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January 16, 2022, 05:32:56 AM

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1
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on Today at 12:48:02 AM »
Good, then reverb is not the source of hum. :tu:

Ok I've looked at the pics all over again,,, :-\
Now one possible Ground loop I can make out is in a second ground right at the input socket.

It looks like the 2 Red ac wires coming from Power Tx go to the PCB BUT!!! the Common seems to be the Yellow wire which goes to the chassis I assume??. (remember I'm only going on what I have in front of me)

Normally the yellow wire would go Direct to the PCB.
This means the PCB has to be mounted for the amp to work.
Ok so far it's valid. BUT you then have the input socket grounded to chassis as well.
It is possible that may cause a hum loop.
I'd try mounting it all back except for the input socket which I'd leave floating, then see if the hum stops.

If so you will need to change the socket to an Isolated unit.
The original socket may have been a failing isolated one (Plastic) and replaced with a metal one.
It's all just a hunch but simple to test.

If it was mine i would rewire that (I assume) Common yellow on the PCB right between the Main Electro Caps.
Phil.
2
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on January 15, 2022, 10:10:24 PM »
I also disconnected the Zorbel bits, and it made no difference. So I reconnected them for now.

I'll do some more signal tracing past the reverb tomorrow, and take better back lit photos of that area.

Cheers!
3
Reminds me, in my shop days, when I had a speaker sitting out and wanted to test it with music.  Just a brief test.  They sound awful sitting on a table, so I would reach over and hold the speaker facing up over my trash can.  That enclosed space under the speaker gave it enough of an enclosure to see if it sounded halfway decent.
4
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on January 15, 2022, 09:50:41 PM »
LOL so much space for confusion. So, on the amp board there's no label of Input/Output, but you said earlier in the this thread:

"The output cable will be the one with the ground wire on the PCB (2 spades), while the drive cable only has one spade conn. (that is the usual setup)"

And I believe I have it right, now 8) The cable with 1 spade is connected to the In RCA connector on the Reverb tank, and the Out RCA connector is connected to the cable with 2 spades.

And I plucked the RC4558P opamp furthest away from the input out, and the buzz remains...

I was shopping for a reverb tank an hour ago. I owe you a beer.

5
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on January 15, 2022, 09:33:02 PM »
The IN is clean, the OUT is dirty.

Just to check we are on the same page  ???

The INPUT RCA on the Tank is the DRIVER end (the signal thsat excites the springs)
The OUTPUT RCA  on Tank is the Pickup end (this picks up the reverberation of the springs and sent back to be mixed with the dry signal)

So Rev drive OUTPUT on Amplifier circuit goes to the Tank Input/ Drive.
And Rev return /Pickup circuit on amplifier goes to Tank Output/ Pickup.

The idea here is to divide and conquer,, you remove as much of the aux circuits as you can to narrow down the problem.
If you remove the Rev chip and the circuit still hums then you know to look elsewhere.
Keep at it you will get there.   ;)
 
6
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on January 15, 2022, 09:26:08 PM »
Hmmm, well the buzz is there wether the Out cable is connected to reverb tank or not, and is present wether or not I short the pin of the out cable.

So the noise may not be coming from the reverb tank after all! What a roller coaster hahaha.

But how can that be if the noise is there on the out, but not on the in??? Could the noise be coming up current? like against the flow of the signal? I'm guessing that could make sense with a ground loop, where there's unexpected potential somewhere.

So, at least I know that the ground problem is downstream from the reverb tank, right? Or is that also not necessarily the case?

Many thanks for all the help!

7
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on January 15, 2022, 08:54:44 PM »
Yes,
Just like an unplugged guitar cord,, they hum a bit and when you short the tip to the sleeve the hum stops.
If there is a dodgy ground connection in the tank then this removes the tank from the circuit.
So pull the RCA plug on the *PICKUP end of Tank (output of tank)* and ground the tip to the chassis on amp, or short tip to outer ring with a screwdriver or something metal.

There is a Drive end and a Pickup End on these tanks and can be confusing.  :loco
So Drive is the input on the tank,, and PU is the Output from tank.
If you touch the drive tip on rca you wont hear anything.
The PU end is an aux input back to Amplifier so you can hear the reverb.
Phil.
8
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on January 15, 2022, 08:43:36 PM »
You said the tank was wired in reverse,,,,,,,,,Well that may well have overheated the coil. 8|

If you suspect the tank is the issue then disconnect the pickup cable and short the pickup end.

You mean short the middle pin to the sleeve on the male RCA connector that's connected to the IN of the reverb tank? Just making sure before I blow up something else -_-
9
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by phatt on January 15, 2022, 08:38:23 PM »
The IN is clean, the OUT is dirty.

So I pulled out the tank and took some measurements inside, the signal wires going into the IN coil is clean, the signal wire coming out of the OUT coil is dirty. The OUT coil does look like it may have overheated, the tape is discoloured in one spot. Is that a thing? Can a small coil overheat if it's overstimulated? Would that indicate the amp was shaken vigorously while On?

So I'm guessing I'm stuck replacing the whole tank, it's all pretty much riveted together...

THANKS GANG!
You said the tank was wired in reverse,,,,,,,,,Well that may well have overheated the coil. 8|

If you suspect the tank is the issue then disconnect the pickup cable and short the pickup end.
Now test the amp to hear if the hum/buzz is still present.
If it greatly reduced the hum then you just found the problem.

If no luck then I would just remove the reverb chip as it could be failing from driving the wrong load.
Phil.
10
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on January 15, 2022, 08:35:55 PM »
OR, or, I will repair this tiny coil, because apparently 8FB2A1B reverbs are hard to find and expensive....
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