Mesa Boogie Preamp Problem

[ooosanteooo]

Hi All,

I forgot my Mesa Boogie Studio preamp on while going out drinking.

I put it in standby when I got back and then turned it off.

The next day I tried to record and I have the following problem:

The sound is about at 15% volume of where it was originally and sounds very hissy. Like a digital distortion unit, and worse.

There is no difference whether I am in standby or in normal mode.

I didn't know there was any sound leakage when in standby mode, and a quick consultation in the user manual confirmed this.

What is broken with my preamp ?

Blown a tube? I have never replaced the tubes so maybe it's time to replace all the tubes anyway, but I want to be sure that this is really the problem.

As I said, the most worrying thing is that there is no difference whether I am in standby or normal mode.. there is tone coming out, but it's very weak and sounds like shit.

Anyone had this before?


Any insight appreciated, I haven't owned this brilliant piece of gear long, and it's already broken

Thanks!

-J

 

[SimonSez]

Could be a preamp tube, if you have any extras lying around then just swap it out. Also I would check the cables, make sure you don't have some cable halfway in or something.

 

[ooosanteooo]

Could be a preamp tube, if you have any extras lying around then just swap it out. Also I would check the cables, make sure you don't have some cable halfway in or something.

What do you mean by some cable halfway in ?

Also, there are 4 preamp tubes. Two for each channel, so theoretically the problem should only be on one of the channels if the tubes are blown...

The real problem is that I can hear the same signal when either in standby or not in standby, which is worrying.

I sure hope it's only a tube issue...

 

[bryan_kilco]

What do you mean by some cable halfway in ?

Also, there are 4 preamp tubes. Two for each channel, so theoretically the problem should only be on one of the channels if the tubes are blown...

The real problem is that I can hear the same signal when either in standby or not in standby, which is worrying.

I sure hope it's only a tube issue...

He means that maybe one of your cables isnt plugged in all the way.

I'd check the tubes. Try tapping them one at a time with a pencil while the amp is on and see if there is any lightshow or noise. I've had nightmares about leaving my amp on overnight/for days.....

also, what preamp is it? Recto Recording Pre?

 

[MarcusGHedwig]

I'd just swap the tubes regardless, they're cheap enough and you're due (I'm a big fan of the Chinese 9th gen tubes)

 

[Omega_Void]

Hi All,
There is no difference whether I am in standby or in normal mode.

The manual says: "The Standby Switch shuts off the signal at the effects send stage."

If there is no difference when this switch is thrown, then the switch is buggered.

If it's a fancy light-dependent resistor thing the fault may lie in the switching circuit. If it's a mechanical switch it needs replacing. Or possibly a wire has come loose from one end of the swich and is making light contact with the other joint.

No fault or failure of other components can explain why the switch doesn't break the audio connection, so fix this first. There's a good chance nothing else is wrong.

 

[ooosanteooo]

The manual says: "The Standby Switch shuts off the signal at the effects send stage."

If there is no difference when this switch is thrown, then the switch is buggered.

If it's a fancy light-dependent resistor thing the fault may lie in the switching circuit. If it's a mechanical switch it needs replacing. Or possibly a wire has come loose from one end of the swich and is making light contact with the other joint.

No fault or failure of other components can explain why the switch doesn't break the audio connection, so fix this first. There's a good chance nothing else is wrong.

Thanks for your reply. Upon inspection of the manual I am still a bit confused.
It shuts of the signal at the effects send stage.

Does that have anything to do with the Effects return output or are we talking about the ouput from the main outs?

It's true, maybe the switch is buggered, I will have a look at it, but it's unlikeley.

I will open it up today and check the switch, if it's not that, then I will replace the tubes (which I should do anyway).

Let's hope it's either, the only problem I have with the loose cables theory is that I haven't moved the amp AT ALL.

Thanks for replies, will keep you updated.

Fingers crossed!!

 

[ooosanteooo]

He means that maybe one of your cables isnt plugged in all the way.

I'd check the tubes. Try tapping them one at a time with a pencil while the amp is on and see if there is any lightshow or noise. I've had nightmares about leaving my amp on overnight/for days.....

also, what preamp is it? Recto Recording Pre?

I tried tapping them with a pencil, no lightshow or noise whatsoever.

The spring from the reverb was unhooked, but it didn't change anything after putting it back into position.

It's a Mesa Studio Preamp (1986).

 

[ooosanteooo]

So I checked if all the cables are in their place (they are) and unplugged the tubes and put them back in. No change.

I will buy a set of new tubes (four 12AX7 and one 12AT7 for the reverb).
Funny thing I noticed, there were three stock Mesa 12AX7 in there, and one no name 12AX7A. What's the difference between 12AX7 and 12AX7A ?

Also I am getting more worried, because I don't think that by changing the tube I will solve my problem. As I said I get an output even if I am in standby - will changing tubes have any effect on this?
Maybe the problem is routed deeper than this.

I paid 300 pounds for it used, and have only had it for a few months DAMN it.

 

[ooosanteooo]

If there is no difference when this switch is thrown, then the switch is buggered.

Upon closer listening, throwing the switch makes a tiny difference in sound. As if there is a lowpass in one position and not in the other. So maybe the switch is alright.

I cannot explain the leakage when in standby though. Wtf ?

 

[bryan_kilco]

Upon closer listening, throwing the switch makes a tiny difference in sound. As if there is a lowpass in one position and not in the other. So maybe the switch is alright.

I cannot explain the leakage when in standby though. Wtf ?

Yeah, I'd send it out to Mesa or a repair shop and get it looked at.

The Studio Pre is basically like a Mark series, correct? with the 5-band EQ and whatnot?

 

[ooosanteooo]

Yeah, I'd send it out to Mesa or a repair shop and get it looked at.

The Studio Pre is basically like a Mark series, correct? with the 5-band EQ and whatnot?

Well I live in the UK, I don't know if there is a MESA repair shop (The address on the manual is from 1990).

Ya, it's like the Mark series, with 4-band EQ though!


To anyone interested.

Here's a schematic:
http://tubefreak.com/studio.gif

Basically, I just discovered that the standby switch (in bottom right of diagram) doesn't disconnect the HT but merely removes the signal from going through output stage. This is weird, in a normal Amp Head you would not have this I guess, but since it's only a preamp.

In any case, the tubes are always driven!!


Furthermore I have discovered something else. The top right of the schematic indicates that the Return inputs (A and B) only go through the simple amplification stage using Tube 4!

This is quite interesting, since I can just hook up a CD or MP3 player on the return A and B and listen to the output. If it's distorted we have a problem!

If the tube is broken (and it's unlikely that all 4 are broken) I can just switch over all four tubes into position V4 and see if I get a clean output from the CD/MP3 player!!

From then I can work backwards!


This will take me some time, as I don't have any suitable equipment at home either

In any case, if anyone has had similar problems, or knows what the hell is going on with my Amp, please let me know!!!

Thanks!

-J

 

[ooosanteooo]

I did what I outlined in my previous post!

All tubes seem to be working fine!

I think it might really be the switch?!

 

[Omega_Void]

What's the difference between 12AX7 and 12AX7A ?

12AX7A is low-noise version. Unbranded I'm guessing is Chinese.

The manual says: "The Standby Switch shuts off the signal at the effects send stage."

Basically, I just discovered that the standby switch (in bottom right of diagram) doesn't disconnect the HT but merely removes the signal from going through output stage.

Glad to see you're keeping up!

There's no need for removing the HT from the preamp because unlike power tubes the triodes don't wear out with use, and because cathode-stripping doesn't happen at all below 300V.

I think it might really be the switch?!

I hate to say I told you so... :danceboy:

Don't mind me, I've had a really crappy day.

 

[ooosanteooo]

I hate to say I told you so... :danceboy:
Don't mind me, I've had a really crappy day.

Hope you feel better soon dude.

Well, the switch. On the one hand I hope it's not the switch, on the other it would be easier if it really is the switch.

But how can it break? Can I repair it? Where can I get a replacement?

Since there are two cables going in to that switch. A quick test would be to just disconnect (i.e. unsolder) one of them then the amp should be in the off (or standby) state right.

If I do this and I don't have any sound leaking then I know it's the switch.

However I am reluctant to unsolder the nicely soldered switch.

Anything else I can do?

 

[Omega_Void]

Well, the switch. On the one hand I hope it's not the switch, on the other it would be easier if it really is the switch.

How can it not be the switch? If throwing the switch previously opened a connection so that no sound passes, and throwing the switch now allows sound to pass, then the switch is now not opening that connection. Switch is definitely buggered. If the problem lay elsewhere the preamp might sound shit, but it would be silent when the switch was thrown to "standby". Switch is definitelydefinitelydefinitelydefinitelydefinitely buggered.

definitely.

Or I'll eat my hat.

But how can it break?

3-way selector switch on my old guitar broke once. Big chunky toggle, looked plenty tough. The actual bits inside looked a bit flimsier. Not like I treated it rough either (er...by my standards ), just one of them things that sometimes happens, I suppose.

Technical datasheets on switches normally state a number of times the switch can typically be operated before failure, so they do go eventually if they see a lot of use. Every now and then one dies early. Manufacturing isn't done to precision tolerances for such a part.

Can I repair it? Where can I get a replacement?

If a local music shop also does repairs they're likely to have spare parts. Or there's Maplin, RS components, Rapid online etc... Switch insulation needs to be good for at least the B+ voltage, which is probably around 300V. I'd recommend 450V or higher.

A quick test would be to just disconnect (i.e. unsolder) one of them then the amp should be in the off (or standby) state right.

Yup.

However I am reluctant to unsolder the nicely soldered switch.

Anything else I can do?

You could solder a connection across the switch so it's permanently closed.

That would be permanently not in standby. If it sounds fine then you know it's the switch, and also that everything else is definitely fine. If you don't actually need a mute switch, you could leave it at that.

If you don't solder, a friendly neighbourhood TV repair shop would like as not do such a simple job for nowt.

 

[ooosanteooo]

definitely.

Or I'll eat my hat.

I HAVE DONE THE WORLD SOME GOOD.

Omega_Void will NOT eat his hat

I used a crocodile clip to short ciruit the switch (leaving it in permanently ON) and guess what.. I immediatly heard the transformer making a noise!

The amp works like before now!!

Many thanks for your input, I actually read this thread after trying it out!!

I'm quite ashamed to admit it, but I'm an electrical engineer, I spent all day today at the lab in my uni to dissect the circuit with a lab technician.

We found some interesting oddities with this MESA circuit! I will continue working on this, since I learned a lot of stuff on the way.

So now that I know it's the switch I have a few questions.

First of all .. do you think the SAME switch is still around? Would like to replace it with the identical part.

Secondly, maybe I can just do without the standby switch.

I mean it's only a preamp, so no power tubes involved. Do you really need to warm it up before putting it on? As I said, from the circuit diagram u can see that B+ is constantly on, just that the switch disconnects the output.

Maybe I will just wiggle a wire around it and do without the standby switch if it has no use. The manual states you should leave it in standby, but upon inspection of the circuit this doesn't stand.

What are your opinions?

If I can get an identical part I will replace it.

Is there maybe a way to 'fix' the existing switch?

In any case many thanks for your input!

I'm happy I have my amp back, because I need to record stuff ASAP.

 

[ahjteam]

Is there maybe a way to 'fix' the existing switch?

I would maybe use the ~5 bucks just to get a new one.

 

[Omega_Void]

Omega_Void will NOT eat his hat

Hah! I was never in any danger, as I don't actually own a hat.

I'm quite ashamed to admit it, but I'm an electrical engineer

We are doomed.

.. do you think the SAME switch is still around? Would like to replace it with the identical part.

No idea. If it has a maker's mark you could Google around, maybe search Digikey and Mouser. You could probably get a spare part direct from Mesa, but you'd almost certainly be paying over the odds for an unremarkable item.

I mean it's only a preamp, so no power tubes involved. Do you really need to warm it up before putting it on? As I said, from the circuit diagram u can see that B+ is constantly on, just that the switch disconnects the output.

Yup, it's just a mute switch. It would prevent any turn-on thump from getting passed on downstream. Beyond that it only strikes me as useful if your guitar lacked a volume control.

Is there maybe a way to 'fix' the existing switch?

I'd expect the attempt to be more trouble than it's worth. I've worked on assembly lines, and stuff just pops easily into place with a purpose-designed jig which would be im-fucking-possible to assemble by hand. And if it died 'cos e.g. an internal leaf-spring lost its springiness, no chance.

I'm happy I have my amp back, because I need to record stuff ASAP.

Glad to have helped.