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May 19, 2022, 07:48:31 AM

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Randall Century 100 schematics?

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

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cin

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!

DrGonz78

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.
"A person who never made a mistake never tried anything new." -Albert Einstein

cin

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!

cin

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!

phatt

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.

cin

#5
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!

phatt

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.

cin

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!



phatt

#8
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.

cin

#9
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.

phatt

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.

cin

#11
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!

phatt

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.

cin

#13
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.



cin

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!