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Messages - cin

#16
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.


#17
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!
#18
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.
#19
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!


#20
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!
#21
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!
#22
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!
#23
Schematics and Layouts / Randall Century 100 schematics?
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!