Marshall Haze 40 shorting out?

[Jazz P Bass]

Have you verified that FX In sounds bad.(trying to isolate the problem to the preamp or the power amp)
We are going in circles hear.
I remember a post that EMU Out sounded bad.
That is a preamp issue.
Now you are in the power amp.
R4 is a dummy load if you run the amp without a speaker. (don't do this).
With a speaker attached, R4 is effectively removed from the circuit. (parallel resistance)

[ricach]

Have you verified that FX In sounds bad.(trying to isolate the problem to the preamp or the power amp)
We are going in circles hear.
I remember a post that EMU Out sounded bad.
That is a preamp issue.
Now you are in the power amp.
R4 is a dummy load if you run the amp without a speaker. (don't do this).
With a speaker attached, R4 is effectively removed from the circuit. (parallel resistance)

Jazz, not sure where the disconnect is but in my previous posts I've mentioned:
FX Out sounds good - ran a cable to another amp
EMU sounds bad - ran a cable to another amp
FX IN (disconnecting preamp to power amp by inserting a plug) sounds bad.
No switches, or pots effect the sound - except the Presence, does change the 'sound' slightly

I've made the wrong assumption the EMU would tap into the power amp section but that doesn't appear so. I'm going to recheck all of this to make certain.

After thinking about that I realized R4 couldn't possibly create that noise.

[g1]

Not sure if it's the schematic or just me, but I can't see a source for the EMUL out. I see the buffer on the FX loop drawing, but can't find a source for it.

[ricach]

Not sure if it's the schematic or just me, but I can't see a source for the EMUL out. I see the buffer on the FX loop drawing, but can't find a source for it.

Good question, I can't seem to find a source either. But checking the marshall forums (from a senior member) apparently it comes from the DSP and it is 'pre-effects loop'. Which I'm finding hard to believe. There appears to be nothing on the DSP chip unaccounted for (provided its a 28 pin chip). Each pin appears to have a function, or is marked with one. I'll follow the trace on the board and see where it goes.

[ricach]

Ok, so EMU does pass a good signal - apparently the cable I was using was bad. Also, now the ENZO thump does quiet the amp. Sometimes it will take 20 or so minutes for the howl to come back, and sometimes 20 seconds. I have the amp oriented upside down now and maybe that's why the thump works now. The one consistency that remains is that it takes almost an hour for the amp to warm up before it howls for the first time.

So, now what? What does that tell me?

[g1]

So EMU out is ok, that makes a lot more sense. Also FX send is ok so you have ruled out the preamp. The fact that it now responds to getting whacked means you have a bad connection somewhere in the power amp (after the FX return). Most likely a bad solder connection. See post #12

[ricach]

So EMU out is ok, that makes a lot more sense. Also FX send is ok so you have ruled out the preamp. The fact that it now responds to getting whacked means you have a bad connection somewhere in the power amp (after the FX return). Most likely a bad solder connection. See post #12

I will resolder connections, but the engineer in me is very curious why a 'break', however so small, could cause such a ruckus. First of all, what creates that sound? DC hitting the grid? I know by poking around with my meter probes I can duplicate that noise in any amplifier. But all of these are the result of a connection occuring, not a break in one.

[Jazz P Bass]

I don't see why an intermittant causing grief should confuse you.
The circuit was not designed that way.
If it is not assembled & soldered correctly, all bets are off as to how it will work.

[ricach]

Intermittant - signal cutting in and out - is not confusing. My question is, what has to cut out of the circuitry to cause such noise? Surely something is not cutting 'in'.

[DrGonz78]

The heat of the amp is cutting "In"... That is enough to make it not perform as it was designed. Solder joints are peculiar buggers and sometimes you think there are 50 other components wrong inside the amp before you find even that one lone assassin solder joint. Maybe others here can explain it better but soldering either way is an art to me. Almost every old used amp I get I solder the boards to make sure that I am in control of something in this regard. However, you might cause the re-flowed solder joint to loose contact in the circuit. That's why when you do re-solder a board it pays off when you spend many hours practicing soldering these boards. To guess what is causing an intermittent problem is a good thought here but it could be anything since we don't even know what exactly the culprit is yet.

Two weeks ago I was messing w/ a cheap ole piece of crap little Esteban amp trying to figure out why it stopped working... Ok don't laugh too hard but I found it was rewarding! Seriously, this one drove me nuts! I kept thinking how can I fix way more complicated amps and this one is just driving me crazy? So, the good news was it was a tiny solder joint on a tone capacitor that was muting out the signal. Now that was easy but it took a long time tinkering around to find the lil bugger. So now you have this huge amp and the Enzo thump is giving you a clue. It would be nice if you finally do pinpoint exactly which solder joint(s) is the culprit, but this is a complicated circuit board. Sometimes you wind up soldering an entire board to find you fixed it and never get to know which exact solder joint was the culprit.

Another attempt could be tapping components w/ a chopstick to see if you can locate the offending solder joint... That is if that is really what is wrong on this amp.

[ricach]

I've fixed many issues by just resoldering the part so I'm aware the problems can be caused by bad solder joints. In each of those cases, it was something - signal - or functionality - that became absent or distorted or dampened. I have never had an issue where it introduced such noise.

Since I don't run a shop I don't have to fix this quickly and that gives me the luxury to dig down deep and try to pinpoint the exact point of failure and I think a big clue is the fact that not only does the signal cut out, but a new loud 'signal' (noise) is introduced concurrently. So I ask myself - given the circuitry at hand - how would I create this issue myself - what trace would I cut? what component leg do I lift? to introduce this problem.

I'll start at the power tubes and work outward. What is the purpose of the diodes D9, D10, D11 and D12? I can't say I've ever come across that before hanging off power tubes.

[g1]

You are getting a very loud hum. You compared it to the momentary sound plugging something into an amp when the volume is up. What creates this is the loss of a ground connection. So a simple bad solder connection in a sensitive part of the circuit can cause this loss of ground reference and give your loud hum.

[ricach]

Finally had the time to remove the PC board and do some resoldering. Not much has changed but there appears to be something happening that's pretty consistant now and was not apparent before. Has to do with the mute circuitry. I can get the amp to start howling by 'exercising' the channel switch, and once it's howling it only appears on channel one. No signal (or howl) passes on channel two.

I've never dealt with a mute circuit before, can somebody walk me through what essentially happens when I engage the channel switch? Although the physical switch has multiple contacts on it, I'm assuming it still operates relays that do the actual switching - evident by the clicks I hear?

[Jazz P Bass]

The Mute relay is controlled by IC2. It appears to be a microcontroller.
It turns on TR9 to give the mute relay ground, which engages it.
The FX, OD Boost & Channel Select relays are controlled by IC1, which is a latching shift register.
And that is controlled by IC2.
I am not certain that a faulty relay contact could make an amp howl, but you could try temporarily 'jumpering' the normally closed channel select contacts (clean channel?)

[J M Fahey]

Intermittant - signal cutting in and out - is not confusing. My question is, what has to cut out of the circuitry to cause such noise? Surely something is not cutting 'in'.

To , say, have a "thump" or a "click" in an amp you do not need to have "something" from "outside" , "injecting" said signal.
Suppose you have a 12AX7 with 150V on plate, and a 270V rail feeding it.
If the cathode track (or soldering) breaks , plate will shoot from 150 to 270V .... nice strong 120V peak pulse, injected on the next grid.
If the grid track opens, tube loses negative biasing and saturates, a 150V negative peak, similar effect on the next grid.
In a nutshell; *any* point which has DC there, will click or thump if interrupted.