Mark IIB Problem.

[BPski]

I have a 1981 MK IIB. 60w, Reverb, EQ with EV12L. Sounds fantastic. The problem is in the Lead channel only, I get a sort of rattle sound along with the note particularly playing high notes. Less than one year ago I changed all the electrolytic caps. I have tried different preamp and power tubes, cleaned the pots and sockets & touched up a few solder joints I thought were suspect. I get this rattle sound with the Lead Master Bright pulled or not pulled. Here is another part of the equation: with the chassis on the bench it does not make the rattle sound at all. Anyone ever have something like this happen?? Thanks in advance.

 

[stokes]

When you say on the bench it doesnt make the rattle sound at all,are you using the same speaker?Is this a combo or a head?If it is a combo it sounds like a vibration induced problem.Maybe retensioning the tube sockets,particularly the #3 & 4 preamp sockets.Or a lose element in one of those two tubes.If it is a combo have you checked that everything in the cab is tight?

 

[Boogiebabies]

Could be reverb tank feedback. If your tank is in a bag, grab the tank and free it from the vinyl. If the bottom of the tank sticks to the vinyl it allows vibration to enter the tank through the chassis. Wiggle it around until it feels like it's moving freely in the bad.

 

[BPski]

It is a combo and does not make this rattle sound with the chassis removed and lying on the bench. The rattle sound is definitely coming through the speaker because when the note decays you can hear it. It sounds a bit like static. It almost sounds like a dirty pot but I cleaned them as well. I will try to re-tension V2 & V3. I have heard a similar sound in an older Fender. It was a bad cathode bypass cap on one of the preamp tubes (first gain stage if I remember correctly). The bypass caps in my amp are tantalum and seldom go bad as compared to the electrolytic ones.

Will the Reverb bag be an issue even if the Reverb control is on "0"? I play almost always with no Reverb.

By the way it makes this sound with the EQ in or out. I think it may be loose tube sockets. That seems to make the most logical sense. It's a vibration thing.

Thanks again!!

 

[Boogiebabies]

In my experience, when you have a loose pre socket pin, when you move the tube you get a loud pop or loud static.

In a IIC+ I worked on, when the reverb was engaged and on 0, with a bad ground it would induce static into the signal.

If a retube does not fix the problem, I do a full detailed cleaning including all input, output and loop jacks.

Does it rattle through that amazing hardwood you have?

 

[stokes]

Like boogiebabies says when you move the tube in the socket you will hear a louder pop or static but the vibration will cause a lesser static sound since the vibration causes less movement.Since it only occurs in the lead mode, it narrows it down to tube 3 & 4.When you clean the socket use the tube you are putting in the socket not an older tube,even new tubes can have some oxidation from sitting on the shelf especially if they are stored in a shop that is in a high humidity environment.I would retension the sockets also.

 

[BPski]

I did try wiggling the tubes after I cleaned the sockets yesterday and did not get any static noise or popping. I even tapped the chassis with a fretting hammer and tapped around V2 & V3 with a Sharpie while the chassis was on the bench. This whole condition got worse over time. I'm thinking now it may be the Reverb. What is the fix for static coming from the Reverb circuit? Clean the Reverb jacks and plugs on the wires? Thank you!

 

[Boogiebabies]

Try the 1 wire mod. It shares the ground with the chassis instead of the tank. Take a 1.5 inch of covered wire and solder it to the sleeves of the reverb cables where they input to the chassis.

The reverb ground can make all sorts of funky noises when it's bad or lifted slightly abouve ground due to corrosion on the jack washer.

 

[stokes]

Wouldnt the reverb grounding problem also appear on the clean channel?Not doubting you BB,I know you know your stuff,just wondering why it wouldnt affect the rhythm mode.Is it a high gain problem?I would assume when he has it on the bench the reverb cables are not connected so that would eliminate the symptom.But why not on the clean channel?

 

[Boogiebabies]

The increase in gain is huge when the lead channel is engaged.
They switch into the circuit by a few LDR's at the 3.3M/20 pf junction.
The loop, reverb and lead channels all intersect here, but IMO it is simply a matter of the increase in amplification and thus increasing the noise.
I had a head that did the same thing. It would kill notes, fuzz out and make all kinds of microphonic static. It would not duplicate it when the chassis and the reverb were removed. I would consider those noises a severe case, but depending on the resistance of the corrosion of the ground, it will hold the reverb circuit slightly above ground and even a few ohms can make the reverb circuit in itself and amplifier within an amplifier by adding two gain stages and an output transformer as well as it's own preamp voicing.

Don't believe it? I have a IIC+ here in my shop that has so much gain it's almost unusable with the reverb know above 3. Oddly, the reverb now acts as a drive pot, so the higher the setting, the more clipping it produced. The clean channel is fine.

The reason, the reverb ground. One wire and the problem was solved.

 

[stokes]

Okay,now the wire fix you describe.I dont understand how if the washer in the chassis is the problem,as I understand it oxidation on the washers causes the jacks to no longer be grounded,is that right?By connecting the two sleeves together where does the new ground reference to the chassis occur?Are we assuming that only one of the reverb jacks on the chassis is bad?Also,looking at the reverb cables on my IIC+ only one has the shielding the other is not grounded,only has the insulated single conductor.I'm just not seeing where the new ground reference is.I dont have a problem with my amp,by the way,I just seem to be missing something here.And I am not doubting this fix,like I said I just seem to be missing something.

 

[Boogiebabies]

Mike Bendinelli came up with this repair. The tank output is the ground for the circuit. All you are doing is using the closest ground. There is nowhere else to ground the output except the output jack, but if that gets corroded you have to do the mod. Now, here is the rub, you can't see the output jack or washer because it is under the preamp board. You just have to assume the washer is bad. For some reason, it never fails to work, unless under rare occasions the input washer is seriously corroded. I have seen this and you could not even get continuity between the washer and the chassis 1 mm away. This jack, however, you can remove and replace then do the 1 wire mod. There are a bunch of oddball tweaks for these amps. Some oscillate at higher volume because Mesa omitted a cap on all of them that is clearly shown in the schematic and would prevent it. Go figure.

BTW,

How is NYC, I left 12 years ago for much better weather. 23 years was enough. I do miss my Yankee's and Jets.

 

[stokes]

So the ground on the output goes bad and is remade at the input jack,okay now I got it.So you could also just get a shielded input cable and accomplish the same effect,making a ground from the tank to the chassis thru the input jack,assuming the input jack is okay.Gotta go take another look at the amp.NYC is okay,the weather is what keeps me here,gotta have my change of seasons.Two friends of mine went to Fla.both came back for the same reason,two years of summer was all they could take.I actually prefer the fall/winter but by mid Feb. we all like the spring.You didnt miss much with the Yanks this year,big choke at the end.Jets are still in it though,but we'll see.

 

[Boogiebabies]

I like 9 months of summer and three of winter with snow every third or fourth year. Although I am a diehard Panther fan, I still wear my Jets hat and Gang Green shirt. I think I am the only guy within 700 miles with a Jets hat. People look at me like I have two heads.


As for Charlotte, there are not too many amp techs around. Fortunately I have a ton of studio work and I see a high volume of Mesa's. I prefer Marshall's because there so straight forward, but Mesa's really keep you up to date and you are always keeping up with difficult circuits.
Right now I am workin on a 12 channel studio preamp based on the Yamaha
PM-1000. XLR with phantom in, TRS and recapping the EQ to the Neve 1073.
What a ball buster.

 

[Boogiebabies]

I like 9 months of summer and three of winter with snow every third or fourth year. Although I am a diehard Panther fan, I still wear my Jets hat and Gang Green shirt. I think I am the only guy within 700 miles with a Jets hat. People look at me like I have two heads.


As for Charlotte, there are not too many amp techs around. Fortunately I have a ton of studio work and I see a high volume of Mesa's. I prefer Marshall's because there so straight forward, but Mesa's really keep you up to date and you are always keeping up with difficult circuits.
Right now I am workin on a 12 channel studio preamp based on the Yamaha
PM-1000. XLR with phantom in, TRS and recapping the EQ to the Neve 1073.
What a ball buster.

 

[stokes]

The amp work is a side thing for me.I got a day job so to speak.When I retire in about 5 yrs I want to open a shop full time.Right now I have a fully equiped repair shop in my basement,and way too many amps if that is possible.I try to stay away from Mesa repairs and stick to the Fender and Marshalls,pretty much try to avoid all pcb amps and dont even touch solid state.I will do the basics on Mesa's- routine maintenance,bias,re-cap and such but major repairs I refer to full time guys like your self.I know my limitations and dont get in over my head.Do a lot of guitar work also.Started tinkering back in the early '70's when I was playing in bands and started leaning more to the tech end for the local bands.I thought I saw a thread you mentioned having akamouzis'amp,how did that turn out?I know he was pulling his hair out trying to find someone in Boston to fix it.You getr to the bottom of it?

 

[BPski]

I will do the Reverb "repair" tomorrow and post results. I am thinking that's going to be it. Too many gigs this weekend to solder at the bench. Thanks again!

 

[Boogiebabies]

The amp work is a side thing for me.I got a day job so to speak.When I retire in about 5 yrs I want to open a shop full time.Right now I have a fully equiped repair shop in my basement,and way too many amps if that is possible.I try to stay away from Mesa repairs and stick to the Fender and Marshalls,pretty much try to avoid all pcb amps and dont even touch solid state.I will do the basics on Mesa's- routine maintenance,bias,re-cap and such but major repairs I refer to full time guys like your self.I know my limitations and dont get in over my head.Do a lot of guitar work also.Started tinkering back in the early '70's when I was playing in bands and started leaning more to the tech end for the local bands.I thought I saw a thread you mentioned having akamouzis'amp,how did that turn out?I know he was pulling his hair out trying to find someone in Boston to fix it.You getr to the bottom of it?

I have a full time day job too, but the amp repair biz has been very buzy.
I pray for a simple retube and bias. I still have Alex's amp and it works perfectly. He wants a devious modification which is still in the works.

 

[stokes]

Oh,somehow I got the idea you were doing the amp thing full time.What di the problem turn out to be?I know he said he took it to a few guys but got nowhere.BPski,sorry we got off topic here.

 

[Boogiebabies]

That amp is now top secret.