Mark IIb - resistors.. HELP

[TattooU]

I just changed new filter caps to my boogie. Although it was major upgrade to sound, there's still something going on when I'm hitting chords in lead mode. A kind of sound like speaker is broken. It's not and and tubes are ok too. ..and while changing filter caps I noticed.. there's 2 resistors missing(!) see upper picture. Looks like somebody has cut them away. I don't know if these 2 missing resistors are causing these problems of lead channel. And i don't understand why would someone remove these from circuit board. what are these missing resistors? any info is appreciated.

 

[sdbthai41]

Hi there, what is the serial number of your Mark IIB?

The short answer is that these particular resistors won't be causing any funny noises that you're describing. The resistors that you're looking are on the bias circuit. There are a few variations in the early Mark IIB? From memory though, it looks like there is the full compliment of required resistors for the bias circuit but I would have to check on the Mark IIB schematic (note there are a few variations of this schematic). Have you ever tested the voltage on the bias supply? It should be around -50VDC - if that's okay then you've got no problems in that area.

It is more likely that the sound that you referred to in the lead mode is associated with some problem with a coupling cap or a bad tube. If it is specific to the lead mode then look at the schematic and look at the likely culprits. Change out the preamp tubes one by one with unknown good 12AX7 and see if there is any change. If that doesn't work then you will need to start looking at coupling caps or dry solder joints.

Another thing you can try is to give the Service team at Mesa Boogie a call. They are incredibly helpful. They are only open from Monday to Thursday and if you're luck is good then you can speak to the chief technician/engineer Mike Bendellini as he can probably pinpoint the problem immediately.

All of the above advice comes with the disclaimer that if you don't know what you're doing then don't screw with it as you could hurt yourself and the amp. Good luck with it

 

[TattooU]

Serial no. is 10727. It's 1983 model, no EQ/reverb. 60 w.

Problem is definitely in preamp section. Adding gain and treble makes it worse. Kinda resonating distorted sound. And only when I hit chords in lead channel. it's not really bad but it is definitely there. I just changed new relay cause it was also causing volume swell problems. I was sure relay was also causing this other problem but new relay didn't fix that. Okay, and it's not tubes, filter caps or speaker.

I'm now thinking of bad solder joint or broken resistor.. and somewhere near V3.

Hi there, what is the serial number of your Mark IIB?

The short answer is that these particular resistors won't be causing any funny noises that you're describing. The resistors that you're looking are on the bias circuit. There are a few variations in the early Mark IIB? From memory though, it looks like there is the full compliment of required resistors for the bias circuit but I would have to check on the Mark IIB schematic (note there are a few variations of this schematic). Have you ever tested the voltage on the bias supply? It should be around -50VDC - if that's okay then you've got no problems in that area.

It is more likely that the sound that you referred to in the lead mode is associated with some problem with a coupling cap or a bad tube. If it is specific to the lead mode then look at the schematic and look at the likely culprits. Change out the preamp tubes one by one with unknown good 12AX7 and see if there is any change. If that doesn't work then you will need to start looking at coupling caps or dry solder joints.

Another thing you can try is to give the Service team at Mesa Boogie a call. They are incredibly helpful. They are only open from Monday to Thursday and if you're luck is good then you can speak to the chief technician/engineer Mike Bendellini as he can probably pinpoint the problem immediately.

All of the above advice comes with the disclaimer that if you don't know what you're doing then don't screw with it as you could hurt yourself and the amp. Good luck with it

 

[Boogiebabies]

I just changed new filter caps to my boogie. Although it was major upgrade to sound, there's still something going on when I'm hitting chords in lead mode. A kind of sound like speaker is broken. It's not and and tubes are ok too. ..and while changing filter caps I noticed.. there's 2 resistors missing(!) see upper picture. Looks like somebody has cut them away. I don't know if these 2 missing resistors are causing these problems of lead channel. And i don't understand why would someone remove these from circuit board. what are these missing resistors? any info is appreciated.

Bias, GEQ and Relay supply.

Clean the relay or replace it. There is a hole in the top to spray in DeOxit.
Then you should rule out tubes. They like to make phantom rattles.

 

[TattooU]

I replaced the relay. It's not tubes. I've tested it with many spare tubes, many times. this thing is driving me nuts. I've spend days now with this crazy little machine. I've opened and cleaned pots also. Found one bad resistor but that didn't fix the problem.. would have been far too easy.

What is that 470uf/16 V capasitor (in lead channel circuit) and what does it do? can i replace it with F&T 470uf/40v?

 

[skizziks]

Bump for interest. My mark llb is doing the same thing when it has fully warmed up. As the note fades on the lead channel, it sounds like paper tearing. I've reconed the speaker, changed tubes, cleaned sockets but no joy. If you are able to solve your issue, please post your solution and I will be forever in your debt. Regards, Skizziks

 

[TattooU]

Yes, I will.

Indeed, seems like we have same problem. By now, I've found, broken channel switch relay and 470uf/16v cap was also burnt. My amp was giving little smoke when problems started and I tought it was power on/off switch (witch was also worn out). I later figured out it was that 470uf cap.

Bump for interest. My mark llb is doing the same thing when it has fully warmed up. As the note fades on the lead channel, it sounds like paper tearing. I've reconed the speaker, changed tubes, cleaned sockets but no joy. If you are able to solve your issue, please post your solution and I will be forever in your debt. Regards, Skizziks

 

[TattooU]

little update.. I still haven't found the problem. Meanwhile.. I re-soldered the whole thing, put some new pots and wires there.

There's one wire that is question mark to me.. Grey (double) cable that comes from V3, socket pins 6 and 7, If I remember correctly. Middle wire (red) goes to middle lug of lead drive pot. The outer one just ends there. Would someone check is that the way it should be or is it possible that it's cut and It also should be connected to lead drive spot. This Boogie is without EQ/REV so V3 is lead drive.

Again, any help is appreciated.

 

[McBarry]

I see a 470/16v electro cap in the relay supply..but can't see one in the lead ch.

If it's this one that was smoked, something has drawn a ton of current through it, or possibly the diode(s) before and after it are broke.

Carefully check the relay voltages both switched and not switched..
Check all voltages around V3, and trace a signal through the lead ch, on a 'scope would be invaluable to SEE the "paper tearing" sound.

Are all tube sockets clean and tight??

There's a few somewhat random ideas that may help..

 

[TattooU]

here's some voltages I measured:

Standby - 470V
relay coil - rhythm ch 8.4V / lead ch 5V
V3 pin 1 - 179V

By now I replaced all diodes except ones near filter caps. I've cleaned tube sockets but if nothing else helps gonna replace those too.

 

[McBarry]

first of all, stop just replacing things without proving they're faulty..

So from your first post it's only in lead channel - yes??

If so, that's a VERY narrow circuit are - V3 mainly. and No GEG - even better.

So study what is going on in V3 area both while active, and inactive..

Given the symptom is subtle, so too will the circuit symptom be subtle..

A voltage might drift A LITTLE as the chord decays, a DC leak might exaccerbate as the chord decays, etc etc etc ad nauseum.

Something is causing that noise..

Maybe get someone with a signal generator and oscilloscope the trace the lead channel..

My point is STOP just replacing stuff (except re-tension sockets rather than just cleaning them..) and measure, measure, measure.. then measure some more..

Be keen to hear your progress..

Dave

 

[TattooU]

Yeah, thanks..

I don't replace stuff just for fun. There's is/was flavor of rust about everywhere when I first started working my amp. If there have been burnt or faulty looking part in circuit board I would have rip it of without delay. There wasn't so, I don't think it is bad idea to replace parts that are rusty or worn out. It is still over 30 years old amp. Diodes cost next to nothing, rusty looking wires, worn out pots,.. Thing sound better now, after I re-soldered it (and with some new pots). Even Lead Channel, even tough it sound bad with that crackling sound..

Thanks for the good tips anyway.

I was hoping that someone here has had similar problem and has came up with solution to it. Anything else is irrelevant.

first of all, stop just replacing things without proving they're faulty..

So from your first post it's only in lead channel - yes??

If so, that's a VERY narrow circuit are - V3 mainly. and No GEG - even better.

So study what is going on in V3 area both while active, and inactive..

Given the symptom is subtle, so too will the circuit symptom be subtle..

A voltage might drift A LITTLE as the chord decays, a DC leak might exaccerbate as the chord decays, etc etc etc ad nauseum.

Something is causing that noise..

Maybe get someone with a signal generator and oscilloscope the trace the lead channel..

My point is STOP just replacing stuff (except re-tension sockets rather than just cleaning them..) and measure, measure, measure.. then measure some more..

Be keen to hear your progress..

Dave

 

[kc_ksom]

I feel ya pain, you have gone a few steps further than I was willing to go, I also had some crackling issues on my Mark III, I diddn't attend to them because Im a broke *** biarch and couldnt afford the repair bill so I done nothing. The crackling eventually caught up with my amp and it went up in smoke & flames, so if its still crackling, save up mate cause its gonna hurt. Get it repaired by someone who knows what the F there doing (they will also know how to rip you a new *******) then and only then will you have a good to go amp.

Regards

Casey

 

[McBarry]

@ KC - hilarious !!

@ TU - if yr not randomly replacing, and following some kinda logic, although it may not be evident here, then cool, and irrespective, I wish U the best.
Re crackling - yr gonna need a ton of luck from the sounds of it..

This may help though..
I have a MkIIA that was crackling like a pork roast just outta the oven..
Turns out was 30year-old crap ON the PCB between tracks. I found it when checking for DC BETWEEN tracks.. sure enough there was tiny amounts leaking through the crap on the PCB between B+ and various signal tracks.
Cleaned with good old methylated spirits, crackling completely disappeared.
Was a royal pain in the arse getting under various components, but paid off bigtime in the end..

Might be worth checking..
Dave

 

[TattooU]

I got it fixed few weeks ago. Today I got finally to try it at full blast and it sure works!

I don't know what was exact failure but.. I replaced those infamous tantalum caps (besides one, 'cause gain boost wouldn't have work) with matching Mallory caps. Replaced three tube sockets; V1, V2 & V3. All sockets looked decent first but we're actually really rusty and some pins we're almost cut. I did lots of cleaning too.

What a pain in *** this was but good thing is - I learned a lot of this amp. And secondly it sounds great now.

Hope this helps ones with similar problems.