Marshall Haze 40 shorting out?

[ricach]

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?)

On the actual switch, or the relay? I guess I should try both.

BTW: Thanks for the time you (and others) are expending to help me out.

[Jazz P Bass]

The mute circuit works by 'grounding out' V1B (mute 1) & the preamp signal (mute 4).
When it mutes is not apparent.
At start up maybe.
Hopefully at channel switching.
IC2 (microcontroller) gets the channel switching input from the switches.
It then turns on the mute relay as it also changes channels via the shift register.
I think before you jumper anything you have to ascertain what is failing.
If you think it is the mute circuit, pull the relay.

[ricach]

[QUOTE=ricach;283494]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.
QUOTE]

Having the above situation allowed me ample time to investigate while the amp was silently howling - no worry's damaging the speaker, tubes or my ears. I traced it to one of the preamp tubes. Slight wiggle and I can get things working fine again for hours. So I resoldered (all) preamp tube sockets and cleaned them. Unbelievably we are now back to original problem of it howling no matter which switches are pushed, and the efx out and emu out have CLEAN signals - or sometimes NOT.

Relay 1 (mute relay) appears to me the only commonality between the stages of the amp - it appears in preamp, efx and power amp stages. Something Jazz mentioned from the get go. Any suggestion where to go from here? Just replace it before going any further? I don't think it's possible to jumper it.

[ricach]

Relay 1 (mute relay) appears to me the only commonality between the stages of the amp - it appears in preamp, efx and power amp stages. Something Jazz mentioned from the get go. Any suggestion where to go from here? Just replace it before going any further? I don't think it's possible to jumper it.

I can't find a replacement relay so I'd disable it and see what happens. I thought if I short the base of TR1 to DGND I could remove it's functionality. I expected a good pop when I channel switch, but what I got was a permanently muted channel 2. I don't follow the logic of the mute circuitry. If you can, please educate me.

FWIW: The only thing that affects the volume of the howl is enabling and disabling the fx loop via the rear panel switch. There is a slight change in volume when bypassing the fx circuitry which tells me the howl must originate before this?

[Jazz P Bass]

They should call this amp "Maze'.
I apologize that I cannot fully follow this schematic without having the amp in front of me
There is no information on what the switches do nor what the true logic of the relays is.
TR1 should activate the mute relay by giving the relay a ground reference when the base goes high.
Why pulling the base permanently low interfered with Ch 2 , I cannot follow.
You really should have a scope or some way of monitoring where this noise is originating from.

[ricach]

They should call this amp "Maze'.
I apologize that I cannot fully follow this schematic without having the amp in front of me
There is no information on what the switches do nor what the true logic of the relays is.
TR1 should activate the mute relay by giving the relay a ground reference when the base goes high.
Why pulling the base permanently low interfered with Ch 2 , I cannot follow.
You really should have a scope or some way of monitoring where this noise is originating from.

I have a scope and I intended to scope this long ago but I wanted to have much better knowledge where to expect the signal path to go as the channel is switched. I guess I should trace a path while it's functioning so I know what to expect when it's howling.

[Jazz P Bass]

What has transpired since Post #32?
It sounds like you where on it by wiggling a tube.
Which tube?

[ricach]

What has transpired since Post #32?
It sounds like you where on it by wiggling a tube.
Which tube?

See Post #33. It was V2. Too sumarize from the beginning, I resoldered most of the power amp section. That then created the situation where switching channels is what caused the howl (instead of a possible heat related issue). Wiggling V2 would fix, and unfix, the problem. So resoldered and cleaned the tube socket - which fixed that problem but that then put me back to the very original issues I experienced in the beginning but without it appearing to be heat induced (once again see post #33).

I don't expect to find just one point in the circuit that the howl originates everytime, but hopefully it occurs on the backside of a relay contact each point. I intend to look at the signal at each of theses points while the amp is operational, and then again when it howls.

Unfortunately this can be like trying to remember the alarm code while the alarm is blaring. lol. It can be disconcerting.

[Jazz P Bass]

Yeah, the trick is to find it.
There is a term 'Divide to Conquer" that is used a lot.
It works.
From the schematic it appears that you can totally bypass the FX section.
That is a start.
If you could narrow it down to preamp or power amp that would help.
On Page 3 of the schematic, left side. Power Amp In.
Get on that point.
If the 'noise' shows up there, than it is a preamp problem.
If not, then it is in the power amp.
Then you 'divide' it down from there, whether or not it is a preamp or power amp issue.

[ricach]

Yeah, the trick is to find it.
There is a term 'Divide to Conquer" that is used a lot.
It works.
From the schematic it appears that you can totally bypass the FX section.
That is a start.
If you could narrow it down to preamp or power amp that would help.
On Page 3 of the schematic, left side. Power Amp In.
Get on that point.
If the 'noise' shows up there, than it is a preamp problem.
If not, then it is in the power amp.
Then you 'divide' it down from there, whether or not it is a preamp or power amp issue.

I kept the mute circuit disabled and traced to where the channel 2 signal disappears. I have signal from both channels up to R79 on FX Loop page (2). From here it goes to the FX card connector. Signal 2 disappears somewhere between there and R117 where the signals come back from the FX card Connector. And this is where the schematic is lacking an FXcard schematic. At least I don't see one.

Tomorrow I'll re-enable the mute circuit and try to get the amp to howl and see where that originates.

**Edit** I went back in to check a few more points coming back from the FXcard and realized there is a signal present for channel 2, but its only about 10percent what I'm getting from channel one. So something still is clamping down the signal. As mentioned I'm going to re-enable the mute and move onto the howl.

[Jazz P Bass]

The FX schematic is on Page 6.
IC2 is a microcontroller for a number of things.
It accepts all of the switch inputs, provides the digital reverb & outputs to the switching circuit.
Busy little guy

[ricach]

The FX schematic is on Page 6.
IC2 is a microcontroller for a number of things.
It accepts all of the switch inputs, provides the digital reverb & outputs to the switching circuit.
Busy little guy

The connector CON4 (page 2) that I was referring to going nowhere has no mate (on the schematics). (The connector CON5 that appears on page 6 (that you refer to) goes to CON2 on the rear panel (page 4)). Looking at the pcboard CON4 is actually a socket connector that the FX board pushes down into. This board is suspended on top of the main board. I'm going to need to take out a meter and actually trace just where R79/R150 leads to on the FX card. Likewise where the signal comes back from the FXcard to R72 and also C54. (These are the points where channel 2signal 'disapppears')

Not to add more wtf?'s but the FX loop bypass switch on the back panel just started working properly. Up to now It never appeared to work properly (or as apparently designed).

[ricach]

I attached a picture of what the howl looks like on the scope. I also attached a picture of what it looks like with the standby on. It's the same. This was taken at the R101 which is where the signal leaves the FX loop. In fact R101 and R100 looks the same no matter if RL4 (fx bypass switch relay) is engaged or not? I suspect it's not working properly (again). There is no howl present on the front side of TR10 or TR12 (at the gate) so its originating either in the preamp power rail or possibly from RL4. Maybe?




FWIW, the signal is still present most way through the FX loop circuitry when its howling, and some howl residue is apparent in the signal - more like a clean sine wave the same frequency - but appears to get completely overwhelmed on the backside of TR10 and TR12.

[Jazz P Bass]

Have we checked the health of the power supply?
Steady Vdc?
Minimum Vac ripple?

[ricach]

Seeing about 175VDC with no ripple. However as the picture shows, get some interesting stuff when it starts to howl.
This is at R73/R91