Single Rectifier - Burnt Resistors in V1 Preamp Stage

[olddawg]

After grinding out the burnt section of the board it's always a good idea to fill in the hole with 5 minute epoxy. A piece of cello tape on one side will keep it from dripping through and you remove the tape when it hardens. If it's a large(ish) area, you can use a piece of junk pcb on top or bottom or both to strengthen it. It can save you a lot of grief in the future. Especially if the board is stressed by removing and installing tubes.

[Pedro Vecino]

This is a better pic. First from another Rectoverb II, and second from a Dual Rect. two channels.
Fourth band is gold. Gold band multiply for 0.1 (silver multiply for 0.01).
392 x 0.1 = 39.2 Ohms (39R).

[drewl]

We have that green filler stuff for PC boards at work.

Great for these type of burns, non conductive.
They sell it at regular places like MCM etc.

[g1]

[ATTACH=CONFIG]35578[/ATTACH]

This is a better pic. First from another Rectoverb II, and second from a Dual Rect. two channels.
Fourth band is gold. Gold band multiply for 0.1 (silver multiply for 0.01).
392 x 0.1 = 39.2 Ohms (39R).

Thanks Pedro, now I can see in your first pic (post #7) it was just a trick of the light that made the gold band look black.

Four bands for resistance and one for tolerance, the last brown stripe for 1%. Five total. Your common three band code usually ha a fourth band for tolerance, dunnit?

I was omitting the tolerance band for my purposes, as I believe Markus was also. No matter as Pedro has solved it.

[Engine09]

That explains it, thanks guys and thanks for the photos Pedro!

Components ordered from Digikey, will clean up the board tomorrow and install the new resistors this week. Will hopefully have the amp up and running this week!

[MarkusBass]

I don't know why I thought that the fourth stripe is black. But it's gold and this is 0.1 multiplier. Being frankly I have Mesa Mark IV in my workshop and it seems that it uses exactly the same resistors - I could have take a look at the amp.
Drewl, you mention some "green filler stuff for PC boards". Can you give some more details about it?

Mark

[DrGonz78]

If not too much trouble... Don't forget to take pictures of the finished repair. I think the before and after pictures go a long way here. Perhaps someone else gets insight to help a similar repair down the road.

[drewl]

The "green filler" is made by Circuit Works called Overcoat Pen.
Basically a tube the size of a magic marker filled with non conductive material for insulation.

It's often used like loctite to keep screws in place.

Digikey should have it.

After scraping all the burnt carbon of of boards I use it to fill in the hole.

Not needed but looks better and helps prevent arcing.

[Engine09]

Found it:

https://www.chemtronics.com/p-710-ci...pen-green.aspx

Digikey does not stock the pens, but they do have the 2 component tubes:

CW2500 Chemtronics | CW2500-ND | DigiKey


Drewl, are the pen overcoats easy to apply? Is it similar to a flux pen where you push down a felt applicator tip and the product slowly releases?


I want to track this stuff down but might get impatient when my components come in. Will definitely post a follow up with some photos.

[drewl]

Yes, easy.

Has like a ball point that when pushed in allows the goo to come out slowly and easily manageable.

Edit: ooh, I like those bigger tubes of it.

I would put some on something and apply it with a toothpick or something until you get used to it, don't just blob it all over the board.

[Engine09]

Damn, the pen sounds good. Going to make some calls and see if anyone local has it. $15 and has a shelf life of 2 years so will likely use it again.

I've used 2 component epoxy a few time for non-electronics repairs, would mix up a small amount on a piece of paper and just like you said use a toothpick to slowly drip onto the surface. If you're careful you can do some really nice work.


Edit: Found a place locally that sells the MG Chemicals version. Going to drive across town today and pick it up. If I'm making the repair, might as well do the job right.

http://www.mgchemicals.com/products/...coat-pen-419d/

[TimmyP1955]

Orange = 3
White = 9
Red = 2 but is third color band so is ignored as a place holder
Blk = 0, but of course it can't be multiply by 0: It is 10 to the Zero Power, which = 1. So it's 39 x1 then stick the third band back in = 39.2

Orange = 3
White = 9
Red = 2 but is third color band so is ignored as a place holder
Red = 2 = 10 to the second power = 100. So it's 3900 then stick the third band back in = 3920

[g1]

Orange = 3
White = 9
Red = 2 but is third color band so is ignored as a place holder
Blk = 0, but of course it can't be multiply by 0: It is 10 to the Zero Power, which = 1. So it's 39 x1 then stick the third band back in = 39.2

Orange = 3
White = 9
Red = 2 but is third color band so is ignored as a place holder
Red = 2 = 10 to the second power = 100. So it's 3900 then stick the third band back in = 3920

That is incorrect. The third band is not ignored as a place holder.
The 39.2 did not have a black band, we were mistaken and it was a gold band for the multiplyer.
So the 39.2 was orange, white,red, gold
and in the picture you can also see the 22.1K and the 100K. The 22.1K is red,red,brown,red. The 100K is brown,black,black,orange

[Kryzsky]

Wrong username for some reason. *Edit*

[g1]

Be aware that the voltages will be higher with no tubes installed. So make sure the filter caps are not subjected to voltages above their rating.
A variac or lamp limiter is helpful for this.