Help troubleshooting a simple buzzy solid state guitar amp (Randall Century-100) - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 05:47:35 am
content: I have a Randall Century 100 solid state guitar amp that has a pretty strong 60 - 80 Hz buzz, strong enough to make the headphone jack unusable. The buzz tracks with the volume pot, and kicks in sort of suddenly at 25% volume. It's present with no instrument cord plugged-in.The input side (left) of the PCB is extremely sensitive to interferences, specifically the 0.01uF 100V cap right at the input. Sensitive as in I was touching the 3.3V calibration square wave connection on my oscilloscope with one hand, and it could easily pickup the signal if I hovered my other hand over that cap (or over the Treble pot for some reason). But even if I make the cap a little grounded tinfoil hat, the amp still buzzes.- I've replaced the electrolytic caps in the power section. Mostly because they looked old and overheated.- I've disconnected the reverb tank, same, and made sure it was connected in the right direction, same.- I've found some signs of slight modifications done to the amp, an extra cap had been added to a resistor which I've removed. There's also a cap-resistor combo to ground from the headphone jack, which looks suspiciously not-stock, but I've left it there because the amp screeches without it. I'd like to bring back the amp to its original state if possible.- Looking for potential ground loops, I disconnected it all from the chassis, leaving only the ground wire back to the transformer as path to ground. That made the amp buzz much worse since it lost all the chassis' shielding. I then grounded the chassis via a single point and it returned to the regular buzz. The AC ground is also tied to the chassis.- I've scoped the In and Out of the LM1875T and the buzz is not really discernible on the In.My next step is to swap that sensitive cap with another one to see if that does it, but I thought I'd post here because I always learn a thing or two from the advice I receiveI haven't found the schematics for this amp, but the circuit is pretty simple and I have this here back lit photo of the whole thing (I'm also posting this here for posterity because there's very little about this amp out there):

author: RoGeorge
author_rank: Super Contributor
timestamp: October 07, 2023, 11:01:13 am
content: Check the ripple on the capacitor after the bridge, maybe the filtering caps dried and became of a much lower capacity.Another thing, if you have the AC transformer nearby (in the same box), then the hum might be from the transformer's field. The E+I transformers escape a lot of magnetic fields around them, more than enough for the nearby electronics to pick up the hum and make it audible (though this would be a design fault, audible even when the amp was brand new).Electric fields are easy to shield, but magnetic fields are very hard to shield against (usually done with a thick permaloy sheet). If the hum comes from the magnetic field of the transformer, then the solution is to move the transformer away (make another box for transformer, and keep it on the floor), or to replace it with a toroidal transformer. Toroidal shaped transformers keep all the field inside, they have theoretically zero field escaped outside.

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 02:43:26 pm
content: Thanks for the tips!The power filtering caps are brand new, I replaced them both, so that's not likely it. (The gold caps in the photo are new. Any other I should change?) I'll check anyway to be sure though!I will check the power transformer, and try powering the amp from a different source to see. It's not far form the board, on the underside of the chassis. But I doubt the buzz was there from day 1: as it is, the phones jack is too noisy to be usable. I'm trying to restore the amp to its original condition, so changing the power supply or moving it are not viable options.The board seems overly sensitive to me. Is there something that can fail in a way that would cause it to become so sensitive?Cheers!

author: themadhippy
author_rank: Super Contributor
timestamp: October 07, 2023, 04:06:11 pm
content: Quotenothing unusual for some guitar amps to hum like hell without an input,the cure often is replacing the input socket with one that shorts signal to ground with no jack inserted

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 04:15:38 pm
content: Well, it also buzzes with a shorted cord plugged-in, and with an instrument plugged-in as well. So it's not just that. I should've been clearer.

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 05:15:19 pm
content: I checked the power for ripples after the 2 large filter caps, and there is a 120 Hz .66V PtoP ripple on there. It's there before and after the filter caps. I just installed those capacitors, and they are fresh new Nichicon Gold (overkill, I know).Is this much ripple normal?Does this suggest a faulty new cap?Could one of the bridge's diode failure cause this?I don't think I'm setup to externally power the amp (bypass the transformer), there's 2 x 20VAC leads coming out of it and I don't have a variac. Past the filter caps it goes straight to the LM1875T Vcc and Vee. I do have a bench DC power supply, but it only has one output... not sure how I can set that up safely for the LM1875T.Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2023, 05:46:49 pm
content: To calculate the "normal" ripple across a filter capacitor, you need to know the DC current drawIof the circuitry to follow, and the capacitanceC.The peak-to-peak ripple voltage should be (for the normal full-wave rectifier, either transformer CT or bridge rectifier)Vpp=Ix (T/C), whereT= 1/(2f) andfis the line frequency (50 or 60 Hz).

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 05:58:53 pm
content: Nice!I haven't measured the actual current draw yet, but for the sake of running the max numbers: The amp has a .5 amp fuse on the AC side, so max would be:Vpp = 0.5A * ((1/(2 * 60Hz))/1000uF) = 0.00000417Did I get this right? If so my .66V ripple is... "abnormal".Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2023, 06:44:20 pm
content: Quote from: cincin on October 07, 2023, 05:58:53 pmYour arithmetic is wrong. The 0.5 A draw is probably too high, but (0.5 A) x (1/120 s) / (1000 uF) = 4.17 V

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 06:59:41 pm
content: I haven't had to use actual math in a bit, so please bare with me, but you wrote Vpp = I x (T/C)So:Vpp = 0.5 * (T/1000) where T = 1/(2f)If T = 1/(2f) = 1/120 = 0.00833333 thenVpp = 0.5 * (0.00833333/1000)Vpp = 0.5 * 0.00000833Vpp = 0.00000417?Is this wrong?I asked Chat GPT, see image.So 4.17V ripple seems way high, but as you said 0.5A is the absolute maximum that could go through this amp, so it's pretty high as an estimate.0.00000417V ripple on the other hand seems excessively low..

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2023, 07:24:55 pm
content: Your "1000" is in microfarads, soC= 0.001 farads, not 1000 farads.As a senator used to say, "a million here, a million there, it adds up".I'm not a fan of ChatGPT, but you did ask aboutC= 1000.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 07, 2023, 07:26:39 pm
content: Quote from: cincin on October 07, 2023, 06:59:41 pmChatGPT is fantasized text. Don't take its blurb as facts. ChatGPT doesn't think. The answer might be correct, or might be wrong as well. Never take ChatGPT as proof.

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 07:27:59 pm
content: @TimFox: That'll do it! hahaha. Ok, I'll go check the actual current draw then...@RoGeorge: Agreed, but it can still solve equations though.Cheers!

author: TimFox
author_rank: Super Contributor
timestamp: October 07, 2023, 08:22:10 pm
content: Your 0.5 A fuse determines the maximum power it can draw from the 120 V line.However, in a tube amp a large fraction of that power goes into the tube heaters.The winding for the plate voltage is at a higher voltage than 120 V, so the current from your filter capacitor will be less than calculated from the remaining power at 120 V.You can estimate the total plate current, lacking a value in the manual, by looking up the tubes athttps://frank.pocnet.net/

author: cincin
author_rank: Regular Contributor
timestamp: October 07, 2023, 08:55:53 pm
content: But not so on solid state amps, right?As far as I know the current it draws on the AC side, where the fuse is, is about the same as what goes through the DC circuit?

author: TimFox
author_rank: Super Contributor
timestamp: October 08, 2023, 03:54:12 am
content: Is there a power transformer in your solid-state amplifier?If so, it has a turns ratio and the input (fused) and output (rectified) currents differ.For the lower voltage on a solid-state amplifier, the output current is higher than the line current.(Sorry I didn't notice your amp was solid-state.)

author: Brumby
author_rank: Unknown
timestamp: October 08, 2023, 04:26:44 am
content: I would have suggested you check all your earthing points - including pot, socket and switch mounts. Never underestimate the insidiousness of oxidation/corrosion under mechanical connections.

author: CaptDon
author_rank: Super Contributor
timestamp: October 08, 2023, 02:23:28 pm
content: Check all grounding points where screws bond grounded traces to chassis ground. Does this amp have a 'master' volume control as well as 'clean gain' and 'overdrive gain'? If you run the master gain wide open then hiss and hum are very normal. Run the channel gains at 5-8 and the master gain at 3-5. Also look for cracked connections around the input connector and where it is soldered to the chassis. Some amplifiers have a line polarity switch and will hum like hell in one polarity but not the other. Does this amplifier have a 3 prong grounded cord? If so try plugging the amp in with one of those little adapters that lifts the ground (3 prong to 2 prong adapter) but be aware this can be very dangerous as you could get a nasty shock when playing barefoot or between your guitar and a house sound system. I played through an ungrounded 2 prong amplifier years ago. It had a .22uf capacitor from 120vac hot to chassis ground. While playing the guitar I got my lips against the house microphone and it felt like someone tore my lips off with a pair of pliers!!

author: BillyO
author_rank: Super Contributor
timestamp: October 08, 2023, 02:35:35 pm
content: Do you have a scope? What does the ripple look like? Also, what is the quiescent current draw on the DC side?AS other have suggested, I'd look carefully at all the ground points (might be difficult without a schematic), but I'd also look to see if one of the diodes in the bridge is either shorted or open.

author: alligatorblues
author_rank: Regular Contributor
timestamp: October 09, 2023, 01:07:52 am
content: The 60-80Hz gives me an idea it's AC mains getting through. What are the 8-dip ICs?

author: cincin
author_rank: Regular Contributor
timestamp: October 09, 2023, 04:14:12 am
content: Ok! It appears it was some sort of ground problem as most of you guessed!I sanded all the contact points between the PCB and the chassis, which is basically just the 2 screws on the LM1875T heatsink, the input jack, and the switch pedal jack. I also sanded the edge of the chassis where it connected to the upper shield in the enclosure because it was a bit rusty. And BOOM clean sound.Amazing!This all happened before I could measure the current draw.Thank you all for chiming in! I could've been looking at components for a loooooooong time....