Modern Marshall Origin 5 no schematic

[Randall]

A Vietnamese Origin 5 single ended combo can in as powers up, but no sound. Both preamp tubes don't light up, so I figured this should be easy. No such luck. They are powered by a DC filament supply, and no schematic is available. I did find the pins 4/5 did not have continuity to the other end of the trace they are connected to, + of a 330uF cap, so I resoldered all the socket pins, but it didn't fix it. I now have 2.8vdc on pins 4/5 to ground with no tube installed, but with tubes installed I have 0.08 volt.

The PT has only two windings, 250vac and 7vac. I think the dc supply is derived from the 7vac. The diodes in that part of the board check good in circuit, and I pulled the three big caps that are on that trace to no avail. That trace also runs down the side of the main board to the other end and feeds something else over there. My guess is something is pulling down the dc supply.

With no schematic and two hours into this, I am cutting my losses and calling it quits. He will have to make the hour + drive to the closest Marshall shop. I woud rather admit I can't fix this than spend more hours tracing out what someone else can read on a schematic.

[nickb]

It sound more like a track/ solder joint/ connection has gone high impedance. If something were loading the DC supply to that extent, the diodes would likely burn out as would the transformer. A favorite place for bad solder joints is on the the diodes as they often run hot in DC heater supplies.

[Pedro Vecino]

It must have an identical or very similar circuit to that of Class 5. It could serve as a reference.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Mick Bailey]

The supply is fused via FS2 and the AC then feeds the EL84. Afterwards it rectified by D5 to D8 for the preamp heaters. Is the fuse good? Is the EL84 powered?

[Randall]

FS2 is good and the EL84 is lit up. The cCass 5 supply is similar, but not the same. I have 5 sectangular .1uF caps that the Class 5 does not, 4 alternating in position with D5 - D8, and one going to ground off one of the big filter caps. First I lifted the diodes to make sure they were good, and they were. After that, the preamp tubes lit up, and I had 6v on pins 4/5, until I put it back together and it went back down to 3v.

Next, I removed the five .1uF capacitors, all measured good, and the tubes were lit with them out of circuit. So I put them back one at a time, measuring each time to see if I could find a bad one. Each time everything looked good up to and including the last one, now the amp is working, and I don't really know why. I can understand if one of those caps was leaking how it would pull down a supply, but why does it work now after simply removing and reinstalling them? And if one was leaking before why doesn't it show itself as leaking now?

Thanks for talking me off the cliff, I have several hours into this that I cannot bill, but at least it is fixed.

Plus, the solder used in this amp is the spawn of the devil. It was miserable to work with, hard to melt and stubborn to wick. I'm just glad I didn't lift any pads or fry any parts trying to get them out.

[Enzo]

That would be the lead-free solder mandated by the Euro people. Use your regular lead solder and it will improve.

Removing and resoldering a part at least reassures the connection, and also potentially removes a tiny short between traces or two other points.

[Mick Bailey]

The four caps C22 to C25 are snubber caps across the diodes. The other is C17, paralleled with with C16 to improve HF filtering. These two are not part of the heater supply, they're fed off it via a 47R resistor R33 and this then powers the transistor FX loop and boost switching functions.

[Dave H]

Plus, the solder used in this amp is the spawn of the devil. It was miserable to work with, hard to melt and stubborn to wick. I'm just glad I didn't lift any pads or fry any parts trying to get them out.

What soldering iron are you using?
I have no trouble with unleaded on PCBs using a 50W Weller TCP and #7 tip (700F, 370C)

[alexradium]

always add leaded solder to the pad and mix it,then suck it away,repeat several times,and yes you need at least 50 watts regulated,and still on ground planes its hard sometimes.

[Mick Bailey]

That's what I do, though I have a desolder station and it's not strictly necessary but makes the job easier. Also, if I need to use solder wick I use additional paste rosin flux on the wick which helps along with the pre-melt with leaded solder.

[Randall]

"What soldering iron are you using?"

I used a 50 watt Weller WES51 soldering station at 750 degrees, a liquid flux pen, copper braid, and added Kester rosin core solder one or more times.

" C17, paralleled with with C16 to improve HF filtering. These two are not part of the heater supply, they're fed off it via a 47R resistor R33 and this then powers the transistor FX loop and boost switching functions."

They may not be part of the heater supply, but if they are fed off it, and one was leaking to ground, couldn't it pull down the heater supply? I thought so, so I looked into them.

[Mick Bailey]

I only mentioned the extended heater circuit to provide a context for the additional cap and its relationship, not to suggest it couldn't be responsible for the problem. Anyhow, I would think that a cap shorting to that extent would get very hot (or something would). Think about it this way - the heaters of the two combined tubes don't pull down the power supply, yet look how hot they get. So if they're not hot and the voltage drops so much then something else must get even hotter as a consequence. A cap shorted to that degree would not likely recover.

Edit: The 47R resistor is the limiting factor in that part of the circuit and even if the cap was dead short it would only dissipate 0.8W

[vintagekiki]

https://www.thegearpage.net/board/index.php?threads/marshall-origin.1906093/
Marshall Origin

1)
http://musiciansroadhouse.com/viewforum.php?f=98
Marshall Origin 50H , 20H, 50C. 20C, 5C

[Randall]

Rats, this amp came back for the same problem except now it will light the two pre tubes when it is cold, but they die out after a few minutes. I found that once they die out they will come back on if I hit the rectifier area with freeze spray.

I might try wholesale replacing that section.

[The Dude]

It's not all that uncommon for a diode to measure ok with a meter, but fail under load. I'd suspect an intermittent/bad rectifier diode and just replace them all.