Mesa boogie DC-3 need help

[angelothewolf]

Hi there, got this mesa with low level on preamp section, injecting silnal through return and pwr amp work as it should.

got levels right from jack input to 680k & 220k resistors before master pot on both channels. wich in theory is right because that resistors and the 25k master pot form a divider.
But the strange thing is that i have a post EQ (not graphic eq) signal from both channels getting into the power amp regardless master pot setting.

I cant find a path on the schem where preamp signal can reach the pwr amp before master pot...

any ideas»»

http://www.thetubestore.com/lib/thet...-Schematic.pdf

[52 Bill]

How much signal are you talking about? Leakage from the relays/optos?

[Enzo]

Not all paths are on the schematic. Traces with high signal levels can radiate signal, and a nearby trace can pick it up. This is an example of crosstalk. The classic example is plug a hot signal into channel 1 of a mixer, then bring up channel 2 fader and hear faintly what is on channel 1. That path will never be on a schematic. Even if not by radiation. a lack of good decoupling can allow signal through the power supply rails.

[angelothewolf]

Not all paths are on the schematic. Traces with high signal levels can radiate signal, and a nearby trace can pick it up. This is an example of crosstalk. The classic example is plug a hot signal into channel 1 of a mixer, then bring up channel 2 fader and hear faintly what is on channel 1. That path will never be on a schematic. Even if not by radiation. a lack of good decoupling can allow signal through the power supply rails.

Well i was thinking about that, the mesa PCBs are pain in the ass btw. So what stops signal flowing trough HT rails from one stage to another?

Another thing is that this schem is not exactly the circuit I'm working with, I found some parts of the circuit (like the switching matrix) that is more like the dc-5 schem...
Thanks for the help, I hope I can figure this out tomorrow...

[Enzo]

Decoupling caps. That is what decoupling means: it does not allow two stages to couple through the power supply.

In a typical tube amp, the first filter after the rectifier is the reservoir cap, and it is usually where they take the B+ for the OT from. The second filter cap usually serves the output tube screens. By this filter stage, pretty much all ripple is gone - or should be. The next couple filters are for the B+ nodes serving the preamp stages. They are more for decoupling than they are for filtering ripple.

[angelothewolf]

Decoupling caps. That is what decoupling means: it does not allow two stages to couple through the power supply.

In a typical tube amp, the first filter after the rectifier is the reservoir cap, and it is usually where they take the B+ for the OT from. The second filter cap usually serves the output tube screens. By this filter stage, pretty much all ripple is gone - or should be. The next couple filters are for the B+ nodes serving the preamp stages. They are more for decoupling than they are for filtering ripple.

I see, in this case the 30uf/450v caps right? those and the resistors are just fine. I think I am missing some path on the circuit that is not in this schem. Also check the optos and relays and are working fine. Its a lot of signal to be some kind of crosstalk.
Im suspecting the EQ and channel switching circuit, that btw is the most common problem i saw in mesas.

All the voltages are fine compared to schematics (really close actually).The only thing that not match is the graphic EQ and FX loop circuit ("preamp part 2").

have one 2200uf/35v cap (that is not on the schem) providing -30VDC directly to the Q1 base the cap was a little bloated btw.

another fact: inserting a p10 only on send jack (with fx mix pot on 100%) does not cut the signal. neither pulling V3 out.

Im trying to understand now what are that -30V doing there...


thanks!

[angelothewolf]

almost there...

well I found that the .1/50v cap at the EQ output really was a 1uf/50v, and it was dead!. replace it and the weird crosstalk went away. Now the master control on each channel works as it should, but still getting a low signal level at EQ output. Now my last problem persists on graphic EQ circuit. Voltages are more like the Mark III/ IV schematics though. My "F" supply is about -30VDC, everything apears to be right, the transistors are healty, but if i pull out Q4 out of the circuit, everything works fine with plenty of power, but no EQ...

DC-3



MarkIII

[MarkusBass]

have one 2200uf/35v cap (that is not on the schem) providing -30VDC directly to the Q1 base the cap was a little bloated btw.

This is rather impossible. Aren't you confusing pinout of the Q1 transistor? The power supply is connected to collector of Q1. The voltages provided on the schematic are also very confusing. On the equalizer page the "F" rail is specified as -13V, but on the power supply page it is specified as -8V. Both cannot be correct. So there are mistakes on the schematic. I also have serious doubts about the 0.1uf/50V electrolytic capacitor. This is very unusual value. Have you seen another capacitor of this capacity in this amp? I would say that the correct value is rather 1uF.

another fact: inserting a p10 only on send jack (with fx mix pot on 100%) does not cut the signal.

This is not a surprise since the loop is parallel. This is how parallel loops work .

neither pulling V3 out.

This is difficult to explain. And if you short the grid of the V3 tube to the ground, do you also have signal on the output?

I'm trying to understand now what are that -30V doing there...

Other versions of Mesa amps have -30V power supply for the equalizer circuit. So maybe -13V and -8V on the schematic are both incorrect (I wouldn't be surprised knowing how many mistakes are on Mesa schematics) .

Mark

[angelothewolf]

yes I confuse the Q1 pinout but i have -19V on Q1 base. This schematics are very confusing indeed. still trying to figure this out, now thats the situation, if I take Q4 out of the circuit everything works fine, but graphic EQ doesnt work. with Q4 on circuit we have low EQ circuit output with a bizarre distortion... got a problem with this simple Eq circuit but i cant find it...

[g1]

Have a look at the DC2 schematic, it has the -30V at EQ transistors.

[MarkusBass]

At least the voltages make sense now. But the 4744 15V Zener diodes in the power supply are marked as 12V. I don't know what they were drinking when they were drawing their schematics . There are so many mistakes on their schematics that they shouldn't be considered a schematic but rather a guideline . Sometimes it matches the actual circuit, sometimes it doesn't.

@angelothewolf: have you tried checking other transistors? What voltage do you have between base and emitter of Q1? You know that for the equalizer to work, the lower end of the filter has to be grounded. Is it?

Mark

[g1]

I don't know what they were drinking when they were drawing their schematics . There are so many mistakes on their schematics that they shouldn't be considered a schematic but rather a guideline . Sometimes it matches the actual circuit, sometimes it doesn't.

This seems to be the case with many mesa schematics. Using them as a guideline that needs to be verified is a good approach.
And in some cases, like this one, you have to combine bits from various models/versions.