Marshall AVT 275 combo --- The OD channels have very little volume

[JimDep]

Marshall Advanced Valvestate Technology 275 One 12AX7 pre amp tube, the rest is solid state 2x12 combo

This is my first post, I was invited here from the Peavey Forums by Enzo...Thanks Enzo!

I bought this Marshall AVT 275 amp cheap knowing that both the OD channels are very weak in volume. Using either the OD1 or OD2 channels, I have to have the master volume dimed and the individual volumes 3/4 and the GAIN at least 1/2 to 3/4 up just for bedroom level. This setting is also very noisy with static and hum. The good news is that the Acoustic simulator channel and clean channel work great. If I didn't reset the master volume to about 9 o'clock (3) before switching back to the clean channel, I'd probably blow the bedroom window out. The Clean channel setting normally I have the Master on 9 o'clock (3) and the individual volume at 10 o'clock (4) and the GAIN varies, but usually 12'oclock (6)..half volume - this is plenty loud for the clean channel in my little practice room.

After getting it home, I replaced the original Marshall preamp tube with a new Groove tube GS 12ax7. It may have brightened up the clean channel a little, but had little impact overall.

I can get by OK with just the clean channel and pedals, but if I want to sell it, I'd like to have the OD channels working properly.

I don't see anything noticeably burned out on the circuit boards, although I didn't visibly check all of them thoroughly. I'm not a tech but willing to follow instructions from somebody who knows more than I do, which is most everybody.

Is is possible that it could just be a loose solder joint or blown resistor or component somewhere?
What would you test first? Thanks.

[Chuck H]

Having trouble locating a schematic that seems accurate. I found one for an avt150/250 but it has no tube in the preamp. Hopefully someone here has one. The schem is the first thing you'll need.

[Jazz P Bass]

Here is a preamp schematic.
The Clean & OD channels are always getting a signal.
Then they are switched with an IC (electronic switch)
You will have to monitor the OD channel IC outputs to verify that they are working.
Then you must test whether or not the signal is passing correctly through the switch IC.
Here is a MEF link to the complete schematic: http://music-electronics-forum.com/t13909/
Post #6. (thankyou Enzo)

[JimDep]

Thanks, I didn't have THAT schematic.... At first glance, I'm wondering what component BOTH OD channels have in common. ASAP, I'll figure out how to test this: " You will have to monitor

the OD channel IC outputs to verify that they are working. Then you must test whether or not the signal is passing correctly through the switch IC." It makes sense, it's just step by step

"how" to do it. Just curious, does the Tube/Valve effect ALL the channels? I can't tell from this. thanks.

[Jazz P Bass]

You really should have a scope for this
If not, you can use a multimeter set to read volts ac.
I would 'look' at the output of the clean IC (IC11a/ pin #1) & compare it to the output of the OD IC.(IC10/ pin #1 & #7).
Put the volume controls at max.
The voltages read should be similar for clean & both od's.
If they are, then do the same test for the switch ic outputs.(IC3).
If that checks o/k, then you have to dig into the OD gain stage.

[Enzo]

There is a mute line from the input jack directly to an input pin on IC8 in that preamp channel, via CNF4. Especially if your input jack board does not have the zener mod, this can lead to static discharge hitting that op amp. Try replacing IC8.

[Enzo]

This is not arcane knowledge, by the way. IN general, it is the parts exposed to the real world that are more likely to fail in things like amps and mixers. The channel inputs on a mixer that go right to pairs of transistors then some ICs and on in... Those first transistors are more likely to be bad than somthing six stages deep. Bad output on a synth? Most likely the chip driving the output rather than one two or three stages back.

Marshall, in the interest of noise abatement, often uses a contact on the input jack to ground off some middle stage in the gain channel. SO when the amp wants to work, but the preamp is unwilling, I usually first look at the input jack and see where any mute line leads, and if it goes to an op amp, that boy gets changed. Majority of cases, that's the cure.


And that is not just electronics. Mechanical systems are involved. Probably the single most common cause of a missing output on a large mixer - I do a lot of Behringers - is the slider. It is the thing the real world can get its grubby fingers on.

Keyboards too. MY synth lost its output. Check the volume slider.

[Chuck H]

This is not arcane knowledge, by the way.

It may be the simple facts of life and systems, but to some of us it sure seems like arcane knowledge. And we're glad to have it.

[JimDep]

Thanks Enzo, I need some common sense logic to use as a platform to start from.

With the EM pinball machines, I was getting to a point where I could look at a schematic (especially Gotliebs) and know where to start from. When pinball machines went solid state, I was way out of my comfort zone. It gave me kind of a phobia....looking at the guts of these transistor amps and the schematics gives me a little of that PTSD from a few years back.. I appreciate you helping me overcome my "El Guapo".

[Enzo]

I am sure I have told this but....

Pinballs all used to be straight relay logic. We called those "mechanical" pins. Or EM for electromechanical. Then in 1976 the world changed and they all became digital electronic systems. They included a self test diagnostic button inside that cycled through varioous modes. Now as I trained techs, I'd go through the test procedures. Fewer and fewer EMs remained in use. New techs saw them less often. Then one magic day, a load of pinballs needing service was in the shop, and a recent hire came to me puzzled. I had trained him on pinball service, but he'd never seen a mechanical before.

"Enzo, where's the self-test switch?"


And that was when I realized the change was complete.