Concert Bass amp - hum/buzz

[brianl-cdn]

My Concert Bass amp and (Fender Jazz) bass guitar have been sitting in my basement since I stopped playing, around 1985. I've decided I might sell them, so I plugged them in to check them out. There were two issues, some crackling/fuzziness when I adjusted volume/tone, and a hum which increased with the amp/guitar volume. FYI the amp has a black front panel with red knobs (no distortion knob), and sticker inside says "tested 11-11-75".

Here are the specifics of the problem:
When I turn on the amp, with the volume off, there is a low hum, but not a lot, and it doesn't increase when the volume is turned up, with the guitar pickups off. When I turn up either pickup volume, there is additional hum that increases with the volume, but when I play the bass, it sounds clean and clear, not distorted or muffled.

I've searched this forum, and one for Fender basses, for similar issues, and have done the following so far:

- I cleaned the volume/tone pots in the bass and amp - they all work smooth and quiet now.
- I reseated all the wire push-connectors on both amplifier boards, and reseated the power amp transistors, and cleaned the input switch contacts with a small file.
- I cleaned and reseated the bass's bridge ground wire, and my meter shows 1-2 Ohms resistance between the bass bridge (and cable) and amplifier chassis.
But, the hum did not reduce much, if at all.

I plugged the bass into another amp for comparison, and also the "power amp in" of the Sunn,  and with both there was less hum than when plugged into the "normal" or "bright" input of the Sunn at the same sound level (about volume 3).

In previous threads about hum/buzzing with Concert amps, it's usually recommended to solder all the push-connectors, and replace all capacitors, but is it possible to "guess" at the most likely (cheapest and simplest) fix I could try next?

Thanks.

[george]

So this is a hum (60 to 120 Hz) that was not there in 1985? The power supply filter capacitors may be the issue (kind of doubt it unless) 1) they look like they are leaking - rare 2) you can test the ESR of these components and verify if they are okay.

There is a possibilty the the bypass caps on one or both boards has failed (see 1 & 2 above)

There is also a possibility that your bass is causing the problem - what do you mean with "guitar pick ups off"? Volume control on bass turned down? Did you try another bass, another cord, etc?

[brianl-cdn]

For the hum frequency, I just checked it, and it matches low B on the bass (60Hz?). With the volumes off, it's fairly muted, then with the amp and pickup volumes turned up, it's "buzzier", but at the same B pitch.
I do remember back when I played, the low-level hum when the amp was turned on, but I'm pretty sure the "buzz" at playing volumes is worse now. Which components are most likely to degrade in storage?

Yes, when I said "pickups off", I meant the bass guitar volume controls. When either pickup volume is turned up by itself, there is a more distinct hum than if both are turned up (apparently it's a known issue that Jazz basses hum somewhat since they have single-pole pickups), but I plugged the bass (with the same cord) into the "power amp in" (and another amp), and it had less hum, so the Sunn preamp seems to be adding to it. I'm trying to borrow another bass guitar in the next day or two, for more comparison.

There's no visible leakage on any of the capacitors.
What does ESR stand for? I don't have a scope or signal generator etc, but I can solder components if I know which ones to replace.

Can you point me to a circuit diagram that matches the later Concert Bass amp? The ones I have seen on this forum, like this one:
http://sunn.ampage.org/site/schematics/concertbass.gif
appear to be the earlier model with distortion, and have different component layouts than mine.
I've attached a couple pictures showing the pre/main amp boards and power caps.

[loudthud]

Are there any light dimmers anywhere in the house? They can cause a buzzy sounding hum. Turn them off. Same with some older florescent light fixtures.

[brianl-cdn]

I have one ceiling light/fan controlled by a dimmer, which was turned off, and turning it on didn't seem to change the buzz.

I just tested again - with the bass plugged into "power amp in", it has an acceptable low hum, but plugged into either front input, the hum/buzz is quite noticeable, and gets progressively louder as the volume is increased.

[MarkG]

Hum on Fender Jazz pickups is not uncommon. I have Bartolini now but when on the originals, my 72 JBass with both pickups at full volume not much hum,but if either ONE of them are turned down,the hum increased considerably,and was pretty bad. Did not seem to matter about grounding,etc. Nothing seemed to help.

Although it's not the main reason I replaced them,when I switched out to the Barts all hum was eliminated. 

[brianl-cdn]

That's what I remember too, but right now, with both pickups wide open, hum is almost as loud as either one by itself. I want to compare it with another guitar plugged in, to know whether the hum follows the bass or amp.

[mckinnon audio]

   Hi there,I've had a similar problem and tried this trick,might not be the cause,but it's worth the effort.Take all the 1/4" jacks off the chassis,normal,brite,preamp in / out and spkr.Clean all the jack parts,positive tang,ground barrel,switch tang,(may or may not have one),and threaded ground body.Also clean rust off washers and spin nuts and area where jacks connect to chassis.This really helped in my case,but like I said,it might not be the cause of your prob.,good luck,Mel.

[brianl-cdn]

Thanks Mel. I had cleaned the tangs and switches, but didn't think to remove the jack bodies and washers. FYI, the normal/brite and speaker jacks have non-metallic washers on both sides, so I left them alone and cleaned the preamp out, power amp in jacks' grounds, but got no reduction in the hum.

I removed the amp front panel so I could look under the preamp board to see if there are any cold solder joints, loose components etc, and they look ok, except for a missing connection (trace with empty hole), where I noticed that the volume pot is different - it doesn't have a ground strap like the original pots, and its solder points under the board look like a repair. Is this a common replacement pot, or should I try and find one similar to the originals?

Does anyone have access to the circuit diagram for the 2nd Gen Concert Bass preamp, with the specs for the volume pot?

Am I wasting time, and should I just replace all the capacitors?

[george]

Go look at the Schematic section on this site - the "Concert Lead Power Amp" schematic is the correct one for your amp's power amp board.

[brianl-cdn]

Go look at the Schematic section on this site - the "Concert Lead Power Amp" schematic is the correct one for your amp's power amp board.

Ok, thanks George, but is there a schematic showing my pre-amp? Or is the Bass pre-amp the same as the Lead, minus a few circuits?

On the Lead schematic, it shows the Volume control is a 500K ohm pot - can anyone confirm if that's the same for my amp, and is it linear or audio taper? I'd like to find the closest match to spec to replace my non-original volume pot, in case it's causing my problem.

[brianl-cdn]

I've been getting mixed results, so I figured I would verify the amp grounding, then work my way from there.

I put my ohmmeter across the ground pin of the power cord to the chassis ground, and I could not get a stable reading, especially when I flexed the cable near the plug, it jumps from 1-2 ohms to the mega-ohms, so it looks like a (partially or mostly) broken ground wire, and the hots are iffy too.

So, are replacement power plugs from a hardware store a good repair?

[loudthud]

So, are replacement power plugs from a hardware store a good repair?

Results can vary but I've had good luck with the orange or yellow one piece designs that meet current safety standards. The old round ones with cardboard insulators are obsolete.

[brianl-cdn]

Thanks, I replaced the original power cord plug with one of those solid yellow "industrial grade" plugs, and there's good continuity from the bass bridge to the plug ground pin now (around 1 ohm).

But, the hum is still there - quite loud (from either guitar) when facing towards the amp, even 6 feet away, and a lot less hum if I turn 90 degrees to the amp. The hum goes up or down with the pickup volume(s). Yes the Jazz bass is known for hum, but not with both pickups turned up, like I'm getting.

Does that sound most likely like a bad capacitor, or something else?