Help needed - MB on the bench

[rzenc]

Hi!!


Weird situation going on here. I have a combo 1x12” Mesa Boogie Single Rectifier 50W series 1 on the bench. It belongs to my neighbor. He bought it used, maybe third/fourth hand. Amp is dated 1998. Also, he bought it without testing.


He tried it on his apartment at very low volumes. The amp seemed to work, but he complained it sounded kind of anemic. At first that is. Then he took it to rehearsal. Then things went bad.


When he cranked the volume, the amp started to motorboat.   Only way to stop it was shutting it down/stand by. So he brought it to me.


We were able to repeat it and sure enough it motorboat again. So I opened the amp up. First thing I noticed was an unsoldered screen resistor on one of the power tubes 6L6 in this case. It simply fell off the PCB. Fixed it and the amp kept motorboating.


So I thought an e-cap job was in order. Replaced all e-caps inside the amp, including bias, cathode bypass and DC heaters as well as power supply rail.


During this process, we discovered that alignment pins on 6L6 bases were broken. Since all tubes seemed like originals, we changed all them, preamp and power as well.


Put the amp back together. Powered and all voltages seemed ok. Got bias, B+, heaters and switching going on. Installed power tubes and no smoke!!


Powered the amp and surprise, motorboating was still present, less pronounced but still there.


I did some research on the net and it seems like these amps are prone to motorboat and oscillate. Found some posts about methods of fixing it. So I gave a shot.
These are the mods I did to the amp:
1- Replaced 30uF B+2 screen node with 100uF;
2- Replaced 10uF subsequent power supply filter caps with 47uF – that’s what I had hand;
3- Replaced 680nF FX send cap with 100nF.


According to other people who tried to solve it, these things cured the problem. However it did no good in this amp.


There is still one suggestion to try: adding a 470K from send jack tip to ground, which I will try on the weekend.


Do you guys have experience with these amps?


I really need help with this one. The owner is a long time friend of mine and I don’t want to let him down.


Hope you guys can help me out. Do you have a suggestion as where to start? It seems like this problem is around the FX loop driver/return. I do have a scope and signal generator and will be forever glad to your willingness to help us out!!!


P.S.: I could not find an actual schematic of this amp but only series 2. I attached the pdf file I found online.


Many thanks in Advance!!
Best Regards
R.

[rzenc]

I was probing inside the amp with scope and signal generator and found noise on V1 pin 6. I found the same noise on V1 pin 7. Strange fact is that it happens with more intensity on rhythm (clean) channel than lead channel.


I took the picture attached to show how it looks like. Also, it happens with no signal injected on V1 pin 2. I did not find noise on V1 pin 1. IMHO, it seems that this noise is appering thru rhythm channel tonestack. Rotating tonestack pots does change it's shape a bit.


Took voltage readings on V1 socket pins:


pin 1: 139V
pin 2: 0V
pin 3: 2.31V
pin 6: 200V
pin 7: 0V
pin 8: 3.51V


I got in touch with MB customer service in I'm still waiting for their reply. I asked for a schematic and mentioned what's happening to the amp.


If you guys have a suggestion as what might be causing this disturbance, do not hesitate, fire ahead


I've never seem it before.


Thanks in advance.
Best regards,


R.

[terminalgs]

If all channels ultimately go through the FX loop,  plug an alternate signal source into the return (signal generator, a pre-amp, etc... something that you turn up the +dB.  See if you have the noise.  this will at least cut the amp in half and tell you which half to investigate.


You can also take a large cap,  like a .1uf, or a 1uf-10uf lytic,   put the (neg) side to ground, and a probe lead on the (pos) side.   with the amp powered on and motorboating, touch and hold the probe to each side of each plate resistor in the preamp.  this might help find the source if the noise is a B+ power supply issue...  (PCB traces?)

[rzenc]

Thanks terminalgs! Will do that.


I was probing again with scope. It seems like this woobling is between V1a and V1b rhythm channel. I don't get woobling on V1 pin 6 when lead channel is active.


Just for the sake of curiosity, I switched the amp to stand by while monitoring V1 pin 6 on scope. I still get wooble even when B+ is below 40V.


I did change all e-caps inside the amp, that is, B+ caps, bias, DC heaters and cathode caps. It does not mean one of them could be bad. 


I will keep posting the 'progress' on the amp...


Many Thanks for your help!!


Best Regards


R.

[terminalgs]

Thanks terminalgs! Will do that.
I did change all e-caps inside the amp, that is, B+ caps, bias, DC heaters and cathode caps. It does not mean one of them could be bad. 

or a bad trace on the PCB.


by motorboating, you mean 120Hz or 60Hz noise?

[rzenc]

by motorboating, you mean 120Hz or 60Hz noise?

I believe it's lower than that. not sure the exact frequency but it seems like a thumbing tremolo.


I did the tests you asked for. Results:


I took a film Solen 47uF@630V and inserted it on plate side of plate resistor (100Kohms), it bled to ground all signal; when I changed it to B+ side of resistor, the woobling kept exactly as before, no change in amplitude, shape or frequency.


As I mentioned before, when scoping V1 pin 1, there is no woobling on either channel, rhythm or lead. When I scope V1 pin 6, there is a lot of woobling on rhythm channel, but none on lead channel.  I will try to upload a video to show what I mean.


Thanks terminalgs. If you have other suggestions to try, please, tell me what you think.


Best Regards


R.




EDIT: uploaded a video showing what's happening. The video starts on the lead channel and as you can see there is no woobling, however, when I flip to rhythm channel, you will see the signal woobling up and down. It does not happen on V1 pin 1, but V1 pin 6 only.


youtube link: https://www.youtube.com/watch?v=5OcegLED-ak

[shooter]

try looking at the power rail in AC, ripple, make sure you get out into the hi kHz, even low mhz.
try removing all the tubes downstream of V1 n see what you get at V1, add them back in 1 at a time.

sounds like you'll nail it soon, good luck

[rzenc]

Thanks shooter!! Your help is greatly appreciated!!


I did what you asked:


Took all preamp tubes out, except V1. Power tubes were already out. I scoped V1 pin 6 and even without signal applied I get this 'noise' out of it. Picture attached. Upper picture.


The smaller, non altering signal is 400Hz reference applied to guitar input, V1 pin 2. The bigger signal is what I get when scoping V1 pin 6. Lower picture.
V1 is the only tube left in.


Thanks!


Best Regards


R.

[terminalgs]

I took a film Solen 47uF@630V and inserted it on plate side of plate resistor (100Kohms), it bled to ground all signal; when I changed it to B+ side of resistor, the woobling kept exactly as before, no change in amplitude, shape or frequency.


As I mentioned before, when scoping V1 pin 1, there is no woobling on either channel, rhythm or lead. When I scope V1 pin 6, there is a lot of woobling on rhythm channel, but none on lead channel.

maybe the noise is coming in on the cathode or grid of V1b (6,7,8).  the problem with finding it there is the amplitude would be 20X-60X smaller than the amplified version of it on the plate, so you'll need to magnify your scope accordingly (if possible). 

• Ground the grid of V1b. that'll tell you if the noise is coming in via the grid.
• Check the grounding of V1b's cathode, make sure its 0ohms to the filter cap grounds (good test for all cathode resistors' ground side).

If the noise is on the plate of V1b, the noise can also be coming from the load area to the "right" (in terms of direction on the schematic).  i.e. something on the right side of the coupling cap,  Since its only when the rhythm switch is thrown,  maybe its coming from the circuit between the RV2B switch and V2A triode, including the reverb input.   You could try grounding different parts on that circuit to see if the noise still exists on V1b's plate (like, ground the grid of V2A, or disconnect the input to the reverb section, if possible).

[SILVERGUN]

Since its only when the rhythm switch is thrown,  maybe its................

How about the relay itself ?....
Maybe you're seeing / hearing the engaged relay coil noise into the signal stream?

Strickly a guess 

[terminalgs]

How about the relay itself ?....
Maybe you're seeing / hearing the engaged relay coil noise into the signal stream?

+1 to examine relay.

[shooter]

I'll 2nd the other comments, I was gonna suggest a big 470k to 1meg grid stop on v1b but grounding it out will give the same results, since there isn't anything downstream you can unsolder the coupler from v1b.  Your pin 6 volts is a little low 200 vs schematic 272, the same for v1a 139 vs 206.
Since it's in the rhythm section n relay might be a cause unsolder the relay from v1a plate and the 100ohm going into v1b grid, jumper a coupling cap from v1a plate to the 100ohm v1b grid, bypassing everything in between.  I would add a grid stop to v1b also.

[rzenc]

Many thanks for your kind attention gentlemen.


I will try your ideas/methods and will get back soon with results.


Again, many thanks!!!!


Best Regards


R.

[shooter]

what are you using for a signal source?  I would think a cleaner signal may help down the road.

[rzenc]

what are you using for a signal source?

400Hz @ 500mVpp


The owner has a EMG (not sure which model) equipped guitar.


Do you have other signal amplitude/frequency standard?


Best Regards


R

[eleventeen]

http://onlinetonegenerator.com/




Figures!   LOLOL

[shooter]

My face has prema-grin!!!  n it ain't green beer:)