Mod 102 amp buzz controlled by volume - leaky coupling cap?

[blindopher]

Hi,

 

 

I'm trying to trouble shoot a buzz that has recently developed in my amp after a year of being very quiet.

 

The buzz gets louder as the volume hits 12 oclock, then starts fading and disappears just after 3 oclock when amp is basically dimed.

 

The amp in question is the Mod 102 amp kit. A 5W SE non master volume amp with a 12AX7 and EL84.

 

The link below contains all info and schematics.

 

 

http://www.modkitsdiy.com/kit/mod_102_guitar_amp_kit

 

 

nothing else besides the volume control has any effect on the buzz.

 

The amp is buzzing with nothing plugged in or with an input signal - same buzz. It's not the guitar.

 

 

The buzz is omnipresent at all locations.

 

It is definitely a buzz and not a hum. It's not the tubes either - i swapped them with the same results. I'm quite a noob having only 2 amp projects under my belt, but that volume phenomena is very suspicious. Any clues and educated guesses would be greatly appreciated before i go inside the chassis.

 

 

thanks!

 

 

 

 

[Mr.Grumpy]

They give AC and DC voltage readings at the power supply (HV, B1, B2), have you checked those yet? Besides the tone stack, there's only one signal coupling cap in the whole circuit. The DC voltages should be close to the stated values, the AC voltages should be at their stated values OR LESS.

[blindopher]

Well my DC voltages at the power supply are lower by about 8% then spec. Assuming there is no leakage in coupling caps, could this be related to a buzz problem?

[WRGKMC]

I'd check the DC voltage for AC ripple. It may be the power supply caps are crapping out and leaking AC into the DC circuit. It could also be one of the rectifiers are partly damaged and you're not getting a balanced rectification. Those rectifiers/diodes connect directly to the transformer coil and are going to see the biggest surge when the amps powered up. If the power caps are beginning to leak, the current across the diodes increases.

 

This could all be easily checked with a technicians #1 test tool - The Oscilloscope. A scope lets you look at the voltage to see if the DC voltage is pure and detect any AC ripple getting into the bias circuits. Its especially good for tube circuits because it doesn't load the circuit down.

 

The other tool you'd use in audio is a signal tracer. Its nothing more then a small amplifier. The shielded input lead gets grounded to the chassis. The hot lead acts as a probe. You put a .1uf cap in series with the probe so it doesn't short to ground but will pass an AC signal which you can hear through the signal tracer amp.

 

From there you just run a signal through the amp you're checking and get it to produce the noise you described.

 

Then you take the probe and begin at the input. When you touch the input, you hear the same signal that you're feeding the repair unit. Then you work yourself along each stage of the amplifier circuit. Tube input, tube output, step by step. Eventually you'll find a clean signal going into one component and a distorted signal coming out. That's how you pinpoint the cause of the problem,.

 

You don't guess at this. You use deductive reasoning to rule out as many possibilities you can. If the signals clean going in and comes out dirty, that's a pretty clear indication that component isn't working right. From there you just have to decide if the component is failing because its defective, or if its functioning properly and failing because its not reserving the correct support/bias voltages.

 

Of course in your case, you swapped the tube so it looks like the support/bias voltage is bad/contaminated. You also have the noise with no input so you're limited to possibilities in this amp which is a relatively simple build.

 

First thing I'd do is go through and reflow all your solder joints. A cold solder joint can work fine for awhile. Then vibration and corrosion can get into the joint as the solder tarnishes and all of a sudden you get noise as the solder joint arch's voltage across the poor connection.

 

If that's not it then you have to check and see if the DC voltage has AC mixed in. You can use an AC meter set high and test the DC voltage. You shouldn't see much if any AC riding on the DC. If its over a volt then I'd look at replacing the power supply caps.

 

You can also measure across the two diodes using an AC meter. It will be 1/2 wave voltage but an AC meter will still read it. You should have the same voltage across both diodes. If one is off allot, then its going to be causing AC ripple in the DC supply.

 

Like I said, this is easier to check with a scope, but there's more then one way to skin a cat and narrow the possibilities down. Hopefully its not your output transformer. You could have shorted windings throwing the circuit bias off but that's pretty unlikely. Transformers will usually open up when they see too much current. Same for the power transformer.

 

You could have a bad coupling cap. Its less likely then a bad power cap but it may be the cause. You can lift a leg and test it with a cap meter. Most budget VOM's have cap testers now, if not get one. The old fashioned way of testing a cap was with an ohm meter. You'd connect the leads and watch the cap discharge through the meter, then swap the leads around. If the meter gradually goes to infinity the cap is good. If it reads a resistance, its bad. It wont tell you if the cap is shorting under high voltage or the actual capacitance.

 

A cap can read good using both of these tests and still fail when the high circuit voltage/current is applied. This is where the circuit tracer comes in. You can compare and "hear" the input/output and if its clean going in, dirty coming out, you know that components failing.

 

These are just some of the techniques used by techs like myself troubleshooting circuits. You use the test tools to see and listen to what's going on in the circuit much like a doctor would use a Stethoscope, Cat scan, or X-ray.

 

Just going in an swapping parts based on the most likely suspects can work too. I don't normally advise that because I'm trained to find the actual cause of the problem, but given the fact these parts are cheap and you may not have the test gear or experience to use them it may be best to just wave your hands over the unit, chant, and give it your best guess.

 

Wish there was more I could tell you but I'd need the unit in hand to get you any closer then I have based on the limited facts so far.