UPDATE: FIXED!(few new questions) How to Clean Burnt PCBoard(carbon scoring???)?

[jeff]

Hey guys,


 It's been a while since I've worked on amps. I have a amp with a PCBoard where it looks as if a tube failed and things toasted. What do you recommend to clean up the char/mess? I've been told if not cleaned properly this can leave a 'trace' an cause arcing. I'm going to replace socket and check components, but how do I clean the board?


Thanks
 Jeff

[kagliostro]

http://www.dailymotion.com/video/x3if7rn_pcb-laminate-repair-best-inc_tech

a bit drastic as solution, but .....

Ciao

Franco

[jeff]

Oh wow, you have to cut it out? There's no way to clean it?

[kagliostro]

Show a picture

Franco

[jeff]

Might take a while, gotta figure out how to take then send picture.

[shooter]

Oh wow

one of my "odd" jobs in the Navy was PCB repair, you do want ALL signs of burnt gone, dermal works good. you need to be carful of dust in the lungs, you need to know board material so you can "fill in" the missing stuff.  If you have room you can put in a terminal strip and wire to that and back to the board.

[kagliostro]

Use this free host

http://imgur.com/

Upload there the images, then post here the link, better if you use the image TAG (the small picture of Mona Lisa) you see on the left

when you are writing a post

Franco

[jeff]

ok thanks guys. my take a while, trying to get camera charged, battery may be dead

[jeff]

It looks like I can dremmel the burnt through spots and use jumpers, just got to think it out.


I feel like I want to use a Q-tip to clean the black stuff off the board. Water is probally bad, What should I use. Acetone? Nail polish remover? Something else?

[HotBluePlates]

I feel like I want to use a Q-tip to clean the black stuff off the board. Water is probally bad, What should I use. Acetone? Nail polish remover? Something else?

Think through the issue:

Carbon is bad because it is a conductive path for future leakage current and/or arcing.  So you've got to get rid of all of it.

Best stuff I know of to remove carbon is carburetor cleaner (from the automotive store) or Hoppe's #9 (from the gun shop).  Either one will take off carbon with a wipe.  But will they leave behind a residue that could be conductive?  I don't know... And I'm not sure I'd want to find out (but if anyone knows a great way to clean off any such residue, chime in!).

So the easiest path is to scrape off all carbon built up on the board.  If I were confronted with it, I'd simply take a razor or razor-knife and scrape away all the carbon.  Catch here is that you might also take off the solder mask insulating nearby traces.  That's a pain, as you now need to cover those over with something (more epoxy?).

Remember you ultimate goal is to prevent any future arcing or leakage current, and whatever method you use to repair the area needs to consider how you might compromise existing insulation, or leave behind conductive residue from a cleaning agent.

[jeff]

ok thanks. camera battery won't charge, I'll figure out something to post picks.


So say you drill out the burns, do you have to 'seal' the board. Will drilling a hole leave exposed board material. I think I can cut away where the board burned through and bent the socket pins out, then use jumper wire as missing trace, but d o I need to seal the holes edges?

[jeff]

Here's what burned, I 'think' I know what happened but would love your opinions


EL34
Pin 2(heater)
Pin 3(plate)
470 and 1K resistors from each leg of 6V to ground(artificial c.t., not sure why not the same value)
Jumper wire to 12AX7 heater pin
jumper for foot switch(may have burnt because it was too close)
... and here's the wired one, 10K tail of P.I.(looked crispy, measured 1K)


I gotta dig out my tube tester and check tubes. I'll let you know what I find.

[jeff]

ok so it seems 2 of the 12AX7s heaters are bad, will test power tubes tomorrow and try to get pics up.

[PRR]

I do not think carb cleaner leaves any residue.

Brake kleener is much more aggressive and leaves no residue.

I am not sure even carb cleaner is safe for all electronics. I'm sure brake cleaner would eat some plastic caps.

Been an awful long time since I snorted Hoppe's. (I see they now offer baby-wipes with gun-oil, and other products that DO leave oil behind...)

[silverfox]

Out of an abundance of caution I post here a little information about Brake Cleaner. Most likely the use in this application would not produce Phosgene gas, but since were on the subject of using it I thought I would link this story as an awareness towards future projects involving Break Cleaner. If the link is dead, simply do a search on: "brake cleaner phosgene death" and you'll see other examples. I'm quite surprised at how deadly this stuff is in very small quantities. Until recently I wasn't aware of the different ways break cleaner can react and produce Phosgene. In addition to heat, ultraviolet rays and Argon gas can produce it during some welding process'.

Anyway, just passing on what I discovered recently myself.

http://www.brewracingframes.com/safety-alert-brake-cleaner--phosgene-gas.html

silverfox.

[kagliostro]

This can be an alternative, but I don't know if it can give the hoped results

Clean the surface the better you can, after use something like the brake cleaner or carbon cleaner in small quantity, using a small brush and cotton swab

then wash with acetone or trichlorethylene, when all is dry apply an adhesive paper tape mask around the part that is damaged

and spry with anti spark-paint

(I don't know if you can still find it, it was used at tube television era) or try applying a thin layer using Super Glue

http://www.wilko.com/adhesives/loctite-super-glue-5g/invt/0074309

Franco

[PRR]

Thanks for the info on brake cleaner. Phosgene was probably the most lethal gas used in WWI, wicked stuff.

[jeff]

yeah, that scared the ---- outta me. Decided not to use that. I found some radio shack Electronics cleaner. States "leaves no residue". Took a while but I cleaned it all up. The spots that were burnt I removed the board with a dremmel. Accedentally got Krazy Glue instead of Super Glue but I sealed all exposed board with that. Soldered jumper wires to make connections where board was 'missing' or where pads had lifted. Looks like everythings connected to where it should be. Fire it up tomorrow.


Side note: I don't know if this was the condition it was in when it blew, but the imp. selector was half way between 8 and 16 so the OT was not connected to the speaker jacks.

[HotBluePlates]

Side note: I don't know if this was the condition it was in when it blew, but the imp. selector was half way between 8 and 16 so the OT was not connected to the speaker jacks.

Not sure.  I think the below is much more problematic, and likely to result in arcing/burns:
"Here's what burned, I 'think' I know what happened but would love your opinions

EL34
Pin 2(heater)
Pin 3(plate)"

[jeff]

  THE AMP IS FIXED!!!! Thank you for all the help.

Please see if I got this right. It's a 4xEL34 amp. Voltage is 440V, Plate current is 73.5mA per side(36.5mA per tube)
 The unloaded voltage(no tubes) is 470V and this amp uses diode rect. Does dropping down from 470V to 440V under load with SS rect seem about right? Also I've heard these amps are biased cold. If an amp is designed to be biased cold and you bias hotter will this cause problems?
I guess what has me questioning my self is the bias instruscions for another model amp by the same maker. It says measure voltage from pin 8 to ground and set to .3V. There's a 10 ohm resistor from pin 8 to ground so this is saying it wants to see 30mA. The schematic shows 450V B+. So:

450V x 30mA = 13.5watts
So that's only 56.25% of the EL34's 24watt rating. Is that cold? or is the amp designed to run colder?
 
 Right now, my amp is biased at

440V x 36.75mA = 16.17watts
Which is ~67% of the EL34's max.

What would you do? Should I go a little colder. Am I stressing the Xfmr? I guess I'm just second guessing myself. Love to hear what you guys would do.

  Thanks again for all the help
Jeff

[HotBluePlates]

THE AMP IS FIXED!!!!

Great job!

... The unloaded voltage(no tubes) is 470V and this amp uses diode rect. Does dropping down from 470V to 440V under load with SS rect seem about right?  ...

Yes.  Voltage drop is due to PT and OT winding resistance.

... I've heard these amps are biased cold. If an amp is designed to be biased cold and you bias hotter will this cause problems?  ...

Probably not, unless you take a Class AB amp and try to idle the tubes at 100%.  The issue there is the OT load will be small and the tubes will tend to redplate when you play loudly.

Worst-case for a cool Class AB amp being biased hotter Class AB:  you won't get every clean part-watt you might have gotten biased cooler.  If the amp was designed to run very very cold (close to Class B, with high plate/screen voltage and very low OT load), you could run into redplating when cranked.  The schematic showing 450vdc implies this is NOT a risk for your amp.

[jeff]

great thanks


When I measured the current I was putting my meter across the OT so what I was reading was just the plate current. This is different than the 1ohm resistor from cath to ground method which will give you the total current(plate + screen) through the tube. When calculating bias you use the formula 0.7 X (Wattage of the tube / Voltage) = idle current. Is this the plate current or the total current? It seems depending on which method you use to monitor current you will end up with different a bias


So if you were using EL34s at 440V
.7(24/440)=.038


Would you want to see .038V across your 1ohm resistor to ground or .038mA from OT tap to plate?

[shooter]

Is this the plate current or the total current?

Current in series is equal throughout so from ground to the ot, you have the same current.

[jeff]

I don't know if that's entirely correct with a pentode. Triode, yes. But with a pentode if you placed your current meter between the OT and plate you would be reading the plate current, place it between screen and screen resistor you'd measure screen current. But by placing it between cathode and ground you'd be reading both plate and screen current.


I looked up weber's bias calculator. It says for an EL34 at 444V the current should be 39.3mA. But I don't know if that is the screen current or the total current.

So if I'm aiming for .039v across 1ohm cath resistor then read plate current it will be 39 - screen. If I bias for 39mA between plate and OT, the cathode resistor read .039V + screen.
 
What worries me is I dont know if that calculator, or the formula (Wattage/Volt) X 0.7= Plate current or Total current

There is a difference, right?