Another possible cause of the short between C9 c and C10 c is solder flux left on the board. Solder flux should be removed with isoprophyl alcohol on a cotton ball. Dry before any further powering up. This short could occur anywhere between C7 and C9/C10.
May you be preserved from short in the paper board itself. Mention has been made on other Adcom threads about carbon tracks from shorts in these inferior boards. If that happens, the cheapest repair is drilling out or cutting out the shorted part of the board itself. May be possible, by bridging traces with insulated wire. A more secure solution is replicate the layout on Nema CE bare board, then replacing all the lands with insulated wire. Highly labor intensive. Better to find some other board on ali/ebay/amazon that will use +-81 v supplies, fit in the hole, and use the heat sinks.
I repair Peavey & Crown & QSC products, that use actual glass fiber board. Dynacos used paper boards, but I never encountered a burned board. Dynaco are so pricey these days there is not much use in buying a hulk "for repair or parts".
May you be preserved from short in the paper board itself. Mention has been made on other Adcom threads about carbon tracks from shorts in these inferior boards. If that happens, the cheapest repair is drilling out or cutting out the shorted part of the board itself. May be possible, by bridging traces with insulated wire. A more secure solution is replicate the layout on Nema CE bare board, then replacing all the lands with insulated wire. Highly labor intensive. Better to find some other board on ali/ebay/amazon that will use +-81 v supplies, fit in the hole, and use the heat sinks.
I repair Peavey & Crown & QSC products, that use actual glass fiber board. Dynacos used paper boards, but I never encountered a burned board. Dynaco are so pricey these days there is not much use in buying a hulk "for repair or parts".
Normal flux isn't conductive, but it can hide solder hairs or outright bridges. If you are dealing with very low leakage currents, then you must remove all flux, fingerprints or any other residue. IPA and a clean toothbrush to finish. I use lacquer thinner and a tooth brush first. I may also use Methyl Hydrate followed by a zero residue cleaner spray when it matters.
If you are using good solder with proper flux, you do not have to clean it off. I do only to check for debris and solder whiskers.
If you are using good solder with proper flux, you do not have to clean it off. I do only to check for debris and solder whiskers.
@ anatech: lacquer thinner is extremely aggressive. What precautions should one take so that it does not attack capacitor coatings, resistor coatings snd color codes, and silkscreen ink on pcb? I’ve never used it and am real scared to try.
Lacquer thinner should never be used indoors. I had a garage I worked at one summer burn down, because the body man left a can of laquer thinner open. Nice cinder block building under 5 years old. Neither lacquer thinner nor MEK should be used within 10 m of electric currents switched on and off: As soldering irons do.
@indianajo: I always use my lacquer thinner outside. The fumes!!!! But also flammability risk. But even if I clean PCBs outside, I am very scared to use lacquer thinner for fear of damaging protective coatings, seals and ink on components and the boards themselves.
Obviously the O.P. has no probelm with his amplifier. Driver spreader voltage, 0, no sweat. Enjoy the music!
Hi Halauhula,
I use 1 oz bottles filled from a jug of the stuff, applied normally with a Q-Tip or a tiny bit on a toothbrush. Obviously not on the component side as a rule. I wouldn't go near any component sensitive to it. I've done this for decades on the bench. The top of the bottle is never left off. "Carbon Tet" (old timers would remember this stuff) used to be great as well. That you simply cannot get, probably since the early 1970's, but it was effective.
Lacquer thinner is very aggressive, which is why I like it. Methyl Hydrate is less aggressive, but more so than IPA. There are some things only Methyl will dissolve, like tape binder in a tape machine tape path. Chemicals are tools. Sometimes you need a hammer, other times a small paint brush. Any tool misused is either ineffective or destructive, or both.
So what else do you use lacquer thinner for? Lock bond, thread lock if you can get it in there. Much less destructive than heat in those cases. Or brute force. I'd rather have low exposure to something taking care, than prolonged exposure to chemicals that aren't that effective. By the way, WD-40 eats rubber - seals on capacitors and can wick up into some older transistor cases. So if you are at all concerned with what chemicals might be on your bench, read up on them. Bet you find some with really nasty habits.
Be aware I'm not recommending just anyone use this stuff. You have to have a brain and be aware of what you are doing, what you are using. I use "safe" chemicals that are literally explosive, yet found on service benches. Lacquer thinner isn't nearly as flammable.
Hi indianajo,
Normal electronic flux is not conductive, but can be hygroscopic (absorbs water). By itself under normal circuit conditions is it not a source of high enough leakage to be concerned about. It was carefully designed to be that way. In special circumstances, it can affect circuit performance - of course. So for high impedance, high voltage or very low noise circuits (leakage again), we want a clean, dry PCB that must be maintained that way. In those circuits, dust and fingerprints are a definite problem. A normal amplifier isn't that situation.
Probably the biggest problem with flux is that it can conceal a solder bridge, debris or trace crack. This is an optical problem, and an external (to the flux) issue. I like to clean boards because I can tell instantly when something I worked on has been messed with. Most techs are pigs, messy. There is absolutely no electrical reason to remove proper flux from a normal amplifier that is used in a home atmosphere (normal temperature ranges, normal humidity). Once heated, it is designed to be inert. If you apply liquid flux, you can't be certain it has all been heated, so you remove it.
I use 1 oz bottles filled from a jug of the stuff, applied normally with a Q-Tip or a tiny bit on a toothbrush. Obviously not on the component side as a rule. I wouldn't go near any component sensitive to it. I've done this for decades on the bench. The top of the bottle is never left off. "Carbon Tet" (old timers would remember this stuff) used to be great as well. That you simply cannot get, probably since the early 1970's, but it was effective.
Lacquer thinner is very aggressive, which is why I like it. Methyl Hydrate is less aggressive, but more so than IPA. There are some things only Methyl will dissolve, like tape binder in a tape machine tape path. Chemicals are tools. Sometimes you need a hammer, other times a small paint brush. Any tool misused is either ineffective or destructive, or both.
So what else do you use lacquer thinner for? Lock bond, thread lock if you can get it in there. Much less destructive than heat in those cases. Or brute force. I'd rather have low exposure to something taking care, than prolonged exposure to chemicals that aren't that effective. By the way, WD-40 eats rubber - seals on capacitors and can wick up into some older transistor cases. So if you are at all concerned with what chemicals might be on your bench, read up on them. Bet you find some with really nasty habits.
Be aware I'm not recommending just anyone use this stuff. You have to have a brain and be aware of what you are doing, what you are using. I use "safe" chemicals that are literally explosive, yet found on service benches. Lacquer thinner isn't nearly as flammable.
Hi indianajo,
Normal electronic flux is not conductive, but can be hygroscopic (absorbs water). By itself under normal circuit conditions is it not a source of high enough leakage to be concerned about. It was carefully designed to be that way. In special circumstances, it can affect circuit performance - of course. So for high impedance, high voltage or very low noise circuits (leakage again), we want a clean, dry PCB that must be maintained that way. In those circuits, dust and fingerprints are a definite problem. A normal amplifier isn't that situation.
Probably the biggest problem with flux is that it can conceal a solder bridge, debris or trace crack. This is an optical problem, and an external (to the flux) issue. I like to clean boards because I can tell instantly when something I worked on has been messed with. Most techs are pigs, messy. There is absolutely no electrical reason to remove proper flux from a normal amplifier that is used in a home atmosphere (normal temperature ranges, normal humidity). Once heated, it is designed to be inert. If you apply liquid flux, you can't be certain it has all been heated, so you remove it.
UPDATE: Boy did I pick the wrong shipping option from digikey. USPS 1-5 days took 8 days.
I installed the new c5 today, and hooked up some speakers. Good new is, audio is now passing through (I ramped up slowly on the variac just incase, and use some old small cheap HT rear speakers). I let it pass audio for about 10 min occasionally giving full voltage for longer and longer.
On the channel that was bad, it is sounding very grainy is the best way I can describe it, like fuzz. I captured a small clip at low volumes as it is more noticeable. I wouldn't have gotten this far without the generous help and advice here. Going to go back to you tube repair vids to see some causes of that grainy sound ( I know it can probably be many things...)
EDIT: just found out moving P1 is making the fuzz go away. should I lose the variac and adjust bias to 16mv per the manual?
I installed the new c5 today, and hooked up some speakers. Good new is, audio is now passing through (I ramped up slowly on the variac just incase, and use some old small cheap HT rear speakers). I let it pass audio for about 10 min occasionally giving full voltage for longer and longer.
On the channel that was bad, it is sounding very grainy is the best way I can describe it, like fuzz. I captured a small clip at low volumes as it is more noticeable. I wouldn't have gotten this far without the generous help and advice here. Going to go back to you tube repair vids to see some causes of that grainy sound ( I know it can probably be many things...)
EDIT: just found out moving P1 is making the fuzz go away. should I lose the variac and adjust bias to 16mv per the manual?
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Yes.
Run the amplifier unitil it has become warm, let it cool and stabilize and set the bias. It will probably take about 1/2 an hour between adjustments to settle. You must get the amplifier very warm and let it cool to set bias properly.
Run the amplifier unitil it has become warm, let it cool and stabilize and set the bias. It will probably take about 1/2 an hour between adjustments to settle. You must get the amplifier very warm and let it cool to set bias properly.
Looks like we are good to go! After a few cycles, I got both sides set to 16mv.
IT seems though that If I don't have speakers hooked up (no input), I do not read any mv at all across the emitter resistors. Is this normal?
IT seems though that If I don't have speakers hooked up (no input), I do not read any mv at all across the emitter resistors. Is this normal?
You must set bias current without a load connected!! This is true of all solid state amplifiers that are not capacitive coupled output. Tube amplifiers always need a load, 100% of the time or you may easily damage them seriously.
If you only read current with a load connected, you probably have a high DC offset. That isn't good. You are measuring across one emitter resistor - right? No load connected, AC current draw should be low.
If you only read current with a load connected, you probably have a high DC offset. That isn't good. You are measuring across one emitter resistor - right? No load connected, AC current draw should be low.
DC Offset on newly repaired right channel is 28.1mv, left channel is 20.6mv with no load connected. Black from dmm to neg speaker terminal and red dmm lead to red speaker terminal. This measurement is about 3 minutes after startup.
I noticed the the dc offset goes up when main power switch gets turned off, up to over 400mv (.4 volt) then gradually back down.
I just checked bias again with no load and it was reading about 15.1mv so I moved it up to 16mv.
I noticed the the dc offset goes up when main power switch gets turned off, up to over 400mv (.4 volt) then gradually back down.
I just checked bias again with no load and it was reading about 15.1mv so I moved it up to 16mv.
Hi,
Okay. So you read bias with no load connected then.
You must get the amplifier warmed up, very warm some might say, then wait until it cools down and set the bias current from there. I know it doesn't make sense, but trust me. You have to do it this way.
Okay. So you read bias with no load connected then.
You must get the amplifier warmed up, very warm some might say, then wait until it cools down and set the bias current from there. I know it doesn't make sense, but trust me. You have to do it this way.
Yes, I did that cycle around 3 times, each time letting it cool for 30-45 min. It didn't get too warm with the small test speakers I had connected.
I just wanted to circle back and thank the community again, especially @anatech and @indianajo for guiding me through this. I have a few more pieces of troubleshooting equipment including a variac, new soldering station ( the last one was beat up), a mk-168 toy component tester and even some pomona grabbers so I don't short anything out in the future! Also have spare parts now in case anything goes in this unit or the other backup all original unit!
Test leads can make a big difference! Using the correct ones for the job is a great start. I use heat shrink tubing to insulate meter probes right up to the business end. Saves heartache.
Soldering stations are important. The KSGER stations on AliExpress are great! Never mind the money. Tip selection is very good, avoid conical tips. A screwdriver profile is much better for most work, and you do want a 3mm tip.
Soldering stations are important. The KSGER stations on AliExpress are great! Never mind the money. Tip selection is very good, avoid conical tips. A screwdriver profile is much better for most work, and you do want a 3mm tip.