Marshall Haze 40 frying bacon sound

[hatecheapstuff]

Hi guys,

I have trouble with this amp.
When I took it off standby it makes a frying bacon/crackling sound and I can't hear guitar at all. When I turn the volume/gain knobs all the way up I'm able to hear the guitar just a little bit.
First, I thought this must be worn tubes so I replaced both preamp and power amp tubes. Sadly, it didn't help. My next step was to replace most of the electrolytic caps, because I thought that filtering caps could be old.
This also didn't help

I'm sure there are no broken/cold solder joints in this amp. I even tried removing the effects PCB but no luck with that.

I plugged the emulated line out with my other amp and I could hear the same stuff (crackling) on the other amp as same as form the speaker of the Haze.

Any ideas what could it be?

[Enzo]

My first suspect would be a noisy op amp, my next would be a noisy transistor.

[The Dude]

1) Welcome to the place.
2) Schematic link: http://www.thetubestore.com/lib/thet...e%20MHZ40C.pdf
3) I wouldn't have just randomly replaced filter caps without checking for poor filtering. A more methodical troubleshooting method will get you there faster.
4) That said, start with the power supply. Do you have all of your voltages? Are all of the tubes lit (I've had a few issues with filament supply on these)?

[hatecheapstuff]

1) Welcome to the place.
2) Schematic link: http://www.thetubestore.com/lib/thet...e%20MHZ40C.pdf
3) I wouldn't have just randomly replaced filter caps without checking for poor filtering. A more methodical troubleshooting method will get you there faster.
4) That said, start with the power supply. Do you have all of your voltages? Are all of the tubes lit (I've had a few issues with filament supply on these)?

1) Hello and thank you for your reply.
2) I'm using the same schematic.
3) I understand that the best way to check if the cap job was necessary, was to check up the shape voltage waveform on DSO? Sorry, I'm a newbie. I don't have a scope too
4)I took a photo of the PT label: https://www.dropbox.com/s/fd5zht8y2m....38-1.jpg?dl=0
and took some readings with my DMM:

RED - GREEN 335.17VAC
YELLOW - WHITE 13,528VAC
ORANGE - BLACK 6,899VAC

And that is with the PCB removed and disconnected from the chassis. I think this looks pretty healthy, but it's without any load.

Would you recommend me further measurements with the PCB and tubes back in the place?

I remember that there was about 440VDC plate voltage(?) when I removed both power tubes and switched the standby switch in "playing" position.
I also measured the cathode current (?) of about 39mVDC for each tube at the test point on the PCB. I took a voltage reading between the central pin and one of the outer ones.

BTW, I think that all of the tunes are lit. Some little more some little less...

Please, excuse my poor english and lack of EE knowledge :-)

[The Dude]

I'll assume some of those commas are supposed to be periods. I doubt there's 13 thousand volts there anywhere.

I would check the power supply outputs on page 5 of the schematic, 12V, 5V, etc. I might also check the voltage on the plates of the preamp tubes. You might have an open plate resistor on a preamp tube.

[gbono]

Do you have a scope? Chase the signal all the way through the stages and see what you get at the plates of V4 and V5. If you have a strong clean signal from V1 through V4/5 then you are looking at a damaged OT - rare but....... Build the circuit here Testing a transformer for shorted turns - UK Vintage Radio Repair and Restoration Discussion Forum and test the primary and secondary windings of the OT with either side unconnected to the circuit. If the neon doesn't flash during testing of the windings then you have a shorted turn in the OT. You can also use a DMM to check and see if any windings are open.

[hatecheapstuff]

I'll assume some of those commas are supposed to be periods. I doubt there's 13 thousand volts there anywhere.

I would check the power supply outputs on page 5 of the schematic, 12V, 5V, etc. I might also check the voltage on the plates of the preamp tubes. You might have an open plate resistor on a preamp tube.

Yes, treat commas as periods. This is European writing ;-)


I took some readings:

No plug in the input socket.
W12-W14: 0,27VAC (Power on, Standby Off), 318,53VAC (Power On, Standby On)
W10-W9: 12,85VAC (Power on, Standby Off), 12,78VAC (Power On, Standby On)
W17-W16: 6,35VAC (Power on, Standby Off), 6,30VAC (Power On, Standby On)


Please, could you tell me how can I check the voltage on the plates of the preamp tubes?

You might have an open plate resistor on a preamp tube. Please, could you tell me the resistor marking from the schematic so I can check them out? I really don't see any burnt resistors or anything other.

[hatecheapstuff]

Do you have a scope? Chase the signal all the way through the stages and see what you get at the plates of V4 and V5. If you have a strong clean signal from V1 through V4/5 then you are looking at a damaged OT - rare but....... Build the circuit here Testing a transformer for shorted turns - UK Vintage Radio Repair and Restoration Discussion Forum and test the primary and secondary windings of the OT with either side unconnected to the circuit. If the neon doesn't flash during testing of the windings then you have a shorted turn in the OT. You can also use a DMM to check and see if any windings are open.

No, I don't have a scope, Wish I had...

How can I check the OT with a DMM?
Should I measure the resistance of Primary and Secondary windings?

[The Dude]

...I took some readings:

No plug in the input socket.
W12-W14: 0,27VAC (Power on, Standby Off), 318,53VAC (Power On, Standby On)
W10-W9: 12,85VAC (Power on, Standby Off), 12,78VAC (Power On, Standby On)
W17-W16: 6,35VAC (Power on, Standby Off), 6,30VAC (Power On, Standby On)


Please, could you tell me how can I check the voltage on the plates of the preamp tubes?

You might have an open plate resistor on a preamp tube. Please, could you tell me the resistor marking from the schematic so I can check them out? I really don't see any burnt resistors or anything other.

Those readings are the same as your others- just after the plugs. You need to measure power supply outputs. You've already proved your transformer is good.
Respectfully (and I sincerely mean no disrespect), it sounds like you need to read up on tube amps and troubleshooting before you attempt this repair. I suggest Google searching "tube amp repair primer" or something of the sort before you go any further. Again, not a personal attack, but if you don't know how to read plate voltages and are digging around in a tube amp where high voltages are present, you are asking for trouble and possibly death. There are lethal voltages in there.

[hatecheapstuff]

Those readings are the same as your others- just after the plugs. You need to measure power supply outputs. You've already proved your transformer is good.
Respectfully (and I sincerely mean no disrespect), it sounds like you need to read up on tube amps and troubleshooting before you attempt this repair. I suggest Google searching "tube amp repair primer" or something of the sort before you go any further. Again, not a personal attack, but if you don't know how to read plate voltages and are digging around in a tube amp where high voltages are present, you are asking for trouble and possibly death. There are lethal voltages in there.

I think those readings are a little different, because now the PCB is connected to the PT and makes some load...

I have some basic knowledge about electronics and doing measurements with a DMM plus soldering.
I know about lethal voltage and stuff... I used to do retubing and biasing all by myself but this case is completly new for me and I really need to fix this amp. There aren't any people that have knowledge or tools to do that around me.
I know how to measure plate voltage on the power amp tubes, but I don't know how to do it at preamp tubes.

Please, could you provide me some help? I promise, I won't die

As I said before, I don't see any burnt resistors and stuff like that. No cold or cracked solder joints too.

[The Dude]

Well then,...... Pins 1 and 6 are the preamp tube plates. Check the DC at those pins just like you would the plates on a power tube. Let me also say that Enzo's post #2 is a good one and just as likely. I brought up the preamp plate voltage because it's a quick easy thing to check and certainly could be the problem. Plate resistors often don't look burnt, so don't rule out that one of them might have gone towards open.

[gbono]

did you see this? Tube Amplifier Debugging Page

[hatecheapstuff]

Well then,...... Pins 1 and 6 are the preamp tube plates. Check the DC at those pins just like you would the plates on a power tube. Let me also say that Enzo's post #2 is a good one and just as likely. I brought up the preamp plate voltage because it's a quick easy thing to check and certainly could be the problem. Plate resistors often don't look burnt, so don't rule out that one of them might have gone towards open.

Thanks for your input.

Here are the readings:
First preamp tube from the left (closest to power and standby switch):
Pin 1: 359.75 VDC
Pin 6: 360.9 VDC

Second preamp tube from the left:
Pin 1: 361.28 VDC
Pin 6: 363.11 VDC

Third preamp tube from the left:
Pin 1: 363.69 VDC
Pin 6: 364.3 VDC

I guess they are OK.



I don't have a neon bulb to make a circuit to test the OT, but I measured the resistance.

Here are the pics of the OT itself:
https://www.dropbox.com/s/xos583qqot...33.19.jpg?dl=0
https://www.dropbox.com/s/mjaf2jbph2...33.25.jpg?dl=0

Primary windings:
Brown - White: 76.26 Ohms
Blue - Brown: 153.24 Ohms
Blue - White: 77.04 Ohms

No shorts to the chassis and secondary windings.


Secondary windings:
Black - Red: 0.79 Ohms
Black - Yellow: 1.22 Ohms
Red - Yellow: 0.48 Ohms

No shorts to the chassis and primary windings.

[gbono]

Okay so in your first post you mentioned that you found that the noise/low signal appeared at the FX out for just the preamp. Can you completely bypass the FX (digital effects)? You may have completely functioning "analog" signal chain and issues with the digital side of the circuit only??

Also check out this link for another technique for troubleshooting a signal through the amp Troubleshooting your amp with audio signal tracing |

You can also continue checking the grid voltages on V1/2 to make sure you have proper interstage voltage conditions (the capacitors are behaving).

[hatecheapstuff]

Okay so in your first post you mentioned that you found that the noise/low signal appeared at the FX out for just the preamp. Can you completely bypass the FX (digital effects)? You may have completely functioning "analog" signal chain and issues with the digital side of the circuit only??

Also check out this link for another technique for troubleshooting a signal through the amp Troubleshooting your amp with audio signal tracing |

You can also continue checking the grid voltages on V1/2 to make sure you have proper interstage voltage conditions (the capacitors are behaving).

I have never used the emulated line out before. I believe, you use this output when you want to connect the headphones or feed the PA mixer or something like that.


When I run a cable from the emulated line out to the input of the other guitar amp I have, I got exactly the same sound, as I can hear from the original speaker. It's quiet and I hear the noise/crackling/beacon frying sound too.


Regarding the digital effect, this amp has a separate PCB just for that. You may see it in this picture: http://storage.lauda-audio.pl/fck_fi...hallhaze13.jpg
It provides echo, delay reverb and stuff like that. I have removed this board to have less stuff to check. I removed it by lifting the PCB, because it is connected with the main PCB with connectors/pins. The amp works without it. Of course, there are controls on the front panel to enable or disable those effects.