Randall RG-75 Has me confused...

[unsworthk]

Hi all,

Thanks for allowing me to join the forum.

I am currently struggling with a Randall RG-75 preamp board. The problem I have, is that when I plug the guitar in, the guitar signal does not seem to be making it very far through the preamp. If I tap the spring reverb, I can hear just the reverb which proves that the pre is good from there and beyond.

The strange part is, when I remove the jack connector from the PCB (Neutrik 2-pole switching socket), and solder the guitar cable straight to the board, on the side that the shunt disconnects the guitar from, it works! So there is some fault between the guitar being connected as it is supposed to be, before the reverb circuit.

But it doesn't work with the jack socket soldered in normally.

I looked at the schematic of the amp and it seems to me that the jack socket is shunting AWAY from where the signal should be going? How is this the case? Am I missing something fundamental here?

Please tell me if I am being an idiot!

Many thanks

Kieran

[Jazz P Bass]

Is there a headphone jack?

A Headphone plug inserted will open the speaker circuit.

Check the HP jack.

[g1]

Are you sure the jack is not bad?
The shunt does not disconnect anything, it grounds the input when nothing is plugged in.
Please post the schematic you are using.

[Enzo]

Did this amp EVER work as it is? Or did you first receive it already in this situation?

Do I understand correctly? Plug into the jack and no sound, remove the jack and solder signal wires directly to where the jack was, and the amp works? That sounds like the jack is bad or wrong type.

This looks to be a "Cliff-type" jack, with the metal straps crosswise to the plug shaft for making contact. Note they come up from one side, usually with a mating contact on the other side. I have seen some brands of jack that are backwards from the basic CLiff. SO, look at your headphones jack, is it the same basic type? If so, note which side of the plug shaft the contacts go down into the board from. Is the input jack on the same side?

That covers wrong type, but the jack could simply be bad. SPrung is the usual failure. Look at the contact for the tip of the plug. Is is bent upwards so it doesn;t touch the plug tip?

[unsworthk]

Thanks all for your replies.

I checked the send and return jack inputs and theres nothing in there which has been snapped off, which would open up the circuit.

I have checked with a multimeter for continuity from the guitar end of the jack and the contacts of the socket, and they are making it through to the board on the side which is still touching the jack when it is plugged in.

The guitar jack socket is original and nobody has swapped it for the wrong type.

The amp works when i solder the hot and ground to the opposite two contacts that the jack passes through to the board when the jack is inserted. Which makes it seem like the jack is switching on the wrong side, however it is the original jack. This is what is really confusing me.

From what I can tell so far is that there must be another circuit for the guitar signal when the jack is connected, which by soldering to the board on the other 2 contacts is bypassing. And the fault must lie there.

I have ordered some batteries for my tone generator so that I can trace the signal path with a scope.

I have also tried another like for like jack socket and the problem is the same.

Sigh!

[nevetslab]

Please post the schematic you are using.

While waiting for your batteries for your generator, does the amp's residual noise increase when you plug a cable into the connector (other end not connected anywhere), and turn up the volume, tone controls to hear the noise. Does it change when you unplug the cable? If you hold the tip of the end of the cable that's plugged in, does it make the amp hum? Very peculiar getting it to work without the jack installed, cable soldered into the jack Output path.

With the guitar cable plugged in, measuring from the jack Tip connection to ground, is it measuring as a short or a medium-high resistance (circuit that follows the jack)?

What happens when you plug into the Return jack of the insert send/return?

[g1]

How do you know the jack is original? You are the original owner and it has never been worked on?
Or it was working before with this same jack?
Post some pictures if you can.

[g1]

Like Enzo said in post #4, some look almost exactly the same, but have the contacts reversed from left to right.
What he said rang a bell, and I ran into the exact problem on a Randall before:
https://music-electronics-forum.com/...ad.php?t=43863
The solution I posted there in post #17 has since changed the photo. I'd check with them that the contacts are reversed from normal before ordering.

[nevetslab]

Like Enzo said in post #4, some look almost exactly the same, but have the contacts reversed from left to right.
What he said rang a bell, and I ran into the exact problem on a Randall before:
https://music-electronics-forum.com/...ad.php?t=43863
The solution I posted there in post #17 has since changed the photo. I'd check with them that the contacts are reversed from normal before ordering.

Somehow, I think this amp NEVER did pass signal, now seeing these jacks in your previous post. I've never been burned by that one, but good to know there's something like that out there to really mess with your head.

[unsworthk]

Ok so after some inspection. Y'all and my initial mind**** are correct.

There is a picture on the god damn board which shows which side the shunts are, and this jack has them on the opposite damn side!

I mean I am no electrical genius by all means but when I saw that the jack was shunting away from the input I did wonder whether I should just quit altogether.

and also this jack has shiny contacts on it as opposed to the others which are slightly tarnished.

That said, does anyone know what jacks these amps use for their input?

Thanks

Kieran

[Enzo]

So other jacks on the board seem original? Look on one of them for a brand. Even if you have to remove it to look. Cliff. Rean, Neutrik, WangFoo?

[unsworthk]

Hi Enzo,

I think they are Cliff looking at the style.

I actually just read the link you posted and managed to flip the pins on the Neutrik to match what the original jack was like, success!

Thanks so much for all your help guys, I really appreciate it.