Mesa dc-3 issue

[Orion_99]

Hello! So happy this is working again.

I have a dc-3 that is sick. Low volume, but audible. Rhythm and lead channels are the same. Gain and tone stacks on both channels work, but the channel output knobs don't do anything. Output stage works fine, I ran another amp anythrough the fx return to test it. I have swapped tubes out and checked voltages. Everything is as it should be according to the schematic. I also tried a jumper in the fx loop. I'm thinking maybe something in the reverb circuit? Oh yeah, reverb doesn't work either.

Ideas??

Thanks.

[Orion_99]

So after checking out the voltages again, I have found that pin one of v4, the reverb tube, is 30 volts over what it should be. Pin 6 is about 10 volts under. Also, using the "click" test, I don't get anything when I touch pin 6.

Very frustrating.

[onelife]

Do you get full Reverb volume if you shake the amp Jon Lord style?

 

Do you get regular volume from the record out or is it low as well?

 

There is a solid state section of the amp that the signal goes through as part of the Graphic EQ. It is just before the effects loop in the signal path and it has its own independent power supply of either 8 or 13 volts - if there is a problem with that power supply then I could see it resulting in the symptoms you describe.

 

A cheap and dirty regulated power supply uses a zener diode and a power resistor. Sometimes the resistor gets overheated because of the amount of power it is forced to dissipate and its value or connection to the printed circuit board may become compromised.

[WRGKMC]

You need to signal trace the circuit.

 

You input a tone to the input jack. Then you can use a small battery operated amp of maybe 1W or so.

Its going to be your test probe.

 

What you do with the input of this little amp is make a lead that plugs into it. On the other end that you'll be using as a

probe is extend the shielded ground of the cable and solder an alligator clip to it. This gets clipped to Chassis Ground.

 

The Hot wire will be your probe. You have to add a capacitor to the end of the hot wire so it wont short out any voltages

in the circuit. A .5UF to 1uf cap should be more than sufficient to block circuit voltages while allowing the little battery amp to

pick up the signal being passed through the circuit.

 

You then begin at the input jack with the amp running. Go to the next component, likely an impedance matching resistor, check its

input and output. There is likely a little drop in strength. Simply use the probe amps volume to adjust it as needed.

Next item may be the volume knob. You test the input and output legs of the pot and make sure the signal increases as you

tweak the knob. Next may be a cap resistor combo and a tube input.

 

You go along like this with schematic in hand and trace the signal from beginning to end.

When you get to the point where you have an input and no output, weak output, distorted output, you have

likely localized the problem (if sound goes in one ear and doesn't come out the other it likely met with an obstruction)

 

Then you just have to figure out if the components bad or its not receiving the proper voltages to bring it up to proper operating levels.

If its a tube you can simply swap it. If nothing changes, its likely the voltages that bias it and you'll have to move to the Power supply and check

it using a VOM. (Volt Ohm meter)

[Orion_99]

And this is why I have missed this forum so much. Your wealth of information is priceless.

 

I don't have a small amp to use, although I will have to pick one up as that sounds like a fantastic idea. I did some more voltage testing from the 70 volt lead, which is reading at 48vac from the PT, odd...

There are a couple of points within the circuit where the voltage disappears. I have narrowed it down to one cap, which reads the proper capacitance but is the most likely culprit. The zener diode that is immediately after the cap reads the same in both directions with the cap in place but properly with it removed. The resistor preceding the cap is the proper value as well.

 

I am hoping that I am on the right track. If not, it is a cheap thing to try.

 

Thank you for your help.

 

[onelife]

You need to signal trace the circuit....

 

... You go along like this with schematic in hand and trace the signal from beginning to end.

 

Although I agree with your basic idea, I find this method of troubleshooting quite tedious.

 

What I do is find a point near the middle of the signal path (FX loop for example) and determine if the problem is before or after the point. I'll then find another mid way point in the half of the circuit that has been determined to contain the fault. I only get specific and go component to component when I've narrowed the problem down to a particular area of the circuit.

 

In this case, since we already know the final stage is okay (and therefore so is the B+), I suspect the graphic EQ is where the signal is being blocked so I'd run your test but go from the input right to one of the Master Volume controls to check for signal continuity.

 

The graphic EQ is in between the Master Volumes and the FX loop and, as I mentioned earlier, has its own power supply. The very first thing I would do is check that power supply.

 

I'm still curious as to whether rattling the Reverb springs will result in an output from the amp.

[Orion_99]

I have left the reverb tank disconnected for most of the troubleshooting. I did connect the tank from my dc-2 to test it at one point and it didn't make a difference.

There is definitely something going on in the power supply to the eq. I have -34 volts on one side of a resistor and +.7 on the other side. I'm not sure what would cause this. I suspected a cap, but it tests fine.

Would the lower voltage from the power transformer that feeds the circuit cause all of this, or can something in the circuit cause the lower voltage? The latter doesn't seem to make sense, but at this point I want to cover all of my bases.

Thanks again.

[onelife]

There is definitely something going on in the power supply to the eq. I have -34 volts on one side of a resistor and +.7 on the other side. I'm not sure what would cause this. I suspected a cap, but it tests fine.

 

Do you know what resistor it is? Which side has the -34 volts? I found a schematic online so if you can give me the resistor number I'll have a look.

 

 

[Orion_99]

The schematic that I have doesn't have any resistor or cap numbers. This actually occurs in a couple spots. If you look at the blue 70vac power supply circuit, there is a 4k7 on the left side and a 3k3 after a 1k on the right. On the actual board, the 3k3 is another 4k7, which was there from the factory. On the right one, it reads -34 at the junction between the 1k, 3k3 (4k7) and the negative of one of the caps. it's .7 on the other side of the 3k3 (4k7). On the left one, I can't remember the numbers exactly, but I want to say -40 something volts between the diode and the 4k7 and around 1v after the 4k7.

 

I hope I explained that enough. Maybe you could shoot me a link for the schematic you have if not.

 

I have been going to my dc-2 to see if what I find on the dc-3 is weird, and this is the spot that it is. I show the proper negative voltages on the dc-2 on the sides that I get the very low positives on the dc-3. The circuits do differ in that area, but the are similar.

[onelife]

It looks like something is pulling "the other side of the 3k3 (4k7)" to ground and, as a result, the -34vols is being dropped across the resistor. It could be that the filter capacitor or one of the transistors in the EQ circuit is experiencing a short circuit.

 

the schematic that I found does not have resistor or cap numbers either but your description was more than adequate

[Orion_99]

I decided to double check my readings and I also checked voltages through a few capaitors. More accurately the numbers are -35.2v before and 7.1v after. The 220uf capacitor on the left reads 35.2v and the right is -7.1.

I was way off with the other numbers. I get 59.2v before and 1.05v after. I'm not exactly sure how zener diodes work, but could this low voltage be preventing it from opening up?

I'm still wondering if the lower voltage from this winding may be the whole problem.

[mbengs1]

I find the graphic eq on the mesa amps that have them are set like a 'V'. too much midrange on those mesa's.

[onelife]

I'm not exactly sure how zener diodes work' date=' [/quote']

 

A diode will only pass current in one direction when it is "forward biased". If the polarity is reversed then it will not pass current. A Zener diode will attempt to maintain a constant voltage across itself when it is forward biased.

 

A simple regulated power supply could be a Zener diode in series with a resistor. If a five volt Zener in series with power resistor is placed across a ten volt supply then the Zener will maintain its five volts and the other five volts will be dropped across the resistor. The regulated five volts would then be taken from across the Zener.

 

The required value and power rating of the resistor will be determined by how much current will be drawn through it by the combination of the Zener diode and the load.

 

If there is a short circuit in the load then more current will be drawn through the resistor creating more of a voltage drop. This added load may even affect the output voltage of the transformer winding.

 

 

[Orion_99]

And now I know. Have I mentioned how I love this forum?? Any tips on how to easily find the short? I have read that capacitors that are shorted show low resistance. All that I have checked are in the mega or kilo range. I'm betting that when I do find it, it will be something obvious. Thanks.

[onelife]

I notice that transistors Q1, Q2 and Q3 all have -7.x volts on the emitter. If any of these are shorted collector to emitter then the collector voltage supply may be pulled down to that voltage as well. My initial suspicion is that one of these transistors could be the problem.

 

Transistors consist of PN junctions that are similar to those in diodes. If you measure a diode with an ohm metre you will find a high resistance in one direction and a low resistance in the other direction. You can test a transistor the same way but doing so in circuit is not reliable however it may be worth doing if the transistor is a short circuit - especially in this case where you can compare the results with the other transistors in the same circuit.

 

Another thing you can do is carefully measure the voltages on the individual leads of the transistors. A short circuit will result in the same voltage appearing on two or more of the leads. The circuit diagram that I have has the expected transistor voltages written on it.

[Orion_99]

I have checked out the transistors before and I checked them again, without power. They all tested OL one way and around the same the other, except for one. I checked the same one in my dc-2 and it read the same, so I figure they aren't the problem. I will check them for voltages again after dinner. Why can't there be little lights that light up to show where the problem is?? Lol.

[onelife]

Why can't there be little lights that light up to show where the problem is?? Lol.

 

A common problem with tube amps, especially heavy ones with wheels, is that the power tubes eventually short circuit. I was playing through a Twin Reverb one night when one of the 6L6s shorted out and blew the fuse. I slid the chassis out far enough to see the destroyed screen resistor that had taken the full 400+ volts because of the short. I was able to identify the defective tube, remove it and one from the other side of the push pull, replace the fuse and finish the show.

 

Unfortunately (or fortunately) the five to twenty five volts used in solid state circuits does not leave the same obvious trace as 400 volts when things go wrong.

[Orion_99]

I tried testing the optocoupler in the recording out circuit and it was different than the one from my dc-2, so I thought I would pop the dc-2 one in the dc-3. It didn't make it come to life, but the voltages on the left side of the schematic we've narrowed in on improved to over 11 volts after the 4k7. At least it's something. I will replace it with the proper one and see what happens. Finding one close by may be a challenge though.

[Orion_99]

So I thought what the hell, pull the octocoupler and see what happens. It works!

Now does that mean that octocoupler is the culprit, or does it mean that something beyond it is causing the short?

in the mean time I can finally find out what this puppy sounds like.

[onelife]

Is the optocoupler labelled TL072 in the circuit diagram?

 

If it is then it looks to me like a short circuit in that component would be enough to load down a portion of the power supply. If the amp works without it and you do not intend to use the compensated record out then it looks like you are good to go.

[Orion_99]

That's the one, although im still not getting 70 volts on the blue wire from the PT. I don't need the record out, but I will still replace the optocoupler to make sure that was the issue. Looks like I can get 20 of them for under $3 with free shipping on ebay so why not.

Thank you so much for all of your help. I have learned a lot trying to find this issue.