Help with Marshall Origin 5 mod

[Dave_11]

I have a Marshall Origin 5, which is very similar to the Class 5, from what I understand. Class 5 schematic can be found here.

One common mod is to change the grid leak resistor R20 from 470K to 1 Meg, and remove C10. So I did that, and then could get no sound from a guitar plugged into the input.

All tubes glow.
When I turn the volume up I get a soft hiss.
If I plug a guitar into the effects loop I do get sound.
I replaced V1 but still no sound.
All plug-in connections on the board are plugged in.

Some people said C10 needs to be jumped, so I tried that, but still no sound. I put the original 470K resistor back in, but no sound. I put the cap back in, but still no sound.

Can anyone help me with somewhere to begin troubleshooting this? I can't imagine what I could have done, as I didn't touch anything except R20 and C10.

 

[corliss1]

So you put the parts back that you took out, and now it still doesn't work?

 

[corliss1]

If that's the case, AND the resistors really are the same as that Class 5 schematic, it sounds like you may have damaged a trace on the board. You'll want to check continuity from the jack to all components around that input section that may have gotten hot, just to make sure they really are connected.

 

[Dave_11]

Thanks for the reply. A leg broke off the original cap when I pulled it, so I scavenged one with the same value from an old radio and put that back with the original resistor. Even if both of these are removed, wouldn't I still get some sound from a plugged in guitar?

I'll try checking continuity.

 

[Dave_11]

With a guitar cable inserted in the jack, I tested continuity between Tip and the three red dots.
Tip to R19 = no
Tip to R20 = no
Tip to C10 = yes
I guess I burned something up desoldering. The traces are on the underneath side of the board and I'll need to take it out to have a look.

 

[GlideOn-Designs]

This is why it's considerably more difficult to work on modern amps because you need to be precose and quick so as not to heat damage everything around where you are working.

Lead is specifically used as solder because it has a low 325° melting temp and is a great conductor. The key to it is keeping your tips sharp and wiped clean after every use. You also really don't want to go hotter than 375° on your 40w iron, it just boils everything else around it...

 

[Dave_11]

Thanks for the reply. I ended up using a small mirror to check the traces underneath the board and found that I had burned through one when desoldering. I soldered in a jumper and it works fine now.

Regarding iron temps, I bought a Hakko 70W soldering station and this is the first time I've used it for anything on a board. I was actually wondering what temp I should use for work like this. I just left it at the factory default of 750F, so maybe that was not good...

 

[GlideOn-Designs]

Unless you're building from scratch and grounding new leads to chassis, any iron over 40w and 400° is overkill. Yes the surface temp consistency is better on higher quality irons, but solder tips are easily swapped and disposable, plus they can be cleaned with a damp
sponge every few uses to extend their service life.

Really should have aim to just melt and flow solder, that's it. Lead based solder doesn't require a whole lot of temp or time to get it flowing which is only a good thing considering most ABS plastics & fibers (caps materials too) melt just a couple of degrees higher around 390° and that's if the items are new spec and not several years old already