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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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Tassieviking

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


There are no stupid questions.
There are only stupid mistakes.

g1

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.

LBeast

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.

LBeast

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!

Tassieviking

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.

You cannot view this attachment.
There are no stupid questions.
There are only stupid mistakes.

LBeast

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.

You cannot view this attachment.

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?

Tassieviking

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.
There are no stupid questions.
There are only stupid mistakes.

Tassieviking

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
There are no stupid questions.
There are only stupid mistakes.

LBeast

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.

Tassieviking

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
There are no stupid questions.
There are only stupid mistakes.

Tassieviking

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
There are no stupid questions.
There are only stupid mistakes.

LBeast

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.

LBeast

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.

Tassieviking

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.
There are no stupid questions.
There are only stupid mistakes.

LBeast

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.