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Author Topic: PCB question  (Read 8215 times)

ilyaa

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PCB question
« on: February 26, 2015, 03:51:49 PM »
i have a question about circuit board (potential) damage:

someone gave me a hot rod deville to fix up - one of the tubes is red plating the other barely glowing - figured just might be a shorted tube. i opened it up to take a look, though, and saw some bad looking light brown ash kind of residue over the area on the PCB where the red-plating tube is.

i cleaned it with some alcohol but there is still some dark brownish residue i cant quite get off

(see pics: http://dropcanvas.com/#68fj9h9Dk788l6)

1) any tips for a better solution to clean this crap?
2) i think Roly mentioned before that a burnt PCB can become resistive. the owner of the amp has said its had problems before and im wondering if a resistive PCB in the power amp section might be shorting tubes for him....any thoughts?

Enzo

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Re: PCB question
« Reply #1 on: February 26, 2015, 04:33:32 PM »
A little brown discoloration won't be conductive, it is when a board is burned to black char that it is conductive.  You are looking at solder flux, and it really isn't a problem.

You may have a bad tube, or may not.  I'd look very closely at the solder on the power tube socket pins, in fact I would just resolder them regardless.

Tubes fail all the time, they don't need some reason.

Roly

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Re: PCB question
« Reply #2 on: February 26, 2015, 06:27:57 PM »
Also...

check the valve socket that the contacts are clean and are gripping the valve pins properly, you may need to re-tension them a bit.  If the grid pin should lose contact that valve will turn on flat out.

But yeah, valves (particularly of modern manufacture) can have a mind of their own.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: PCB question
« Reply #3 on: February 26, 2015, 11:37:47 PM »
I see nothing abnormal in your pictures.

Please care to surround what is worrying you.

And try to vary illumination angle to show it better.

DrGonz78

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Re: PCB question
« Reply #4 on: February 26, 2015, 11:40:14 PM »
Yeah that stuff is hard to clean. I some times use scraper tools to work the old dried flux loose and then hit is with an alcohol soaked q-tip. It seems to me that you need just remove many of the solder joints and clean the area up before re-soldering each one. There is one spot on that circuit where the trace is cut and the pad is missing. Was it a mod or something? Clearly this looks like the aftermath of an inexperienced person with a solder iron.
“A person who never made a mistake never tried anything new.” -Albert Einstein

Loudthud

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Re: PCB question
« Reply #5 on: February 27, 2015, 07:27:31 PM »
On small areas I use alcohol and a cotton swab (Q-Tip) but on large areas I use an old tooth brush and alcohol or a commercial cleaner such as Puretronics Flux Remover available in the US at Frys. It usually helpful to mop up after a good scrub with a paper tissue before the cleaner evaporates.

The flux cleaner mentioned contains Toluene, Heptane, Isopropyl Alcohol and 1,1 Difluoroethane.

Roly

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Re: PCB question
« Reply #6 on: February 28, 2015, 04:57:51 AM »
The flux cleaner mentioned contains Toluene, Heptane, Isopropyl Alcohol and 1,1 Difluoroethane.

So don't get it on your skin or breathe the fumes. 
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: PCB question
« Reply #7 on: March 04, 2015, 03:51:41 AM »
the tube socket (of the tube in question) was all gunky inside - hard to even squeeze the tube in and out. i ordered a new one.

i got some better cleaner and cleaned the pcb looks good now.

those missing pads are for the screen grid stoppers. they often fall off in these particular amps, i think, which is why those resistors get resoldered to weird spots.

inspecting stuff i noticed two more fried resistors (r80 and r81 in the attached schematic). 100 ohmers in the heater circuit - what are they for??

phatt

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Re: PCB question
« Reply #8 on: March 04, 2015, 06:56:10 AM »

As the amp does not have a centre tapped filament supply these act as a virtual centre tap. Unless they are black (suggesting a major problem) I'd leave them. :tu:
Phil.

g1

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Re: PCB question
« Reply #9 on: March 04, 2015, 12:12:14 PM »
The heater resistors usually fail if there is a horrible fault in a power tube, or the locating pin in the power tube gets broken off and the tube gets installed with pins offset from correct location.
  The "screen stoppers" as you called them, serve 2 functions.  For their current limit functions, they can be located anywhere.  But for their "stopper" function (preventing oscillation) they are best located right on the tube socket, just like the grid stoppers.
  The vintage Fender way is good for this.

ilyaa

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Re: PCB question
« Reply #10 on: March 04, 2015, 12:35:39 PM »
Quote
Unless they are black (suggesting a major problem) I'd leave them.

oh they are black alright!

Roly

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Re: PCB question
« Reply #11 on: March 04, 2015, 04:15:16 PM »
There are a couple of types of damage that are typical of EL34's;

- fried screen resistors, when the screen has melted and collapsed and internally shorted to the cathode and;

- damage to the heater "hum dinger" (as these resistors or balance pot are known), where there has been a flashover between the anode pin 3 and the heater pin 2.  This only has the OPT primary in series and being DC can be a sustained arc which carbonises the valve base, socket, or both (and once carbonised they really have to be replaced).

Hope this gives you some clues.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: PCB question
« Reply #12 on: March 04, 2015, 06:06:24 PM »
Quote
damage to the heater "hum dinger" (as these resistors or balance pot are known), where there has been a flashover between the anode pin 3 and the heater pin 2.  This only has the OPT primary in series and being DC can be a sustained arc which carbonises the valve base, socket, or both (and once carbonised they really have to be replaced).

that definitely sounds like what happened here. except these are 6L6s

g1

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Re: PCB question
« Reply #13 on: March 05, 2015, 12:19:00 PM »
  A plate to heater short would have the same effect on a 6L6.
Now the trick is to figure out if it was caused by a tube currently in the unit, or was the damage done long ago.
  The amp will continue to function fine without those resistors, it will only affect hum levels.

ilyaa

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Re: PCB question
« Reply #14 on: March 05, 2015, 12:52:52 PM »
Quote
Now the trick is to figure out if it was caused by a tube currently in the unit, or was the damage done long ago.

one of the tubes was red plating when tested. any 'trick' that will tell me? ill replace those resistors anyway -