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January 22, 2022, 07:13:59 PM

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31
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.   ;)
 
32
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!

33
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.
34
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 -_-
35
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.
36
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....
37
If the speakers are not in a box then Bass will be non existent.
Especially being that they are H phone speakers (As Enzo noted)
Even a small cardboard box will improve the bass a little.
Phil.
38
Schematics and Layouts / Re: Randall Century 100 schematics?
« Last post by cin on January 15, 2022, 07:16:50 PM »
Alright, I checked the ground continuity from the earth pin of the power plug to the chassis, and to the sleeve of the input jack, and it's fine. Then I tried another power point, but the buzz is still there. Good things to check though. You wizards are teaching me to troubleshoot and that's awesome.

Instead of removing the reverb opamp, I did a little signal tracing with my oscope. But I quickly got lost after the first opamp, where the signal goes through the channel 2 overdrive pots, which are switched off to ground by the channel switch set to channel 1... So I randomly checked a few points to see if the signal was clean or dirty, and then I checked the reverb in and out...

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...

As for the headphone jack, it is wired with the tip and ring connected, I believe that provides mono to both left and right. @Phil That's probably what you meant. I do use the jack, I would only remove it for troubleshooting purposes, and then put it back. I'd like to bring back this amp to a functioning state that's as close to the original as possible. So I'm looking suspiciously at everything that looks janky, like that Zobel cap/resistor deal on the headphones jack. If it was a Zobel network from the factory, it probably would be done better, closer to the LM1875, would you say that's a pretty good reason to think those components were an additions?

I realize the schematics would be useful in finding out what is original and what isn't...

The 3W resistor is indeed a 0.27R.

I can take better backlit photos of the board if I've jumped to conclusions about the reverb tank being at fault, but that would surprise me at this point.

@g1 Yes, I'm learning with every rookie mistake I make, but I'm having a great time. 8) Thanks for the clarifications. I now understand that. I'm not sure how the empty TRS jack would help though. So as to get the speaker out of the equation? Since the buzz is present in the headphones too that would make sense. I'm still not really sure how to proceed with the "resistance reading" and wiring tracing, I'll google that.

THANKS GANG!
39
My first build was something called the 'Noisy Cricket'.  It could work on a 9V, but I opted to use a Boss power supply.  Sounds great, and gets a little dirty as your turn it up.

I once took it to an audition with a little 1x8 Orange cab. I know it's just one watt (tops), but I thought it was going to be a low-key audition. The guy was pumping backing tracks through a HUGE PA.  I pretty much pegged that little thing, and it just about kept up.  However, he said he wanted me to play 'cleaner'... I told him to turn down his PA :)

Look it up. Fun little amp to build.

I think I have a short demo on YouTube of me using it. But, I used a dirt box the whole time..
40
Low end from tiny speakers?  Not likely.  To make bottom, the speaker cones have to move sufficiently to make the wave fronts.  That is hard for a speaker to do with a movement of a millimeter or so.
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