Mesa Boogie Mark II troubleshooting

[gar13]

Hi all,

I have a Mesa Boogie Mark II on the bench. This version has the EQ, but no Reverb.

I have two problems, the first is the EQ section. It isn't passing any signal when switched into the circuit. The toggle switch seems to be working fine I've traced the circuit with a listening amp. I've got signal through the circuit until I reach the piont circled in Red below. I have replaced all transistors with 3 x 2n3906 and a 2n3904. Schematic mentions that any general purpose transistors should work as replacements, but I'm not good at tran. specs.

Can anyone see why the signal might disappear at this point? My first suspect is the 3.3k to ground, I may blindly replace this evening and check results. Any other ideas?

Second is the lead channel relay.  I can't get the lead channel to work. I see 4volts DC on the relay power supply. I can't find the power supply cap for the relay anywhere on the board, anyone know where it is usually located?

[Willabe]

I believe that there are 3 versions of the Mk. ll, a, b and c. Probably only slight differences, but you never know. Schemo you've posted say's Rev. b.

It shows -dc. at different points along the 4 transistors, did you check those? Are they right? If you find a missing/wrong -dcv you found the bad stage. Then you have to figure out why the -dcv is missing/wrong and fix it.

Also see if there's -dc on the other side of the 10uF@63v. I don't know where that goes but it's supposed to block that dc.


                                Brad    

 

 

[gar13]

Thanks Willabe,

I'm looking at the schemo as a guide, I think there are differences in component values, but the EQ circuit is exact. I didn't have a chance to look at the EQ circuit last night.

I did get the lead channel relay switching working. I but  I found on the end of the board that the relay PS cap was cut out. I replaced the cap, and the relay now works, but I get major buzz with the lead circuit engaged.

Is there anything wrong with the way the circuit is designed that would lend itself to buzzing? Should I be looking in the relay circuit for the source of the buzz, or could it be something with the lead channel?

[Willabe]

I found on the end of the board that the relay PS cap was cut out. I replaced the cap, and the relay now works, but I get major buzz with the lead circuit engaged.

Are you saying someone removed the B+ filter cap for the relay B+ supply? I wonder if they disabled it because it was noisey or causing some other problem?    

the relay now works, but I get major buzz with the lead circuit engaged.

Is there anything wrong with the way the circuit is designed that would lend itself to buzzing? Should I be looking in the relay circuit for the source of the buzz, or could it be something with the lead channel?

          

Maybe another bad B+ filter cap? Did you use the same uF value or a little bigger value cap for the relay?

I wonder if the internal contacts in the relay are bad? That could cause distrotion, maybe a buzz if bad enough?



                      
                                Brad        

[gar13]

Are you saying someone removed the B+ filter cap for the relay B+ supply? I wonder if they disabled it because it was noisey or causing some other problem? 

Yes, the cap circled above in red had been removed. Not sure why, the amp was given to me in current condition.

Maybe another bad B+ filter cap? Did you use the same uF value or a little bigger value cap for the relay?

I'm going to check the 22 uf after the relay tonite. I replaced with 470uf as in the schemo. I may test a 1000uf paralleled as a quick test to see if higher value will help.

Does it seem strange that the relay circuit uses the chassis ground for the diode and cap after the relay coil? I thought best practice was to float relay circuit grouands as in Doug's scheme?

[Willabe]

You might want to check the 30uF@500v for the B+ PSU at point D. that cap feds the pre amp and lead channel tubes.

In fact the _ whole _ amp probably needs a cap job. If that amp was built around 1980 or so, it's time. 

[gar13]

I replace all caps in the B+ already, sorry I should have mentioned that. I haven't replaced smaller electros in the rest of the circuit as of yet.

[Willabe]

Those should be changed too, to be on the safe side. They can cause weird/strange things to happen also.

[gar13]

The relay power supply has some differences from the schematic version. I've drawn it out below.

1) Should I move the diode to the other side of the relay coil as in the Mesa schematic?

2) There isn't a second cap in the amp, should I add it? Before or after the coil as in the Mesa schematic?

[gar13]

I forgot to mention, with the expection of the removal of the 470uf that I replaced, this configuration looks like it came from the factory this way, even though it differs from the schematic.

Can someone explain this type of rectification? It it half wave? If so, what is the diode to ground doing? Does in need the seond 22uf cap to function correctly?

Thanks in advance for any response :)

[Willabe]

gar,

I think it is  1/2 wave.
 
I think that the 2'nd diode, on your drawing, (B+ to ground) is not a rec. diode, but it's a zener diode being used as a voltage reg? Cheap way to limit the voltage for the relay.

First off, I think the Mesa schemo is wrong. It might be a typo or it might have been done on purpose to confuse copy cats? Mesa has been funny this way.

The Mesa schemo shows the the 1'st diode across the 2 heater legs, which I can't see as being right. Then the 2'nd 22uF cap is after the relay, (your's doesn't show it) I don't know why they would do that. I would think it would go right before the relay. The way it's drawn, when you step on the FSW that would short the B+ end of the 22uF cap to ground.

If it was realy before the relay, then the 2'nd cap might not be needed for the relay to function properly all the time. It will smooth out the ripple a little more on the relay B+. Mesa might have decided at some point it was not necessary, so out it goes. Save a little $$ parts and time.

OTOH, it would be interesting to know where that 2'nd 22uF cap was placed, was it close to the relay? If so, they might have placed it there as a little B+dc reserve for the relay?

Since your not building dozens/hundreds of units/amps you can leave it in. I think it can only help.

[gar13]

Thanks Willabe,

It's not a zener, It is a 1N4007, same variety in the rest of the amp.  I'll tack in a 22uf and see if it helps the hum any.

[Willabe]

gar, you better wait for one of the other guys to help you here, cause I'm just shooting in the dark.     

               Brad       

[Willabe]

It's not a zener, It is a 1N4007, same variety in the rest of the amp.

I think then that diode should go directly across the relays coil (low resistance path for the fly back current) and not to ground.

[PRR]

> Does it seem strange that the relay circuit uses the chassis ground

Yes, it is a very strange circuit. It shouldn't hang off-sides like that, though that does not seem to be a killer problem. It does not seem to reach 5V across the relay, and it is a very ugly waveform. 470uFd is really low.

The second diode and 22u cap "do nothing". They probably suppress a transient when the relay turns off. It will work without those. I'm not sure why they are shown in different places.

What is the chance of putting a dedicated transformer in there just for the relay?

Did you ever check DC voltagse through the EQ?

[gar13]

Thanks PRR,

I think I'll tack in a 4700uf and see what happens. I probably could use a separate transformer, but I'd like to see how far I can get with stock. There are few thousand of these amps out there, and the channel switching works ok on them (although its known to POP on switching). If I did try a separate transformer, would I have to remove all connections to chassis in the circuit?

I did check voltages in the EQ before original post. They were within the 20% tolerance mentioned on the schematic, although on the higher end of the range. I'll measure again and report on exact values.

[gar13]

Voltages as measure, some seem on spec, other maybe a little high...

The -41 was measured before the 10uf cap, at the point where -31 is stated.

[PRR]

> some seem on spec

No, it's pretty forkked-up.

Replace Q1. Watch your E-B-C... pinouts vary.

[gar13]

The 4700uf in the relay power supply fixed the switching and noise problem, the lead channel now works flawlessly.

I did finally figure out the EQ. I forgot that I had a B&K 520 transistor tester. I wish I had used this early. It identified two of the original transistors in Q2 and Q3 pinouts and that they were functioning perfectly. I picked up some higher voltage transistors instead of using the 2N3904 and 2n3906. I'm not sure if these weren't up to the job, or I blew them installing them incorrectly. The voltages are still way off, but the circuit works well, so I'm going with it.

Just some cleaning, biasing replacing of the low voltage electros and this one will be ready to go.

Thanx PRR and Willabe for the help! 

[Willabe]

All right, nice job! Glad you got her fixed.