Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Schematics and Layouts => Topic started by: cin on December 13, 2021, 07:36:32 PM

Title: Randall Century 100 schematics?
Post by: cin on December 13, 2021, 07:36:32 PM
Hi. I'm new here.

I'm getting into amp repairs, and I have a Randall Century 100 (c-100), from '92, on the bench right now. I really could use the schematics to help me along, and all I can find is for the RG80 or RG100 series. There may be similar elements but they're different enough to be confusing to a noob such as myself.

Cheers!
Title: Re: Randall Century 100 schematics?
Post by: DrGonz78 on December 14, 2021, 01:34:01 PM
What are the symptoms with the amp? A roadmap helps repair an amp with a major fault but sometimes we get by without the schematic. Only thing I could find Century related was a 200ii preamp schematic.
Title: Re: Randall Century 100 schematics?
Post by: cin on December 19, 2021, 01:58:12 PM
Thanks!

The amp arrived to me with a loud buzz that's independent of any knob.

I found it had been modified, a capacitor had been added to a resistor in series but that solder job had broken at one end, so neither was connected. So I removed the extra capacitor and reconnected the resistor, but that didn't help the buzz, surprisingly.

I tapped around with a chop stick and found the ceramic capacitor just next to that mod to be very sensitive, so I replaced it. That made the buzz throb.

So I'm not sure what the problem is. The PCB doesn't have any markings on it, and I'm not experienced enough to understand the circuit, though it looks pretty simple. My next step would be to follow the buzz along, from the output jack in, with an oscope. With the schematics I could make sure there's no other mods or missing bits: some holes on the PCB don't have components in them, but the contacts are soldered, and it looks like from the factory.

Thanks for the help!
Title: Re: Randall Century 100 schematics?
Post by: cin on December 19, 2021, 03:50:32 PM
Looking at this schematic, I think the area I just tinkered with is different, on my c-100:
- The 0.005uF cap I replaced is connected directly from the treble pot to pin 5 (2IN+) of one of the RC4558P, no 22KΩ resistor.
- The 6.8KΩ resistor that was connected to the extra 1uF capacitor I removed, is the second resistor before coming into pin 6 (2IN-) of the same RC4558P. The resistor closest to the pin being a 100KΩ. Not 4.7KΩ and 470KΩ as per the c-200 schematics. And no 0.22uF cap between those and the reverb pot (there's no reverb footswitch either).

So I'm not sure there's much I can get from it.

I found another potential modification around the phone jack, it looks like it may have been replaced, the ground pin has a leftover leg in the solder. And the reverb pot is connected at the chassis, to the signal pins of the phone jack, via a .1uF cap and a 10Ω resistor, what is that about? I can't find a phone jack on the c-200 schematics, I'm assuming it's on the other page.

Recommended course of action?

Cheers!
Title: Re: Randall Century 100 schematics?
Post by: phatt on December 19, 2021, 09:25:52 PM
Quote from: cin on December 19, 2021, 01:58:12 PM

The amp arrived to me with a loud buzz that's independent of any knob.
Forget all the mods for now because you most likely have a power stage failure.
Set your meter to DC and measure the DC voltage at the speaker terminals.
It should read very low DC voltage i.e. less than 1/2 a volt, the less DC the better.
If the DC reading is high turn off the Amp and disconnect the speaker as you could burn out the Vcoil.

Report findings and the minds here will be able to steer you in the right direction. :tu:
Phil.
Title: Re: Randall Century 100 schematics?
Post by: cin on December 20, 2021, 12:14:15 PM
Quote from: phatt on December 19, 2021, 09:25:52 PM
Set your meter to DC and measure the DC voltage at the speaker terminals.
It should read very low DC voltage i.e. less than 1/2 a volt, the less DC the better.

Thanks for the tip. I did just that and found only -0.004V DC across the speaker terminals with the speaker plugged-in and buzzing.

However, while I was trying to do that with the amp only partially assembled, on cardboard to prevent shorts, I found that the amp wasn't making any sound at one point and then the PCB shifted and made proper contact with the chassis at the ground plate, which caused a small spark at the chassis, and then the buzz was back while the chassis was in contact. So I'm thinking I may have a ground problem? Or it normal for amps not to work unless they have physical contact with the chassis?

Cheers!
Title: Re: Randall Century 100 schematics?
Post by: phatt on December 20, 2021, 09:25:42 PM
Quote from: cin on December 20, 2021, 12:14:15 PM
Quote from: phatt on December 19, 2021, 09:25:52 PM
Set your meter to DC and measure the DC voltage at the speaker terminals.
It should read very low DC voltage i.e. less than 1/2 a volt, the less DC the better.

Thanks for the tip. I did just that and found only -0.004V DC across the speaker terminals with the speaker plugged-in and buzzing.

I found that the amp wasn't making any sound at one point and then the PCB shifted and made proper contact with the chassis at the ground plate,


Great news 4mV is a good sign.
Q,
At any time while the chassis was not contacting the pcb did the amp pass any audio signal?

Likely there is a ground loop issue, as to exactly what/where is anyone's guess.
Ground plane issues are a pain as there are just so many ways one can wire up the grounding.
Schematics are helpful but grounding is always implied, it's left to the layout design.
The old style point to point wiring is often easier to decipher when trying to work out ground paths.
With PCB it can be a jungle tracing all the possible copper tracks back to ground.

Could be someone has worked on the unit before and forgot to connect something or connected the wrong points.

Quote from: cin on December 20, 2021, 12:14:15 PM
Or it normal for amps not to work unless they have physical contact with the chassis?
Cheers!

Depends on the design,,, but Normally yes if Circuit is NOT grounded to chassis there will be more hum/buzz.

Be aware Some PCBs use a mounting post as ground back to chassis.
While others might Isolate that point with a fiber washer.  (again, depends on the layout design used)
I've been caught once forgetting to put the isolation washer back which gave me a hum and baffled me for days.
Sorry i can't be more help others here might be able to help.

If you can take a picture of the PCB (both sides) and a pic of Chassis as best you can to show what we are looking at might help.
Phil.
Title: Re: Randall Century 100 schematics?
Post by: cin on December 22, 2021, 05:49:04 PM
Quote from: phatt on December 20, 2021, 09:25:42 PM
Q,
At any time while the chassis was not contacting the pcb did the amp pass any audio signal?

If you can take a picture of the PCB (both sides) and a pic of Chassis as best you can to show what we are looking at might help.

No, I don't think there was any audio coming out until the ground plate made contact with the chassis, then the buzz returned with the signal. Looking at the PCB I'm pretty sure the ground goes to the chassis.

Here's some photos of it all, I can provide more, or closeups, if need be.

You're all being very generous with your help, thanks!


Title: Re: Randall Century 100 schematics?
Post by: phatt on December 23, 2021, 09:07:09 AM
Hi Cin,
Well done, Good pics and the light behind is a big plus. :tu:
Arrh huh, You have a spring reverb tank.  They can cause a big hum if someone plugged it in reverse.
The tanks are often marked input/output so wise to check them.
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)
If that is correct and you still have hum then use the meter to check ground continuity from the tank case all the way back to the cable and PCB spades.

Also sometimes the tank picks up the mains hum from the power transformer and you have to remount it away from the Tx for less hum. Make sure the PU end of the tank is the furtherest away from the TX as that end picks up the hum.

Phil.
Title: Re: Randall Century 100 schematics?
Post by: cin on December 25, 2021, 02:11:45 PM
Good call, I had plugged the tank back in backwards... With it connected properly the throb is gone, but not the buzz. Continuity checks fine between the ground connector of the tank and the spade on the PCB. The tank is mounted on the opposite corner from the transformer, I moved it even further but it didn't seem to change anything. The buzz is also present in the phones jack, even when the amp is disconnected from the speaker and reverb tank.

So, the 1st step in untangling this, if it's a ground problem, would be to identify all the areas of the circuit that should be grounded, and make sure they are indeed at 0V with continuity to the ground prong of the power cable?

Googling around, it sounds like chasing amp problems is a bit of a dark art. I have so much to learn.

Thanks again for the assistance.
Title: Re: Randall Century 100 schematics?
Post by: phatt on December 27, 2021, 06:17:05 PM
Hi Cin,
Yes many factors can cause the issue.
If the Ground/Common has many return paths it can cause what is called a ground loop,
often picks up the 50Hz hum.
If it's more of a buzz it might be a shielding issue.

Is the Buzz still present with guitar plugged in and the volume knob on guitar set to Zero?
Also does the buzz increase as you turn up the gain knob on the amp?

Just be aware that a lot of amps will have some residual noise and in a QUITE room near computers, Fluro lights and many other sources of RFI/EMI it will be noticed.
If the buzz/hum is louder than your guitar playing then yes there is a major problem, other wise you may have to live with it.
These are the low budget models and unlikely they do much R&D 8|
Phil.
Title: Re: Randall Century 100 schematics?
Post by: cin on December 29, 2021, 09:16:36 PM
Quote from: phatt on December 27, 2021, 06:17:05 PM
Is the Buzz still present with guitar plugged in and the volume knob on guitar set to Zero?
Also does the buzz increase as you turn up the gain knob on the amp?

Just be aware that a lot of amps will have some residual noise and in a QUITE room near computers, Fluro lights and many other sources of RFI/EMI it will be noticed.
If the buzz/hum is louder than your guitar playing then yes there is a major problem, other wise you may have to live with it.

The buzz is present when a guitar is plugged-in and the volume set to zero.
The buzz does not increase with the gain.

It is way louder than a residual noise, it's quite loud, louder than the guitar.

Thanks!
Title: Re: Randall Century 100 schematics?
Post by: phatt on December 29, 2021, 11:25:35 PM
Well as the output is stable then read the DC voltages that supply the preamp.
Those 2 rails are regulated via 2 Zener diodes and filtered via 2 ElCaps.

Using the first picture posted they are top left corner next to the mounting post at the edge of the PCB. The 2 small Blue Elcaps are just near the Silver Zeners.

Use your meter to check you have both rail voltages +12v & -12v. they will likely be 12 Volt but could be 15V rails, both rails should be very close to the same.
likely easy to check voltages at the pig tails on the Blue ElCaps rather than the Zeners.

If a Zener is blown then the voltage will be almost equal to the main voltage which can be read on the bottom side of those 470R resistors below the zeners.
Also the Capacitors could be dying causing hum.
Phil.
Title: Re: Randall Century 100 schematics?
Post by: cin on December 30, 2021, 11:25:55 PM
I'm assuming ElCap is short for "electrolytic capacitor" and not another specific type of cap, or brand.

One of those 2 ElCaps has its anode to ground, the other its cathode to ground. Comparing to chassis ground, the ElCap that has its anode connected to ground reads -12.36V at the cathode, the other reads 12.34V at the anode. At the Zeners I find 12.32V and -12.35V. The resistors by the Zeners read 12.34V and -12.38V on one side and 25.6V -25.6V on the other. All looks normal to my tourist eyes so far.

Going down the line, both opamp ICs are supplied with -12.xV at pin 4, and 12.xV at pin 8. I've reseated them to be sure. No change.

I put the 1OUT and 2OUT pins of both opamps ICs on the oscope and found nothing (with no guitar plugged in and no signal), though I'm new to using this oscope so it could be user error.

Looking for another good place to put my oscope ground probe using my MM, I found that all 5 prongs on the phone jack are grounded (when there's nothing plugged into it).

That surely can't be right, can it?

The phone jack is one of those I believe: https://www.switchcraft.com/Drawings/ra49b_Series_cd.pdf

It's setup like this <see photo>

Thanks for guiding me through this... I'm learning a lot, and it's tons of fun.


Title: Re: Randall Century 100 schematics?
Post by: cin on December 31, 2021, 01:14:40 AM
So there actually was some user error with the oscope... There's a tiny bit of noise (<10mV) on OUT1 of the IC closest to the power end of things, while there's no such noise on the other OUT pins. Still, I'm not sure that's the noise I'm looking for, at under 10mV on the out of a 12V fed opamp, it wouldn't be that loud? But again, I really know nothing. Making assumptions.

I put my oscope on the phone jack, knowing that it was all grounded, and it showed what I think is probably the buzz I'm looking for. 60Hz 157mV peak to peak. Which makes sense since I can hear it loud in the phone jack... captain obvious here.

I made sure the square capacitor and resistor between the phone jack and the reverb pot were good, they check out. And there's no continuity across the square cap, so that's not how that path gets grounded.

My guess right now is that neither of the 2 white wires on the phone jack should be grounded, the black wire should be the ground. I'm assuming one of those 2 wires carries the signal, the other one acts as a switch to turn off the speaker when headphones are connected, but not sure how. One of those 2 white wires goes to the speaker terminal, and that confuses me... I don't really understand the signal path here. I did check that there was nothing stuck in the phones jack connecting it all together.

I may be getting ahead of myself... getting late, I will investigate further tomorrow.

Cheers!

Title: Re: Randall Century 100 schematics?
Post by: willpirkle on December 31, 2021, 09:13:54 AM
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.

Will
Title: Re: Randall Century 100 schematics?
Post by: cin on December 31, 2021, 02:04:19 PM
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)
Title: Re: Randall Century 100 schematics?
Post by: phatt on January 02, 2022, 03:08:49 AM
 @*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.
Phil.
Title: Re: Randall Century 100 schematics?
Post by: g1 on January 02, 2022, 10:36:26 AM
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.
Title: Re: Randall Century 100 schematics?
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!
Title: Re: Randall Century 100 schematics?
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....
Title: Re: Randall Century 100 schematics?
Post by: phatt on January 15, 2022, 08:38:23 PM
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...

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.
Title: Re: Randall Century 100 schematics?
Post by: cin on January 15, 2022, 08:43:36 PM
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 -_-
Title: Re: Randall Century 100 schematics?
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.
Title: Re: Randall Century 100 schematics?
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!

Title: Re: Randall Century 100 schematics?
Post by: phatt on January 15, 2022, 09:33:02 PM
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.   ;)
 
Title: Re: Randall Century 100 schematics?
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.

Title: Re: Randall Century 100 schematics?
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!
Title: Re: Randall Century 100 schematics?
Post by: phatt on January 16, 2022, 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.
Title: Re: Randall Century 100 schematics?
Post by: cin on January 16, 2022, 12:25:38 PM
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.
Title: Re: Randall Century 100 schematics?
Post by: cin on January 16, 2022, 03:21:58 PM
Alright, here's another backlit photo. Hopefully it's a little clearer. I labeled the pot connections and such.

I redid a couple of soldering points on the pots, like on the treble pot, and it removed the sensitivity but not the buzz.

I figured signal tracing is pointless if the noise can be coming from anywhere downstream of the reverb tank. The noise would likely show up everywhere past that.

So then, apart from the input jack, which may be part of the problem, I'm looking for places that should be grounded but aren't, correct? Spots that have voltage potential on the ground trace, where it is not supposed to be? So I can go around testing all the ground points against a known good one, looking for potential?

Title: Re: Randall Century 100 schematics?
Post by: cin on January 16, 2022, 05:56:16 PM
I apologize for the many posts. I imagine I am as confusing as I am confused.  :dbtu:

I've been reading about ground loops, and if I understand correctly having many paths to ground is bad because there can be potential differences between the many points, which would cause current flow between the two. But how can there a be differences in potential between two points that are connected? Which is why I concluded that I was looking for a disconnect in the ground.

I thought I'd trace it out on my photo of the circuit, to make it easier to test everything. I also traced out some parts of the circuit that are not obvious in the backlit shots.

Ground in blue, missing circuits in red, yellow dots are the pins of the power amp. It's very likely wrong in some places, I can see spots where it may not make sense, like that ceramic cap in the middle that's not connected to anything??

Title: Re: Randall Century 100 schematics?
Post by: cin on January 16, 2022, 07:27:37 PM
I've gone back over what I thought was GND with my multimeter, and made some corrections to the image (attached). I've also made some corrections to the live circuit in red. And I've added, in dark blue, all the places I've checked to be <.4 Ohm when compared to the yellow common wire at the chassis (ie properly grounded?). Again, I'm assuming this means those spots can't have unexpected voltage potential, unless the chassis isn't grounded.

Where else should I check?

Title: Re: Randall Century 100 schematics?
Post by: phatt on January 16, 2022, 07:48:47 PM
Great work you are doing well. :tu:

Ok isolating the in socket reduced the hum so you know ground path is an issue.
I did look at the first PCB pics again last night and Yes there are many ground paths. ouch!
It's likely that this being a budget model there was no R&D done so the issue may well be a factory fluff up,,, I've seen that a few times even in higher end products.

I've got things to do today but later tonight I'll edit some of your pics with possible re routing of the ground paths.
thanks for the new pics it will help us understand just where all the ground paths run.

the issue of ground loop hum is not the voltage diff but more about *high Current and low Current* mixing in the wrong place. Input ground path is very sensitive and can be easy pick up the hum if the ground paths mix.
something like that,,better minds will explain it better. lol.
More tonight, Phil.
Title: Re: Randall Century 100 schematics?
Post by: phatt on January 17, 2022, 12:15:49 AM
Before we go rewiring the whole ground path, just start with one mod at a time, that way you can assess progress.
Ok isolating the input socket has helped so leave it isolated and Re route that Yellow Com wire
to the Common/Ground junction of the 2 main filter caps on the PCB. (see pic)
You will need to drill a hole for that wire and scrape off the lacquer to solder it.
(while you are there drill 3 holes as you may need it for other mods).

Note the common junction of the main filter caps becomes Com 1, ideally every other com point
should go back to Com1 on a separate path and then Com1 junction is wired back to the chassis
case. (where the yellow wire was) Don't worry about inserting a chassis wire right now as the PCB
is already grounded via the mounting posts and could lead to more problems.
I'm just saying this to show you how it should be done.
Google star grounding for clues.

Rewiring the yellow will alter the Current path and might be enough, if not then you may have to alter the path for com 2. (blue wire on picture) At the moment it seems to run all the way around the out side of the PCB then back to Com1,, which is not a good design. xP
To do the Com2 rewire you will need to cut a pcb track so meantime, Can you tell me if the ground trace runs all the way along the front edge of the PCB and back around to the main filter caps?
I can't clearly see the front edge with all the pots . It looks like it does but i need to be sure.

So Yellow wire mod first and see if it improves,, then move on from there. :tu:

You seem keen to learn so maybe sit down and draw out the whole schematic,, ,yeah it will be slow but you will learn a lot.
BTW, be aware electronics can be very addictive. :duh
Phil.
Title: Re: Randall Century 100 schematics?
Post by: phatt on January 17, 2022, 12:34:42 AM
Oh adding this to show the idea of ground paths in amplifiers.
Note, How Com 1 is the Central Com point for the whole Amp, High current paths come first (The Lm1875)
THEN the low current paths from the sensitive inputs come last and preferably on separate tracks.
Title: Re: Randall Century 100 schematics?
Post by: cin on January 17, 2022, 12:59:08 AM
Amazing! I'll review all of this carefully tomorrow.

Moving away from looking for faults, and into changing the circuit, pre-supposes that the amp is basically buzzy by design. It is a fairly low-cost unit, and it shows in the simplicity of the circuit compared to other amps I've seen, but I'd be unimpressed if it came out of the factory sounding like that. The buzz is about as loud as an electric toothbrush at any volume (a very scientific alternative unit to the decibel), it pretty much makes the amp unusable at anything lower than "quite loud" (like 500 or so electric toothbrushes).

I was also hoping to bring back the amp to a working state while removing all the modifications to make it as close to the original as possible. I won't be keeping it, and it's a lot easier to sell an amp that sounds good and has just been re-capped, than one with mods.

That being said, the primary objective is to get rid of that buzz. And bettering a flawed design is appealing. But I'm wondering if there isn't still a fault somewhere, other than a bad design. I can test re-routing the yellow wire to one of the cap's ground terminal without having to drill anything, so I'll try that to see if that does it, and report back.

Again, many thanks for the patient guidance and knowledge.
Title: Re: Randall Century 100 schematics?
Post by: phatt on January 17, 2022, 07:09:28 AM
If the amp makes exceptable sound and all the controls work as one would expect then it's highly unlikely there are other faults. 8|

The fact you isolated the input and found it reduced the hum tells you the issue is most likely a ground path problem.
And after studying the pcb pictures it's a poor layout and I'm not surprised it has a hum.
As far as mods go without another Amp or schematic of same you have no way to know what was stock. ???

If your intention is to sell it then just fix the hum and don't waste your time.

If you keep it and use it as a learning tool it will help you down the track. :-X

Long before I fully understood Amplifiers
I once had a little 10 watt amp and tried to modify it. By luck more than brains I fluked it into a great little sounding amp. For the first time I actually sounded like Clapton with my strat.

Silly me sold it for another rig that had reverb but I never drew up the schematic. I still regret not keeping it and drawing up the circuit before I sold it. That was 40 years ago and I still regret it now. :-[
Phil.
Title: Re: Randall Century 100 schematics?
Post by: cin on January 17, 2022, 11:16:25 AM
Well, my primary intention is to learn, and this little amp has been great for that so far... with your coaching that is.

But once I'm done it's got to go. I only have so much space, and I have another amp that I prefer (for now). So the plan was: repair it if I can, make it as close to original as possible, resell it.

But I hear you, it'll be hard to bring it back to the original state without the schematics or another one to compare to. There's another forum where I've found some hard-to-find schematics before.... music-electronics-forum.com I'll check there before giving up on the original design.

Will test the yellow wire re-route in a bit. I'm assuming I'm leaving the green earth-ground wire coming out of the power cable tied to the chassis here, for safety?
Title: Re: Randall Century 100 schematics?
Post by: cin on January 17, 2022, 07:50:35 PM
I've rerouted the yellow common wire to one of the power cap's ground leg and the buzz is gone!

And that does look like an elegant enough solution to my problem. I'll just have to do a good job at wiring the new ground path, so it looks all pro and such.

Splendid!

This is great news for the amp, but above all else it's what I've learned along the way... That sounds cheezy.
But you get what I'm saying. Thank you for showing me the way there. I now have a much bigger bag of tricks to attack my next amp project with, thanks largely to you.

This is certainly very addictive. Pretty sure I got some endorphins when I flipped the switch and the noise was... not there! very good.

Cheers!
Title: Re: Randall Century 100 schematics?
Post by: phatt on January 17, 2022, 08:38:36 PM
Great to hear you have resolved the issue, I know how hard it is to understand the intricacies of these things when you are new to it all. :dbtu: :dbtu:
And YES leave the Mains Earth wire connected to the Chassis, It's always the first thing I check when working on gear.
Keep at it, the next time you have a bad hum you will know what to look for :tu:
Phil.