Marshall Origin 50H

[jondoe]

Hi All,

I have a Marshall Origin 50 which is misbehaving, I don't often touch valve amps so I was hoping I could share the problem with the great and good of the forum and ask for some help please.

I can't find an official schematic, I did find a DIY reverse engineered schematic which appears to be mostly correct, that said I found at least one resistor which is incorrectly marked, so there may be more errors.

The problem manifests itself when the amplifier is in HIGH mode, V4 will red plate and draw so much current it pops the 630ma fuse FS1. I've swapped the existing output valves around and the fault doesn't follow to the other socket, it still red plates only on V4. It does not red plate when the out put is on LOW or MID. As I'm not a valve person I don't have any spare EL34's at the moment, but as the fault doesn't follow the valve, am I safe to assume it's not the valve? (They are fairly new and little played)


Measuring the voltage at the resistors R38/R39 and switching between modes I see the following;

R38 (V4)

LOW 0.3v MID 1.8v HIGH 6.8v

R39 (V5)

LOW 0.6v MID 7.9v HIGH 34v


The pin voltage readings on V4 while in LOW are;

1 0.1v
2 0.1v
3 0v
4 425v
5 63
6 NC
7 0.3
0 0


The pin voltage readings on V5 while in LOW are;

1 0.1v
2 0.1v
3 0v
4 433v
5 63
6 NC
7 0.6
0 0



Measuring the bias at point CN32 shows 126 mV, I've read that it should be around half that. Adjusting the bias trim pot has no effect on the reading.


I'm not sure where to go from here, so any pointers are gratefully received.

Thanks.

[x-pro]

Wow.
I guess a lot of weed was smoked in the development of the scheme.
They made a lot of switching elements, with a lot of small parts to maintain them.

....Measuring the bias at point CN32 shows 126 mV, I've read that it should be around half that. .

Where is this point?

[Helmholtz]

Grid (pin 5) voltages must be negative.
Lift one end of C26 and see if this changes V4 voltages.

[jondoe]

Wow.
I guess a lot of weed was smoked in the development of the scheme.
They made a lot of switching elements, with a lot of small parts to maintain them.



Where is this point?

Oh dear, it appears to be missing from the DIY schematic



It sits on R3, which IS on the schematic Google/Youtube shows people using CN32 for bias adjustments.

Grid (pin 5) voltages must be negative.
Lift one end of C26 and see if this changes V4 voltages.

I'll double check my pins, looking at 8 pins in the socket and 7 pins under the PCB gets a little confusing I'll see if I can lift the leg of C26 without popping the board out and report back.




Thanks guys.

[g1]

The other thing wrong in your measurements was the pin3 voltages of 0. Should be more than screen voltage there (or at least equal to).

[jondoe]

The other thing wrong in your measurements was the pin3 voltages of 0. Should be more than screen voltage there (or at least equal to).

You could well be right I've just tried to continuity test the top of the socket to the pins under the PCB, as some of them are tied together I may have mistaken pins. Does anyone have a transposed pin numbering from these 8 sockets to a 7 pin PCB mount? I might have to pull the board again to be sure

Would it be useful to measure the voltages without the valves in place?

[jondoe]

I've taken readings from the top side minus the valves, at least I know what pin I'm on


1 0
2 0
3 487
4 300
5 -0.25
6 N/A
7 0
8 0

Both sockets measure the same, switching the LOW/MID/HIGH switch on the front makes no changes to the BIAS, from the DIY schematic it suggests there should be three voltages for the bias, which I assume I would see on pin 5? Or do I need the valves in the socket to get a true measurement?

[Helmholtz]

From post #1:

R38 (V4)
LOW 0.3v MID 1.8v HIGH 6.8v

This means the bias voltages changes with Low/Mid/High settings. I assume these are negative voltages. Values are too low.

One end of R38 connects to the bias supply, the other one to V4 pin 5.
The voltages at either end of R38 should be the same (so pin 5 voltage should vary as well).
If they're are different either R38 is bad (gone high resistance), the socket is conductive or your meter is unsuitable to measure grid voltage.

Did you try disconnecting C26?

[g1]

Does anyone have a transposed pin numbering from these 8 sockets to a 7 pin PCB mount? I might have to pull the board again to be sure

The blank spot is pin 6.

[jondoe]

From post #1:



This means the bias voltages changes with Low/Mid/High settings. I assume these are negative voltages. Values are too low.

One end of R38 connects to the bias supply, the other one to V4 pin 5.
The voltages at either end of R38 should be the same (so pin 5 voltage should vary as well).
If they're are different either R38 is bad (gone high resistance), the socket is conductive or your meter is unsuitable to measure grid voltage.

Did you try disconnecting C26?

I have yet to remove C26, it's too tight to the board to lift a leg so requires the board out to remove it, which I will try once I've manged to take relaible readings, I seem to have made a pigs ear of it so far

I made a schoolboy error taking the readings with the power valves out, I didn't plug in the dummy load, which of course left the protection circuit working, which means the LMH switch has no effect on the bias.

So when plugged it in , I see identical readings on both sockets. 480v on pin 3, 296v on pin 4 and the voltages on pin 5 vary with the L/M/H settings, -0.2 / -8.3 / -44 respectively.


Thanks for the annotation g1, that's really useful, I'll put the valves back in and take readings with them in circuit on the low and mid settings.

[jondoe]

Having taken the readings with the valves in place I noticed that the V5 socket was acting up, this lead to me to think I may not have swapped the valves around like I thought I had, so I swapped them around marking the "bad" tube so I didn't mix them up. The fault moved with the valve, so I'm now of the opinion the valve is the problem The bias voltage collapses on the bad tube as you switch from LOW to HIGH and the valve red plates.

I've ordered a new pair

[g1]

Once you get the new tubes in, could you post the voltages measured at EL34 pins 4 and 5 for each of the 3 switch positions? I think there are errors in the voltages listed on that schematic.

[jondoe]

Once you get the new tubes in, could you post the voltages measured at EL34 pins 4 and 5 for each of the 3 switch positions? I think there are errors in the voltages listed on that schematic.

I replaced both valves, this has solved the issue The bias was been set to approx 79mv across CN32 in HIGH.

Pin 4

L 60v
M 165v
H 445v

Pin 5

L -0.7v
M -8.2v
H -38.5v

Thanks again for input everyone