DC - 3 Hum problems on Rhythm Channel

[Jimmy74]

Here's the problem: After a first observation of the pre-amp tubes which were all very used, I replaced all 6 of them with new ones but the problem on the rhythm channel is the same, I also tried changing out the power tubes with new ones but still the same problem. When I turn output volume to 4-5 master to 3-4 and gain up to 3-4 I get a hiss/hum (not a squeel just hiss) that only occurs on the rhythm channel, the lead positions are much quieter just that classic volume hum when turn up.

My question is..... is this normal for a DC-3 to behave that way? If not what could the problem be?
thanks for any help at all

 

[MusicManJP6]

It's definitely strange that your RHYTHM channel hums and your LEAD channel is not worse. It's definitely not normal, but aside from tubes I wouldn't have any idea what the problem could be. A bad cable maybe, but that doesn't explain your lack of hum in the Lead channel...

 

[adam972]

Do the normal run down. Start pulling tubes one by one, starting with V1. Pull the tube, take the amp off stand by...any hum? if you can pull a tube and kill the hum, then you can narrow down where the problem is occuring. if you have all the pots at zero, and raise the output level to 5, and then the rhythm master to 5, do you get hum then? or do you need to increase the gain of the rhythm as well? does this happen when a guitar is not plugged in?

 

[Jimmy74]

Ok here goes, I'll try answering all the questions:
1) Pull the tube, take the amp off stand by...any hum?
-----I pulled V1 and no hiss or hum whatsoever, I put V1 back in and pulled V2 and got hiss, I put V2 back in and pulled V4 and go no hiss or hum whatsoever.

2)if you have all the pots at zero, and raise the output level to 5, and then the rhythm master to 5, do you get hum then?
----- I put output volume up to 5 and master volume on rhythm channel up to 5 and get normal volume hum.... when I increase the Gain volume.... I get hiss.

3)or do you need to increase the gain of the rhythm as well?
------ Yes

4)does this happen when a guitar is not plugged in?
----- there's no guitar plugged in

What do these symptoms tell you?

I took a closer look at some of the smaller sized electrolytic caps and there's one on the circuit board situated near V6 (between V5 and V6) that looks a little suspicious with the -ve end popping out.... the code is 1uf 50v but taking its readings it measures around 1.3uf.... I've taken a look at the schematic and the only electrolytic caps that match are the 1 uf caps on pin 8 of V1b, pin 3 on V2a, pin 8 on V2b, pin 3 on V3a and pin 8 on V3b..... could this be the culprit?

thanks

 

[Jimmy74]

I've done somemore circuit checking comparing it to the schematic and here's what I found:

Looking at the rhythm channel pre-amp section the input is wired directly the 1.5Mohm resistor (in parallel with the 180pf cap).

Looking at the circuit board the input wire is soldered to a mystery resistor which colour code is :
Grey White Yellow (or Orange but it looks more like Gold) Red Silver which should be around 89kohms right??

That mystery resistor is then soldered to the 1.5Mohm resistor in series. The mystery resistor measures 2 ohms on my multi-metre and the 1.5 Mohm resistor measures around 600kohms.

Could that mysery resistor be the culprit?

thanks for any help at all

 

[adam972]

i gotta get the dc-5 schematic out of my head and look at your DC3. if you problem was on your input chain, then you should hear the problem on both channels. So you pull V1 and that kills the hum, that is a good indication. I think the v4 of the DC3 is the same as the dc5, and if so, pulling it will kill the end of the rhythem channel circuit, so yes it should be quiet.

ah, just looks at your schematic...yea, the extra resistors seems a bit od....and it does look like the lead channels pulls the input before this. I would call mesa in the morning, they open at 9 pacific, and get a hold of a tech, just call and ask for customer support, and then explain the extra resistor to marcus, he will prob hand you off to a tech. They might have included that might not...but they will be able to tell you whether it should be there or not.

 

[Jimmy74]

Well since I'm writing from Italy I don't think I could call them up without paying big $$$, that's why I wrote up my problem here. If anyone knows about that mystery resistor being or not being stock please let me know.

 

[Monsta-Tone]

My DC-3's rythm channel was dead silent with the gain under 8 or so.

I would think that you have a bad preamp tube still. Or at least, that is where I would start, just like adam972 said.


Here is what I would do next:
Swap V1 & V2
Does the problem shift to channel 2?
Put V1 back in it's original spot

Swap V4 with the original V2
Does the problem shift to channel 2



As for the 'Mystery' resistor, Mesa had a few versions of each model of the DC, not just 2. There were some transitional measures taken between the 'A' & the 'B' versions. I've seen quite a few things that did not necessarily look exactly like the schematic, but still sounded really great.

Let us know if this helps.

 

[Jimmy74]

Well I did follow your tube swapping instructions but the problem is always only present on the rhythm channel, the tubes are new.

I'll try bypassing that mystery resistor and see what happens

thanks

 

[Jimmy74]

Well that bypass didn't do much for the hiss on rhythm channel.... but I discovered that it did make both lead channels noisier (not hiss but a loud hum) so that could be a lesson to all those who are having hum problems

As for my problem well it's still a mystery.... another thing I noticed is that when I turn the amp on I hear a rattling sound but I'm not sure where it's coming from, could it be some sort of relay switch? I'm pretty sure it isn't the power tubes.... does anyone else experience this?

I'm not sure if this would be the cause but I unsoldered one end of the 1.5megaohm resistors and took it's readings and it still results in being 600 kohms..... I just tried turning the amp on with that resistor unsoldered and there wasn't any difference at all!!

I soldered everything back in place as it was, and just for doubts sake I tried pulling V5 which is the reverb tube right? Here's what happened:
1) the rhythm channel didn't change whatsoever
2) the lead channels weren't working anymore!

Now I'm figuring that the tube layout on this amp is different from that shown in the instruction manual, infact the power transformer, output transformer and reverb sockets are opposite the way they're shown on the instruction manual.... could that mean that some tubes are in different positions?

This could be an indication, the hiss is mainly caused by the gain volume on the rhythm channel but the strange thing is when I turn the gain up to max the hiss disappears!!

 

[Monsta-Tone]

Well...

Maybe we are over analyzing this a little.
Return the circuit to the way it was with the Mystery Resistor. I think that needs to be there for now.

Try looking at all of the solder connections with a magnifying glass. It could be a simple bad solder connection.

Then, I would inspect all of the capacitors in the circuit. It could be that one of them is letting a little bit of DC through, although you are not experiencing any hum.

Also look at the resistors, particularly these:
100k at Pin 1 of V1
100k at Pin 1 of V4
100k at Pin 6 of V4


Usually, with Vintage Fenders, the Plate Load resistors (100k's) are the cause of great hiss and hum and crackle. The ones in your DC are much higher quality, but I've still had to replace a few of them.

Try this and let us know what happened.

 

[Jimmy74]

Well I just checked all the resistors in the rhythm channel and they all test ok except for the following 3:

1) 1.5megaohm resistor on pin 2 of V1a tests 600kohms
2) 1 megaohm resistor on pin 2 of V1a tests 594kohms
3) 150k resistor in the treble, bass & mid control measures 134k

I couldn't find the 3.3megaohm resistor in the reverb input on the rhythm channel, maybe it's hidden?

Testing all the caps will be a real PITA to do so I'm going to test the 120pf cap on the gain pot and some of the caps that are mainly connected to the gain section.

Something else I've noticed, though I'm not going to bother about that right now, is that the lead + lead&rhythm switch on the rear panel doesn't seem to be doing the right job.... that is the equalizer is engaged only on the lead channel in both selections.

I'll let you know what I find out on the caps in the gain section

thanks

 

[Monsta-Tone]

Did you lift one side of each resistor when you tested them?

Sometimes you will be testing a parrallel circuit and will not be able to get the correct values if it is still in the circuit.

Also, make sure you have a good battery in your meter. I chased my tail over this one once!



I would simply replace the 3 100k resistors and see if that cures the problem.

 

[Jimmy74]

Well where the resistors were reachable I did take one end out, and the 1st 2 of the suspect list were tested that way.

I also tested the 2x 220uf 63v caps (one for the bias and the other for low voltage) and they are giving me around 160uf..... time to change??

The battery in my tester is fine, and so is the cap on the gain pot. How do I know if the pot is not worn out?

thanks

 

[Monsta-Tone]

Not sure about the caps. My tester doesn't test electrolytics that well.
I usually compare the readings to a new cap or 2 and they are right on the money almost always.

As for the resistors, make sure that you are not touching both sides of them with your fingers.
I always use aligator clips on the ends of my meter probes. If you're sweaty or just touching them, the readings can be false.

 

[adam972]

Jimmy, as for calling mesa, you could download skype, shouldnt cost you to much. They open at 9 am pacific time in the US, and i would say the best time to call them is around 10:30am pacific time US. They are generally pretty quick at getting calls through to support, and the skype charge should be minimal.

Given the age of the DC series amps, if you are already in the board, it found its easier just to go a head and replace all the electrolytic capacitors. my DC-5, amongst other problems, would go microphoic on the lead channel, no matter what tubes i put in there. After replacing all the caps this went away...not sure what the true culprit was, but those electos are only going to get worse with time. ordering from mouser i think my total was 15 bucks and took about an hour to desolder and reinstall.

one thing i meant to ask you. Have you ever noticed that the controls of the rythem channel affect the way the lead channel sounds when it is engaged? or vice versa. For instance, if you are playing on the lead channel, and you start increasing settings on the rhythm channel, do you hear a change?

 

[Monsta-Tone]

one thing i meant to ask you. Have you ever noticed that the controls of the rythem channel affect the way the lead channel sounds when it is engaged? or vice versa. For instance, if you are playing on the lead channel, and you start increasing settings on the rhythm channel, do you hear a change?

You may have a failing LDR. I have made it a point to replace all of the LDR's in every DC I buy. There is just something about the DC (higher voltage or current than the LDR's like) that takes them out eventually.

 

[adam972]

ya i have replaced mine as well during all of this. Just trying to give him a couple more things to replace on the cheap while he is already in there!


i gotta say...i am getting really close to selling my dc-5 and building a fender super amp....it just keeps drawing me closer! i love my dc-5, but not having been able to play it for weeks....absence is not making my heart grow fonder....sigh

 

[Jimmy74]

Well thanks for all the tips, most of the electrolytics in the signal path are actually tantalums so they should be ok, there is only 1 electrolytic 1uf 50v on V6 that looks suspicious.

Anyways got another one up for grabs.... I changed out those 100k plate resistors on V1a, V4a & V4b and when I turned the amp on one of the power tubes started to red-plate.... so I tried swapping them around but no matter what tube I put in there it would red-plate. No I'm suspecting the bias caps, would it be one of the 22uf 63v electros on the board or is it tucked underneath the power tube board?

I did try getting some readings on the disc caps around the gain area and all was ok!!

Which LDR's should I look at?

 

[adam972]

the bias caps are on top of the board, i dont remember which pair of two it is....near the very large filter caps on the right side. I would just replace all 4.

There are two LDR's for the channel switching. They appear right near each other nead V1. There is a third that is located on the main board, near the GEQ, and it is used for the GEQ. I would suspect if you had a hum from a bad LDR, it would be the channel switching pair, but might as well replace them all. There are ways to test LDR's...i am not sure exactly how it is done, but the an online search, even Monsta may be able to help you. I think you just need to make sure the resistance of the LDR is changing properly when the amp is changing channels