Mesa Boogie Mark: Master Volume, Gain and power tube switch problems

[JCK Bavaria]

Hi there,

we are Hans and Christian from germany and working a bit with guitar amps.
I (Chris) am the musican and doing the communication and research in the net and Hans is the Tech. He built up a Marshall 2204 modell successful and knows how to measure the amp. Now we are on a point where we need your helpt to know how to find out a error in the amp, by analyzing single components in the amp.

At the moment we have two boogies, which have mysteriously the same problem.
The one is a Mark IIB and the other a Mark III blue stripe.

Both has three errors:

1) Power switch 60/100 Watts

If you switching, there is no effect to the amp.

2) Gain

There is less and very undifferentiated gain in the amp.

3) Master Volume

The amp has a very low loudness. Far away from 100 W power, which destroy your ears.
A very strange phenomenon is that in clean channel we can adjust the whole master volume from 0-10 and in lead channel we just can adjust from 0-2 and then is there no effect to the amp.

Its very weird. We adjust the amps with exactly the same setting to have a comparision.

The Mark IIB sounded very bad and we've done some replacements.
We changed one of the big 30 caps, which was leak and replaced the old  preamp tubes (tube by tube to hear the improvement) and tried out the power tubes.
This better the amp sound, but dont patch the bug.

Then we gave external voltage to the relais in the IIB. This was functional.

Maybe you have a clue where we can start to find the error.

Viele Grüße
Christian
 


 

[shooter]

Start with the schematic, I linked one from dougs library, believe the other is there also
http://el34world.com/charts/Schematics/files/mesa_boogie/boogie_mkiib.pdf

compare the voltages on the schematic to what you measure on your amp, note what's different by say 20%
then you have a start

[JCK Bavaria]

The vaultage points are pretty correct...

[shooter]

there is no effect to the amp

that would appear 2 of the tubes aren't doing their thing, but even at 60W it should be loud, so I'd guess you don't have enough drive to make any difference. 

If all your volts are *good*, then it's probably something in the signal path.  Do you have access to a scope, maybe build dougs "listening amp" so you can trace the signal path.

do you have a PA or some other amp where you can take the signal from "pre out" jack, that should get you half way.  You could even plug in an MP3 player or other *signal* source to the "return" jack and see what you hear.

[JCK Bavaria]

Yes, wenn Do have a Scope. Also we have a signal simulator.

[JCK Bavaria]

Well, we've done some expierience with signals.

First we tried to get a signal from our "listening amp", but there was a problem with mass, because the signal disapears after jacking in.
Then we get a signal out of a metronome. The first try was the guitar input, the second the send input.

The result was the following:

1) The sound was very much louder on the send input. Master on 2 and the ears begun to bleed. Correct volume. ;)

2) As we used the power switch there was effect on the volume.

3) As we pulled the lead master, the master had no effect to the amp.

Viele Grüße
Hans & Christian

[shooter]

The sound was very much louder on the send input

do you mean send OUT, or return IN?  guessing you're jacking a signal into the RETURN and you've proved the *PA* section is good enough for now.

but there was a problem with mass, because the signal disapears after jacking in

Not sure the meaning of mass.  if you can't get a signal IN the input jack to pin 2 of V1A, then I'd probably look for broke wire, bad jack, etc.

[JCK Bavaria]

There are 3 jacks behind the Mark IIB.
Send, Slave and Return. We'd tried all of em and got just a signal at "send" and connected the amp to a cab. 
The signal was much more louder at "send" as in the guitar input.

Mass = ground...Hans said this is a problem between the listening amp and the amp. Nothing to do with the problem.

[shooter]

We'd tried all of em and got just a signal at "send" and connected the amp to a cab

So you put a cable into SEND, then ran it to a 2nd AMP?, or you plugged the signal generator into Send and the MkII sounded louder?

[JCK Bavaria]

We put the signal generator (metronome sound) into send and the amp sounded louder yes...

(The signal generator dont work for now at the amp...we have to find out how to use it and avoid the ground problems)

[shooter]

into send and the amp sounded louder yes

so we can *assume* - for now - the PA section works.  Your next step should be injecting your sig gen into the input jack and use the scope - meter to figure out where the signal goes missing.  from your earlier post, you seemed to indicate there was nothing on V1A pin 2. - fix that and you can move to pin 1, then v1B pin 7 etc.

[drew]

If you have something in the effects loop -- either stompboxes or just a patch cord connecting SEND and RETURN -- does it make any difference?

[JCK Bavaria]

Update:

We solved the Problem with the ground and got the Signal in the Amp and are able get the Signal from the Pins.
Today we will Report you bout our result.

Viele Grüße
Christian

[JCK Bavaria]

Well here is the result on the scope: (Volume: 2)
Additional we found a 100 nF cap which has a uneven surface. It is going to the left side of the treble poti.

V1:

Pin 1: 2V
Pin 2: 50mV
Pin 3: 0V
Pin 6: 100mv
Pin 7: 0V
Pin 8: 0V

V2:

Pin 1: 200mV
Pin 2: 10mV
Pin 3: distorted signal
Pin 6: 100mV
Pin 7: 10mV
Pin 8: 0V

V3:

Pin 1: 0 V
Pin 2: 0 V
Pin 3: 0 V
Pin 6: 10mV: distorted & weak signal
Pin 7: 10mV: distorted & weak signal
Pin 8: 10mV: distorted & weak signal

--------------------------------------------------

V3 (with pulled Lead Drive)

Pin 1: 0 V
Pin 2: 0 V
Pin 3: 0 V
Pin 6: 10 mV
Pin 7: 10 mV
Pin 8: 10 mV

[shooter]

These are signal volts?  If so, V1a looks correct, but I'm thinking v1b (pin 7) should have more than 0. 
does playing with the tone knobs or vol 1 get any signal to the grid?  I'm used to seeing v1b with similar #'s as v1a, so say 50-100mV grid, 2-5v plate.  Re-verify your DC volts on V1b just for fun, spend some time trying to get more signal outta the TS

[JCK Bavaria]

Well, todays results:

1) DC Volts are correct

2) Input signal adjusted at 100 mV (V1 pin2)

3) Signal with Volume Poti adjusted to 100 mV (V1 pin7). Volume at 8.

4) signal from wiper contact Leadmaster (5V), from Master (2V; full until 10V)

5) Lead Drive pulled until 2V: V3 Pin 7, (adjustment Lead Drive: 10: Voltage 2 V)

6) Lead Master pulled; Signal at wiper contact until 50V (full ajustment)

7) Signal at Master Poti: Adjustment 9/ Volume: 9/Treble: 9 = 40 V

8) Signal V5 Pin2 until 7,5 V (full adjustment)

9) Signal V5 Pin7 until 5 V (full adjustment)

10) Power amp grid 1 120 V (full adjustment)

[shooter]

10) Power amp grid 1 120 V (full adjustment)

That *should* make your ears bleed AND distort the crap outta the amp!

I'm guessing you have the EQ circuit out - bypassed? If not try that.

Pin 6: 10mV: distorted & weak signal  Pin 7: 10mV: distorted & weak signal Pin 8: 10mV: distorted & weak signal

Is this still the case?  If so, does it "clean up" by adjusting lead drive?   Is the signal "bad" the rest of the way to the power tubes?
pull V4 and set REV pot to 0, do things get better?

[JCK Bavaria]

We tried signal from guitar, too.
The measure is nearly the same as the signal amp.
Its far away from ears bleeding...you not even get buzzing in the ears being on 10 with master.

As said in the incoming post, the master reacts 0-10 in clean and 0-3/4 in the Leadmodus. Dont know if this is concept of this Boogie amp.

Signal on the osciloscope is minimum 1/3 more using the 60-100V switch, but in the outgoing tune there is no difference to hear.

The EQ is bypassed by disable the EQ-Switch.

We tried pulling V4 and Reverb by 0. No difference.

The measure at V3 is a bit confusing. At Pin1, Pin2, Pin3 there is no right signal to measure. And it doesnt matter if the Leadmaster is enabled or not...what are the correct values at this tube???

[shooter]

At Pin1, Pin2, Pin3 there is no right signal to measure

V3A is your reverb recovery so I wouldn't spend time there - yet, unless it's messing up the main signal somehow.  V3B is important, you should have good signal in and out using a single frequency a guitar will just make things complex.

I'm still confused if you can put a signal into the FX and make the amp loud, why isn't it loud with 120Vac driving tubes that are biased around 50vdc.

With the amp off and discharged try making resistance measurements and cap measurements around V3B and verify they are with say 20% of rated value.

you can un-solder the wire coming off the REV pot wiper and eliminate V3A as a problem

try un-soldering the .001uF that is in parallel with the 330k on V3B's plate.

[Ritchie200]

Are you sure the EQ is being bypassed?  Can you hear or see the relay activate?

Jim

[Ritchie200]

You said BOTH amps have the same problems.  Did they BOTH go bad at the same time - at a gig or practice?  Or are they two different owners with the problem happening at different times?

Jim

[JCK Bavaria]

@shooter

we will try this, now...

@Ritchie

We bypassed it just with EQ-Switch.

Both amps have exactly the same problems, but the defect occurs on two different moments.
The Mark IIB (we working on now) was sent broken from the US. (Broken Speaker, broken on/off) and the defect on the Mark III happend in the rehearsal room. I remember I was pulling the Lead Master as the error occurs...

[JCK Bavaria]

@shooter

the values around the V3B are correct...

Do you mean just unsold the 0.001uF or replace it?

We noticed now something by playing around, changing all tubes and using different guitars.
There is no clear clean channel!

[shooter]

There is no clear clean channel

Like Ritchie said, verify you're switching the EQ also verify the lead relay is switching, verify the power for the relay.  In the 1st post you said you swapped preamp tubes, was there something that caused you to want to swap them again?   

just unsold the 0.001uF

just unsolder for now, if it's resistive, leaky, it *could* cause issues

[shooter]

this might be shooting in the dark, with amp off, unplugged and discharged, put your meter on resistance, clip the neg lead to the grounded center tap of the power transformer then use the other lead to probe EVERY ground point on the schematic, you should ideally have zero ohms, but some meters only get to 1 or 2, don't forget jacks, pots etc.  I usually print out the schematic, mark every ground in yellow, then X it once I ohm it.

[drew]

The switching jack in the effects loop of my Mesa .22+ (built in the same era as the two subject amps) after many years became flaky and wouldn't pass the preamp signal along cleanly and at full strength if nothing was plugged into the loop.  Hence my question above about trying a patch cable in the loop -- it bypasses the switch component.

[Ritchie200]

+1 That is a common issue on a LOT of amp problems!

Jim

[Ritchie200]

@Ritchie

We bypassed it just with EQ-Switch.

Check and make sure the relay is activating when you switch it.

Jim

[shooter]

make sure the relay is activating

I *think* the EQ is just a switch(foot?) - no relay, the lead switching is relay.  Least I can only find 1 relay coil, but the switch should be verified.  Also, looks like the ac signal runs through connectors, just after the lead/master pot, the EQ section, then into the PA, any corrosion there will muck up the signal.

[JCK Bavaria]

As we can see the Relais (for lead) is working. We proved Master volume Poti also. 0.001uF will bei unsoldered tomorrow...Tubes was changed, because the New ones was old ones...EQ has. no Relais just a switch in front

[Ritchie200]

Ooops, sorry about that.  I saw that the lead and reverb relay symbol looked the same as the EQ switch, I just assumed.

Jim

[JCK Bavaria]

Well...

1) We will unsold the 0.001uF tomorrow and look what happens.

2) We did measure the switches and the relais. Relais is switching optical and with external voltage.

3) We made some mp3 to make the defect listenable.

[shooter]

tomorrow and look what happens

look closely at any connectors in the signal path, unplug/replug them, verify they have 0ohms across the connectors, the schematic indicates maybe as many as 3

[JCK Bavaria]

Okay, to be sure we can it unsolder it from above?!

So here are the mp3 as .zip.
As you hear the clean channel is much more louder then the lead and also very dirty. Also there is less distortion in lead.
The adjustment on amp is: Volume: 8, Treble: 3, Bass: 1, Mids: 2,5, Master: 5 (dont know if it was on 3 in clean), Lead Drive: 5, Lead Master: 2,5...Presence: 3, Reverb: 0
In M2b 7, the leaddrive and leadmaster are on 7.

https://www.dropbox.com/s/kmit50smahiwhz4/M2b%20JCK%20Bavaria.zip?dl=0

[JCK Bavaria]

We did unsolder the 0.001uF. No effect.
Also we did the measure of the ground points. There are all correct.

What should we try next?

[shooter]

What should we try next?

I don't know your Techs skill set, but if it's me, I'm about ready for some more drastic measures;

look at the schematic and the sketch I attached, I would do a temporary - rewire.

1 photograph everything in it's original state as it is now
2. on V2A pin 2 unsolder and tape off everything you unsoldered
3. solder a 1M Resistor from V2a pin 2 to ground
4. on V1B pin 6, identify the .05uF cap, unsolder the *left* side in the schematic so it's still connected at the plate - verify with meter.
5. the *free end* of the .o5 cap, tack a wire to the cap and the other end to v2A pin 2.
6. pull V3 out
I call this *shotgun* troubleshooting

Verify your work till you are 100% sure nothing will short, arc, etc .

IF THERE ARE ANY DOUBTS, don't do any of this till you are SURE

power up and test

[drew]

+1 That is a common issue on a LOT of amp problems!

Jim

And yet, it is one which these gentlemen seem unwilling to even consider.   

I guess I'm done here.

[Willabe]

+1 That is a common issue on a LOT of amp problems!

Jim

And yet, it is one which these gentlemen seem unwilling to even consider.   

I agree with these 2 guys! 

We've seen this happen MANY times here, and I'm sure they have seen the same thing at other forums. VERY common problem.

When you trouble shoot an amp you try to start with the most common problem(s) and easiest to test. Rule those out and then go on.

A scope, signal generator, listening amp........  are great when you run all the basic/easy tests and still haven't found the problem(s). Then you use those, not 1st. 

[shooter]

I agree with these 2 guys

I believe there are still 4? things not checked, verified, but if they want to get in the deep weeds i'll go there 

[JCK Bavaria]

Hooray,

thank you very much for your mod and your support.
Think we have now a clue where to search the defect.

We got a very much improvement. The volume is at normal state now and we got a big ball boogie distortion.
Hans said after studying the shematic after work today, we avoid the lead section and its stuff and got a 1channel amp now.

We are happy bout this success! :)
What we should do next.

Danke und viele Grüße
Christian & Hans

[shooter]

we avoid the lead section and its stuff

So your problem could be parts associated with V3B - lead section, OR, the reverb section, or both, or.....

here's a "shopping list";

1 take a serious look at your relay, verify it works 10 out of 10 times BOTH contacts A and B
2 completely *isolate* the reverb section, pull V4, look at attachment; temporarily  remove the 150k resistor  that comes off the *left side* of the .05uF and goes to a .02uF
3 ohm the reverb pot's wiper to ground adjust pot so it's 0 or very close.
4 put the lead section back the way is was originally - OR just replace all the resistors and caps that "connect" to V3B - lead section (maybe $5 American)

again, take pics, tape off loose wires, be absolutely sure you understand what you're doing!!

glad you're making progress!

[JCK Bavaria]

well we are back now. We changed the caps who effect the V4 tube. We bought an cap measure gear and all caps had correct values...We had no effect

[shooter]

we are back now

So you do have both rhythm and lead working?
V4 is your reverb drive tube, that circuit also needs the transformer and tank to get the signal to V3a.
double check that you have something close to 335vDC on V4 pins1 & 6

you have a 2nd mesa?, if so does it have reverb, maybe swap tanks and see if you get verb sound.  I am pretty weak in the reverb circuitry, so maybe someone else can point out things I'm missing

welcome back

[JCK Bavaria]

Sorry, my mistake meant V3...What about the 60-100 switch error? At Oszilloskop we have an effect..but at the cab we dont hear an effect

[shooter]

the 60/100 switch is still "downstream" from your problem.  Need to get your lead and rhythm working.  Did you verify the relay?  What is working? Look at post #40, how much did you do?  What were the results?

[JCK Bavaria]

Ok, Voltage not correct (400V DC) at plate Pin1 & 6. after this wed disconnected V4 the reverb circuit. Resistor 150k disconnected. Result better than before. clean and distorted Sound better differenced.

[shooter]

(400V DC) at plate Pin1 & 6

This is on V3?  If so that's pretty high, schematic shows 220 - 250.  400 indicates you probably have something open?  What are your voltage readings on V3's cathodes (pins 8 and 3)?

[JCK Bavaria]

well, we did a measure about all pins:

V4 - Pin 1 & 6: (406V)
V4 - Pin 3 & 8: 3,4V

V3 - Pin 1: (285V)
V3 - Pin 3: 2,1 V
V3 - Pin 6: 221 V
V3 - Pin 8: 1,98V

V2 - Pin 1: 224V
V2 - Pin 2: 1,3 V
V2 - Pin 6: 250V
V2 - Pin 8: 1,75V

V1 - Pin 1: 273V
V1 - Pin 3: 2V
V1 - Pin 6: 155V
V1 - Pin 8: 1V

[shooter]

those voltages in () are high, but I'm not sure they're "bad" - yet.
So can we re-cap what's working and what's not?

When you are playing and you push the lead footswitch, there is a definite change?
when you are playing without pushing the footswitch the lead master volume has NO effect?

You have NO reverb effect at all either in lead or rhythm ?

when you flip the EQ IN it works, EQ OUT it doesn't?

You said the 60 - 100 switch has no effect in "loudness", does the amp sound like it's putting out 100w or 60w?

[JCK Bavaria]

1. if i pull lead function it has effect, if i push lead on footswitch it has effect.

2. now we have no reverb effect, because we disconnected reverb. If we has reverb connected there was no reverb effect as the knob was on 0.

3. EQ works if i flip EQ switch

4. Now without reverb, we can hear a bit effect to loudness, amp does NOT sound its putting out 100w/60w...