Troubleshooting Amp

GIBSON Les Paul · Post by **GIBSON Les Paul** » Sat Sep 17, 2005 7:02 pm

Well I finished my kit.
Tubes glow, but the indicator lamp doesnt light up and there is no sound. Any suggestions?

I will try to get voltage measurements and pics up soon.

GIBSON Les Paul · Post by **GIBSON Les Paul** » Sat Sep 17, 2005 7:36 pm

With no tubes in, I get the proper VAC on pins 2 and 7 on the output tube sockets, and the same reading on pins 4, 5 and 9 of the preamp tube sockets. And I get the proper VDC readings on pin 5 of the output tube sockets.

I looked at the fuses. The mains fuse looks fine, but the H.T. fuse is blown.

Flames1950 · Post by **Flames1950** » Sun Sep 18, 2005 12:05 am

What kit were you doing, the 100-watt? Or was it a PTP board in an actual Marshall?
If your tubes were in, leave them out and try a new HT fuse just to prove whether it was the tubes or not. If the fuse lives you may have bad tubes (likely power tubes and not preamp tubes.)
If the fuse blows with no tubes in you've got a short that needs to be tracked down inthe rectifier circuit or the filtering. We can go through voltages, but pics may speed things up even more.

GIBSON Les Paul · Post by **GIBSON Les Paul** » Sun Sep 18, 2005 1:29 am

I took the tubes out and put in a new HT fuse and fired it up. Then I put the tubes back in. The HT didnt blow, but still no noise. I think I blew the fuse when checking voltages (saw a spark).

Hopefully I will get around to post some pics tomorrow. Thanks.

GIBSON Les Paul · Post by **GIBSON Les Paul** » Sun Sep 18, 2005 4:18 pm

Here are some pics.

Sorry about how messy it is.

Flames1950 · Post by **Flames1950** » Sun Sep 18, 2005 4:32 pm

Well I'm sure the no sound issue is probably caused by the green grid wire going to the second preamp tube -- it's not connected.

GIBSON Les Paul · Post by **GIBSON Les Paul** » Sun Sep 18, 2005 4:57 pm

Flames1950 wrote:Well I'm sure the no sound issue is probably caused by the green grid wire going to the second preamp tube -- it's not connected.

I connected it and still nothing. Ugh.

flemingmras · Post by **flemingmras** » Tue Sep 20, 2005 1:33 am

Your indicator lamp problem is an easy one. It's a 115VAC lamp,yet you have it hooked to the heater supply, which is only 6.3VAC. The lamp needs to be connected with one wire going to the red wire of the mains switch, and the other wire going to the orange wire on the mains fuse.

For the rest of the problems, call me at (559)292-6274. If you call me we can get this thing up and running before I get off the phone with you.

Jon

sub · Post by **sub** » Tue Sep 20, 2005 9:34 am

Flames1950 wrote:Well I'm sure the no sound issue is probably caused by the green grid wire going to the second preamp tube -- it's not connected.

Cool!!!
You have eyes like a hawk!

Sub

Flames1950 · Post by **Flames1950** » Tue Sep 20, 2005 4:59 pm

I know you've had this up on Plexi Palace too and I haven't been keeping up well. Anything from there to update us?
Did you try both channels? The green wire going from the bright channel's two 68K resistors to the first preamp tube doesn't look like it's joined to the turret very well right where those resistors join (it's the inside pair of 68K's.)
You said the HT fuse did not blow without tubes, and survived a short while with tubes in? Let's get DC voltage readings at all of the following points, with the black common lead stuck in one of the chassis mounting nuts for ground:
All EL34 sockets, pins 3 and 6.
All 12AX7 sockets, pins 1 and 6.
Give us readings from the first 12AX7 to the last EL34 on these pins and we might spot where you're going wrong. I'm not seeing much else in the pics but I can't see how that filter cap under the board is wired and this should clear up whether it is wrong or not.

GIBSON Les Paul · Post by **GIBSON Les Paul** » Tue Sep 20, 2005 7:28 pm

Ill probably be able to do that tommorow. I have been advised to go over my sloppy solder work too.

And about the HT fuse, I am almost certain I blew it checking voltages, because i saw a spark (the lead was touching 2 pins).

Thanks guys.

Flames1950 · Post by **Flames1950** » Wed Sep 21, 2005 12:21 am

The reason I wonder about the filter underneath the board is that the wiring instructions were a little vague and I remember one of the guys wiring the red B+ wired to ground under there, I don't think his fuse blew but he lost all his B+ voltage halfway across the board.
I do remember seeing the guy's soldering comments -- not that everyone shouldn't learn to solder well, but I kinda thought he was a bit of a dick about it.......it does take practice after all.

GIBSON Les Paul · Post by **GIBSON Les Paul** » Wed Sep 21, 2005 6:21 am

Flames1950 wrote: I do remember seeing the guy's soldering comments -- not that everyone shouldn't learn to solder well, but I kinda thought he was a bit of a dick about it.......it does take practice after all.

Well actually my solder work is REAL bad. Im 95% thats where the problem lies. And even if it isnt, the amp vibrating on the cabinet is bound to knock something out of the circuit.