Marshall JCM800. pics of signal....

[Garydean]

Hi,
I'm working on a JCM800 4010. I replaced the large cans re-tubed and biased a month ago. All seemed fine at the time. The amp came back yesterday. Now (or maybe still?) when the preamp is turned up to 8 and the master is on 2 or higher, I'm getting a hum and a little static. When the mid control is raised the volume raises significantly, or perhaps the hum. Customer is complaining that when he raises the volume it's waaaay to bright, yet it breaks up to his liking.

The schematic calls for EL34's yet there were 6550's in there when I originally got the amp on the bench last month. I put new 6550's back in. FYI.

I injected a sine wave and scoped the output and this is what I got.




Only when I fiddled with the tone controls on the amp was I able to turn the signal into a round cornered square wave, on the sine wave setting. I traced the signal from the input and got a perfect sine wave halfway thru the first preamp stage. I'm not proficient with a scope yet, so I could use some guidance.

Few preliminary thoughts I have are.

1) Change to EL34's and bias, as per the schemo.
2) Swap in new tubes, same values and bias.
3) touching the strings on the guitar lessens the static....ground loop issue.

What are your thought and tips?

Gary

[loudthud]

Most function generators put out way too much signal for guitar amps. I build an attenuator on a phone plug to reduce the output to something usable. Try a voltage divider with a 1K and a 10 ohm.

Looks like you are using a Tektronix 465 scope. Do you have a 10X probe? Do you know how to adjust the probe compensation?

[Garydean]

Thank You,

I'm using a 10x probe on a Tek. 475, with the same setup I've used in the past on a few other amps, and have gotten a clean signal at the output (If the amp is working 100%).

[loudthud]

The signal looks like a square wave going through an intermittent connection or an intermittent cap. Check J2, C3, C7, C11 and C9. Do all the DC voltages look ok?

[Garydean]

Thanks "LT", I will check that tonight.

[Enzo]

Or consider this: there is a break in the signal path, and all you are amplifying is the remaining crosstalk. In this scenario, something like an interstage coupling cap is open, or moer likely broken free of the board. In that case, the following stages have enough gain to pick up what the earlier stages are doing without the direct coupling through the cap. This will result in a real thin trebly sound, and distortion.

Since you are scoping the amp, run a test signal into the front, and scope stage to stage and see where the signal goes haywire.

[Garydean]

OK..... I should send a signal at the input (1,000Hz), and probe everything? Both sides of every component, caps, resistors...etc...? Should I be getting a clean signal at the grids and plates of every 12AX7?

May not be relative: (presence function WAS working)
The NFB wire looked sketchy on the board so I re-worked that last night.

I did find out that USA 4010's came out of the factory with 6550 tubes.

Thank you!!

Gary

[Garydean]

I was able to get a fairly nice sine wave today after replacing the 2 6550's and biasing to a really nice blue haze. I did replace the high input jack as it was very sloppy. Still though, when I plug in any guitar, there is a highly audible buzz and hum when I plug into the high input jack. It responds to the volume pot so I figure there is an issue in the first gain stage. I'm looking for the issue to be with C1 or C3. I didn't have the correct value orange drop for C3 or the square-ish .68 C1 cap. What can I use to replace C1? Can I use a .1mf 630vdc lemon drop or is this something special? I lifted the resistors in that stage and measured and found everything in spec. there are 2, 10uf/100vdc electrolytics by the rectifier, what are these and should I replace them?

Another thought,.... I thought that maybe the low input jack may be a problem as well because when I plug in a cord to that jack as well as a cord with my guitar into the high input, the buzz and hum almost goes away. Could someone explain that to me?

thanks for all your help.

Gary

[Alex R]

Probably the jack is supposed to ground the input when you pull out the plug. Old ones often fail, very common problem. ungrounded inputs will often hum. Try grounding the tip contact, or give it a shove against its ground contact and see if the hum stops.