Mesa Mark III purple stripe Humming.

[diydidi]

Hi
I have this amp for repair. Its a purple stripe. No Graphic EQ, and non- simul-class. It has 2x 6L6's.
Cleaned it up, re-valved it, clean controls, etc...
Lead channel wasn't working, found an open-circuit 470 ohm res in the LDR led drive circuit. Now works like a charm.
Only one problem.
The amp now has a hum when turning up the master volume. If i engage the lead channel pull switch, it goes away.
If I plug a signal into the return socket and turn up Master Vol, its gone and clean.
I have narrowed it down to Valve V3 section. Before V3 no hum. After V3 where it goes into V2 again, on the Pin 1 of V2, I get this annoying loud hum.
FYI, I used a signal tracer amp to check this.
I'm not hearing it on V3 plates, but this might just be that there isn't much gain there?
I have checked all the filter caps for ESR and Capacitance. All check fine.
In the schematic they use 1k resistors between the B, C, D supply points. I even went so far as to replace the first 1k with a 5k6 as in the mark 2 schematic. Doesn't really make a difference. That tells me the fault isn't in the power supply; I hope...
Another thing I forgot to mention, is that this amp has the reverb tank removed. I tried shorting-out the RCA in/outs to the tank, but no difference.
I played around with different valves in position V2 & V3 still no difference.
Thoughts??

[diydidi]

Here is the schematic for the standard Mark 3.

[Audiotexan]

Lead channel wasn't working, found an open-circuit 470 ohm res in the LDR led drive circuit. Now works like a charm.

Nice!

The amp now has a hum when turning up the master volume. If i engage the lead channel pull switch, it goes away.

To me, that should say, there's no problem in the V3 section...As while you're in lead mode, everything is A-Ok right?

Are you in Rhythm 1 (normal) or Rhythm 2 (crunch) channel when you have the hum, or does it hum in both of those channels?

If I plug a signal into the return socket and turn up Master Vol, its gone and clean.

Ok, that (reconfirms and) clears the power amp (again)!
But we could safely assume that before, given that you said that when in lead mode, that there was no hum! Always nice to confirm though!

I have narrowed it down to Valve V3 section. Before V3 no hum. After V3 where it goes into V2 again, on the Pin 1 of V2, I get this annoying loud hum.

Personally, my train of thought at the moment is, that something is either:
a. injecting slight PSU ac into the signal path at v2, while not in lead mode.
b. something in the v3 portion of the ckt is somehow providing a 'missing' ground while in lead mode, for something that's spilling over into v2 while v3 is NOT engaged (thus able to take up the slack when it is.)

FYI, I used a signal tracer amp to check this.
I'm not hearing it on V3 plates, but this might just be that there isn't much gain there?

No, v3 is where the majority of the gain comes from, it's the /lead/ channel after all. So trust your readings, and I'd believe it that you're not seeing it on v3 plates. Again, no hum on lead mode (which is all v3 does!)

I even went so far as to replace the first 1k with a 5k6 as in the mark 2 schematic. Doesn't really make a difference. That tells me the fault isn't in the power supply; I hope...

I'd pop that 1k back to where it was, and look elsewhere.

Another thing I forgot to mention, is that this amp has the reverb tank removed. I tried shorting-out the RCA in/outs to the tank, but no difference.

Just for reference, on the MkIII's, the input side of the rev.tank is grounded to chassis. The return is insulated.

I played around with different valves in position V2 & V3 still no difference.
Thoughts??

I'd check the caps/grounds in the area between v1b and v2a. Also the 100k to ground (even though that one should be ok, as it's helping to tie the feed to a set level for the input to the lead channel, which you say works.)

[diydidi]

Just a follow up.
Checked all of the resistors all fine. I noticed that if I remove valve 3, the hum is almost gone. Signal to hum ratio MUCH better. Of course Lead channel doesn't work then.
ALSO, with the Lead Drive pushed in (lead not activated& V3 back), It still has an effect when turned. This tells me LDR2 &LDR4 either on or leaky. Shouldn't LDR2&4 only be on when Lead is activated?
Confused

[Audiotexan]

Ok, from the sounds of it, this thing has multiple problems.
But I'd recommend addressing them one at a time.
-We know there's been supply issues to the LDR section. Especially Lead channel related...
-Hum is (potentially/probably) another issue.

Just a follow up.
Checked all of the resistors all fine. I noticed that if I remove valve 3, the hum is almost gone. Signal to hum ratio MUCH better. Of course Lead channel doesn't work then.

Ok. Scrap everything I said before for the moment (as that was based on my understanding that I thought you said when you pull the lead channel switch that everything worked without hum. -assuming you were NOT talking about "Pull Bright" on the Lead Master pot-).

The amp now has a hum when turning up the master volume. If i engage the lead channel pull switch, it goes away.

First question: So, with all valves in place, does it hum when in the lead mode?

ALSO, with the Lead Drive pushed in (lead not activated& V3 back), It still has an effect when turned. This tells me LDR2 &LDR4 either on or leaky. Shouldn't LDR2&4 only be on when Lead is activated?

This is a separate issue. However, yes, those two LDR's should turn the lead channel on/off.

Did you check the voltages in the LDR section after you replaced the blown 470 ohm resistor?

[diydidi]

Ok, from the sounds of it, this thing has multiple problems.
But I'd recommend addressing them one at a time.
-We know there's been supply issues to the LDR section. Especially Lead channel related...
-Hum is (potentially/probably) another issue.



Ok. Scrap everything I said before for the moment (as that was based on my understanding that I thought you said when you pull the lead channel switch that everything worked without hum. -assuming you were NOT talking about "Pull Bright" on the Lead Master pot-).



First question: So, with all valves in place, does it hum when in the lead mode?
No It doesn't.



This is a separate issue. However, yes, those two LDR's should turn the lead channel on/off.

Did you check the voltages in the LDR section after you replaced the blown 470 ohm resistor?

With all the valved in, and lead activated, the problem goes away.
With the Lead Off, the led's of the LDR's are getting a reverse voltage of about 2,3V.

[diydidi]

Found the fault.
Some asshole replaced one of the Optocouplers with another type, and also had the Led side of it soldered-in with wrong polarity.
Replaced with a new one and problem solved. Nice separation between lead and rhythm channels, and also no more HUM!!!!
Thanx for all the help.
Just one thing, I think the LED's on the schematic are drawn the wrong way round. In my books the line is the cathode, at least that's how I was taught.
Will have to make a note on my schematic.

[Audiotexan]

Wow. GREAT catch!!

Bad enough that they used the wrong one, but reversed LED leads on the LDR would definitely jack things up. Will have to add this one to the books.

Congrats on the fix =D

[Audiotexan]

Sorry I missed answering this earlier. :x

Just one thing, I think the LED's on the schematic are drawn the wrong way round. In my books the line is the cathode, at least that's how I was taught.

You're absolutely correct, the cathode is still the side with the line.

Assume you're talking about LDR2/LDR4? There's really no 'orientation' seen there, as that's the 'switch' (resistor) portion of the LDR being applied and completing the circuit. The LED section (that holds the cathode/anode) is shown in the PSU section of the circuit where you replaced the 470 ohm. That's where the direction is important!

[diydidi]

Don't say sorry. You helped plenty!!
Yes, I'm referring to the power supply section where the LED section of the optocouplers are. They are drawn wrong in the reverse.

[Audiotexan]

Don't say sorry. You helped plenty!!
Yes, I'm referring to the power supply section where the LED section of the optocouplers are. They are drawn wrong in the reverse.

They're just showing you that it's LDR2/4 that IS the switch, as without having the LDR/LED notation there, we'd be looking for a physical switch.

The non-LED side of the LDR has no 'polarity'. It's just a resistor in essence, that varies in value dependant upon how much light hits it.

That's why you were getting 'bleed thru' because the voltages were being tossed around on the LED side, and not hitting '0v' (to shut the signal off completely) which is why the 'Lead' control was affecting things even when you were not in 'Lead' mode. [read as: 0v on LED side = infinite resistance on other side of LDR, or the 'switch' at LDR2 is open]

But yeah, the LED is shown "backwards" in the preamp section, and would be blocking signal IF the LED connection was made at that point, except it's not.
Hopefully you follow me.

EDIT: wait. It juust hit me you were talking about the PSU section. Like a dog with a bone, I locked in on the preamp section...*goes to take another look*

EDIT2: I believe you're correct. But its also way late for me, and I know I'm overthinking it at the moment, as I've already confused myself twice at this point trying to make sense of it as drawn. I've been up over 17 hours now, so maybe fresh eyes/mind later will help. lol

[Audiotexan]

I'm referring to the power supply section where the LED section of the optocouplers are. They are drawn wrong in the reverse.

Confirmed. You are 100% correct!

It seems intentionally drawn up to mislead folks. *shakes fist at R.S.*

For reference:


I had to actually dig back into gutshots I took of my purple stripe to confirm.


If I took my notes correctly while talking to Mike B some time ago...
In my pic, the bottom right is the EQ Vactrol for the EQ (VTL5C4 aka LDR5), the one directly left of that (and behind the Lead Master pot) is the "Lead Out" LDR (VTL5C4 aka LDR3) to the left of that (directly behind the Master pot) is the "Lead In" LDR (VTL5C1 aka LDR2), and then to the immediate left of that is a double-stacked LDR pair that I'm iffy on...I know one's Rhy2 for the relay...but I didn't take notes as to whether it was the top or bottom, and I didn't note at all what the other one is in that stack. :x

Anyways, if you look between the "Lead In" and "Lead Out" LDR's, you'll notice the zener with it's cathode tied to the 'inner' rail. With the 'slant or notch' on the LDR (indicating cathode) being on the outer rail, that means the inner has to be anode of the LDR. So it would seem that the cathode of that ZD goes to the anode of the LDR. As you suspected.

As you go on across the bottom of that pic, they seem to keep it uniform (as would be expected) and each cathode from an LDR goes to the outer rail (or closest to the edge of the pcb).

Just for reference, it would appear that LDR5 (for the EQ) is drawn backwards on the schem as well. Since it's definitely showing the 680 ohm being tied to the anode of that LDR.

Interestingly enough, if I'm reading it right, that's a 4.7Meg that's coming off of the cathode of the EQ LDR. Not 470 ohm as shown on the schem. But on the plus side, the anode of the 4006 that goes to the EQ footswitch is being shown correctly.

So it seems there's far more discrepancies than I remembered on this schem. I recall when I had to get heavily into one 10-15 years ago, that you really had to take Mesa schems with a grain, as they were famous for intentional errors/ommisions, but MAN.

If this doesn't serve as a potent reminder to take their stuff as a guideline and not "gospel" I don't know what does.

edit 2: had anode on the brain evidently when I was talking about the "4.7meg" in question..

[52 Bill]

Interestingly enough, if I'm reading it right, that's a 4.7Meg that's coming off of the anode of the EQ LDR. Not 470 ohm as shown on the schem.

There are a number of 4 band 1% resistors in the photo, so could it be a 475 ohm resistor?

[diydidi]

There are a number of 4 band 1% resistors in the photo, so could it be a 475 ohm resistor?

You are correct. It is a 475ohm resistor.

[Audiotexan]

You are correct. It is a 475ohm resistor.

Good catch Bill, and ty for the confirm Didi
Precision resistors still mess with me on occasion. (Obviously) lol