical failures with time.
Plus the schematic says 1 watt and that looks like a 5 watt resistor to me.
If you use a torch to light up under the PCB you can sometimes see the copper tracks from the top, see what it connects to.
The link in post #8 shows a partial picture of the tracks as well.
Cheers
Mick
Ok, now I can see the pictures in post #14. The ones in post #4 and #5 just open a blank page for me.
To drop the 58V down to the 12V zener (46V drop), that 1K5 has to run at 1.4W, so the 1W on the schematic is wrong. My guess is even a 2W there would run pretty hot and maybe cause some browning of the board, so they just went with the 5W.
Quote from: saturated on November 21, 2023, 09:50:54 PMHi 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
Much thanks to Saturated for helping me figure out the attachment process.
So here is a picture of my working RG150ES head PCB board, also built in 1990. I have never opened this head before. I see the same powder on the same resistor. I did this to determine if the resistor in question in the RG50-112 was factory. From this picture it appears that it is, as both 50 watt PCBs are most likely identical.
BTW, this head sounds amazing with an overdrive pedal to push the front end. I play it through a Marshall 4x12 with English made greenbacks. Mmmmmm.........Butter!
That 5W resistor must be original if both have it I guess.
I had a quick look at the PCB track picture I found on SSGuitar, I painted in the relay and resistor to see how it was set out.
I think the small relay might be similar to an Omron G5V-1 relay.
Omron-g5v_1 relay.pdf
Quote from: Tassieviking on November 26, 2023, 12:24:51 PMThat 5W resistor must be original if both have it I guess.
I had a quick look at the PCB track picture I found on SSGuitar, I painted in the relay and resistor to see how it was set out.
I think the small relay might be similar to an Omron G5V-1 relay.
Omron-g5v_1 relay.pdf
Tassieviking,
Thanks! That is some jam up PCB detective work.
Any ideas on where I could purchase a new relay if it is the culprit in this situation?
That is a very common relay so most mayor electronic companies have them.
Mouser, RS-Electronics, Element-14, Digikey, etc. just google G5V-1 relay.
If it was the relay not working then the LED's should still work.
I still think you are loosing power somewhere between the 58V (B)and the earth end of the 2k2 resistor.
If I am correct that the LED's are swapped on the schematic and terminal X has to be connected to earth for the overdrive to be on then the problem would be between the 58V and the grounding on the treble pot switch or the foot switch jack.
Are you using the treble pot or the foot switch to select the overdrive channel ?
If you are using the treble pot then try the foot switch instead and see what happens.
If you do not have a foot switch then insert a normal mono plug into the footswitch jack as this should work as well. (guitar lead will do)
Try the opposite of what you have been using to try to eliminate the fault.
It might be a bad part or component, but more likely a solder joint that has gone bad over time from vibrations.
I redrew the schematic around the channel swiching so you can see it all in one place.
I included the red wire so i put 2 connectors in the circuit.
It might be easier to follow from the switches to +58v
Quote from: Tassieviking on November 27, 2023, 08:06:25 AMThat is a very common relay so most mayor electronic companies have them.
Mouser, RS-Electronics, Element-14, Digikey, etc. just google G5V-1 relay.
If it was the relay not working then the LED's should still work.
I still think you are loosing power somewhere between the 58V (B)and the earth end of the 2k2 resistor.
If I am correct that the LED's are swapped on the schematic and terminal X has to be connected to earth for the overdrive to be on then the problem would be between the 58V and the grounding on the treble pot switch or the foot switch jack.
Are you using the treble pot or the foot switch to select the overdrive channel ?
If you are using the treble pot then try the foot switch instead and see what happens.
If you do not have a foot switch then insert a normal mono plug into the footswitch jack as this should work as well. (guitar lead will do)
Try the opposite of what you have been using to try to eliminate the fault.
It might be a bad part or component, but more likely a solder joint that has gone bad over time from vibrations.
Thanks for the relay info. Sounds like it may not be the problem, as you surmised.
I was using the amp with treble pot pulled out. I thought the same as you, that I could possibly use a foot switch instead to switch channels, so I tried that and there was no change in operation; still no LEDs/distortion channel.
I have been busy with other more serious life related things this week and may not get to firing up the soldering iron until this weekend. I also need to get a loupe or other type of magnifying equipment to try and track down any breaks in solder.
I really appreciate all you have done so far. You really know your stuff about electronics and schematics. That was a killer relay schematic you drew, I assume using some sort of CAD program. You really went above and beyond there. Looks like drawings my dad use to do when he was a designer using CAD. Sweet.
I use KiCad to make schematics, I use it mainly to make up PCB's for stomp boxes and amps i want to build for myself. It is freeware so it costs nothing and is really good and easy to use.
I know the basics of audio circuits but nowhere near as much as many others on this forum, I'm just a retired electrician so I know more about industrial electronics.
cheers
Mick
Would you be able to check the reverb tank for the model ?
I might build myself one of these and the reverb tank information is not on the schematic.
If you could provide all information you can see on the tank I should be able to work it out.
Thanks
Mick
Quote from: Tassieviking on December 01, 2023, 12:42:31 PMWould you be able to check the reverb tank for the model ?
I might build myself one of these and the reverb tank information is not on the schematic.
If you could provide all information you can see on the tank I should be able to work it out.
Thanks
Mick
Hey Mick,
Be glad to. I'll pull her apart tomorrow and send you some pics/info. It is an Accutronics brand just like Fender uses. That much I know of off hand.
The reverb tank is an Accutronics 9" model#: 1FB2A1B. The stamped model number on the reverb tank doesn't show up well in the photo. Here is the pic of the back of the tank.
Thanks for the Reverb Tank info, I am definitely putting this amp on my to-do build list.
If you have the lid of the amp you could measure the voltage across the 12v Zener diode, I think it's the one behind the big resistor so it might be a bit tight to get at.
You can also tap some components in that area with a plastic or wood chopstick to try to make the LED's flicker on, it is an easy way to look for bad connections.
Make sure the chopstick is plastic or wood only and it should be safe, plastic is the preferred type of course.
No problem.
I opened up the amp, removed the board, and took some pics (see attached).
It appears to me that this amp has had some work done to it previously. The 2" green wire was very curious and the scrub through the board below where the wire attaches to the relay area made me go "Hmmm".
Let me know your thoughts.
As far as measuring values, I'm not gonna lie, never done it before and am a bit wary of doing that on a live amp for fear of frying myself. I did try reheating some of the solder joints which appeared to possibly have issues, but the amp has been sitting unplugged for a week so I was pretty sure the caps were no longer live. Nothing changed in the amp when I put it back together. So it didn't hurt anything, but I also didn't make any headway really.
That is one of the better pictures I have seen of the track side of a PCB, it really helps to see the shadow of the components through the board.
I often use a torch to shine through the board when I take the photo.
Would you be able to take some good clear shots of the component side, it helps to see where the tracks go to.
Perhaps you can open your 150 amp and see if the same link is on the bottom of that PCB ?
It seems to be the same amp basically and the PCB seems to be the same type.
If you do that then you might as well take the photos of that amp.
Sorry for the delayed response. Been battling Covid for the past couple of weeks. See if this component side pic will work.
I don't think that green wire under the PCB is at fault, they just relocated the wire from the relay to the Master Volume pot.
The wire goes from the relay to a point after the Gain Pot in the Distortion Channel.
It goes to a junction between a 1k resistor, JFET, 22pF capacitor (20pF).
I am still tracing it slowly for you.
This is what I have come up with from the pictures of your RG50-112 PCB.
I copied a picture of the tracks (mirrored) into KiCad, drew up the original schematic and made up the components.
I had to give the components names in KiCad so I made them up as I went. (Ie. R1, C1 etc)
I placed the components on the picture of the PCB tracks and drew the tracks in as close as I could.
I did find some differences compared to Randalls schematic so I changed my schematic to match what was on the PCB.
I think my version might be helpful for anyone who is trying to fault-find one of these amps as the components have names on the schematic and on the PCB drawing.