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Randall Century 100 schematics?

Started by cin, December 13, 2021, 07:36:32 PM

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For the caps, set the DVM on AC volts (preferably AC mV) and read the AC voltage *across* the caps, from one plate to the other with power on. The AC voltage should be 0.0 since caps pass AC. If the AC voltage is higher than a few mV then the cap is leaky and should be replaced. You will need a DVM that can measure relatively low voltages (i.e. not the cheapo ones). The absolute DC voltages on one plate won't tell the whole story. There is not a particular voltage threshold above which the cap is considered bad (thus the vague "a few mV") but if it is in the 100's of mV or in the volt range, then it's definitely bad.

The nice thing about the AC voltage test is that you can set the DVM and then very quickly test every cap on the PCB. My old boss would blindly test every cap (even the little 0.1uF bypass caps on op-amps and especially logic chips) as soon as he opened a unit, and very often had the device fixed within a few minutes since this is such a common failure point. For the big electrolytics in the PS, you can usually hear the 60 or 120Hz noise from the speaker as well.



That's a great trick, Will. I went through all the ElCaps with my good MM on AC V and there's no AC voltage across any of them. I checked the film capacitor next to the phones jack (or is it a paper cap?) and it leaks about 30mV, but then I'm not sure those caps are polarized, so that might be normal.

I may be wrong, again... about the phones jack. So all pins on the phones jack have continuity to ground (duh), but the signal pins are at -4mV compared to ground, and test with 7.1 Ohm of resistance to ground, so I don't think they are "grounded". I was expecting the ground pin of the phones jack to have very little resistance to ground (to be actually "grounded"), but it actually measures at 10 Ohm. And.. looking again the datasheet for that jack, the pairs of signal pins are connected, I believe, so then the second wire is probably not for switching purpose but actually the signal coming in to the jack, then leaving for the speaker.

Would bypassing the phones jack, and cap/resistor to ground assembly, be a worthwhile test?
Or, should I be following the buzz upstream on the signal path?
Or, should I be looking for something that's connected to ground that shouldn't be?

I'm probably thinking about this wrong, so yeah, your help is greatly appreciated hahah.

Cheers, and a happy new year to you all!

EDIT: Ok, so when I said the "ground pin" of the phones jack I meant the sleeve pin. Turns out that's probably not ground or it would be connected to the reverb knob chassis right beside it... I don't yet understand the use of negative voltage in DC circuits, outside AC for signal and such. Can you tell?  8)


 @*Will* thanks good call.
@ *Cin*
As the DC voltages read normal and if the Caps are ok then you are back to tracking down what seems like a grounding issue.
(Yes Elcap means Electrolytic Capacitor)

Other possible causes,
Check that there is continuity all they way from the Earth pin on the power plug through to the chassis and onto the sleeve contact at the input socket.
If ok then try another power point in the house just in case that power point is faulty.
(I recently got caught with an intermittent failing power point at my place which had me chasing my tail for a few weeks)

There is still the possibility the reverb is not right.
Although you checked the wiring, if the internal ground connection of the tank has broken it can induce hum.

It looks like the reverb utilizes one dual opamp for Drive and Pickup so you could remove that chip which (As I see it?) will render the reverb out of the circuit. A sure way to find out if the reverb is causing an issue.
The pic with light behind was helpful except there is a pot over the reverb section, so hard to tell.

As to the Headphone socket, I can't see clearly but looks like Tip and sleeve are tied so you get mono sound through both ears when TRS phone plug is used, While disconnecting the internal speaker.
As to the cap/resistor from socket to pot. Hum??  :-\ well maybe that is a poor attempt at what could be a Zobel network. (google it if you're not sure what that means)
From my reading A zobel network should be as close to the output of the LM1875 power chip as possible.

And yes if you don't use the phone plug you can bypass it but that Zobel setup would need to be rewired.

Be aware that the Neg speaker node may not be at ground potential as a lot of these amps use what is called current Feedback. That is likely the large Cement 3Watt resistor near the Lm1875, can't read it but likely 0.22R or 0.27R
Sorry although the pictures help without more info it's hard to trace where the wiring goes.

Regards to the single/dual supply Q;
If you have a circuit that runs on a 30VDC single supply then Obviously zero Volt is common and often Ground/Chassis. So you have 30volts potential to swing the audio signal.
If you have a dual +15/0/-15VDC supply then all you have done is move *Common* to the middle of  what is still a 30Volt potential.  The Audio cares little as it's just floating on a DC potential.

Whenever you look at Amp schematics like yours you are actually looking at 2 circuits intertwined.
There is the DC parts and then there are the AC parts.
You setup the DC bias points through each stage so as to pass the best Audio.


So you have found out that a continuity 'beep' is not the same as a short.  That is a good thing to know, as it leads to many errors for novice users.  Different brands have different ranges that will cause the beep, some anything below 40 ohms, some as high as 200 ohms!
If you know the spec for your meter, continuity function can be useful.  But usually for things like looking for an automotive wire that is shorting to the chassis or something.
When reporting resistance always use resistance range.

The 7 ohms you measure at the headphone jack will be the speaker in parallel.  And the common way to disconnect the speaker for headphone use is to disconnect the ground side of the speaker via a switch contact in the phones jack. (with attention to what Phil said about current feedback resistor)
If you have an empty TRS plug to put in the phones jack, it might help with the resistance readings and wiring tracing.


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.



OR, or, I will repair this tiny coil, because apparently 8FB2A1B reverbs are hard to find and expensive....


Quote from: cin on January 15, 2022, 07:16:50 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...

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.


Quote from: phatt on January 15, 2022, 08:38:23 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 -_-


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.


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!


Quote from: cin on January 15, 2022, 07:16:50 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.   ;)


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.


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.



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.


Good morning!

The 2 red wires coming out of the power tx do go to the PCB. And the yellow wire coming out of the power tx does indeed go to the chassis.

The ground trace on the PCB leads to the mounting screws that go into the chassis. The PCB definitely has to be mounted for the amp to work. So I've been screwing the PCB and the pots back in every time I need to test it.

The input socket is grounded to the chassis, but then so are most of the pots? I unscrewed the input jack and left it floating (everything else grounded) and the buzz is half as loud as when the jack is screwed in, but not gone. So this could be part of the problem, but not the only source of buzz.

I tried unscrewing the headphones jack and reverb pot from the chassis ground too, and the buzz is still there. I unscrewed the treble and volume pots, buzz is still there.

Poking around the pots, the treble pot, which is a 50K linear pot that's bigger than all the other pots except the volume pot (so maybe a replacement?), is especially sensitive to being touched, I can hear changes in the buzz when I touch the metal parts. The solder joints look fine at bother ends, but I might just redo them to be sure.

So a ground loop can be caused by many paths to ground? I thought it was mostly about some section of the ground trace actually having unexpected voltage potential. There are many paths to the chassis ground on this PCB. The 3 mounting screws, the input jack, most of the pots seem to be grounded via the chassis, etc.

Still on first coffee, might make more sense after the second cup.