Marshall 1959HW has low volume

[parky]

I am working on a Marshall 1959HW (re-issue) 100W super lead for a friend. It is about 2-3 years old, I think (he bought it used 6 months ago) The volume suddenly dropped down to about 20-30% of normal. A complete swap of tubes made no inprovement, and all the originals test strong.
I have a schematic with no voltages for the SLP model but not the hand wired model. I have spotted a few differences between the schematic I have and the HW amp.
The EL34's are showing 480V on the plates and 460V on the screens. Bias is at 41-42 volts. These seem normal. The plate voltages on the 12AX7's seem to be within range also, ranging from 200-250 volts. I do not know what they are supposed to be,though.
The amp is not noisy at all, just low in volume. With the volume set at 2-3, you can still talk over the output, and he informs me that it should be blowing us out of the room at that level.
Are there any caps in the audio path that can cause this? I can get my hands on a scope and audio generator if needed.
Any help would be greatly appreciated !! Parky

[Bruce / Mission Amps]

I am working on a Marshall 1959HW (re-issue) 100W super lead for a friend. It is about 2-3 years old, I think (he bought it used 6 months ago) The volume suddenly dropped down to about 20-30% of normal. A complete swap of tubes made no inprovement, and all the originals test strong.
I have a schematic with no voltages for the SLP model but not the hand wired model. I have spotted a few differences between the schematic I have and the HW amp.
The EL34's are showing 480V on the plates and 460V on the screens. Bias is at 41-42 volts. These seem normal. The plate voltages on the 12AX7's seem to be within range also, ranging from 200-250 volts. I do not know what they are supposed to be,though.
The amp is not noisy at all, just low in volume. With the volume set at 2-3, you can still talk over the output, and he informs me that it should be blowing us out of the room at that level.
Are there any caps in the audio path that can cause this? I can get my hands on a scope and audio generator if needed.
Any help would be greatly appreciated !! Parky

Is it distorted audio or very clean but no volume?
I haven't worked on many of these myself but I have had to replace the output transformer twice.

[Enzo]

Isolate the problem.

Are both channels equally affected? Then it is likely after the first tube. If one channel only, then the first stage is where it lies.

FLip the impedance selector for the speaker. Does the level come back at a "wrong" setting? Bad switch.


At this reduced output, do the tone controls still seem to work like they should?

Since both channels are affected - assuming this at this point anyway - and knowing that V2 is a direct coupled cathode follower out, there are not very many caps that would cause this problem. C10 maybe, and the unlikely C13 across the PI plates.

A cracked solder joint comes to mind, but I would just apply a test signal and trace it through the stages, this is not a complex amp.

I posted the schematic in my gallery at www.ampix.org

[ddm]

Parky

My ampegs having the same problem. .Did you ever find out what was wrong with your amp?

Thanks
Dennis

[bigdrums925]

Enzo: I am also trying to repair a Marshall 1959HW with the same issue. I tried to follow the link above to Ampix.org and get only fatal error on the page. Any suggestions?

[pdf64]

See http://music-electronics-forum.com/t40521/ nevetslab had fun with one of these.
The HW schematic should be pretty close to http://bmamps.com/Schematics/marshal..._100w_1959.pdf

[bigdrums925]

THANKS GOT IT