Dirty circuit board... how bad is it?

[JoeDotCom]

Hola Friends,

I picked up a Technics SU-8600 from AKer jengland which was previously owned (I am guessing here) by AKer von.ah. (I only mention the pedigree because I am basing some of my info on posts i found by these gentlemen regarding this unit.)

Anywhoo, i got this integrated with the intent to tinker, learn and possibly restore. I am pretty new to circuit board work. I just finished my first receiver project over in the pioneer forum (http://www.audiokarma.org/forums/showthread.php?t=389163). I am getting ready to order some parts for the SU-8600, but i am concerned about the circuitboard(s). They are pretty gunky. In some spots, i cant tell if it is old glue, or char marks. Can any of you who are more experienced w/ circuitboards chime in and let me know how bad the situation is with these please?

I am OK with replacing a few parts, etc. but I would really like to initially just get this back into working condition. But if the boards are trashed, then i do not want to invest too much in replacement parts.

 

[avionic]

That area around those cement block resistors sure looks like charred pc board.Definitely need to scrap all that carbon off.Carbon is a conductor of electricity.

 

[JoeDotCom]

That area around those cement block resistors sure looks like charred pc board.Definitely need to scrap all that carbon off.Carbon is a conductor of electricity.

a conductor? Good to know, thanks will defiantly do some scraping. That resistor does not looked burnt at all though. Would you guess that the cement resistor caused the charring and has already been replaced?

 

[terra1]

a conductor? Good to know, thanks will defiantly do some scraping. That resistor does not looked burnt at all though. Would you guess that the cement resistor caused the charring and has already been replaced?

First, were you told it works at all or is it sold as a project?

The cause could be any number of things. As you see, the burnt areas are spread out. Hard to say it's the cause or just a casualty.

You might check underneath to check the condition of the board's underside as well.

I was checking the bias or DC on my SU-8600, don't remember which. I shorted something by the test points - spark and pop. Then the area to the right of the capacitors started to smoke. I couldn't determine exactly where it was coming from. It actually looked like it might have been coming from under the board. The tech replaced a couple of transistors.

If you get to that point of adjusting the amp, electric tape the surrounding area or the shaft of your probes. It was an awkward congested area where I found it difficult to use my minigrabbers.

It's too late now, but it looks like somebody drilled holes in your volume pot. I was able to remove the volume pot and the 2 screws and separate the segments just enough without drilling.

 

[Ruxman]

It looks like a typical old circuit board. Nothing to worry about, other than what has been previously mentioned.

 

[JoeDotCom]

Awesome, super valuable reply here, thanks!

First, were you told it works at all or is it sold as a project?

The cause could be any number of things. As you see, the burnt areas are spread out. Hard to say it's the cause or just a casualty.

It works. After checking it all out (below) i connected some sacrificial speakers to it and they sounded fine.

I checked the DC offset (at the speaker terminals) and it was within acceptable range. Using the service manual, I was able to check and adjust the "alignment" as the manual calls it. It calls for the setting the voltage to 10mV, so I am assuming alignment = idle current. I coudl not find any instructions for setting DC offset. Perhaps you have a better service manual than I do. Care to share? ) Mine is from Hifiengine.

If you get to that point of adjusting the amp, electric tape the surrounding area or the shaft of your probes. It was an awkward congested area where I found it difficult to use my minigrabbers.

the "alignment" pins worked well with my little clip on leads. Perhaps once i find the CD offset test points, things will get interesting.

It's too late now, but it looks like somebody drilled holes in your volume pot. I was able to remove the volume pot and the 2 screws and separate the segments just enough without drilling.

The unit works, but the volume pot is jacked up. Previous owner drilled in an attempt to flush it with deoxit without success. I am trying to assess the circuit board to see if it is worth perusing the volume pot issue. Basically the pot is experiencing a connection problem w/ the left channel until you turn it up past the 11 o'clock position. anything less than 11 o'clock and the output is about 50% less than it should be, while the right side is fine. (issue is still present on headphones, multiple sources, outputs, etc. I m pretty sure it is the volume pot that is the problem.) There is also a notch/bump in the pot somewhere around 8 o'clock that i can feel as i turn it.

Thanks for your advice on checking the underside fo the board as well. i will see if i can peek down there.

I was thinking I would try to find a replacement pot if it was worth fixing. But i am DEFINITELY interested in hearing more about how you got yours apart. If I could get mine apart and clean / repair that would be wonderful. You dont have any photos of your apart do you?

-Joe

 

[JoeDotCom]

It looks like a typical old circuit board. Nothing to worry about, other than what has been previously mentioned.

great to hear. I guess the boards in my sx-980 just happen to be in great shape.. because while they looked old, they did not look so gunky.

 

[terra1]

I couldn't remember if it was the DC or bias I adjusted. I believe it was only the "idle" current as you mention and there is no DC adjustment.

I don't have any pics.

But here's what I basically did:

You remove the volume knob and remove the pot from the face plate.

While removed from the face plate, I then install the volume control knob to provide enough torque for easy rotation during Deoxit/Faderlube instead of twisting the bare shaft.

Before I remove the two long bolts. I place a paper towel underneath to catch the tiny nuts on the other side if they slip and fall.

The three sections separate about an 1/8" along the top. I used a jeweler's screw driver to initially pry them apart. I use a little bigger thin flat head screw driver to slightly twist and separate and hold the sections about an 1/8". Enough room to peek and spray. Do not separate sections completely as the lower section is still soldered to the little board it is attached to.

Within each section, there is a piece of masonite-liike board separating each of the three sections. You can actually see the shaft if you have a good enough overhead light.

I spritz one side of the mason-like board on one side and then the other side using the flat head screw driver to expose each section.

The paper towel I leave underneath to catch drippings. I turn the volume control face up and right side up to use gravity to help flow of Deoxit and faderlube.

 

[zenith2134]

That's a restorable amp. Good advice to scrape the carbonized crap off...while you're at it touch up all the solder joints and deflux the foil side of the board. Standard procedure whenever I do work in a vintage SS unit.
You should see what the high voltage boards in 70's-80's tv sets look like--BLACK. This causes arcing and can potentially be a fire hazard as well.

 

[JoeDotCom]

here's what I basically did:

Thanks for the details. I hope to attempt this in teh next few days. I will report back.

That's a restorable amp. Good advice to scrape the carbonized crap off...while you're at it touch up all the solder joints and deflux the foil side of the board. Standard procedure whenever I do work in a vintage SS unit.
You should see what the high voltage boards in 70's-80's tv sets look like--BLACK. This causes arcing and can potentially be a fire hazard as well.

restorable: great to hear!!
Scrape and solder reflowing, good call. I m not sure how to deflux. I am sure there is info on AK about it here, so I will look it up and learn.

thanks!

 

[avionic]

Every hear of carbon composite and carbon film resistors..:scratch2:

 

[JoeDotCom]

Every hear of carbon composite and carbon film resistors..:scratch2:

Yes, i suppose I have. good point. :scratch2:

 

[Doug G.]

I don't think anybody mentioned this yet but charred boards don't necessarily indicate a specific problem.

Some components run hot if they are being used near their ratings and won't always LOOK like they are running hot.

But the PC board around those components will eventually turn dark or even black from the heat.

The main considerations are, if the board has actually turned to carbon as has been said, it can conduct which is not good and the circuit tracings which can come loose from the board if the heat has been high enough.

Doug

 

[century tek]

Wait 'til you get a piece of equipment from a smoker.

Aside from the obvious charred or burnt parts, the underside or foil side of the board where everything is soldered and connected to one another, is the main concern as there could be places where a short occured and blew the foil completely off of the board, or, where someone tried to solder something and solder bleeded across another trace causing a direct short.

 

[qsaudio]

You may have to carve a hole and reroute the foil to get rid of the carbon areas on that board.(I done it many times, no other way but to scrape ALL black away even if it goes all the way though, using epoxy to hold things for support)

I had an Orion car amp once, I was tracking down a 11 ohm short.for some reason I still don't know what made me do it, must be God.I pulled out a small electrolytic cap, and looked through the board with bright light and seen a dark patch, about 1/8 inch in diameter.

Both board surfaces where smooth and clean and not discolored, sure enough under that cap, looked perfect both sides of the board, but it shorted out inside the board, friggenn 11 ohms. !!!!!!!

amp tested good once I got it going,

 

[JoeDotCom]

I couldn't remember if it was the DC or bias I adjusted. I believe it was only the "idle" current as you mention and there is no DC adjustment.

I don't have any pics.

But here's what I basically did:

You remove the volume knob and remove the pot from the face plate.

While removed from the face plate, I then install the volume control knob to provide enough torque for easy rotation during Deoxit/Faderlube instead of twisting the bare shaft.

Before I remove the two long bolts. I place a paper towel underneath to catch the tiny nuts on the other side if they slip and fall.

The three sections separate about an 1/8" along the top. I used a jeweler's screw driver to initially pry them apart. I use a little bigger thin flat head screw driver to slightly twist and separate and hold the sections about an 1/8". Enough room to peek and spray. Do not separate sections completely as the lower section is still soldered to the little board it is attached to.

Within each section, there is a piece of masonite-liike board separating each of the three sections. You can actually see the shaft if you have a good enough overhead light.

I spritz one side of the mason-like board on one side and then the other side using the flat head screw driver to expose each section.

The paper towel I leave underneath to catch drippings. I turn the volume control face up and right side up to use gravity to help flow of Deoxit and faderlube.

So I removed the volume pot last night and cleaned it best I could. Sadly this had ZERO effect on the the problem.

I went ahead and unsoldered the pot from the board. It was nice to be able to get actually inside a bit to clean. I was able to clean the rear contact with deoxit and a toothbrush. all that white gunk in the photos has been removed. The inner contacts are another story. I was able to pry the plastic far enough apart to get a straw in there (canned air, and deoxit) but could not do any real scrubbing or cleaning. Is there a way to get this apart even further? I coudl not see a way to separate the shaft from the front plate.

I gave the rest of the pots/switches a deoxit bath (without removing them) and them popped the volume pot back onto the circuit board and tested things out. No improvement.

I also scraped off the charred stuff from the board and reflowed the solder on a couple of connections on the underside of the board.

Can I test the volume pot using a Multimeter? if i pull it out of the circuit, can i measure its resistance as i rotate the knob will i see the resistance make a sharp drop in the same spot that the sound always jumps?

(pics will be in next post so i can upload from my android)

 

[JoeDotCom]

Technics SU-8600 Volume pot disassemble
EWC-F 150K Ohm

Sent using a mobile device.

 

[JoeDotCom]

UPDATE:
THREAD CONTINUED HERE: http://www.audiokarma.org/forums/showthread.php?t=399543


Sent using a mobile device.

 

[Emarisma]

Hola Friends,

I picked up a Technics SU-8600 from AKer jengland which was previously owned (I am guessing here) by AKer von.ah. (I only mention the pedigree because I am basing some of my info on posts i found by these gentlemen regarding this unit.)

Anywhoo, i got this integrated with the intent to tinker, learn and possibly restore. I am pretty new to circuit board work. I just finished my first receiver project over in the pioneer forum (http://www.audiokarma.org/forums/showthread.php?t=389163). I am getting ready to order some parts for the SU-8600, but i am concerned about the circuitboard(s). They are pretty gunky. In some spots, i cant tell if it is old glue, or char marks. Can any of you who are more experienced w/ circuitboards chime in and let me know how bad the situation is with these please?

I am OK with replacing a few parts, etc. but I would really like to initially just get this back into working condition. But if the boards are trashed, then i do not want to invest too much in replacement parts.

Hi Joe, I am new in this forum. Have SU-9070 with same potmeter issue. Saw your post, but I cannot download the JPG pics. Any way I can receive these pics ?

 

[Bill Ferris]

Hi Joe, I am new in this forum. Have SU-9070 with same potmeter issue. Saw your post, but I cannot download the JPG pics. Any way I can receive these pics ?

JoeDotCom hasn`t been active on A/K since April 18, 2021, so I wouldn`t get you hopes up about receiving a reply from a member in a currently inactive 2011 thread!
Maybe someone knowledgeable currently on A/K can provide you with the SU-9070 information you seek, Emarisma ?

BTW, Welcome to Audiokarma.org, Sir.
Good luck.