background noise in DC-3 - help needed!

[Jimmy74]

Hi just a few straightfoward questions:

1) Clean channel: background noise sounds like hum or bad jack noise but I just can't figure out where it's coming from. No instruments attached, turn up 1/4 output volume, 1/4 master volume and gain up 1/2. The hum comes up strongly after I turn the Bass volume up. When I touch the gain pot or the mounting nut for that pot the hum gets even stronger, same thing happens when I touch the solid wire that connects the gain pot to the treble pot. Here's the good part, I've changed out just about all the hum prone resistors and caps in the clean channel but that obviously didn't fix it. I've also changed out all the LDR's, 4N33 chips and also the NE5532 opamp, all the transistors with original replacements, all the electrolytic caps in the HV, bias and LV sections and the voltage dropper resistors in the HV. The only thing that I'm puzzled about is the resistor that comes straight off the input jack before going to the circuit, the original one broke and I replaced it with a 2.2 ohm resistor, could that be the problem?

2)The 4 resistors in parallel just after the rectifier diode in the LV supply get very very hot, I tried replacing them but that didn't change anything, same thing for the voltage dropper resistors in the HV supply.

Here's a schematic of the LV supply that I have been drawing up:

http://www.flickr.com/photos/73835524@N00/3592697774/

I've tested all voltages on all the tubes and everything is fine, and all the tubes are new original mesa replacements.

any help at all is welcome.

thanks

 

[Jimmy74]

I might have found where the problem may be, as you can see in the LV supply schematic above there are 2 x 1N4007's feeding the LV supply circuit. I tested the voltages across these diodes and I'm getting 52vAC from the power tranny though they are letting through 10-16vAC ito the circuit.

any ideas?

 

[DougM_TX]

Do you have a copy of this ?

http://www.schematicheaven.com/boogieamps/boogie_dual_caliber_dc3.pdf

 

[Jimmy74]

Hi yes I do have the full official mesa boogie schematic for this model but the LV supply and many other sections of the amp are totaly different, that's why I ended up drawing up my own.

http://www.flickr.com/photos/73835524@N00/3592697774/

 

[Jimmy74]

Can any one do a simple dc resistance test on the power transformer connector for me? All you need to do is:

1) unplug the connector from the circuit board
2) measure the dc resistance across the LV voltage supply wire( blue wire which is 5th wire from the right on the plug - having the control panel facing you) to the 2 red wires on each side of that wire. Please tell me the reading you get across the blue wire to each red wire.

thanks for any help at all

Saverio

 

[DougM_TX]

I can try and measure that later today.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I am not an amp repair person, but not totally ignorant about electronic circuits either.

So let me make an observation or two and and ask a question or two also. Feel free to tell me to STFU and I'll butt back out, no harm, no foul. But I'm thinking I might learn something from your efforts.

First, my understanding is that it is only on the rhythm channel.

Observation : Wouldn't the LV supplies be common to both channels? Expect noise on both channels?

Observation/Question : If you turn the gain on the rhythm channel all the way down you are basically grounding the input to V4b, if the noise goes away, wouldn't that indicate that the problem is around V1a or the tone stack?

Observation/Question : Seems to me problem has some similarities to a grounding problem/loop. Have you checked the solder joints/grounding points along the ground rail for the rhythm channel.

BTW, my (quite possibly wrong) schematic shows that resistor off the input is a 1MOhm (in parallel with 180 pF cap). I'll try to check the value when I pull my chassis out.


Doug

 

[Jimmy74]

To answer Dougs questions:

1) yes the hum is only on the rhythm channel

2) I don't think the LV supply problems have anything to do with the hum I'm getting, here are some more tests that you can do for me if you have the time, measure the AC voltage on the resistor side of the LV supply diodes (look at my schematic to get a better idea of what I'm talking about) if you get 10vAC and 16 to 18vAC off both diodes then that will confirm that all is normal on my amp. Also once you've turned the amp off check if those 4 resistors are hot to touch. I'll check the gain pot suggestion and let you know.

3) I'll check the grounding rail for the ryhthem channel once again but I'd say that everything is normal.

thanks for your help and time

 

[DougM_TX]

Ok,

I did not pull that big Xformer connection loose to measure the DC resistance. I don't like those knife edge connections and it worries me that I could make them intermittent, BUT, the voltages are what really matter, right?

So, from the Xfmr secondaries:

Blue to ground : 49 VAC stdby / 47 VAC full on
Red(1) to ground : 340 VAC stdby / 327 VAC full on
Red(2) to ground : 341 VAC stdby / 329 VAC full on

In the LV supply :

Above the 3.3 K resistor 9.2 VAC stdby / 8.8 VAC full on
Above the 4 parallel resistors 18.5 VAC stdby / 17.5 VAC full on

The 4 resistors do get hot.

I'll keep the chassis out a while if you need more readings. ( I just got a GNX4 and its keeping me occupied )

Doug

 

[Jimmy74]

Hey thanks for that, great news, sounds like our readings are pretty much the same that's making me feel better. Just to sum things up:

AC voltage on the resistor end of the LV supply diodes:
Diode 1 to 3.3k resistor : around 10vAC
Diode 2 to parallel resistors: around 16-18vAC

There's really no need to unhook the PT connector, just meant to exclude the PT completely from the circuit. You can measure DC resistance between the blue wire and the red wires on each side of the blue wire, I'm getting readings under 200 ohms for each red wire. Make sure your unplug the amps power cable just to make sure there's no VAC to disturb the readings and avoid getting buzzed!!!

keeping my fingers crossed and tomorrow I'll be going through the grounding rail of the clean channel.

thanks for your fantastic help!!

 

[Jimmy74]

Well I went through the grounding and the only doubts are right off the input jack:

1) Am I supposed to be getting ground off both input wires?
2) the mystery resistor before the 1.5mohm input resistor is probably blocking grounding to one of the LDR's.... could this be true?

thanks

 

[DougM_TX]

1) Am I supposed to be getting ground off both input wires?

Isn't the input shunted to ground by the input jack when there is NO CABLE inserted ?

When I plug a cable in and measure the input resistance on it...

On Ryth Chan : 2.49M Ohm
On Lead Chan : 399K Ohm

Or, am I missing your point all together on the "ground off both wires".

 

[Jimmy74]

Yeah sorry if I didn't explain my question properly, there are 2 wires coming off the input jack, one goes to ground and the other goes to the circuit. Looking at the original schematic, one end of the 1.5Mohm resistor and also one leg on the LDR should test to ground when there's no cable inserted..... right?

If this is correct.... the difference in this amp being that between the input jack and the LDR-1.5mohm resistor, there is a mystery resistor that broke and which I replaced with a 100k resistor. No my question is should I ground the junction between the LDR, 1.5Mohm resistor and the mystery resistor using a separate wire to ground?

Here's a photo of the broken mystery resistor:

http://www.flickr.com/photos/73835524@N00/3356870910/

 

[Jimmy74]

I tried grounding the junction between the 3 components but that only took a little cable hum away, the mid-bass pot hum is still present. So whatever that hum problem is, it has nothing to do with tube voltages, wrong LV or HV voltages or grounding problems. I also tried the "bad jack test" but that didn't change anything either.

though I'm wondering if the positive DC heater voltage point for V1 (pin 9) should be grounded?

 

[DougM_TX]

I'm wondering if the positive DC heater voltage point for V1 (pin 9) should be grounded?

Is the pdf schematic that wrong ???

It shows the heaters are wired parallel @ 6VAC. Pin 9 of U1 should have about 3VAC measured to ground (half of the full heater voltage seen on pins 4/5 to 9).

 

[Jimmy74]

Nope it's not wrong, well if you look at the last page of the official schematic you'll find that the AC heater line gets rectified to DC for the heaters on V1 (to provide a quieter first stage). The rectifying diodes in the schematic are designed with errors but it shows exactly how the heaters on V1 work, negative DC 3.2v to pins 4 & 5 and positive DC 3.2v to pin 9. What I needed to know is if pin 9 on V1 in your amp is grounded?

thanks

 

[DougM_TX]

Ah, I see the DC for the U1 heater now, missed it before.

I can can pull my chassis back out later today and check.

I'll check U1 Pin9 to ground resistance.

While it's out I'll check the DC heater voltages: 9 to ground and 4/5 to ground and how much AC ripple is on each as well.

It will be a few hours before I'll have time, so if there is anything else you want a reference measurement on let me know.

 

[Jimmy74]

Well the only other thing I would like to know is if you get ground on the very left pin of the left LDR, the pin that is connected to the 1.5Mohm input resistor.

thanks again for all your help

 

[DougM_TX]

OK,

1. The 1.5MOhm/LDR junction : When I first measured it (nothing plugged into front panel input jack), I measured a little over 6 Ohms. Should be zero. Cleaned input jack with a little DeOxit and it went to zero.

2. Measured U1Pin9 resistance to ground. Measured about 700KOhms. Drained +/-3V filter caps. Measured resistance again and showed the normal capacitor charging behavior of very low resistance increasing with time. NO short pin 9 to ground.

3. Measured U1 heater voltages to ground with amp in stdby.

U1Pin9 to Gnd -> 3.07 VDC and 0.088VAC (88mVAC)
Pins 4/5 to Gnd -> 3.07 VDC and 0.083VAC (83mVAC)

Good Luck!

 

[Jimmy74]

Thanks again for all the help, and sorry about taking your time away.... I still have no idea where the hum is coming from but I'm starting to think of the pots themselves but that would be really strange, I just changed both gain pots with original mesa replacements and that took away the crackle and unbalanced gain increase off the gain channel, but then this strange hum came out.... could also be the 120pf disc cap on the rhythm channel gain pot.