1st timer board install help

clarkydaz · Post by **clarkydaz** » Thu Jan 27, 2005 11:25 am

with the slight burning, maybe the replaced resistors were not heat resistant and the ones george uses are.

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 12:07 pm

The wattage rating of a resistor is really a measurement of how much heat it can dissipate (watts are used as a unit of power in the ol' English measurement system but they are somewhat a measure of heat as well.)
If the resistor is the correct 2 watt rating and it's getting too hot, and we are losing all our voltage on the other side of it, then the bottom line is that there is a short somewhere right around that end of that resistor. It's surprising that we don't pop the HT fuse but we surely must be close to doing so!! Pulling the tubes and re-checking the voltages will eliminate the tubes from possibility, or prove that there's a bad preamp tube (usually preamp tubes don't croak that way but you just can't be sure unless you have a tube tester to check for shorts.) If tubes don't pan out then the short is in the wiring somewhere close to that point, either from that resistor to the tube socket, to the resistor next to it, or possibly the filter cap mounted above the board is shorted in some way. The trick is that I may not be able to find a wiring short from the pictures.
If pulling the tubes doesn't pan out my last-ditch effort in finding the short may be to yank the chassis of my '78 Marshall, pull the tubes and start taking resistance measurements from different points in that area to ground, then have you do the same and compare. But I'm not sure if I can find enough points to do that to so that we can prove exactly where the short is. But we'll try if that's what it comes to.

Dax · Post by **Dax** » Thu Jan 27, 2005 3:16 pm

I had the same problems, i converted a 2203 to a 1987, i just followed every wire and wired it like the diagram. Maybe you overlooked some wiring, wish i could look in person. If you can't get it, send it to george, you will be amazed when the amp is finished!!

Dax Good Luck.

clarkydaz · Post by **clarkydaz** » Thu Jan 27, 2005 4:03 pm

on the last page i mentioned that all the test voltages i have taken up to now have been with no tubes present, i didnt want to chance smoking my mullards! i'd love to send it george but i'm in the uk so that would be pricey and probably impractical. everything just seems to stop at the top of the 10k resistor voltagewise. i,ve emailed george a few times this week but he seems very busy, and probably would only say what you guys have said already so far. i dont know if redoing the board would be worthwhile as you guys have scanned it and nothing jumps out, so would again likely end up where i am now.

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 7:40 pm

I missed the post where you said the tubes were yanked. The Mullards are probably crap anyway, send them to me for disposal........

So we know the tubes aren't the short. That's almost bad news in this case.
I'm still staring at your pics right now. If there's no tubes in and the voltage ends at that 10K the short is there somewhere, you shouldn't even draw enough current to get those resistors hot without tubes for a load.
I can't see the connections to the filter cap mounted above the board, you might see if you can lift the board enough to double check your work and connections there. The two red wires at each end of the next 10K resistor go there, with a black wire attached to the third cap terminal going to ground, kinda like this:

Red, Red

Black

It's gotta be there or that tube socket wiring somehow I'm thinking, but I can't find it in the pics.

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 8:19 pm

Here's the pic of the current preamp wiring:

I notated the wire that needs moving if you haven't already done that yet.
Dax, here's the original Park PCB. Check it out, some funky stuff going on if you're used to looking at the Marshall variants!

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 8:23 pm

On the original Park PCB pic, over to the top right there is a bundle of a yellow, thick red and thin red wires. Where are they currently, and are they well-insulated from shorting out? (I think that terminal is just a convenient place to join them together but I'd have to check my '78 pcb to be sure.

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 8:26 pm

My '78 PCB doesn't even have the terminal there, from the pic I'm looking at it looks to be just a junction point that doesn't lead elsewhere on the PCB. What are these wires going to and where are they now?

Dax · Post by **Dax** » Thu Jan 27, 2005 8:34 pm

Yeah, thats just a junction point and i don't see them on his new board.

Dax

Dax · Post by **Dax** » Thu Jan 27, 2005 11:31 pm

This shit is so confusing, My amp isn't wired that way. I don't have that junction of 3 wires- 1OT 1Choke 1Fuse Holder. The OT and Choke go to the fuse Holder. The Filter cap goes to the Fuse holder and a wire comes off and goes to the diodes. I am confused

Dax · Post by **Dax** » Thu Jan 27, 2005 11:53 pm

Here is a pic of my 76' 1987 (converted from 2204)

clarkydaz · Post by **clarkydaz** » Fri Jan 28, 2005 6:21 am

hey guys! must mention that pcb is the 100 watt park i used to have (same year, model), not the present 50watt i'm having trouble with. the extra wires at the junction are not present on the 50watt. i threw it in so you could see the general variations.

Flames1950 · Post by **Flames1950** » Fri Jan 28, 2005 6:31 am

Yeah, my '78 Marshall isn't wired in that fashion either:

It's really probably a case of a different wiring method that, electrically, comes out the same. I'd just be sure that bundle didn't get folded under the board and forgotten or something, causing a short at the middle of the turret board. Our problem has really been proven to be past that point in the B+ circuit but it's worth double-checking.

clarkydaz · Post by **clarkydaz** » Fri Jan 28, 2005 6:47 am

regarding the filter cap, i have

2 red wires
1black from filter lug
1 black from input jack
1 black from tone strip

all at the filter ground point

confirm thats correct again?

Flames1950 · Post by **Flames1950** » Fri Jan 28, 2005 6:50 am

Should be right as long as the two reds are directly across from each other at the middle of the cap, and the black is on the edge as the "oddball" wire.